485BPOS

As filed with the Securities and Exchange Commission on May 17, 2018

Securities Act Registration No. 2-34223

Investment Company Act File No. 811-1829

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM N-1A

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

Post-Effective Amendment No. 109

and

REGISTRATION STATEMENT

UNDER

THE INVESTMENT COMPANY ACT OF 1940

Amendment No. 84

COLUMBIA ACORN TRUST

227 West Monroe Street, Suite 3000

Chicago, Illinois 60606

Telephone number: 312.634.9200

Ryan C. Larrenaga c/o Columbia Management Investment Advisers, LLC 225 Franklin Street Boston, MA 02110	P. Zachary Egan Columbia Acorn Trust 227 West Monroe Street, Suite 3000 Chicago, Illinois 60606	Mary C. Moynihan Perkins Coie LLP 700 13th Street, N.W., Suite 600 Washington, D.C. 20005
(Agents for service)

It is proposed that this filing will become effective:

☒ immediately upon filing pursuant to rule 485(b)

☐ on                      pursuant to rule 485(b)

☐ 60 days after filing pursuant to rule 485(a)(1)

☐ on                      pursuant to rule 485(a)(l)

☐ 75 days after filing pursuant to rule 485(a)(2)

☐ on                      pursuant to rule 485(a)(2).

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933 and the Investment Company Act of 1940, the Registrant certifies that it meets all the requirements for effectiveness of this post-effective amendment pursuant to Rule 485(b) under the Securities Act of 1933 and has duly caused this registration statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Chicago and State of Illinois on May 17, 2018.

COLUMBIA ACORN TRUST
By	/S/    P. ZACHARY EGAN
	P. Zachary Egan, President

Pursuant to the requirements of the Securities Act of 1933, this amendment to the registration statement has been signed below by the following persons in the capacities and on the dates indicated.

Name	Title		Date
/S/    LAURA M. BORN	Trustee and Chair	)
Laura M. Born		)
		)
/S/    MAUREEN M. CULHANE	Trustee	)
Maureen M. Culhane		)
		)
/S/    MARGARET M. EISEN	Trustee	)
Margaret M. Eisen		)
		)
/S/    JOHN C. HEATON	Trustee	)
John C. Heaton		)
		)
/S/    CHARLES R. PHILLIPS	Trustee	)
Charles R. Phillips		)
		)
/S/    DAVID J. RUDIS	Trustee	)	May 17, 2018
David J. Rudis		)
		)
/S/    P. ZACHARY EGAN	Trustee and President (principal executive	)
P. Zachary Egan	officer)	)
		)
/S/    JOHN M. KUNKA	Vice President and Treasurer (principal financial and accounting officer)	) )
John M. Kunka		)

Exhibit Index

Exhibit No.	Description
EX-101.INS	XBRL Instance Document
EX-101.SCH	XBRL Taxonomy Extension Schema Document
EX-101.CAL	XBRL Taxonomy Extension Calculation Linkbase
EX-101.DEF	XBRL Taxonomy Extension Definition Linkbase
EX-101.LAB	XBRL Taxonomy Extension Labels Linkbase
EX-101.PRE	XBRL Taxonomy Extension Presentation Linkbase

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Document Type	dei_DocumentType	485BPOS
Document Period End Date	dei_DocumentPeriodEndDate	Dec. 31, 2017
Registrant Name	dei_EntityRegistrantName	COLUMBIA ACORN TRUST
Central Index Key	dei_EntityCentralIndexKey	0000002110
Amendment Flag	dei_AmendmentFlag	false
Document Creation Date	dei_DocumentCreationDate	Apr. 30, 2018
Document Effective Date	dei_DocumentEffectiveDate	May 01, 2018
Prospectus Date	rr_ProspectusDate	May 01, 2018

XML

Columbia Acorn Emerging Markets Fund

SUMMARY OF THE FUND

Investment Objective

Columbia Acorn Emerging Markets Fund^SM (the Fund) seeks long-term capital appreciation.

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. An investor transacting in a class of Fund shares without any front-end sales charge, contingent deferred sales charge, or other asset-based fee for sales or distribution may be required to pay a commission to the financial intermediary for effecting such transactions. Such commission rates are set by the financial intermediary and are not reflected in the tables or the example below. You may qualify for sales charge discounts if you and members of your immediate family invest, or agree to invest in the future, at least $50,000 in certain classes of shares of eligible funds distributed by Columbia Management Investment Distributors, Inc. (the Distributor). More information is available about these and other sales charge discounts and waivers from your financial intermediary, and can be found in the Choosing a Share Class section beginning on page 24 of the Fund’s prospectus, in Appendix A to the prospectus beginning on page A-1 and in Appendix S to the Statement of Additional Information (SAI) under Sales Charge Waivers beginning on page S-1.

Shareholder Fees (fees paid directly from your investment)

Shareholder Fees - Columbia Acorn Emerging Markets Fund	Class A		Class C		Class Adv	Class Inst	Class Inst2	Class Inst3
Maximum sales charge (load) imposed on purchases (as a % of offering price)	5.75%		none		none	none	none	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	1.00%	[1]	1.00%	[2]	none	none	none	none

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)

Annual Fund Operating Expenses - Columbia Acorn Emerging Markets Fund		Class A	Class Adv	Class C	Class Inst	Class Inst2	Class Inst3
Management fees		1.24%	1.24%	1.24%	1.24%	1.24%	1.24%
Distribution and/or service (12b-1) fees		0.25%	none	1.00%	none	none	none
Other expenses	[1]	0.49%	0.49%	0.49%	0.49%	0.42%	0.36%
Total annual Fund operating expenses		1.98%	1.73%	2.73%	1.73%	1.66%	1.60%
Fee waivers and/or expense reimbursements	[2]	(0.43%)	(0.43%)	(0.43%)	(0.43%)	(0.47%)	(0.46%)
Total annual Fund operating expenses after fee waivers and/or expense reimbursements		1.55%	1.30%	2.30%	1.30%	1.19%	1.14%

Example

The following example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example illustrates the hypothetical expenses that you would incur over the time periods indicated, and assumes that:

• you invest $10,000 in the applicable class of Fund shares for the periods indicated,

• your investment has a 5% return each year, and

• the Fund’s total annual operating expenses remain the same as shown in the Annual Fund Operating Expenses table above.

Since the waivers and/or reimbursements shown in the Annual Fund Operating Expenses table above expire as indicated in the preceding table, they are only reflected in the 1 year example and the first year of the other examples. Although your actual costs may be higher or lower, based on the assumptions listed above, your costs would be:

Expense Example - Columbia Acorn Emerging Markets Fund - USD ($)	1 year	3 years	5 years	10 years
Class A	724	1,121	1,544	2,716
Class Adv	132	503	898	2,005
Class C	333	807	1,407	3,030
Class Inst	132	503	898	2,005
Class Inst2	121	478	858	1,926
Class Inst3	116	460	827	1,861

Expense Example, No Redemption - Columbia Acorn Emerging Markets Fund - USD ($)	1 year	3 years	5 years	10 years
Class A	724	1,121	1,544	2,716
Class Adv	132	503	898	2,005
Class C	233	807	1,407	3,030
Class Inst	132	503	898	2,005
Class Inst2	121	478	858	1,926
Class Inst3	116	460	827	1,861

Portfolio Turnover

The Fund may pay transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 47% of the average value of its portfolio.

Principal Investment Strategies

Under normal circumstances, the Fund invests at least 80% of its net assets (plus any borrowing for investment purposes) in companies located in emerging market countries, including frontier market countries. Emerging market countries are those countries whose economies are developing or emerging from underdevelopment (for example, China, India, Poland and Turkey). Frontier market countries generally have smaller economies and even less developed capital markets than traditional emerging market countries (for example, Vietnam, Colombia, Nigeria and Kazakhstan). For purposes of the Fund's policies, the Fund may invest in a company if (i) it is domiciled in, or the principal trading market for its securities is in, an emerging market country, (ii) it derives 50% or more of its economic value from goods produced, sales made or services performed or has at least 50% of its assets in an emerging market country or countries or (iii) it is a holding company that predominantly holds shares in such companies. The Fund may invest in a variety of countries, industries and sectors and does not attempt to invest a specific percentage of its assets in any given country, industry or sector.

Under normal circumstances, the Fund (i) invests a majority of its net assets in the common stock of small- and mid-sized companies with market capitalizations under $10 billion at the time of initial investment (“Focus Stocks”) and (ii) may also invest in companies with market capitalizations above $10 billion, provided that immediately after that investment a majority of the Fund’s net assets would be invested in Focus Stocks. The Fund may continue to hold, and make additional investments in, Focus Stocks whose market capitalizations have grown to exceed $10 billion, regardless of whether the Fund’s investments in Focus Stocks are a majority of the Fund’s net assets.

Columbia Wanger Asset Management, LLC, the Fund's investment adviser (the Investment Manager), believes that stocks of small- and mid-sized companies, which generally are not as well known by financial analysts as larger companies, may offer higher return potential than stocks of larger companies.

The Fund takes advantage of the Investment Manager's research and stock-picking capabilities to initially invest in a limited number of companies (generally under 100), offering the potential to provide above-average growth over time.

Generally, the Investment Manager will determine which countries are emerging market countries by reference to the countries included in the MSCI Emerging Markets SMID Cap Index (Net). In addition, the Fund may invest in certain developing market countries that are not included in the MSCI Emerging Markets SMID Cap Index (Net), but which are included on another independent third-party listing of emerging market and/or frontier market countries. The Investment Manager will make all determinations as to whether a company is an emerging market company at the time of investment.

The Investment Manager typically seeks companies with:

• A strong business franchise that offers growth potential.

• Products and services in which the company has a competitive advantage.

• A stock price the Investment Manager believes is reasonable relative to the assets and earning power of the company.

The Investment Manager may sell a portfolio holding if the security reaches the Investment Manager's price target, if the company has a deterioration of fundamentals, such as failing to meet key operating benchmarks, or if the Investment Manager believes other securities are more attractive. The Investment Manager also may sell a portfolio holding to fund redemptions.

Principal Risks

An investment in the Fund involves risks, including those described below. There is no assurance that the Fund will achieve its investment objective and you may lose money. The value of the Fund’s holdings may decline, and the Fund’s net asset value (NAV) and share price may go down. An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.

Active Management Risk. The Investment Manager’s active management of the Fund could cause the Fund to underperform its benchmark index and/or other funds with similar investment objectives and/or strategies.

Liquidity and Trading Volume Risk. Due to market conditions, including uncertainty regarding the price of a security, it may be difficult for the Fund to buy or sell portfolio securities at a desirable time or price, which could result in investment losses. This risk of portfolio illiquidity is heightened with respect to small- and mid-cap securities, generally, and foreign small- and mid-cap securities in particular. The Fund may have to lower the selling price, liquidate other investments, or forego another, more appealing investment opportunity as a result of illiquidity in the markets. As a result of significant and sustained reductions in emerging and developed international market trading volumes in the wake of the 2007-2009 financial crisis, it may take longer to buy or sell securities, which can exacerbate the Fund’s exposure to volatile markets. The Fund may also be limited in its ability to execute favorable trades in portfolio securities in response to changes in share prices and fundamentals, and may be forced to dispose of securities under disadvantageous circumstances and at a loss. As the Fund grows in size or, conversely, if it faces significant redemption pressure, these considerations take on increasing significance and may adversely impact performance.

Foreign Securities Risk. Investments in or exposure to foreign securities involve certain risks not associated with investments in or exposure to securities of U.S. companies. Foreign securities subject the Fund to the risks associated with investing in the particular country of an issuer, including political, regulatory, economic, social, diplomatic and other conditions or events (including, for example, military confrontations, war and terrorism), occurring in the country or region, as well as risks associated with less developed custody and settlement practices. Foreign securities may be more volatile and less liquid than securities of U.S. companies, and are subject to the risks associated with potential imposition of economic and other sanctions against a particular foreign country, its nationals or industries or businesses within the country. In addition, foreign governments may impose withholding or other taxes on the Fund’s income, capital gains or proceeds from the disposition of foreign securities, which could reduce the Fund’s return on such securities.

Operational and Settlement Risks of Foreign Securities. The Fund’s foreign securities are generally held outside the United States in the primary market for the securities in the custody of foreign sub-custodians. Some countries have limited governmental oversight and regulation, which increases the risk of corruption and fraud and the possibility of losses to the Fund. In particular, under certain circumstances, foreign securities may settle on a delayed delivery basis, meaning that the Fund may be required to make payment for securities before the Fund has actually received delivery of the securities or deliver securities prior to the receipt of payment. As a result, there is a risk that the security will not be delivered to the Fund or that payment will not be received. Losses can also result from lost, stolen or counterfeit securities; defaults by brokers and banks; failures or defects of the settlement system; or poor and improper record keeping by registrars and issuers.

Share Blocking. In certain non-U.S. markets, an issuer’s securities are blocked from trading for a specified number of days before and, in certain instances, after a shareholder meeting. The blocking period can last up to several weeks. Share blocking may prevent the Fund from buying or selling securities during this period. As a consequence of these restrictions, the Investment Manager, on behalf of the Fund, may abstain from voting proxies in markets that require share blocking.

Emerging Market Securities Risk. Securities issued by foreign governments or companies in emerging market countries, such as China, Russia and certain countries in Eastern Europe, the Middle East, Asia, Latin America or Africa, are more likely to have greater exposure to the risks of investing in foreign securities that are described in Foreign Securities Risk. In addition, emerging market countries are more likely to experience instability resulting, for example, from rapid changes or developments in social, political, economic or other conditions. Their economies are usually less mature and their securities markets are typically less developed with more limited trading activity (i.e., lower trading volumes and less liquidity) than more developed countries. Emerging market securities tend to be more volatile than securities in more developed markets. Many emerging market countries are heavily dependent on international trade and have fewer trading partners, which makes them more sensitive to world commodity prices and economic downturns in other countries, and some have a higher risk of currency devaluations.

Operational and Settlement Risks of Securities in Emerging Markets. Foreign sub-custodians in emerging markets may be recently organized, lack extensive operating experience or lack effective government oversight or regulation. In addition, there may be legal restrictions or limitations on the ability of the Fund to recover assets held in custody by a foreign sub-custodian in the event of the bankruptcy of the sub-custodian. There may also be a greater risk that settlement may be delayed and that cash or securities of the Fund may be lost because of failures of or defects in the system, including fraud or corruption. Settlement systems in emerging markets also have a higher risk of failed trades.

Risks Related to Currencies and Corporate Actions in Emerging Markets. Risks related to currencies and corporate actions are also greater in emerging market countries than in developed countries. Emerging market currencies may not have an active trading market and are subject to a higher risk of currency devaluations.

Risks Related to Corporate and Securities Laws in Emerging Markets. Securities laws in emerging markets may be relatively new and unsettled and, consequently, there is a risk of rapid and unpredictable change in laws regarding foreign investment, securities regulation, title to securities and shareholder rights.

Frontier Market Risk. Frontier market countries generally have smaller economies and even less developed capital markets than traditional emerging market countries (which themselves have increased investment risk relative to more developed market countries) and, as a result, the Fund’s exposure to the risks associated with investing in emerging market countries are magnified when the Fund invests in frontier market countries. Increased risks include: the potential for extreme price volatility and illiquidity in frontier market countries; government ownership or control of parts of the private sector and of certain companies; trade barriers, exchange controls, managed adjustments in relative currency values and other protectionist and similar measures imposed or negotiated by the countries with which frontier market countries trade; and the relatively new and unsettled securities laws in many frontier market countries.

Foreign Currency Risk. The performance of the Fund may be materially affected positively or negatively by foreign currency strength or weakness relative to the U.S. dollar, particularly if the Fund invests a significant percentage of its assets in foreign securities or other assets denominated in currencies other than the U.S. dollar.

Geographic Focus Risk. The Fund may be particularly susceptible to economic, political, regulatory or other events or conditions affecting issuers and countries within the specific geographic regions in which the Fund invests. The Fund’s NAV may be more volatile than the NAV of a more geographically diversified fund.

Asia Pacific Region. Many of the countries in the Asia Pacific region are considered underdeveloped or developing, including from a political, economic and/or social perspective, and may have relatively unstable governments and economies based on limited business, industries and/or natural resources or commodities. Events in any one country within the region may impact other countries in the region or the region as a whole. As a result, events in the region will generally have a greater effect on the Fund than if the Fund were more geographically diversified. This could result in increased volatility in the value of the Fund’s investments and losses for the Fund. Also, securities of some companies in the region can be less liquid than U.S. or other foreign securities, potentially making it difficult for the Fund to sell such securities at a desirable time and price.

Growth Securities Risk. Growth securities typically trade at a higher multiple of earnings than other types of equity securities. Accordingly, the market values of growth securities may never reach their expected market value and may decline in price. In addition, growth securities, at times, may not perform as well as value securities or the stock market in general, and may be out of favor with investors for varying periods of time.

Small- and Mid-Cap Company Securities Risk. Investments in small- and mid-capitalization companies (small- and mid-cap companies) often involve greater risks than investments in larger, more established companies (larger companies) because small- and mid-cap companies tend to have less predictable earnings and may lack the management experience, financial resources, product diversification and competitive strengths of larger companies. Securities of small- and mid-cap companies may be less liquid and more volatile than the securities of larger companies.

Sector Risk. At times, the Fund may have a significant portion of its assets invested in securities of companies conducting business in a related group of industries within a sector. Companies in the same sector may be similarly affected by economic, regulatory, political or market events or conditions, which may make the Fund more vulnerable to unfavorable developments in that sector than funds that invest more broadly. Generally, the more the Fund diversifies its investments, the more it spreads risk and potentially reduces the risks of loss and volatility.

Issuer Risk. An issuer in which the Fund invests may perform poorly, and the value of its securities may therefore decline, which would negatively affect the Fund’s performance. Poor performance may be caused by poor management decisions, competitive pressures, breakthroughs in technology, reliance on suppliers, labor problems or shortages, corporate restructurings, fraudulent disclosures, natural disasters or other events, conditions or factors.

Market Risk. Market risk refers to the possibility that the market values of securities or other investments that the Fund holds will fall, sometimes rapidly or unpredictably, or fail to rise. An investment in the Fund could lose money over short or long periods.

Performance Information

The following bar chart and table show you how the Fund has performed in the past, and can help you understand the risks of investing in the Fund. The bar chart shows how the Fund’s Class Inst share performance has varied for each full calendar year shown. The table below the bar chart compares the Fund’s returns for the periods shown with the MSCI Emerging Markets SMID Cap Index (Net). The MSCI Emerging Markets SMID Cap Index (Net) captures mid- and small-cap representation across 24 emerging market countries. It has 2,204 constituents (as of March 31, 2018) and covers approximately 29% of the free float adjusted market capitalization in each country.

The performance of one or more share classes shown in the Average Annual Total Returns table below includes the Fund’s Class Inst share returns (adjusted to reflect the higher class-related operating expenses of such classes, where applicable) for periods prior to the indicated inception date of such share classes. Except for differences in fees and expenses, all share classes of the Fund would have substantially similar annual returns because all share classes of the Fund invest in the same portfolio of securities.

The after-tax returns shown in the Average Annual Total Returns table below are calculated using the highest historical individual U.S. federal marginal income tax rates in effect during the period indicated in the table and do not reflect the impact of state, local or foreign taxes. Your actual after-tax returns will depend on your personal tax situation and may differ from those shown in the table. In addition, the after-tax returns shown in the table do not apply to shares held in tax-advantaged accounts such as 401(k) plans or Individual Retirement Accounts (IRAs). The after-tax returns are shown only for Class Inst shares and will vary for other share classes.

The Fund’s past performance (before and after taxes) is no guarantee of how the Fund will perform in the future. Updated performance information can be obtained by calling toll-free 800.345.6611 or visiting columbiathreadneedle.com/us.

Year by Year Total Return (%)
as of December 31 Each Year

Best and Worst Quarterly Returns
During the Period Shown in the Bar Chart

Best                                                      1st Quarter 2012                                                      18.00%

Worst                                                   3rd Quarter 2015                                                     -17.21%

Average Annual Total Returns After Applicable Sales Charges (for periods ended December 31, 2017)

Average Annual Total Returns - Columbia Acorn Emerging Markets Fund	Share Class Inception Date	1 Year	5 Years	Life of Fund
Class Inst	Aug. 19, 2011	34.92%	2.86%	5.47%
Class Inst | returns after taxes on distributions	Aug. 19, 2011	35.06%	2.76%	5.38%
Class Inst | returns after taxes on distributions and sale of Fund shares	Aug. 19, 2011	19.90%	2.30%	4.39%
Class A	Aug. 19, 2011	26.82%	1.43%	4.21%
Class Adv	Nov. 08, 2012	34.88%	2.92%	5.51%
Class C	Aug. 19, 2011	32.57%	1.87%	4.43%
Class Inst2	Nov. 08, 2012	35.02%	2.96%	5.55%
Class Inst3	Jun. 13, 2013	35.13%	3.00%	5.58%
MSCI Emerging Markets SMID Cap Index (Net) (reflects reinvested dividends net of withholding taxes but reflects no deductions for fees, expenses or other taxes)		34.53%	4.21%	3.73%

[1] This charge is imposed on certain investments of between $1 million and $50 million redeemed within 18 months after purchase, as follows: 1.00% if redeemed within 12 months after purchase, and 0.50% if redeemed more than 12, but less than 18, months after purchase, with certain limited exceptions.

[2] This charge applies to redemptions within 12 months after purchase, with certain limited exceptions.

[1] Other expenses have been restated to reflect current transfer agency fees paid by the Fund.

[2] Columbia Wanger Asset Management, LLC (the Investment Manager) has contractually agreed to waive fees and reimburse certain expenses of the Fund, through June 30, 2019, so that ordinary operating expenses (excluding transaction costs and certain other investment-related expenses, interest and fees on borrowings and expenses associated with the Fund’s investment in other investment companies, if any) do not exceed the annual rates of 1.55% for Class A shares, 1.30% for Class Adv shares, 2.30% for Class C shares, 1.30% for Class Inst shares, 1.19% for Class Inst2 shares and 1.14% for Class Inst3 shares. This arrangement may only be amended or terminated with approval from the Fund’s Board of Trustees and the Investment Manager. The fee waivers and/or expense reimbursements shown in the table also reflect the contractual agreement of the Fund’s transfer agent, Columbia Management Investment Services Corp. (the Transfer Agent), to waive a portion of its fees through June 30, 2019, such that the Fund’s transfer agency fees do not exceed the annual rates of 0.05% of the average daily net assets of Class Inst2 shares and 0.00% of the average daily net assets of Class Inst3 shares.

[1] Year to Date return as of March 31, 2018: 1.73%

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	COLUMBIA ACORN TRUST
Prospectus Date	rr_ProspectusDate	May 01, 2018
Columbia Acorn Emerging Markets Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	SUMMARY OF THE FUND
Objective [Heading]	rr_ObjectiveHeading	Investment Objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	Columbia Acorn Emerging Markets FundSM (the Fund) seeks long-term capital appreciation.
Expense [Heading]	rr_ExpenseHeading	Fees and Expenses of the Fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. An investor transacting in a class of Fund shares without any front-end sales charge, contingent deferred sales charge, or other asset-based fee for sales or distribution may be required to pay a commission to the financial intermediary for effecting such transactions. Such commission rates are set by the financial intermediary and are not reflected in the tables or the example below. You may qualify for sales charge discounts if you and members of your immediate family invest, or agree to invest in the future, at least $50,000 in certain classes of shares of eligible funds distributed by Columbia Management Investment Distributors, Inc. (the Distributor). More information is available about these and other sales charge discounts and waivers from your financial intermediary, and can be found in the Choosing a Share Class section beginning on page 24 of the Fund’s prospectus, in Appendix A to the prospectus beginning on page A-1 and in Appendix S to the Statement of Additional Information (SAI) under Sales Charge Waivers beginning on page S-1.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder Fees (fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	June 30, 2019
Portfolio Turnover [Heading]	rr_PortfolioTurnoverHeading	Portfolio Turnover
Portfolio Turnover [Text Block]	rr_PortfolioTurnoverTextBlock	The Fund may pay transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 47% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	47.00%
Expense Breakpoint Discounts [Text]	rr_ExpenseBreakpointDiscounts	You may qualify for sales charge discounts if you and members of your immediate family invest, or agree to invest in the future, at least $50,000 in certain classes of shares of eligible funds distributed by Columbia Management Investment Distributors, Inc. (the Distributor). More information is available about these and other sales charge discounts and waivers from your financial intermediary, and can be found in the Choosing a Share Class section beginning on page 24 of the Fund’s prospectus, in Appendix A to the prospectus beginning on page A-1 and in Appendix S to the Statement of Additional Information (SAI) under Sales Charge Waivers beginning on page S-1.
Expense Breakpoint, Minimum Investment Required [Amount]	rr_ExpenseBreakpointMinimumInvestmentRequiredAmount	$ 50,000
Expenses Restated to Reflect Current [Text]	rr_ExpensesRestatedToReflectCurrent	Other expenses have been restated to reflect current transfer agency fees paid by the Fund.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	The following example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example illustrates the hypothetical expenses that you would incur over the time periods indicated, and assumes that: you invest $10,000 in the applicable class of Fund shares for the periods indicated, your investment has a 5% return each year, and the Fund’s total annual operating expenses remain the same as shown in the Annual Fund Operating Expenses table above. Since the waivers and/or reimbursements shown in the Annual Fund Operating Expenses table above expire as indicated in the preceding table, they are only reflected in the 1 year example and the first year of the other examples. Although your actual costs may be higher or lower, based on the assumptions listed above, your costs would be:
Strategy [Heading]	rr_StrategyHeading	Principal Investment Strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	Under normal circumstances, the Fund invests at least 80% of its net assets (plus any borrowing for investment purposes) in companies located in emerging market countries, including frontier market countries. Emerging market countries are those countries whose economies are developing or emerging from underdevelopment (for example, China, India, Poland and Turkey). Frontier market countries generally have smaller economies and even less developed capital markets than traditional emerging market countries (for example, Vietnam, Colombia, Nigeria and Kazakhstan). For purposes of the Fund's policies, the Fund may invest in a company if (i) it is domiciled in, or the principal trading market for its securities is in, an emerging market country, (ii) it derives 50% or more of its economic value from goods produced, sales made or services performed or has at least 50% of its assets in an emerging market country or countries or (iii) it is a holding company that predominantly holds shares in such companies. The Fund may invest in a variety of countries, industries and sectors and does not attempt to invest a specific percentage of its assets in any given country, industry or sector. Under normal circumstances, the Fund (i) invests a majority of its net assets in the common stock of small- and mid-sized companies with market capitalizations under $10 billion at the time of initial investment (“Focus Stocks”) and (ii) may also invest in companies with market capitalizations above $10 billion, provided that immediately after that investment a majority of the Fund’s net assets would be invested in Focus Stocks. The Fund may continue to hold, and make additional investments in, Focus Stocks whose market capitalizations have grown to exceed $10 billion, regardless of whether the Fund’s investments in Focus Stocks are a majority of the Fund’s net assets. Columbia Wanger Asset Management, LLC, the Fund's investment adviser (the Investment Manager), believes that stocks of small- and mid-sized companies, which generally are not as well known by financial analysts as larger companies, may offer higher return potential than stocks of larger companies. The Fund takes advantage of the Investment Manager's research and stock-picking capabilities to initially invest in a limited number of companies (generally under 100), offering the potential to provide above-average growth over time. Generally, the Investment Manager will determine which countries are emerging market countries by reference to the countries included in the MSCI Emerging Markets SMID Cap Index (Net). In addition, the Fund may invest in certain developing market countries that are not included in the MSCI Emerging Markets SMID Cap Index (Net), but which are included on another independent third-party listing of emerging market and/or frontier market countries. The Investment Manager will make all determinations as to whether a company is an emerging market company at the time of investment. The Investment Manager typically seeks companies with: A strong business franchise that offers growth potential. Products and services in which the company has a competitive advantage. A stock price the Investment Manager believes is reasonable relative to the assets and earning power of the company. The Investment Manager may sell a portfolio holding if the security reaches the Investment Manager's price target, if the company has a deterioration of fundamentals, such as failing to meet key operating benchmarks, or if the Investment Manager believes other securities are more attractive. The Investment Manager also may sell a portfolio holding to fund redemptions.
Risk [Heading]	rr_RiskHeading	Principal Risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the Fund involves risks, including those described below. There is no assurance that the Fund will achieve its investment objective and you may lose money. The value of the Fund’s holdings may decline, and the Fund’s net asset value (NAV) and share price may go down. An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Active Management Risk. The Investment Manager’s active management of the Fund could cause the Fund to underperform its benchmark index and/or other funds with similar investment objectives and/or strategies. Liquidity and Trading Volume Risk. Due to market conditions, including uncertainty regarding the price of a security, it may be difficult for the Fund to buy or sell portfolio securities at a desirable time or price, which could result in investment losses. This risk of portfolio illiquidity is heightened with respect to small- and mid-cap securities, generally, and foreign small- and mid-cap securities in particular. The Fund may have to lower the selling price, liquidate other investments, or forego another, more appealing investment opportunity as a result of illiquidity in the markets. As a result of significant and sustained reductions in emerging and developed international market trading volumes in the wake of the 2007-2009 financial crisis, it may take longer to buy or sell securities, which can exacerbate the Fund’s exposure to volatile markets. The Fund may also be limited in its ability to execute favorable trades in portfolio securities in response to changes in share prices and fundamentals, and may be forced to dispose of securities under disadvantageous circumstances and at a loss. As the Fund grows in size or, conversely, if it faces significant redemption pressure, these considerations take on increasing significance and may adversely impact performance. Foreign Securities Risk. Investments in or exposure to foreign securities involve certain risks not associated with investments in or exposure to securities of U.S. companies. Foreign securities subject the Fund to the risks associated with investing in the particular country of an issuer, including political, regulatory, economic, social, diplomatic and other conditions or events (including, for example, military confrontations, war and terrorism), occurring in the country or region, as well as risks associated with less developed custody and settlement practices. Foreign securities may be more volatile and less liquid than securities of U.S. companies, and are subject to the risks associated with potential imposition of economic and other sanctions against a particular foreign country, its nationals or industries or businesses within the country. In addition, foreign governments may impose withholding or other taxes on the Fund’s income, capital gains or proceeds from the disposition of foreign securities, which could reduce the Fund’s return on such securities. Operational and Settlement Risks of Foreign Securities. The Fund’s foreign securities are generally held outside the United States in the primary market for the securities in the custody of foreign sub-custodians. Some countries have limited governmental oversight and regulation, which increases the risk of corruption and fraud and the possibility of losses to the Fund. In particular, under certain circumstances, foreign securities may settle on a delayed delivery basis, meaning that the Fund may be required to make payment for securities before the Fund has actually received delivery of the securities or deliver securities prior to the receipt of payment. As a result, there is a risk that the security will not be delivered to the Fund or that payment will not be received. Losses can also result from lost, stolen or counterfeit securities; defaults by brokers and banks; failures or defects of the settlement system; or poor and improper record keeping by registrars and issuers. Share Blocking. In certain non-U.S. markets, an issuer’s securities are blocked from trading for a specified number of days before and, in certain instances, after a shareholder meeting. The blocking period can last up to several weeks. Share blocking may prevent the Fund from buying or selling securities during this period. As a consequence of these restrictions, the Investment Manager, on behalf of the Fund, may abstain from voting proxies in markets that require share blocking. Emerging Market Securities Risk. Securities issued by foreign governments or companies in emerging market countries, such as China, Russia and certain countries in Eastern Europe, the Middle East, Asia, Latin America or Africa, are more likely to have greater exposure to the risks of investing in foreign securities that are described in Foreign Securities Risk. In addition, emerging market countries are more likely to experience instability resulting, for example, from rapid changes or developments in social, political, economic or other conditions. Their economies are usually less mature and their securities markets are typically less developed with more limited trading activity (i.e., lower trading volumes and less liquidity) than more developed countries. Emerging market securities tend to be more volatile than securities in more developed markets. Many emerging market countries are heavily dependent on international trade and have fewer trading partners, which makes them more sensitive to world commodity prices and economic downturns in other countries, and some have a higher risk of currency devaluations. Operational and Settlement Risks of Securities in Emerging Markets. Foreign sub-custodians in emerging markets may be recently organized, lack extensive operating experience or lack effective government oversight or regulation. In addition, there may be legal restrictions or limitations on the ability of the Fund to recover assets held in custody by a foreign sub-custodian in the event of the bankruptcy of the sub-custodian. There may also be a greater risk that settlement may be delayed and that cash or securities of the Fund may be lost because of failures of or defects in the system, including fraud or corruption. Settlement systems in emerging markets also have a higher risk of failed trades. Risks Related to Currencies and Corporate Actions in Emerging Markets. Risks related to currencies and corporate actions are also greater in emerging market countries than in developed countries. Emerging market currencies may not have an active trading market and are subject to a higher risk of currency devaluations. Risks Related to Corporate and Securities Laws in Emerging Markets. Securities laws in emerging markets may be relatively new and unsettled and, consequently, there is a risk of rapid and unpredictable change in laws regarding foreign investment, securities regulation, title to securities and shareholder rights. Frontier Market Risk. Frontier market countries generally have smaller economies and even less developed capital markets than traditional emerging market countries (which themselves have increased investment risk relative to more developed market countries) and, as a result, the Fund’s exposure to the risks associated with investing in emerging market countries are magnified when the Fund invests in frontier market countries. Increased risks include: the potential for extreme price volatility and illiquidity in frontier market countries; government ownership or control of parts of the private sector and of certain companies; trade barriers, exchange controls, managed adjustments in relative currency values and other protectionist and similar measures imposed or negotiated by the countries with which frontier market countries trade; and the relatively new and unsettled securities laws in many frontier market countries. Foreign Currency Risk. The performance of the Fund may be materially affected positively or negatively by foreign currency strength or weakness relative to the U.S. dollar, particularly if the Fund invests a significant percentage of its assets in foreign securities or other assets denominated in currencies other than the U.S. dollar. Geographic Focus Risk. The Fund may be particularly susceptible to economic, political, regulatory or other events or conditions affecting issuers and countries within the specific geographic regions in which the Fund invests. The Fund’s NAV may be more volatile than the NAV of a more geographically diversified fund. Asia Pacific Region. Many of the countries in the Asia Pacific region are considered underdeveloped or developing, including from a political, economic and/or social perspective, and may have relatively unstable governments and economies based on limited business, industries and/or natural resources or commodities. Events in any one country within the region may impact other countries in the region or the region as a whole. As a result, events in the region will generally have a greater effect on the Fund than if the Fund were more geographically diversified. This could result in increased volatility in the value of the Fund’s investments and losses for the Fund. Also, securities of some companies in the region can be less liquid than U.S. or other foreign securities, potentially making it difficult for the Fund to sell such securities at a desirable time and price. Growth Securities Risk. Growth securities typically trade at a higher multiple of earnings than other types of equity securities. Accordingly, the market values of growth securities may never reach their expected market value and may decline in price. In addition, growth securities, at times, may not perform as well as value securities or the stock market in general, and may be out of favor with investors for varying periods of time. Small- and Mid-Cap Company Securities Risk. Investments in small- and mid-capitalization companies (small- and mid-cap companies) often involve greater risks than investments in larger, more established companies (larger companies) because small- and mid-cap companies tend to have less predictable earnings and may lack the management experience, financial resources, product diversification and competitive strengths of larger companies. Securities of small- and mid-cap companies may be less liquid and more volatile than the securities of larger companies. Sector Risk. At times, the Fund may have a significant portion of its assets invested in securities of companies conducting business in a related group of industries within a sector. Companies in the same sector may be similarly affected by economic, regulatory, political or market events or conditions, which may make the Fund more vulnerable to unfavorable developments in that sector than funds that invest more broadly. Generally, the more the Fund diversifies its investments, the more it spreads risk and potentially reduces the risks of loss and volatility. Issuer Risk. An issuer in which the Fund invests may perform poorly, and the value of its securities may therefore decline, which would negatively affect the Fund’s performance. Poor performance may be caused by poor management decisions, competitive pressures, breakthroughs in technology, reliance on suppliers, labor problems or shortages, corporate restructurings, fraudulent disclosures, natural disasters or other events, conditions or factors. Market Risk. Market risk refers to the possibility that the market values of securities or other investments that the Fund holds will fall, sometimes rapidly or unpredictably, or fail to rise. An investment in the Fund could lose money over short or long periods.
Risk Lose Money [Text]	rr_RiskLoseMoney	There is no assurance that the Fund will achieve its investment objective and you may lose money.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance Information
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The following bar chart and table show you how the Fund has performed in the past, and can help you understand the risks of investing in the Fund. The bar chart shows how the Fund’s Class Inst share performance has varied for each full calendar year shown. The table below the bar chart compares the Fund’s returns for the periods shown with the MSCI Emerging Markets SMID Cap Index (Net). The MSCI Emerging Markets SMID Cap Index (Net) captures mid- and small-cap representation across 24 emerging market countries. It has 2,204 constituents (as of March 31, 2018) and covers approximately 29% of the free float adjusted market capitalization in each country. The performance of one or more share classes shown in the Average Annual Total Returns table below includes the Fund’s Class Inst share returns (adjusted to reflect the higher class-related operating expenses of such classes, where applicable) for periods prior to the indicated inception date of such share classes. Except for differences in fees and expenses, all share classes of the Fund would have substantially similar annual returns because all share classes of the Fund invest in the same portfolio of securities. The after-tax returns shown in the Average Annual Total Returns table below are calculated using the highest historical individual U.S. federal marginal income tax rates in effect during the period indicated in the table and do not reflect the impact of state, local or foreign taxes. Your actual after-tax returns will depend on your personal tax situation and may differ from those shown in the table. In addition, the after-tax returns shown in the table do not apply to shares held in tax-advantaged accounts such as 401(k) plans or Individual Retirement Accounts (IRAs). The after-tax returns are shown only for Class Inst shares and will vary for other share classes. The Fund’s past performance (before and after taxes) is no guarantee of how the Fund will perform in the future. Updated performance information can be obtained by calling toll-free 800.345.6611 or visiting columbiathreadneedle.com/us.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The following bar chart and table show you how the Fund has performed in the past, and can help you understand the risks of investing in the Fund. The bar chart shows how the Fund’s Class Inst share performance has varied for each full calendar year shown. The table below the bar chart compares the Fund’s returns for the periods shown with the MSCI Emerging Markets SMID Cap Index (Net). The MSCI Emerging Markets SMID Cap Index (Net) captures mid- and small-cap representation across 24 emerging market countries. It has 2,204 constituents (as of March 31, 2018) and covers approximately 29% of the free float adjusted market capitalization in each country.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	800.345.6611
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	columbiathreadneedle.com/us
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The Fund’s past performance (before and after taxes) is no guarantee of how the Fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Year by Year Total Return (%) as of December 31 Each Year
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Best and Worst Quarterly Returns During the Period Shown in the Bar Chart Best                                                      1st Quarter 2012                                                      18.00% Worst                                                   3rd Quarter 2015                                                     -17.21%
Performance Table Heading	rr_PerformanceTableHeading	Average Annual Total Returns After Applicable Sales Charges (for periods ended December 31, 2017)
Performance Table Uses Highest Federal Rate	rr_PerformanceTableUsesHighestFederalRate	The after-tax returns shown in the Average Annual Total Returns table below are calculated using the highest historical individual U.S. federal marginal income tax rates in effect during the period indicated in the table and do not reflect the impact of state, local or foreign taxes.
Performance Table Not Relevant to Tax Deferred	rr_PerformanceTableNotRelevantToTaxDeferred	In addition, the after-tax returns shown in the table do not apply to shares held in tax-advantaged accounts such as 401(k) plans or Individual Retirement Accounts (IRAs).
Performance Table One Class of after Tax Shown [Text]	rr_PerformanceTableOneClassOfAfterTaxShown	The after-tax returns are shown only for Class Inst shares and will vary for other share classes.
Columbia Acorn Emerging Markets Fund | Class A
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	5.75%
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	1.00%	[1]
Management fees	rr_ManagementFeesOverAssets	1.24%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Other expenses	rr_OtherExpensesOverAssets	0.49%	[2]
Total annual Fund operating expenses	rr_ExpensesOverAssets	1.98%
Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.43%)	[3]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	1.55%
Expenses Deferred Charges [Text Block]	rr_ExpensesDeferredChargesTextBlock	This charge is imposed on certain investments of between $1 million and $50 million redeemed within 18 months after purchase, as follows: 1.00% if redeemed within 12 months after purchase, and 0.50% if redeemed more than 12, but less than 18, months after purchase, with certain limited exceptions.
1 year	rr_ExpenseExampleYear01	$ 724
3 years	rr_ExpenseExampleYear03	1,121
5 years	rr_ExpenseExampleYear05	1,544
10 years	rr_ExpenseExampleYear10	2,716
1 year	rr_ExpenseExampleNoRedemptionYear01	724
3 years	rr_ExpenseExampleNoRedemptionYear03	1,121
5 years	rr_ExpenseExampleNoRedemptionYear05	1,544
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 2,716
1 Year	rr_AverageAnnualReturnYear01	26.82%
5 Years	rr_AverageAnnualReturnYear05	1.43%
Life of Fund	rr_AverageAnnualReturnSinceInception	4.21%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Aug. 19, 2011
Columbia Acorn Emerging Markets Fund | Class Adv
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	1.24%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.49%	[2]
Total annual Fund operating expenses	rr_ExpensesOverAssets	1.73%
Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.43%)	[3]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	1.30%
1 year	rr_ExpenseExampleYear01	$ 132
3 years	rr_ExpenseExampleYear03	503
5 years	rr_ExpenseExampleYear05	898
10 years	rr_ExpenseExampleYear10	2,005
1 year	rr_ExpenseExampleNoRedemptionYear01	132
3 years	rr_ExpenseExampleNoRedemptionYear03	503
5 years	rr_ExpenseExampleNoRedemptionYear05	898
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 2,005
1 Year	rr_AverageAnnualReturnYear01	34.88%
5 Years	rr_AverageAnnualReturnYear05	2.92%
Life of Fund	rr_AverageAnnualReturnSinceInception	5.51%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 08, 2012
Columbia Acorn Emerging Markets Fund | Class C
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	1.00%	[4]
Management fees	rr_ManagementFeesOverAssets	1.24%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	1.00%
Other expenses	rr_OtherExpensesOverAssets	0.49%	[2]
Total annual Fund operating expenses	rr_ExpensesOverAssets	2.73%
Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.43%)	[3]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	2.30%
Expenses Deferred Charges [Text Block]	rr_ExpensesDeferredChargesTextBlock	This charge applies to redemptions within 12 months after purchase, with certain limited exceptions.
1 year	rr_ExpenseExampleYear01	$ 333
3 years	rr_ExpenseExampleYear03	807
5 years	rr_ExpenseExampleYear05	1,407
10 years	rr_ExpenseExampleYear10	3,030
1 year	rr_ExpenseExampleNoRedemptionYear01	233
3 years	rr_ExpenseExampleNoRedemptionYear03	807
5 years	rr_ExpenseExampleNoRedemptionYear05	1,407
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 3,030
1 Year	rr_AverageAnnualReturnYear01	32.57%
5 Years	rr_AverageAnnualReturnYear05	1.87%
Life of Fund	rr_AverageAnnualReturnSinceInception	4.43%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Aug. 19, 2011
Columbia Acorn Emerging Markets Fund | Class Inst
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	1.24%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.49%	[2]
Total annual Fund operating expenses	rr_ExpensesOverAssets	1.73%
Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.43%)	[3]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	1.30%
1 year	rr_ExpenseExampleYear01	$ 132
3 years	rr_ExpenseExampleYear03	503
5 years	rr_ExpenseExampleYear05	898
10 years	rr_ExpenseExampleYear10	2,005
1 year	rr_ExpenseExampleNoRedemptionYear01	132
3 years	rr_ExpenseExampleNoRedemptionYear03	503
5 years	rr_ExpenseExampleNoRedemptionYear05	898
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 2,005
2012	rr_AnnualReturn2012	31.35%	[5]
2013	rr_AnnualReturn2013	11.92%	[5]
2014	rr_AnnualReturn2014	(4.12%)	[5]
2015	rr_AnnualReturn2015	(17.98%)	[5]
2016	rr_AnnualReturn2016	(3.04%)	[5]
2017	rr_AnnualReturn2017	34.92%	[5]
Year to Date Return, Label	rr_YearToDateReturnLabel	Year to Date return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Mar. 31, 2018
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	1.73%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Mar. 31, 2012
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	18.00%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Sep. 30, 2015
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(17.21%)
1 Year	rr_AverageAnnualReturnYear01	34.92%
5 Years	rr_AverageAnnualReturnYear05	2.86%
Life of Fund	rr_AverageAnnualReturnSinceInception	5.47%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Aug. 19, 2011
Columbia Acorn Emerging Markets Fund | Class Inst2
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	1.24%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.42%	[2]
Total annual Fund operating expenses	rr_ExpensesOverAssets	1.66%
Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.47%)	[3]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	1.19%
1 year	rr_ExpenseExampleYear01	$ 121
3 years	rr_ExpenseExampleYear03	478
5 years	rr_ExpenseExampleYear05	858
10 years	rr_ExpenseExampleYear10	1,926
1 year	rr_ExpenseExampleNoRedemptionYear01	121
3 years	rr_ExpenseExampleNoRedemptionYear03	478
5 years	rr_ExpenseExampleNoRedemptionYear05	858
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 1,926
1 Year	rr_AverageAnnualReturnYear01	35.02%
5 Years	rr_AverageAnnualReturnYear05	2.96%
Life of Fund	rr_AverageAnnualReturnSinceInception	5.55%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 08, 2012
Columbia Acorn Emerging Markets Fund | Class Inst3
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	1.24%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.36%	[2]
Total annual Fund operating expenses	rr_ExpensesOverAssets	1.60%
Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.46%)	[3]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	1.14%
1 year	rr_ExpenseExampleYear01	$ 116
3 years	rr_ExpenseExampleYear03	460
5 years	rr_ExpenseExampleYear05	827
10 years	rr_ExpenseExampleYear10	1,861
1 year	rr_ExpenseExampleNoRedemptionYear01	116
3 years	rr_ExpenseExampleNoRedemptionYear03	460
5 years	rr_ExpenseExampleNoRedemptionYear05	827
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 1,861
1 Year	rr_AverageAnnualReturnYear01	35.13%
5 Years	rr_AverageAnnualReturnYear05	3.00%
Life of Fund	rr_AverageAnnualReturnSinceInception	5.58%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Jun. 13, 2013
Columbia Acorn Emerging Markets Fund | returns after taxes on distributions | Class Inst
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	35.06%
5 Years	rr_AverageAnnualReturnYear05	2.76%
Life of Fund	rr_AverageAnnualReturnSinceInception	5.38%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Aug. 19, 2011
Columbia Acorn Emerging Markets Fund | returns after taxes on distributions and sale of Fund shares | Class Inst
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	19.90%
5 Years	rr_AverageAnnualReturnYear05	2.30%
Life of Fund	rr_AverageAnnualReturnSinceInception	4.39%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Aug. 19, 2011
Columbia Acorn Emerging Markets Fund | MSCI Emerging Markets SMID Cap Index (Net) (reflects reinvested dividends net of withholding taxes but reflects no deductions for fees, expenses or other taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	34.53%
5 Years	rr_AverageAnnualReturnYear05	4.21%
Life of Fund	rr_AverageAnnualReturnSinceInception	3.73%

[1] This charge is imposed on certain investments of between $1 million and $50 million redeemed within 18 months after purchase, as follows: 1.00% if redeemed within 12 months after purchase, and 0.50% if redeemed more than 12, but less than 18, months after purchase, with certain limited exceptions.

[2] Other expenses have been restated to reflect current transfer agency fees paid by the Fund.

[3] Columbia Wanger Asset Management, LLC (the Investment Manager) has contractually agreed to waive fees and reimburse certain expenses of the Fund, through June 30, 2019, so that ordinary operating expenses (excluding transaction costs and certain other investment-related expenses, interest and fees on borrowings and expenses associated with the Fund’s investment in other investment companies, if any) do not exceed the annual rates of 1.55% for Class A shares, 1.30% for Class Adv shares, 2.30% for Class C shares, 1.30% for Class Inst shares, 1.19% for Class Inst2 shares and 1.14% for Class Inst3 shares. This arrangement may only be amended or terminated with approval from the Fund’s Board of Trustees and the Investment Manager. The fee waivers and/or expense reimbursements shown in the table also reflect the contractual agreement of the Fund’s transfer agent, Columbia Management Investment Services Corp. (the Transfer Agent), to waive a portion of its fees through June 30, 2019, such that the Fund’s transfer agency fees do not exceed the annual rates of 0.05% of the average daily net assets of Class Inst2 shares and 0.00% of the average daily net assets of Class Inst3 shares.

[4] This charge applies to redemptions within 12 months after purchase, with certain limited exceptions.

[5] Year to Date return as of March 31, 2018: 1.73%

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Columbia Acorn European Fund

SUMMARY OF THE FUND

Investment Objective

Columbia Acorn European Fund^SM (the Fund) seeks long-term capital appreciation.

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. An investor transacting in a class of Fund shares without any front-end sales charge, contingent deferred sales charge, or other asset-based fee for sales or distribution may be required to pay a commission to the financial intermediary for effecting such transactions. Such commission rates are set by the financial intermediary and are not reflected in the tables or the example below. You may qualify for sales charge discounts if you and members of your immediate family invest, or agree to invest in the future, at least $50,000 in certain classes of shares of eligible funds distributed by Columbia Management Investment Distributors, Inc. (the Distributor). More information is available about these and other sales charge discounts and waivers from your financial intermediary, and can be found in the Choosing a Share Class section beginning on page 23 of the Fund’s prospectus, in Appendix A to the prospectus beginning on page A-1 and in Appendix S to the Statement of Additional Information (SAI) under Sales Charge Waivers beginning on page S-1.

Shareholder Fees (fees paid directly from your investment)

Shareholder Fees - Columbia Acorn European Fund	Class A		Class C		Class Adv	Class Inst	Class Inst2	Class Inst3
Maximum sales charge (load) imposed on purchases (as a % of offering price)	5.75%		none		none	none	none	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	1.00%	[1]	1.00%	[2]	none	none	none	none

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)

Annual Fund Operating Expenses - Columbia Acorn European Fund		Class A	Class Adv	Class C	Class Inst	Class Inst2	Class Inst3
Management fees		1.19%	1.19%	1.19%	1.19%	1.19%	1.19%
Distribution and/or service (12b-1) fees		0.25%	none	1.00%	none	none	none
Other expenses	[1]	0.48%	0.48%	0.48%	0.48%	0.43%	0.39%
Acquired fund fees and expenses		0.01%	0.01%	0.01%	0.01%	0.01%	0.01%
Total annual Fund operating expenses	[2]	1.93%	1.68%	2.68%	1.68%	1.63%	1.59%
Fee waivers and/or expense reimbursements	[3]	(0.47%)	(0.47%)	(0.47%)	(0.47%)	(0.49%)	(0.50%)
Total annual Fund operating expenses after fee waivers and/or expense reimbursements		1.46%	1.21%	2.21%	1.21%	1.14%	1.09%

Example

The following example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example illustrates the hypothetical expenses that you would incur over the time periods indicated, and assumes that:

• you invest $10,000 in the applicable class of Fund shares for the periods indicated,

• your investment has a 5% return each year, and

• the Fund’s total annual operating expenses remain the same as shown in the Annual Fund Operating Expenses table above.

Since the waivers and/or reimbursements shown in the Annual Fund Operating Expenses table above expire as indicated in the preceding table, they are only reflected in the 1 year example and the first year of the other examples. Although your actual costs may be higher or lower, based on the assumptions listed above, your costs would be:

Expense Example - Columbia Acorn European Fund - USD ($)	1 year	3 years	5 years	10 years
Class A	715	1,103	1,516	2,663
Class Adv	123	484	868	1,948
Class C	324	788	1,378	2,978
Class Inst	123	484	868	1,948
Class Inst2	116	466	840	1,892
Class Inst3	111	453	818	1,847

Expense Example, No Redemption - Columbia Acorn European Fund - USD ($)	1 year	3 years	5 years	10 years
Class A	715	1,103	1,516	2,663
Class Adv	123	484	868	1,948
Class C	224	788	1,378	2,978
Class Inst	123	484	868	1,948
Class Inst2	116	466	840	1,892
Class Inst3	111	453	818	1,847

Portfolio Turnover

The Fund may pay transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 34% of the average value of its portfolio.

Principal Investment Strategies

Under normal circumstances, the Fund invests at least 80% of its net assets (plus any borrowing for investment purposes) in European companies. Under normal circumstances, the Fund invests at least 70% of its total assets in companies in Western European countries (for example, the United Kingdom, Germany, France and Italy), but also may invest up to 30% of its total assets in companies in emerging Central and Eastern European countries (for example, Poland, the Czech Republic, Turkey and Cyprus), including up to 10% of its total assets in companies in Russia and the Ukraine. For purposes of the Fund's policies, the Fund may invest in a company if (i) it is domiciled in, or the principal trading market for its securities is in, a European country, (ii) it derives 50% or more of its economic value from goods produced, sales made or services performed or has at least 50% of its assets in a European country or countries or (iii) it is a holding company that predominantly holds shares in such companies. The Fund may invest in a variety of countries, industries and sectors and does not attempt to invest a specific percentage of its assets in any given country, industry or sector.

Under normal circumstances, the Fund (i) invests a majority of its net assets in the common stock of small- and mid-sized companies with market capitalizations under $5 billion at the time of initial investment (“Focus Stocks”) and (ii) may also invest in companies with market capitalizations above $5 billion, provided that immediately after that investment a majority of the Fund’s net assets would be invested in Focus Stocks. The Fund may continue to hold, and make additional investments in, Focus Stocks whose market capitalizations have grown to exceed $5 billion, regardless of whether the Fund’s investments in Focus Stocks are a majority of the Fund’s net assets.

Columbia Wanger Asset Management, LLC, the Fund's investment adviser (the Investment Manager), believes that stocks of small- and mid-sized companies, which generally are not as well known by financial analysts as larger companies, may offer higher return potential than stocks of larger companies.

The Fund takes advantage of the Investment Manager's research and stock-picking capabilities to initially invest in a limited number of companies (generally under 100), offering the potential to provide above-average growth over time.

The Investment Manager typically seeks companies with:

• A strong business franchise that offers growth potential.

• Products and services in which the company has a competitive advantage.

• A stock price the Investment Manager believes is reasonable relative to the assets and earning power of the company.

The Investment Manager may sell a portfolio holding if the security reaches the Investment Manager's price target, if the company has a deterioration of fundamentals, such as failing to meet key operating benchmarks, or if the Investment Manager believes other securities are more attractive. The Investment Manager also may sell a portfolio holding to fund redemptions.

Principal Risks

An investment in the Fund involves risks, including those described below. There is no assurance that the Fund will achieve its investment objective and you may lose money. The value of the Fund’s holdings may decline, and the Fund’s net asset value (NAV) and share price may go down. An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.

Active Management Risk. The Investment Manager’s active management of the Fund could cause the Fund to underperform its benchmark index and/or other funds with similar investment objectives and/or strategies.

Liquidity and Trading Volume Risk. Due to market conditions, including uncertainty regarding the price of a security, it may be difficult for the Fund to buy or sell portfolio securities at a desirable time or price, which could result in investment losses. This risk of portfolio illiquidity is heightened with respect to small- and mid-cap securities, generally, and foreign small- and mid-cap securities in particular. The Fund may have to lower the selling price, liquidate other investments, or forego another, more appealing investment opportunity as a result of illiquidity in the markets. As a result of significant and sustained reductions in emerging and developed international market trading volumes in the wake of the 2007-2009 financial crisis, it may take longer to buy or sell securities, which can exacerbate the Fund’s exposure to volatile markets. The Fund may also be limited in its ability to execute favorable trades in portfolio securities in response to changes in share prices and fundamentals, and may be forced to dispose of securities under disadvantageous circumstances and at a loss. As the Fund grows in size or, conversely, if it faces significant redemption pressure, these considerations take on increasing significance and may adversely impact performance.

Small- and Mid-Cap Company Securities Risk. Investments in small- and mid-capitalization companies (small- and mid-cap companies) often involve greater risks than investments in larger, more established companies (larger companies) because small- and mid-cap companies tend to have less predictable earnings and may lack the management experience, financial resources, product diversification and competitive strengths of larger companies. Securities of small- and mid-cap companies may be less liquid and more volatile than the securities of larger companies.

Sector Risk. At times, the Fund may have a significant portion of its assets invested in securities of companies conducting business in a related group of industries within a sector. Companies in the same sector may be similarly affected by economic, regulatory, political or market events or conditions, which may make the Fund more vulnerable to unfavorable developments in that sector than funds that invest more broadly. Generally, the more the Fund diversifies its investments, the more it spreads risk and potentially reduces the risks of loss and volatility.

Foreign Securities Risk. Investments in or exposure to foreign securities involve certain risks not associated with investments in or exposure to securities of U.S. companies. Foreign securities subject the Fund to the risks associated with investing in the particular country of an issuer, including political, regulatory, economic, social, diplomatic and other conditions or events (including, for example, military confrontations, war and terrorism), occurring in the country or region, as well as risks associated with less developed custody and settlement practices. Foreign securities may be more volatile and less liquid than securities of U.S. companies, and are subject to the risks associated with potential imposition of economic and other sanctions against a particular foreign country, its nationals or industries or businesses within the country. In addition, foreign governments may impose withholding or other taxes on the Fund’s income, capital gains or proceeds from the disposition of foreign securities, which could reduce the Fund’s return on such securities.

Operational and Settlement Risks of Foreign Securities. The Fund’s foreign securities are generally held outside the United States in the primary market for the securities in the custody of foreign sub-custodians. Some countries have limited governmental oversight and regulation, which increases the risk of corruption and fraud and the possibility of losses to the Fund. In particular, under certain circumstances, foreign securities may settle on a delayed delivery basis, meaning that the Fund may be required to make payment for securities before the Fund has actually received delivery of the securities or deliver securities prior to the receipt of payment. As a result, there is a risk that the security will not be delivered to the Fund or that payment will not be received. Losses can also result from lost, stolen or counterfeit securities; defaults by brokers and banks; failures or defects of the settlement system; or poor and improper record keeping by registrars and issuers.

Share Blocking. In certain non-U.S. markets, an issuer’s securities are blocked from trading for a specified number of days before and, in certain instances, after a shareholder meeting. The blocking period can last up to several weeks. Share blocking may prevent the Fund from buying or selling securities during this period. As a consequence of these restrictions, the Investment Manager, on behalf of the Fund, may abstain from voting proxies in markets that require share blocking.

Emerging Market Securities Risk. Securities issued by foreign governments or companies in emerging market countries, such as China, Russia and certain countries in Eastern Europe, the Middle East, Asia, Latin America or Africa, are more likely to have greater exposure to the risks of investing in foreign securities that are described in Foreign Securities Risk. In addition, emerging market countries are more likely to experience instability resulting, for example, from rapid changes or developments in social, political, economic or other conditions. Their economies are usually less mature and their securities markets are typically less developed with more limited trading activity (i.e., lower trading volumes and less liquidity) than more developed countries. Emerging market securities tend to be more volatile than securities in more developed markets. Many emerging market countries are heavily dependent on international trade and have fewer trading partners, which makes them more sensitive to world commodity prices and economic downturns in other countries, and some have a higher risk of currency devaluations.

Operational and Settlement Risks of Securities in Emerging Markets. Foreign sub-custodians in emerging markets may be recently organized, lack extensive operating experience or lack effective government oversight or regulation. In addition, there may be legal restrictions or limitations on the ability of the Fund to recover assets held in custody by a foreign sub-custodian in the event of the bankruptcy of the sub-custodian. There may also be a greater risk that settlement may be delayed and that cash or securities of the Fund may be lost because of failures of or defects in the system, including fraud or corruption. Settlement systems in emerging markets also have a higher risk of failed trades.

Risks Related to Currencies and Corporate Actions in Emerging Markets. Risks related to currencies and corporate actions are also greater in emerging market countries than in developed countries. Emerging market currencies may not have an active trading market and are subject to a higher risk of currency devaluations.

Risks Related to Corporate and Securities Laws in Emerging Markets. Securities laws in emerging markets may be relatively new and unsettled and, consequently, there is a risk of rapid and unpredictable change in laws regarding foreign investment, securities regulation, title to securities and shareholder rights.

Foreign Currency Risk. The performance of the Fund may be materially affected positively or negatively by foreign currency strength or weakness relative to the U.S. dollar, particularly if the Fund invests a significant percentage of its assets in foreign securities or other assets denominated in currencies other than the U.S. dollar.

Market Risk. Market risk refers to the possibility that the market values of securities or other investments that the Fund holds will fall, sometimes rapidly or unpredictably, or fail to rise. An investment in the Fund could lose money over short or long periods.

Issuer Risk. An issuer in which the Fund invests may perform poorly, and the value of its securities may therefore decline, which would negatively affect the Fund’s performance. Poor performance may be caused by poor management decisions, competitive pressures, breakthroughs in technology, reliance on suppliers, labor problems or shortages, corporate restructurings, fraudulent disclosures, natural disasters or other events, conditions or factors.

Geographic Focus Risk. The Fund may be particularly susceptible to economic, political, regulatory or other events or conditions affecting issuers and countries within the specific geographic regions in which the Fund invests. The Fund’s NAV may be more volatile than the NAV of a more geographically diversified fund.

Europe. The Fund is particularly susceptible to economic, political, regulatory or other events or conditions affecting issuers and countries in Europe. In addition, the private and public sectors’ debt problems of a single European Union (EU) country can pose significant economic risks to the EU as a whole. As a result, the Fund’s NAV may be more volatile than the NAV of a more geographically diversified fund. If securities of issuers in Europe fall out of favor, it may cause the Fund to underperform other funds that do not focus their investments in this region of the world. The impact of any partial or complete dissolution of the EU on European economies could be significant, resulting in negative impacts on currency and financial markets generally, such as increased volatility and illiquidity, and potentially lower economic growth in markets in Europe, which may adversely affect the value of your investment in the Fund.

Growth Securities Risk. Growth securities typically trade at a higher multiple of earnings than other types of equity securities. Accordingly, the market values of growth securities may never reach their expected market value and may decline in price. In addition, growth securities, at times, may not perform as well as value securities or the stock market in general, and may be out of favor with investors for varying periods of time.

Performance Information

The following bar chart and table show you how the Fund has performed in the past, and can help you understand the risks of investing in the Fund. The bar chart shows how the Fund’s Class Inst share performance has varied for each full calendar year shown. The table below the bar chart compares the Fund’s returns for the periods shown with the MSCI AC Europe Small Cap Index (Net). The MSCI AC Europe Small Cap Index (Net) captures small-cap representation across 21 countries in Europe. It has 1,081 constituents (as of March 31, 2018) and covers approximately 14% of the free float adjusted market capitalization across each country in Europe.

The performance of one or more share classes shown in the Average Annual Total Returns table below includes the Fund’s Class Inst share returns (adjusted to reflect the higher class-related operating expenses of such classes, where applicable) for periods prior to the indicated inception date of such share classes. Except for differences in fees and expenses, all share classes of the Fund would have substantially similar annual returns because all share classes of the Fund invest in the same portfolio of securities.

The after-tax returns shown in the Average Annual Total Returns table below are calculated using the highest historical individual U.S. federal marginal income tax rates in effect during the period indicated in the table and do not reflect the impact of state, local or foreign taxes. Your actual after-tax returns will depend on your personal tax situation and may differ from those shown in the table. In addition, the after-tax returns shown in the table do not apply to shares held in tax-advantaged accounts such as 401(k) plans or Individual Retirement Accounts (IRAs). The after-tax returns are shown only for Class Inst shares and will vary for other share classes.

The Fund’s past performance (before and after taxes) is no guarantee of how the Fund will perform in the future. Updated performance information can be obtained by calling toll-free 800.345.6611 or visiting columbiathreadneedle.com/us.

Year by Year Total Return (%)
as of December 31 Each Year

Best and Worst Quarterly Returns
During the Period Shown in the Bar Chart

Best                                                        1st Quarter 2012                                                     17.69%

Worst                                                     3rd Quarter 2014                                                     -9.07%

Average Annual Total Returns After Applicable Sales Charges (for periods ended December 31, 2017)

Average Annual Total Returns - Columbia Acorn European Fund	Share Class Inception Date	1 Year	5 Years	Life of Fund
Class Inst	Aug. 19, 2011	38.35%	11.60%	12.11%
Class Inst | returns after taxes on distributions	Aug. 19, 2011	38.03%	11.45%	11.95%
Class Inst | returns after taxes on distributions and sale of Fund shares	Aug. 19, 2011	22.14%	9.36%	9.90%
Class A	Aug. 19, 2011	29.98%	10.00%	10.79%
Class Adv	Jun. 25, 2014	38.29%	11.61%	12.12%
Class C	Aug. 19, 2011	35.95%	10.50%	11.01%
Class Inst2	Nov. 08, 2012	38.32%	11.61%	12.12%
Class Inst3	Mar. 01, 2017	38.46%	11.62%	12.13%
MSCI AC Europe Small Cap Index (Net) (reflects reinvested dividends net of withholding taxes but reflects no deductions for fees, expenses or other taxes)		35.50%	13.58%	13.65%

[1] This charge is imposed on certain investments of between $1 million and $50 million redeemed within 18 months after purchase, as follows: 1.00% if redeemed within 12 months after purchase, and 0.50% if redeemed more than 12, but less than 18, months after purchase, with certain limited exceptions.

[2] This charge applies to redemptions within 12 months after purchase, with certain limited exceptions.

[1] Other expenses have been restated to reflect current fees paid by the Fund.

[2] “Total annual Fund operating expenses” include acquired fund fees and expenses (expenses the Fund incurs indirectly through its investments in other investment companies) and may be higher than “Total Net Expenses” shown in the Financial Highlights section of this prospectus because “Total Net Expenses” do not include acquired fund fees and expenses.

[3] Columbia Wanger Asset Management, LLC (the Investment Manager) has contractually agreed to waive fees and reimburse certain expenses of the Fund, through June 30, 2019, so that ordinary operating expenses (excluding transaction costs and certain other investment-related expenses, interest and fees on borrowings and expenses associated with the Fund’s investment in other investment companies, if any) do not exceed the annual rates of 1.45% for Class A shares, 1.20% for Class Adv shares, 2.20% for Class C shares, 1.20% for Class Inst shares, 1.13% for Class Inst2 shares and 1.08% for Class Inst3 shares. This arrangement may only be amended or terminated with approval from the Fund’s Board of Trustees and the Investment Manager. The fee waivers and/or expense reimbursements shown in the table also reflect the contractual agreement of the Fund’s transfer agent, Columbia Management Investment Services Corp. (the Transfer Agent), to waive a portion of its fees through June 30, 2019, such that the Fund’s transfer agency fees do not exceed the annual rates of 0.05% of the average daily net assets of Class Inst2 shares and 0.00% of the average daily net assets of Class Inst3 shares.

[1] Year to Date return as of March 31, 2018: 0.10%

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Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	COLUMBIA ACORN TRUST
Prospectus Date	rr_ProspectusDate	May 01, 2018
Columbia Acorn European Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	SUMMARY OF THE FUND
Objective [Heading]	rr_ObjectiveHeading	Investment Objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	Columbia Acorn European FundSM (the Fund) seeks long-term capital appreciation.
Expense [Heading]	rr_ExpenseHeading	Fees and Expenses of the Fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. An investor transacting in a class of Fund shares without any front-end sales charge, contingent deferred sales charge, or other asset-based fee for sales or distribution may be required to pay a commission to the financial intermediary for effecting such transactions. Such commission rates are set by the financial intermediary and are not reflected in the tables or the example below. You may qualify for sales charge discounts if you and members of your immediate family invest, or agree to invest in the future, at least $50,000 in certain classes of shares of eligible funds distributed by Columbia Management Investment Distributors, Inc. (the Distributor). More information is available about these and other sales charge discounts and waivers from your financial intermediary, and can be found in the Choosing a Share Class section beginning on page 23 of the Fund’s prospectus, in Appendix A to the prospectus beginning on page A-1 and in Appendix S to the Statement of Additional Information (SAI) under Sales Charge Waivers beginning on page S-1.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder Fees (fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	June 30, 2019
Portfolio Turnover [Heading]	rr_PortfolioTurnoverHeading	Portfolio Turnover
Portfolio Turnover [Text Block]	rr_PortfolioTurnoverTextBlock	The Fund may pay transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 34% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	34.00%
Expense Breakpoint Discounts [Text]	rr_ExpenseBreakpointDiscounts	You may qualify for sales charge discounts if you and members of your immediate family invest, or agree to invest in the future, at least $50,000 in certain classes of shares of eligible funds distributed by Columbia Management Investment Distributors, Inc. (the Distributor). More information is available about these and other sales charge discounts and waivers from your financial intermediary, and can be found in the Choosing a Share Class section beginning on page 23 of the Fund's prospectus, in Appendix A to the prospectus beginning on page A-1 and in Appendix S to the Statement of Additional Information (SAI) under Sales Charge Waivers beginning on page S-1.
Expense Breakpoint, Minimum Investment Required [Amount]	rr_ExpenseBreakpointMinimumInvestmentRequiredAmount	$ 50,000
Expenses Restated to Reflect Current [Text]	rr_ExpensesRestatedToReflectCurrent	Other expenses have been restated to reflect current fees paid by the Fund.
Expenses Not Correlated to Ratio Due to Acquired Fund Fees [Text]	rr_ExpensesNotCorrelatedToRatioDueToAcquiredFundFees	“Total annual Fund operating expenses” include acquired fund fees and expenses (expenses the Fund incurs indirectly through its investments in other investment companies) and may be higher than “Total Net Expenses” shown in the Financial Highlights section of this prospectus because “Total Net Expenses” do not include acquired fund fees and expenses.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	The following example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example illustrates the hypothetical expenses that you would incur over the time periods indicated, and assumes that: you invest $10,000 in the applicable class of Fund shares for the periods indicated, your investment has a 5% return each year, and the Fund’s total annual operating expenses remain the same as shown in the Annual Fund Operating Expenses table above. Since the waivers and/or reimbursements shown in the Annual Fund Operating Expenses table above expire as indicated in the preceding table, they are only reflected in the 1 year example and the first year of the other examples. Although your actual costs may be higher or lower, based on the assumptions listed above, your costs would be:
Strategy [Heading]	rr_StrategyHeading	Principal Investment Strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	Under normal circumstances, the Fund invests at least 80% of its net assets (plus any borrowing for investment purposes) in European companies. Under normal circumstances, the Fund invests at least 70% of its total assets in companies in Western European countries (for example, the United Kingdom, Germany, France and Italy), but also may invest up to 30% of its total assets in companies in emerging Central and Eastern European countries (for example, Poland, the Czech Republic, Turkey and Cyprus), including up to 10% of its total assets in companies in Russia and the Ukraine. For purposes of the Fund's policies, the Fund may invest in a company if (i) it is domiciled in, or the principal trading market for its securities is in, a European country, (ii) it derives 50% or more of its economic value from goods produced, sales made or services performed or has at least 50% of its assets in a European country or countries or (iii) it is a holding company that predominantly holds shares in such companies. The Fund may invest in a variety of countries, industries and sectors and does not attempt to invest a specific percentage of its assets in any given country, industry or sector. Under normal circumstances, the Fund (i) invests a majority of its net assets in the common stock of small- and mid-sized companies with market capitalizations under $5 billion at the time of initial investment (“Focus Stocks”) and (ii) may also invest in companies with market capitalizations above $5 billion, provided that immediately after that investment a majority of the Fund’s net assets would be invested in Focus Stocks. The Fund may continue to hold, and make additional investments in, Focus Stocks whose market capitalizations have grown to exceed $5 billion, regardless of whether the Fund’s investments in Focus Stocks are a majority of the Fund’s net assets. Columbia Wanger Asset Management, LLC, the Fund's investment adviser (the Investment Manager), believes that stocks of small- and mid-sized companies, which generally are not as well known by financial analysts as larger companies, may offer higher return potential than stocks of larger companies. The Fund takes advantage of the Investment Manager's research and stock-picking capabilities to initially invest in a limited number of companies (generally under 100), offering the potential to provide above-average growth over time. The Investment Manager typically seeks companies with: A strong business franchise that offers growth potential. Products and services in which the company has a competitive advantage. A stock price the Investment Manager believes is reasonable relative to the assets and earning power of the company. The Investment Manager may sell a portfolio holding if the security reaches the Investment Manager's price target, if the company has a deterioration of fundamentals, such as failing to meet key operating benchmarks, or if the Investment Manager believes other securities are more attractive. The Investment Manager also may sell a portfolio holding to fund redemptions.
Risk [Heading]	rr_RiskHeading	Principal Risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the Fund involves risks, including those described below. There is no assurance that the Fund will achieve its investment objective and you may lose money. The value of the Fund’s holdings may decline, and the Fund’s net asset value (NAV) and share price may go down. An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Active Management Risk. The Investment Manager’s active management of the Fund could cause the Fund to underperform its benchmark index and/or other funds with similar investment objectives and/or strategies. Liquidity and Trading Volume Risk. Due to market conditions, including uncertainty regarding the price of a security, it may be difficult for the Fund to buy or sell portfolio securities at a desirable time or price, which could result in investment losses. This risk of portfolio illiquidity is heightened with respect to small- and mid-cap securities, generally, and foreign small- and mid-cap securities in particular. The Fund may have to lower the selling price, liquidate other investments, or forego another, more appealing investment opportunity as a result of illiquidity in the markets. As a result of significant and sustained reductions in emerging and developed international market trading volumes in the wake of the 2007-2009 financial crisis, it may take longer to buy or sell securities, which can exacerbate the Fund’s exposure to volatile markets. The Fund may also be limited in its ability to execute favorable trades in portfolio securities in response to changes in share prices and fundamentals, and may be forced to dispose of securities under disadvantageous circumstances and at a loss. As the Fund grows in size or, conversely, if it faces significant redemption pressure, these considerations take on increasing significance and may adversely impact performance. Small- and Mid-Cap Company Securities Risk. Investments in small- and mid-capitalization companies (small- and mid-cap companies) often involve greater risks than investments in larger, more established companies (larger companies) because small- and mid-cap companies tend to have less predictable earnings and may lack the management experience, financial resources, product diversification and competitive strengths of larger companies. Securities of small- and mid-cap companies may be less liquid and more volatile than the securities of larger companies. Sector Risk. At times, the Fund may have a significant portion of its assets invested in securities of companies conducting business in a related group of industries within a sector. Companies in the same sector may be similarly affected by economic, regulatory, political or market events or conditions, which may make the Fund more vulnerable to unfavorable developments in that sector than funds that invest more broadly. Generally, the more the Fund diversifies its investments, the more it spreads risk and potentially reduces the risks of loss and volatility. Foreign Securities Risk. Investments in or exposure to foreign securities involve certain risks not associated with investments in or exposure to securities of U.S. companies. Foreign securities subject the Fund to the risks associated with investing in the particular country of an issuer, including political, regulatory, economic, social, diplomatic and other conditions or events (including, for example, military confrontations, war and terrorism), occurring in the country or region, as well as risks associated with less developed custody and settlement practices. Foreign securities may be more volatile and less liquid than securities of U.S. companies, and are subject to the risks associated with potential imposition of economic and other sanctions against a particular foreign country, its nationals or industries or businesses within the country. In addition, foreign governments may impose withholding or other taxes on the Fund’s income, capital gains or proceeds from the disposition of foreign securities, which could reduce the Fund’s return on such securities. Operational and Settlement Risks of Foreign Securities. The Fund’s foreign securities are generally held outside the United States in the primary market for the securities in the custody of foreign sub-custodians. Some countries have limited governmental oversight and regulation, which increases the risk of corruption and fraud and the possibility of losses to the Fund. In particular, under certain circumstances, foreign securities may settle on a delayed delivery basis, meaning that the Fund may be required to make payment for securities before the Fund has actually received delivery of the securities or deliver securities prior to the receipt of payment. As a result, there is a risk that the security will not be delivered to the Fund or that payment will not be received. Losses can also result from lost, stolen or counterfeit securities; defaults by brokers and banks; failures or defects of the settlement system; or poor and improper record keeping by registrars and issuers. Share Blocking. In certain non-U.S. markets, an issuer’s securities are blocked from trading for a specified number of days before and, in certain instances, after a shareholder meeting. The blocking period can last up to several weeks. Share blocking may prevent the Fund from buying or selling securities during this period. As a consequence of these restrictions, the Investment Manager, on behalf of the Fund, may abstain from voting proxies in markets that require share blocking. Emerging Market Securities Risk. Securities issued by foreign governments or companies in emerging market countries, such as China, Russia and certain countries in Eastern Europe, the Middle East, Asia, Latin America or Africa, are more likely to have greater exposure to the risks of investing in foreign securities that are described in Foreign Securities Risk. In addition, emerging market countries are more likely to experience instability resulting, for example, from rapid changes or developments in social, political, economic or other conditions. Their economies are usually less mature and their securities markets are typically less developed with more limited trading activity (i.e., lower trading volumes and less liquidity) than more developed countries. Emerging market securities tend to be more volatile than securities in more developed markets. Many emerging market countries are heavily dependent on international trade and have fewer trading partners, which makes them more sensitive to world commodity prices and economic downturns in other countries, and some have a higher risk of currency devaluations. Operational and Settlement Risks of Securities in Emerging Markets. Foreign sub-custodians in emerging markets may be recently organized, lack extensive operating experience or lack effective government oversight or regulation. In addition, there may be legal restrictions or limitations on the ability of the Fund to recover assets held in custody by a foreign sub-custodian in the event of the bankruptcy of the sub-custodian. There may also be a greater risk that settlement may be delayed and that cash or securities of the Fund may be lost because of failures of or defects in the system, including fraud or corruption. Settlement systems in emerging markets also have a higher risk of failed trades. Risks Related to Currencies and Corporate Actions in Emerging Markets. Risks related to currencies and corporate actions are also greater in emerging market countries than in developed countries. Emerging market currencies may not have an active trading market and are subject to a higher risk of currency devaluations. Risks Related to Corporate and Securities Laws in Emerging Markets. Securities laws in emerging markets may be relatively new and unsettled and, consequently, there is a risk of rapid and unpredictable change in laws regarding foreign investment, securities regulation, title to securities and shareholder rights. Foreign Currency Risk. The performance of the Fund may be materially affected positively or negatively by foreign currency strength or weakness relative to the U.S. dollar, particularly if the Fund invests a significant percentage of its assets in foreign securities or other assets denominated in currencies other than the U.S. dollar. Market Risk. Market risk refers to the possibility that the market values of securities or other investments that the Fund holds will fall, sometimes rapidly or unpredictably, or fail to rise. An investment in the Fund could lose money over short or long periods. Issuer Risk. An issuer in which the Fund invests may perform poorly, and the value of its securities may therefore decline, which would negatively affect the Fund’s performance. Poor performance may be caused by poor management decisions, competitive pressures, breakthroughs in technology, reliance on suppliers, labor problems or shortages, corporate restructurings, fraudulent disclosures, natural disasters or other events, conditions or factors. Geographic Focus Risk. The Fund may be particularly susceptible to economic, political, regulatory or other events or conditions affecting issuers and countries within the specific geographic regions in which the Fund invests. The Fund’s NAV may be more volatile than the NAV of a more geographically diversified fund. Europe. The Fund is particularly susceptible to economic, political, regulatory or other events or conditions affecting issuers and countries in Europe. In addition, the private and public sectors’ debt problems of a single European Union (EU) country can pose significant economic risks to the EU as a whole. As a result, the Fund’s NAV may be more volatile than the NAV of a more geographically diversified fund. If securities of issuers in Europe fall out of favor, it may cause the Fund to underperform other funds that do not focus their investments in this region of the world. The impact of any partial or complete dissolution of the EU on European economies could be significant, resulting in negative impacts on currency and financial markets generally, such as increased volatility and illiquidity, and potentially lower economic growth in markets in Europe, which may adversely affect the value of your investment in the Fund. Growth Securities Risk. Growth securities typically trade at a higher multiple of earnings than other types of equity securities. Accordingly, the market values of growth securities may never reach their expected market value and may decline in price. In addition, growth securities, at times, may not perform as well as value securities or the stock market in general, and may be out of favor with investors for varying periods of time.
Risk Lose Money [Text]	rr_RiskLoseMoney	There is no assurance that the Fund will achieve its investment objective and you may lose money.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance Information
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The following bar chart and table show you how the Fund has performed in the past, and can help you understand the risks of investing in the Fund. The bar chart shows how the Fund’s Class Inst share performance has varied for each full calendar year shown. The table below the bar chart compares the Fund’s returns for the periods shown with the MSCI AC Europe Small Cap Index (Net). The MSCI AC Europe Small Cap Index (Net) captures small-cap representation across 21 countries in Europe. It has 1,081 constituents (as of March 31, 2018) and covers approximately 14% of the free float adjusted market capitalization across each country in Europe. The performance of one or more share classes shown in the Average Annual Total Returns table below includes the Fund’s Class Inst share returns (adjusted to reflect the higher class-related operating expenses of such classes, where applicable) for periods prior to the indicated inception date of such share classes. Except for differences in fees and expenses, all share classes of the Fund would have substantially similar annual returns because all share classes of the Fund invest in the same portfolio of securities. The after-tax returns shown in the Average Annual Total Returns table below are calculated using the highest historical individual U.S. federal marginal income tax rates in effect during the period indicated in the table and do not reflect the impact of state, local or foreign taxes. Your actual after-tax returns will depend on your personal tax situation and may differ from those shown in the table. In addition, the after-tax returns shown in the table do not apply to shares held in tax-advantaged accounts such as 401(k) plans or Individual Retirement Accounts (IRAs). The after-tax returns are shown only for Class Inst shares and will vary for other share classes. The Fund’s past performance (before and after taxes) is no guarantee of how the Fund will perform in the future. Updated performance information can be obtained by calling toll-free 800.345.6611 or visiting columbiathreadneedle.com/us.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The following bar chart and table show you how the Fund has performed in the past, and can help you understand the risks of investing in the Fund. The bar chart shows how the Fund’s Class Inst share performance has varied for each full calendar year shown. The table below the bar chart compares the Fund’s returns for the periods shown with the MSCI AC Europe Small Cap Index (Net). The MSCI AC Europe Small Cap Index (Net) captures small-cap representation across 21 countries in Europe. It has 1,081 constituents (as of March 31, 2018) and covers approximately 14% of the free float adjusted market capitalization across each country in Europe.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	800.345.6611
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	columbiathreadneedle.com/us
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The Fund’s past performance (before and after taxes) is no guarantee of how the Fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Year by Year Total Return (%) as of December 31 Each Year
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Best and Worst Quarterly Returns During the Period Shown in the Bar Chart Best                                                        1st Quarter 2012                                                     17.69% Worst                                                     3rd Quarter 2014                                                     -9.07%
Performance Table Heading	rr_PerformanceTableHeading	Average Annual Total Returns After Applicable Sales Charges (for periods ended December 31, 2017)
Performance Table Uses Highest Federal Rate	rr_PerformanceTableUsesHighestFederalRate	The after-tax returns shown in the Average Annual Total Returns table below are calculated using the highest historical individual U.S. federal marginal income tax rates in effect during the period indicated in the table and do not reflect the impact of state, local or foreign taxes.
Performance Table Not Relevant to Tax Deferred	rr_PerformanceTableNotRelevantToTaxDeferred	In addition, the after-tax returns shown in the table do not apply to shares held in tax-advantaged accounts such as 401(k) plans or Individual Retirement Accounts (IRAs).
Performance Table One Class of after Tax Shown [Text]	rr_PerformanceTableOneClassOfAfterTaxShown	The after-tax returns are shown only for Class Inst shares and will vary for other share classes.
Columbia Acorn European Fund | Class A
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	5.75%
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	1.00%	[1]
Management fees	rr_ManagementFeesOverAssets	1.19%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Other expenses	rr_OtherExpensesOverAssets	0.48%	[2]
Acquired fund fees and expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.01%
Total annual Fund operating expenses	rr_ExpensesOverAssets	1.93%	[3]
Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.47%)	[4]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	1.46%
Expenses Deferred Charges [Text Block]	rr_ExpensesDeferredChargesTextBlock	This charge is imposed on certain investments of between $1 million and $50 million redeemed within 18 months after purchase, as follows: 1.00% if redeemed within 12 months after purchase, and 0.50% if redeemed more than 12, but less than 18, months after purchase, with certain limited exceptions.
1 year	rr_ExpenseExampleYear01	$ 715
3 years	rr_ExpenseExampleYear03	1,103
5 years	rr_ExpenseExampleYear05	1,516
10 years	rr_ExpenseExampleYear10	2,663
1 year	rr_ExpenseExampleNoRedemptionYear01	715
3 years	rr_ExpenseExampleNoRedemptionYear03	1,103
5 years	rr_ExpenseExampleNoRedemptionYear05	1,516
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 2,663
1 Year	rr_AverageAnnualReturnYear01	29.98%
5 Years	rr_AverageAnnualReturnYear05	10.00%
Life of Fund	rr_AverageAnnualReturnSinceInception	10.79%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Aug. 19, 2011
Columbia Acorn European Fund | Class Adv
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	1.19%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.48%	[2]
Acquired fund fees and expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.01%
Total annual Fund operating expenses	rr_ExpensesOverAssets	1.68%	[3]
Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.47%)	[4]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	1.21%
1 year	rr_ExpenseExampleYear01	$ 123
3 years	rr_ExpenseExampleYear03	484
5 years	rr_ExpenseExampleYear05	868
10 years	rr_ExpenseExampleYear10	1,948
1 year	rr_ExpenseExampleNoRedemptionYear01	123
3 years	rr_ExpenseExampleNoRedemptionYear03	484
5 years	rr_ExpenseExampleNoRedemptionYear05	868
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 1,948
1 Year	rr_AverageAnnualReturnYear01	38.29%
5 Years	rr_AverageAnnualReturnYear05	11.61%
Life of Fund	rr_AverageAnnualReturnSinceInception	12.12%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Jun. 25, 2014
Columbia Acorn European Fund | Class C
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	1.00%	[5]
Management fees	rr_ManagementFeesOverAssets	1.19%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	1.00%
Other expenses	rr_OtherExpensesOverAssets	0.48%	[2]
Acquired fund fees and expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.01%
Total annual Fund operating expenses	rr_ExpensesOverAssets	2.68%	[3]
Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.47%)	[4]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	2.21%
Expenses Deferred Charges [Text Block]	rr_ExpensesDeferredChargesTextBlock	This charge applies to redemptions within 12 months after purchase, with certain limited exceptions.
1 year	rr_ExpenseExampleYear01	$ 324
3 years	rr_ExpenseExampleYear03	788
5 years	rr_ExpenseExampleYear05	1,378
10 years	rr_ExpenseExampleYear10	2,978
1 year	rr_ExpenseExampleNoRedemptionYear01	224
3 years	rr_ExpenseExampleNoRedemptionYear03	788
5 years	rr_ExpenseExampleNoRedemptionYear05	1,378
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 2,978
1 Year	rr_AverageAnnualReturnYear01	35.95%
5 Years	rr_AverageAnnualReturnYear05	10.50%
Life of Fund	rr_AverageAnnualReturnSinceInception	11.01%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Aug. 19, 2011
Columbia Acorn European Fund | Class Inst
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	1.19%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.48%	[2]
Acquired fund fees and expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.01%
Total annual Fund operating expenses	rr_ExpensesOverAssets	1.68%	[3]
Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.47%)	[4]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	1.21%
1 year	rr_ExpenseExampleYear01	$ 123
3 years	rr_ExpenseExampleYear03	484
5 years	rr_ExpenseExampleYear05	868
10 years	rr_ExpenseExampleYear10	1,948
1 year	rr_ExpenseExampleNoRedemptionYear01	123
3 years	rr_ExpenseExampleNoRedemptionYear03	484
5 years	rr_ExpenseExampleNoRedemptionYear05	868
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 1,948
2012	rr_AnnualReturn2012	25.66%	[6]
2013	rr_AnnualReturn2013	33.98%	[6]
2014	rr_AnnualReturn2014	(7.52%)	[6]
2015	rr_AnnualReturn2015	4.43%	[6]
2016	rr_AnnualReturn2016	(3.29%)	[6]
2017	rr_AnnualReturn2017	38.35%	[6]
Year to Date Return, Label	rr_YearToDateReturnLabel	Year to Date return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Mar. 31, 2018
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.10%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Mar. 31, 2012
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	17.69%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Sep. 30, 2014
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(9.07%)
1 Year	rr_AverageAnnualReturnYear01	38.35%
5 Years	rr_AverageAnnualReturnYear05	11.60%
Life of Fund	rr_AverageAnnualReturnSinceInception	12.11%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Aug. 19, 2011
Columbia Acorn European Fund | Class Inst2
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	1.19%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.43%	[2]
Acquired fund fees and expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.01%
Total annual Fund operating expenses	rr_ExpensesOverAssets	1.63%	[3]
Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.49%)	[4]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	1.14%
1 year	rr_ExpenseExampleYear01	$ 116
3 years	rr_ExpenseExampleYear03	466
5 years	rr_ExpenseExampleYear05	840
10 years	rr_ExpenseExampleYear10	1,892
1 year	rr_ExpenseExampleNoRedemptionYear01	116
3 years	rr_ExpenseExampleNoRedemptionYear03	466
5 years	rr_ExpenseExampleNoRedemptionYear05	840
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 1,892
1 Year	rr_AverageAnnualReturnYear01	38.32%
5 Years	rr_AverageAnnualReturnYear05	11.61%
Life of Fund	rr_AverageAnnualReturnSinceInception	12.12%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 08, 2012
Columbia Acorn European Fund | Class Inst3
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	1.19%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.39%	[2]
Acquired fund fees and expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.01%
Total annual Fund operating expenses	rr_ExpensesOverAssets	1.59%	[3]
Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.50%)	[4]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	1.09%
1 year	rr_ExpenseExampleYear01	$ 111
3 years	rr_ExpenseExampleYear03	453
5 years	rr_ExpenseExampleYear05	818
10 years	rr_ExpenseExampleYear10	1,847
1 year	rr_ExpenseExampleNoRedemptionYear01	111
3 years	rr_ExpenseExampleNoRedemptionYear03	453
5 years	rr_ExpenseExampleNoRedemptionYear05	818
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 1,847
1 Year	rr_AverageAnnualReturnYear01	38.46%
5 Years	rr_AverageAnnualReturnYear05	11.62%
Life of Fund	rr_AverageAnnualReturnSinceInception	12.13%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Mar. 01, 2017
Columbia Acorn European Fund | returns after taxes on distributions | Class Inst
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	38.03%
5 Years	rr_AverageAnnualReturnYear05	11.45%
Life of Fund	rr_AverageAnnualReturnSinceInception	11.95%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Aug. 19, 2011
Columbia Acorn European Fund | returns after taxes on distributions and sale of Fund shares | Class Inst
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	22.14%
5 Years	rr_AverageAnnualReturnYear05	9.36%
Life of Fund	rr_AverageAnnualReturnSinceInception	9.90%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Aug. 19, 2011
Columbia Acorn European Fund | MSCI AC Europe Small Cap Index (Net) (reflects reinvested dividends net of withholding taxes but reflects no deductions for fees, expenses or other taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	35.50%
5 Years	rr_AverageAnnualReturnYear05	13.58%
Life of Fund	rr_AverageAnnualReturnSinceInception	13.65%

[1] This charge is imposed on certain investments of between $1 million and $50 million redeemed within 18 months after purchase, as follows: 1.00% if redeemed within 12 months after purchase, and 0.50% if redeemed more than 12, but less than 18, months after purchase, with certain limited exceptions.

[2] Other expenses have been restated to reflect current fees paid by the Fund.

[3] “Total annual Fund operating expenses” include acquired fund fees and expenses (expenses the Fund incurs indirectly through its investments in other investment companies) and may be higher than “Total Net Expenses” shown in the Financial Highlights section of this prospectus because “Total Net Expenses” do not include acquired fund fees and expenses.

[4] Columbia Wanger Asset Management, LLC (the Investment Manager) has contractually agreed to waive fees and reimburse certain expenses of the Fund, through June 30, 2019, so that ordinary operating expenses (excluding transaction costs and certain other investment-related expenses, interest and fees on borrowings and expenses associated with the Fund’s investment in other investment companies, if any) do not exceed the annual rates of 1.45% for Class A shares, 1.20% for Class Adv shares, 2.20% for Class C shares, 1.20% for Class Inst shares, 1.13% for Class Inst2 shares and 1.08% for Class Inst3 shares. This arrangement may only be amended or terminated with approval from the Fund’s Board of Trustees and the Investment Manager. The fee waivers and/or expense reimbursements shown in the table also reflect the contractual agreement of the Fund’s transfer agent, Columbia Management Investment Services Corp. (the Transfer Agent), to waive a portion of its fees through June 30, 2019, such that the Fund’s transfer agency fees do not exceed the annual rates of 0.05% of the average daily net assets of Class Inst2 shares and 0.00% of the average daily net assets of Class Inst3 shares.

[5] This charge applies to redemptions within 12 months after purchase, with certain limited exceptions.

[6] Year to Date return as of March 31, 2018: 0.10%

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Columbia Acorn Fund

SUMMARY OF THE FUND

Investment Objective

Columbia Acorn^® Fund (the Fund) seeks long-term capital appreciation.

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. An investor transacting in a class of Fund shares without any front-end sales charge, contingent deferred sales charge, or other asset-based fee for sales or distribution may be required to pay a commission to the financial intermediary for effecting such transactions. Such commission rates are set by the financial intermediary and are not reflected in the tables or the example below. You may qualify for sales charge discounts if you and members of your immediate family invest, or agree to invest in the future, at least $50,000 in certain classes of shares of eligible funds distributed by Columbia Management Investment Distributors, Inc. (the Distributor). More information is available about these and other sales charge discounts and waivers from your financial intermediary, and can be found in the Choosing a Share Class section beginning on page 22 of the Fund’s prospectus, in Appendix A to the prospectus beginning on page A-1 and in Appendix S to the Statement of Additional Information (SAI) under Sales Charge Waivers beginning on page S-1.

Shareholder Fees (fees paid directly from your investment)

Shareholder Fees - Columbia Acorn Fund	Class A		Class C		Class Adv	Class Inst	Class Inst2	Class Inst3
Maximum sales charge (load) imposed on purchases (as a % of offering price)	5.75%		none		none	none	none	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	1.00%	[1]	1.00%	[2]	none	none	none	none

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)

Annual Fund Operating Expenses - Columbia Acorn Fund		Class A	Class Adv	Class C	Class Inst	Class Inst2	Class Inst3
Management fees		0.67%	0.67%	0.67%	0.67%	0.67%	0.67%
Distribution and/or service (12b-1) fees		0.25%	none	1.00%	none	none	none
Other expenses	[1]	0.18%	0.18%	0.18%	0.18%	0.16%	0.09%
Acquired fund fees and expenses		0.08%	0.08%	0.08%	0.08%	0.08%	0.08%
Total annual Fund operating expenses	[2]	1.18%	0.93%	1.93%	0.93%	0.91%	0.84%
Less: Fee waivers and/or expense reimbursements	[3]	(0.07%)	(0.07%)	(0.07%)	(0.07%)	(0.07%)	(0.05%)
Total annual Fund operating expenses after fee waivers and/or expense reimbursements		1.11%	0.86%	1.86%	0.86%	0.84%	0.79%

Example

The following example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example illustrates the hypothetical expenses that you would incur over the time periods indicated, and assumes that:

• you invest $10,000 in the applicable class of Fund shares for the periods indicated,

• your investment has a 5% return each year, and

• the Fund’s total annual operating expenses remain the same as shown in the Annual Fund Operating Expenses table above.

Since the waivers and/or reimbursements shown in the Annual Fund Operating Expenses table above expire as indicated in the preceding table, they are only reflected in the 1 year example and the first year of the other examples. Although your actual costs may be higher or lower, based on the assumptions listed above, your costs would be:

Expense Example - Columbia Acorn Fund - USD ($)	1 year	3 years	5 years	10 years
Class A	682	922	1,180	1,919
Class Adv	88	289	508	1,137
Class C	289	599	1,035	2,248
Class Inst	88	289	508	1,137
Class Inst2	86	283	497	1,113
Class Inst3	81	263	461	1,033

Expense Example, No Redemption - Columbia Acorn Fund - USD ($)	1 year	3 years	5 years	10 years
Class A	682	922	1,180	1,919
Class Adv	88	289	508	1,137
Class C	189	599	1,035	2,248
Class Inst	88	289	508	1,137
Class Inst2	86	283	497	1,113
Class Inst3	81	263	461	1,033

Portfolio Turnover

The Fund may pay transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 72% of the average value of its portfolio.

Principal Investment Strategies

Under normal circumstances, the Fund (i) invests a majority of its net assets in the common stock of small- and mid-sized companies with market capitalizations under $5 billion at the time of initial investment (“Focus Stocks”) and (ii) may also invest in companies with market capitalizations above $5 billion, provided that immediately after that investment a majority of the Fund’s net assets would be invested in Focus Stocks. The Fund may continue to hold, and make additional investments in, Focus Stocks whose market capitalizations have grown to exceed $5 billion, regardless of whether the Fund’s investments in Focus Stocks are a majority of the Fund’s net assets.

Columbia Wanger Asset Management, LLC, the Fund's investment adviser (the Investment Manager), believes that stocks of small- and mid-sized companies, which generally are not as well known by financial analysts as larger companies, may offer higher return potential than stocks of larger companies.

The Fund invests the majority of its assets in U.S. companies, but also may invest up to 33% of its total assets in foreign companies in developed markets (for example, Japan, Canada and the United Kingdom) and in emerging markets (for example, China, India and Brazil).

The Investment Manager typically seeks companies with:

• A strong business franchise that offers growth potential.

• Products and services in which the company has a competitive advantage.

• A stock price the Investment Manager believes is reasonable relative to the assets and earning power of the company.

The Investment Manager may sell a portfolio holding if the security reaches the Investment Manager's price target, if the company has a deterioration of fundamentals, such as failing to meet key operating benchmarks, or if the Investment Manager believes other securities are more attractive. The Investment Manager also may sell a portfolio holding to fund redemptions.

Principal Risks

An investment in the Fund involves risks, including those described below. There is no assurance that the Fund will achieve its investment objective and you may lose money. The value of the Fund’s holdings may decline, and the Fund’s net asset value (NAV) and share price may go down. An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.

Active Management Risk. The Investment Manager’s active management of the Fund could cause the Fund to underperform its benchmark index and/or other funds with similar investment objectives and/or strategies.

Small- and Mid-Cap Company Securities Risk. Investments in small- and mid-capitalization companies (small- and mid-cap companies) often involve greater risks than investments in larger, more established companies (larger companies) because small- and mid-cap companies tend to have less predictable earnings and may lack the management experience, financial resources, product diversification and competitive strengths of larger companies. Securities of small- and mid-cap companies may be less liquid and more volatile than the securities of larger companies.

Issuer Risk. An issuer in which the Fund invests may perform poorly, and the value of its securities may therefore decline, which would negatively affect the Fund’s performance. Poor performance may be caused by poor management decisions, competitive pressures, breakthroughs in technology, reliance on suppliers, labor problems or shortages, corporate restructurings, fraudulent disclosures, natural disasters or other events, conditions or factors.

Market Risk. Market risk refers to the possibility that the market values of securities or other investments that the Fund holds will fall, sometimes rapidly or unpredictably, or fail to rise. An investment in the Fund could lose money over short or long periods.

Growth Securities Risk. Growth securities typically trade at a higher multiple of earnings than other types of equity securities. Accordingly, the market values of growth securities may never reach their expected market value and may decline in price. In addition, growth securities, at times, may not perform as well as value securities or the stock market in general, and may be out of favor with investors for varying periods of time.

Foreign Securities Risk. Investments in or exposure to foreign securities involve certain risks not associated with investments in or exposure to securities of U.S. companies. Foreign securities subject the Fund to the risks associated with investing in the particular country of an issuer, including political, regulatory, economic, social, diplomatic and other conditions or events (including, for example, military confrontations, war and terrorism), occurring in the country or region, as well as risks associated with less developed custody and settlement practices. Foreign securities may be more volatile and less liquid than securities of U.S. companies, and are subject to the risks associated with potential imposition of economic and other sanctions against a particular foreign country, its nationals or industries or businesses within the country. In addition, foreign governments may impose withholding or other taxes on the Fund’s income, capital gains or proceeds from the disposition of foreign securities, which could reduce the Fund’s return on such securities.

Operational and Settlement Risks of Foreign Securities. The Fund’s foreign securities are generally held outside the United States in the primary market for the securities in the custody of foreign sub-custodians. Some countries have limited governmental oversight and regulation, which increases the risk of corruption and fraud and the possibility of losses to the Fund. In particular, under certain circumstances, foreign securities may settle on a delayed delivery basis, meaning that the Fund may be required to make payment for securities before the Fund has actually received delivery of the securities or deliver securities prior to the receipt of payment. As a result, there is a risk that the security will not be delivered to the Fund or that payment will not be received. Losses can also result from lost, stolen or counterfeit securities; defaults by brokers and banks; failures or defects of the settlement system; or poor and improper record keeping by registrars and issuers.

Share Blocking. In certain non-U.S. markets, an issuer’s securities are blocked from trading for a specified number of days before and, in certain instances, after a shareholder meeting. The blocking period can last up to several weeks. Share blocking may prevent the Fund from buying or selling securities during this period. As a consequence of these restrictions, the Investment Manager, on behalf of the Fund, may abstain from voting proxies in markets that require share blocking.

Emerging Market Securities Risk. Securities issued by foreign governments or companies in emerging market countries, such as China, Russia and certain countries in Eastern Europe, the Middle East, Asia, Latin America or Africa, are more likely to have greater exposure to the risks of investing in foreign securities that are described in Foreign Securities Risk. In addition, emerging market countries are more likely to experience instability resulting, for example, from rapid changes or developments in social, political, economic or other conditions. Their economies are usually less mature and their securities markets are typically less developed with more limited trading activity (i.e., lower trading volumes and less liquidity) than more developed countries. Emerging market securities tend to be more volatile than securities in more developed markets. Many emerging market countries are heavily dependent on international trade and have fewer trading partners, which makes them more sensitive to world commodity prices and economic downturns in other countries, and some have a higher risk of currency devaluations.

Operational and Settlement Risks of Securities in Emerging Markets. Foreign sub-custodians in emerging markets may be recently organized, lack extensive operating experience or lack effective government oversight or regulation. In addition, there may be legal restrictions or limitations on the ability of the Fund to recover assets held in custody by a foreign sub-custodian in the event of the bankruptcy of the sub-custodian. There may also be a greater risk that settlement may be delayed and that cash or securities of the Fund may be lost because of failures of or defects in the system, including fraud or corruption. Settlement systems in emerging markets also have a higher risk of failed trades.

Risks Related to Currencies and Corporate Actions in Emerging Markets. Risks related to currencies and corporate actions are also greater in emerging market countries than in developed countries. Emerging market currencies may not have an active trading market and are subject to a higher risk of currency devaluations.

Risks Related to Corporate and Securities Laws in Emerging Markets. Securities laws in emerging markets may be relatively new and unsettled and, consequently, there is a risk of rapid and unpredictable change in laws regarding foreign investment, securities regulation, title to securities and shareholder rights.

Liquidity and Trading Volume Risk. Because the Fund may invest a percentage of its assets in foreign securities, it may be subject to the liquidity and trading volume risks associated with international investing. Due to market conditions, including uncertainty regarding the price of a security, it may be difficult for the Fund to buy or sell foreign portfolio securities at a desirable time or price, which could result in investment losses and may affect the value of your investment in the Fund. This risk of portfolio illiquidity is heightened with respect to small- and mid-cap securities, generally, and foreign small- and mid-cap securities in particular. The Fund may have to lower the selling price, liquidate other investments, or forego another, more appealing investment opportunity as a result of illiquidity in the markets. As a result of significant and sustained reductions in emerging and developed international market trading volumes in the wake of the 2007-2009 financial crisis, it may take longer to buy or sell these securities, which can exacerbate the Fund’s exposure to volatile markets. The Fund may also be limited in its ability to execute favorable trades in foreign portfolio securities in response to changes in share prices and fundamentals, and may be forced to dispose of securities under disadvantageous circumstances and at a loss. As the Fund grows in size or, conversely, if it faces significant redemption pressure, these considerations take on increasing significance and may adversely impact performance.

Foreign Currency Risk. The performance of the Fund may be materially affected positively or negatively by foreign currency strength or weakness relative to the U.S. dollar, particularly if the Fund invests a significant percentage of its assets in foreign securities or other assets denominated in currencies other than the U.S. dollar.

Sector Risk. At times, the Fund may have a significant portion of its assets invested in securities of companies conducting business in a related group of industries within a sector. Companies in the same sector may be similarly affected by economic, regulatory, political or market events or conditions, which may make the Fund more vulnerable to unfavorable developments in that sector than funds that invest more broadly. Generally, the more the Fund diversifies its investments, the more it spreads risk and potentially reduces the risks of loss and volatility.

Performance Information

The following bar chart and table show you how the Fund has performed in the past, and can help you understand the risks of investing in the Fund. The bar chart shows how the Fund’s Class Inst share performance has varied for each full calendar year shown. The table below the bar chart compares the Fund’s returns for the periods shown with the Russell 2500 Growth Index.

The performance of one or more share classes shown in the Average Annual Total Returns table below includes the Fund’s Class Inst share returns (adjusted to reflect the higher class-related operating expenses of such classes, where applicable) for periods prior to the indicated inception date of such share classes. Except for differences in fees and expenses, all share classes of the Fund would have substantially similar annual returns because all share classes of the Fund invest in the same portfolio of securities.

The after-tax returns shown in the Average Annual Total Returns table below are calculated using the highest historical individual U.S. federal marginal income tax rates in effect during the period indicated in the table and do not reflect the impact of state, local or foreign taxes. Your actual after-tax returns will depend on your personal tax situation and may differ from those shown in the table. In addition, the after-tax returns shown in the table do not apply to shares held in tax-advantaged accounts such as 401(k) plans or Individual Retirement Accounts (IRAs). The after-tax returns are shown only for Class Inst shares and will vary for other share classes.

The Fund’s past performance (before and after taxes) is no guarantee of how the Fund will perform in the future. Updated performance information can be obtained by calling toll-free 800.345.6611 or visiting columbiathreadneedle.com/us.

Year by Year Total Return (%)
as of December 31 Each Year

Best and Worst Quarterly Returns
During the Period Shown in the Bar Chart

Best                                                        2nd Quarter 2009                                                      20.90%

Worst                                                     4th Quarter 2008                                                      -25.11%

Average Annual Total Returns After Applicable Sales Charges (for periods ended December 31, 2017)

Average Annual Total Returns - Columbia Acorn Fund	Share Class Inception Date	1 Year	5 Years	10 Years
Class Inst	Jun. 10, 1970	25.24%	12.42%	8.13%
Class Inst | returns after taxes on distributions	Jun. 10, 1970	18.84%	6.33%	4.90%
Class Inst | returns after taxes on distributions and sale of Fund shares	Jun. 10, 1970	18.52%	8.87%	6.09%
Class A	Oct. 16, 2000	17.77%	10.78%	7.18%
Class Adv	Nov. 08, 2012	25.19%	12.35%	8.09%
Class C	Oct. 16, 2000	23.10%	11.28%	7.00%
Class Inst2	Nov. 08, 2012	25.21%	12.44%	8.14%
Class Inst3	Nov. 08, 2012	25.31%	12.51%	8.17%
Russell 2500 Growth Index (reflects no deductions for fees, expenses or taxes)		24.46%	15.47%	9.62%

[1] This charge is imposed on certain investments of between $1 million and $50 million redeemed within 18 months after purchase, as follows: 1.00% if redeemed within 12 months after purchase, and 0.50% if redeemed more than 12, but less than 18, months after purchase, with certain limited exceptions.

[2] This charge applies to redemptions within 12 months after purchase, with certain limited exceptions.

[1] Other expenses have been restated to reflect current transfer agency fees paid by the Fund.

[2] “Total annual Fund operating expenses” include acquired fund fees and expenses (expenses the Fund incurs indirectly through its investments in other investment companies) and may be higher than “Total Net Expenses” shown in the Financial Highlights section of this prospectus because “Total Net Expenses” do not include acquired fund fees and expenses.

[3] Columbia Wanger Asset Management, LLC (the Investment Manager) has contractually agreed to waive fees and reimburse certain expenses of the Fund, through April 30, 2019, so that ordinary operating expenses (excluding transaction costs and certain other investment-related expenses, interest and fees on borrowings, if any) do not exceed the annual rates of 1.11% for Class A shares, 0.86% for Class Adv shares, 1.86% for Class C shares, 0.86% for Class Inst shares, 0.84% for Class Inst2 shares and 0.79% for Class Inst3 shares. This arrangement may only be amended or terminated with approval from the Fund’s Board of Trustees and the Investment Manager. The fee waivers and/or expense reimbursements shown in the table also reflect the contractual agreement of the Fund’s transfer agent, Columbia Management Investment Services Corp. (the Transfer Agent), to waive a portion of its fees through June 30, 2019, such that the Fund’s transfer agency fees do not exceed the annual rate of 0.07% of the average daily net assets of each share class other than Class Inst2 and Class Inst3. With respect to Class Inst2 and Class Inst3 shares, the Transfer Agent has contractually agreed to waive a portion of its fees such that the Fund's transfer agency fees do not exceed the annual rates of 0.05% of the average daily net assets of Class Inst2 shares and 0.00% of the average daily net assets of Class Inst3 shares.

[1] Year to Date return as of March 31, 2018: 3.57%

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	COLUMBIA ACORN TRUST
Prospectus Date	rr_ProspectusDate	May 01, 2018
Columbia Acorn Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	SUMMARY OF THE FUND
Objective [Heading]	rr_ObjectiveHeading	Investment Objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	Columbia Acorn® Fund (the Fund) seeks long-term capital appreciation.
Expense [Heading]	rr_ExpenseHeading	Fees and Expenses of the Fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. An investor transacting in a class of Fund shares without any front-end sales charge, contingent deferred sales charge, or other asset-based fee for sales or distribution may be required to pay a commission to the financial intermediary for effecting such transactions. Such commission rates are set by the financial intermediary and are not reflected in the tables or the example below. You may qualify for sales charge discounts if you and members of your immediate family invest, or agree to invest in the future, at least $50,000 in certain classes of shares of eligible funds distributed by Columbia Management Investment Distributors, Inc. (the Distributor). More information is available about these and other sales charge discounts and waivers from your financial intermediary, and can be found in the Choosing a Share Class section beginning on page 22 of the Fund’s prospectus, in Appendix A to the prospectus beginning on page A-1 and in Appendix S to the Statement of Additional Information (SAI) under Sales Charge Waivers beginning on page S-1.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder Fees (fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	April 30, 2019 June 30, 2019
Portfolio Turnover [Heading]	rr_PortfolioTurnoverHeading	Portfolio Turnover
Portfolio Turnover [Text Block]	rr_PortfolioTurnoverTextBlock	The Fund may pay transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 72% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	72.00%
Expense Breakpoint Discounts [Text]	rr_ExpenseBreakpointDiscounts	You may qualify for sales charge discounts if you and members of your immediate family invest, or agree to invest in the future, at least $50,000 in certain classes of shares of eligible funds distributed by Columbia Management Investment Distributors, Inc. (the Distributor). More information is available about these and other sales charge discounts and waivers from your financial intermediary, and can be found in the Choosing a Share Class section beginning on page 22 of the Fund’s prospectus, in Appendix A to the prospectus beginning on page A-1 and in Appendix S to the Statement of Additional Information (SAI) under Sales Charge Waivers beginning on page S-1.
Expense Breakpoint, Minimum Investment Required [Amount]	rr_ExpenseBreakpointMinimumInvestmentRequiredAmount	$ 50,000
Expenses Restated to Reflect Current [Text]	rr_ExpensesRestatedToReflectCurrent	Other expenses have been restated to reflect current transfer agency fees paid by the Fund.
Expenses Not Correlated to Ratio Due to Acquired Fund Fees [Text]	rr_ExpensesNotCorrelatedToRatioDueToAcquiredFundFees	“Total annual Fund operating expenses” include acquired fund fees and expenses (expenses the Fund incurs indirectly through its investments in other investment companies) and may be higher than “Total Net Expenses” shown in the Financial Highlights section of this prospectus because “Total Net Expenses” do not include acquired fund fees and expenses.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	The following example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example illustrates the hypothetical expenses that you would incur over the time periods indicated, and assumes that: you invest $10,000 in the applicable class of Fund shares for the periods indicated, your investment has a 5% return each year, and the Fund’s total annual operating expenses remain the same as shown in the Annual Fund Operating Expenses table above. Since the waivers and/or reimbursements shown in the Annual Fund Operating Expenses table above expire as indicated in the preceding table, they are only reflected in the 1 year example and the first year of the other examples. Although your actual costs may be higher or lower, based on the assumptions listed above, your costs would be:
Strategy [Heading]	rr_StrategyHeading	Principal Investment Strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	Under normal circumstances, the Fund (i) invests a majority of its net assets in the common stock of small- and mid-sized companies with market capitalizations under $5 billion at the time of initial investment (“Focus Stocks”) and (ii) may also invest in companies with market capitalizations above $5 billion, provided that immediately after that investment a majority of the Fund’s net assets would be invested in Focus Stocks. The Fund may continue to hold, and make additional investments in, Focus Stocks whose market capitalizations have grown to exceed $5 billion, regardless of whether the Fund’s investments in Focus Stocks are a majority of the Fund’s net assets. Columbia Wanger Asset Management, LLC, the Fund's investment adviser (the Investment Manager), believes that stocks of small- and mid-sized companies, which generally are not as well known by financial analysts as larger companies, may offer higher return potential than stocks of larger companies. The Fund invests the majority of its assets in U.S. companies, but also may invest up to 33% of its total assets in foreign companies in developed markets (for example, Japan, Canada and the United Kingdom) and in emerging markets (for example, China, India and Brazil). The Investment Manager typically seeks companies with: A strong business franchise that offers growth potential. Products and services in which the company has a competitive advantage. A stock price the Investment Manager believes is reasonable relative to the assets and earning power of the company. The Investment Manager may sell a portfolio holding if the security reaches the Investment Manager's price target, if the company has a deterioration of fundamentals, such as failing to meet key operating benchmarks, or if the Investment Manager believes other securities are more attractive. The Investment Manager also may sell a portfolio holding to fund redemptions.
Risk [Heading]	rr_RiskHeading	Principal Risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the Fund involves risks, including those described below. There is no assurance that the Fund will achieve its investment objective and you may lose money. The value of the Fund’s holdings may decline, and the Fund’s net asset value (NAV) and share price may go down. An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Active Management Risk. The Investment Manager’s active management of the Fund could cause the Fund to underperform its benchmark index and/or other funds with similar investment objectives and/or strategies. Small- and Mid-Cap Company Securities Risk. Investments in small- and mid-capitalization companies (small- and mid-cap companies) often involve greater risks than investments in larger, more established companies (larger companies) because small- and mid-cap companies tend to have less predictable earnings and may lack the management experience, financial resources, product diversification and competitive strengths of larger companies. Securities of small- and mid-cap companies may be less liquid and more volatile than the securities of larger companies. Issuer Risk. An issuer in which the Fund invests may perform poorly, and the value of its securities may therefore decline, which would negatively affect the Fund’s performance. Poor performance may be caused by poor management decisions, competitive pressures, breakthroughs in technology, reliance on suppliers, labor problems or shortages, corporate restructurings, fraudulent disclosures, natural disasters or other events, conditions or factors. Market Risk. Market risk refers to the possibility that the market values of securities or other investments that the Fund holds will fall, sometimes rapidly or unpredictably, or fail to rise. An investment in the Fund could lose money over short or long periods. Growth Securities Risk. Growth securities typically trade at a higher multiple of earnings than other types of equity securities. Accordingly, the market values of growth securities may never reach their expected market value and may decline in price. In addition, growth securities, at times, may not perform as well as value securities or the stock market in general, and may be out of favor with investors for varying periods of time. Foreign Securities Risk. Investments in or exposure to foreign securities involve certain risks not associated with investments in or exposure to securities of U.S. companies. Foreign securities subject the Fund to the risks associated with investing in the particular country of an issuer, including political, regulatory, economic, social, diplomatic and other conditions or events (including, for example, military confrontations, war and terrorism), occurring in the country or region, as well as risks associated with less developed custody and settlement practices. Foreign securities may be more volatile and less liquid than securities of U.S. companies, and are subject to the risks associated with potential imposition of economic and other sanctions against a particular foreign country, its nationals or industries or businesses within the country. In addition, foreign governments may impose withholding or other taxes on the Fund’s income, capital gains or proceeds from the disposition of foreign securities, which could reduce the Fund’s return on such securities. Operational and Settlement Risks of Foreign Securities. The Fund’s foreign securities are generally held outside the United States in the primary market for the securities in the custody of foreign sub-custodians. Some countries have limited governmental oversight and regulation, which increases the risk of corruption and fraud and the possibility of losses to the Fund. In particular, under certain circumstances, foreign securities may settle on a delayed delivery basis, meaning that the Fund may be required to make payment for securities before the Fund has actually received delivery of the securities or deliver securities prior to the receipt of payment. As a result, there is a risk that the security will not be delivered to the Fund or that payment will not be received. Losses can also result from lost, stolen or counterfeit securities; defaults by brokers and banks; failures or defects of the settlement system; or poor and improper record keeping by registrars and issuers. Share Blocking. In certain non-U.S. markets, an issuer’s securities are blocked from trading for a specified number of days before and, in certain instances, after a shareholder meeting. The blocking period can last up to several weeks. Share blocking may prevent the Fund from buying or selling securities during this period. As a consequence of these restrictions, the Investment Manager, on behalf of the Fund, may abstain from voting proxies in markets that require share blocking. Emerging Market Securities Risk. Securities issued by foreign governments or companies in emerging market countries, such as China, Russia and certain countries in Eastern Europe, the Middle East, Asia, Latin America or Africa, are more likely to have greater exposure to the risks of investing in foreign securities that are described in Foreign Securities Risk. In addition, emerging market countries are more likely to experience instability resulting, for example, from rapid changes or developments in social, political, economic or other conditions. Their economies are usually less mature and their securities markets are typically less developed with more limited trading activity (i.e., lower trading volumes and less liquidity) than more developed countries. Emerging market securities tend to be more volatile than securities in more developed markets. Many emerging market countries are heavily dependent on international trade and have fewer trading partners, which makes them more sensitive to world commodity prices and economic downturns in other countries, and some have a higher risk of currency devaluations. Operational and Settlement Risks of Securities in Emerging Markets. Foreign sub-custodians in emerging markets may be recently organized, lack extensive operating experience or lack effective government oversight or regulation. In addition, there may be legal restrictions or limitations on the ability of the Fund to recover assets held in custody by a foreign sub-custodian in the event of the bankruptcy of the sub-custodian. There may also be a greater risk that settlement may be delayed and that cash or securities of the Fund may be lost because of failures of or defects in the system, including fraud or corruption. Settlement systems in emerging markets also have a higher risk of failed trades. Risks Related to Currencies and Corporate Actions in Emerging Markets. Risks related to currencies and corporate actions are also greater in emerging market countries than in developed countries. Emerging market currencies may not have an active trading market and are subject to a higher risk of currency devaluations. Risks Related to Corporate and Securities Laws in Emerging Markets. Securities laws in emerging markets may be relatively new and unsettled and, consequently, there is a risk of rapid and unpredictable change in laws regarding foreign investment, securities regulation, title to securities and shareholder rights. Liquidity and Trading Volume Risk. Because the Fund may invest a percentage of its assets in foreign securities, it may be subject to the liquidity and trading volume risks associated with international investing. Due to market conditions, including uncertainty regarding the price of a security, it may be difficult for the Fund to buy or sell foreign portfolio securities at a desirable time or price, which could result in investment losses and may affect the value of your investment in the Fund. This risk of portfolio illiquidity is heightened with respect to small- and mid-cap securities, generally, and foreign small- and mid-cap securities in particular. The Fund may have to lower the selling price, liquidate other investments, or forego another, more appealing investment opportunity as a result of illiquidity in the markets. As a result of significant and sustained reductions in emerging and developed international market trading volumes in the wake of the 2007-2009 financial crisis, it may take longer to buy or sell these securities, which can exacerbate the Fund’s exposure to volatile markets. The Fund may also be limited in its ability to execute favorable trades in foreign portfolio securities in response to changes in share prices and fundamentals, and may be forced to dispose of securities under disadvantageous circumstances and at a loss. As the Fund grows in size or, conversely, if it faces significant redemption pressure, these considerations take on increasing significance and may adversely impact performance. Foreign Currency Risk. The performance of the Fund may be materially affected positively or negatively by foreign currency strength or weakness relative to the U.S. dollar, particularly if the Fund invests a significant percentage of its assets in foreign securities or other assets denominated in currencies other than the U.S. dollar. Sector Risk. At times, the Fund may have a significant portion of its assets invested in securities of companies conducting business in a related group of industries within a sector. Companies in the same sector may be similarly affected by economic, regulatory, political or market events or conditions, which may make the Fund more vulnerable to unfavorable developments in that sector than funds that invest more broadly. Generally, the more the Fund diversifies its investments, the more it spreads risk and potentially reduces the risks of loss and volatility.
Risk Lose Money [Text]	rr_RiskLoseMoney	There is no assurance that the Fund will achieve its investment objective and you may lose money.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance Information
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The following bar chart and table show you how the Fund has performed in the past, and can help you understand the risks of investing in the Fund. The bar chart shows how the Fund’s Class Inst share performance has varied for each full calendar year shown. The table below the bar chart compares the Fund’s returns for the periods shown with the Russell 2500 Growth Index. The performance of one or more share classes shown in the Average Annual Total Returns table below includes the Fund’s Class Inst share returns (adjusted to reflect the higher class-related operating expenses of such classes, where applicable) for periods prior to the indicated inception date of such share classes. Except for differences in fees and expenses, all share classes of the Fund would have substantially similar annual returns because all share classes of the Fund invest in the same portfolio of securities. The after-tax returns shown in the Average Annual Total Returns table below are calculated using the highest historical individual U.S. federal marginal income tax rates in effect during the period indicated in the table and do not reflect the impact of state, local or foreign taxes. Your actual after-tax returns will depend on your personal tax situation and may differ from those shown in the table. In addition, the after-tax returns shown in the table do not apply to shares held in tax-advantaged accounts such as 401(k) plans or Individual Retirement Accounts (IRAs). The after-tax returns are shown only for Class Inst shares and will vary for other share classes. The Fund’s past performance (before and after taxes) is no guarantee of how the Fund will perform in the future. Updated performance information can be obtained by calling toll-free 800.345.6611 or visiting columbiathreadneedle.com/us.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The following bar chart and table show you how the Fund has performed in the past, and can help you understand the risks of investing in the Fund. The bar chart shows how the Fund’s Class Inst share performance has varied for each full calendar year shown. The table below the bar chart compares the Fund’s returns for the periods shown with the Russell 2500 Growth Index.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	800.345.6611
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	columbiathreadneedle.com/us
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The Fund’s past performance (before and after taxes) is no guarantee of how the Fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Year by Year Total Return (%) as of December 31 Each Year
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Best and Worst Quarterly Returns During the Period Shown in the Bar Chart Best                                                        2nd Quarter 2009                                                      20.90% Worst                                                     4th Quarter 2008                                                      -25.11%
Performance Table Heading	rr_PerformanceTableHeading	Average Annual Total Returns After Applicable Sales Charges (for periods ended December 31, 2017)
Performance Table Uses Highest Federal Rate	rr_PerformanceTableUsesHighestFederalRate	The after-tax returns shown in the Average Annual Total Returns table below are calculated using the highest historical individual U.S. federal marginal income tax rates in effect during the period indicated in the table and do not reflect the impact of state, local or foreign taxes.
Performance Table Not Relevant to Tax Deferred	rr_PerformanceTableNotRelevantToTaxDeferred	In addition, the after-tax returns shown in the table do not apply to shares held in tax-advantaged accounts such as 401(k) plans or Individual Retirement Accounts (IRAs).
Performance Table One Class of after Tax Shown [Text]	rr_PerformanceTableOneClassOfAfterTaxShown	The after-tax returns are shown only for Class Inst shares and will vary for other share classes.
Columbia Acorn Fund | Class A
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	5.75%
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	1.00%	[1]
Management fees	rr_ManagementFeesOverAssets	0.67%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Other expenses	rr_OtherExpensesOverAssets	0.18%	[2]
Acquired fund fees and expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.08%
Total annual Fund operating expenses	rr_ExpensesOverAssets	1.18%	[3]
Less: Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.07%)	[4]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	1.11%
Expenses Deferred Charges [Text Block]	rr_ExpensesDeferredChargesTextBlock	This charge is imposed on certain investments of between $1 million and $50 million redeemed within 18 months after purchase, as follows: 1.00% if redeemed within 12 months after purchase, and 0.50% if redeemed more than 12, but less than 18, months after purchase, with certain limited exceptions.
1 year	rr_ExpenseExampleYear01	$ 682
3 years	rr_ExpenseExampleYear03	922
5 years	rr_ExpenseExampleYear05	1,180
10 years	rr_ExpenseExampleYear10	1,919
1 year	rr_ExpenseExampleNoRedemptionYear01	682
3 years	rr_ExpenseExampleNoRedemptionYear03	922
5 years	rr_ExpenseExampleNoRedemptionYear05	1,180
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 1,919
1 Year	rr_AverageAnnualReturnYear01	17.77%
5 Years	rr_AverageAnnualReturnYear05	10.78%
10 Years	rr_AverageAnnualReturnYear10	7.18%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Oct. 16, 2000
Columbia Acorn Fund | Class Adv
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.67%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.18%	[2]
Acquired fund fees and expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.08%
Total annual Fund operating expenses	rr_ExpensesOverAssets	0.93%	[3]
Less: Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.07%)	[4]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	0.86%
1 year	rr_ExpenseExampleYear01	$ 88
3 years	rr_ExpenseExampleYear03	289
5 years	rr_ExpenseExampleYear05	508
10 years	rr_ExpenseExampleYear10	1,137
1 year	rr_ExpenseExampleNoRedemptionYear01	88
3 years	rr_ExpenseExampleNoRedemptionYear03	289
5 years	rr_ExpenseExampleNoRedemptionYear05	508
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 1,137
1 Year	rr_AverageAnnualReturnYear01	25.19%
5 Years	rr_AverageAnnualReturnYear05	12.35%
10 Years	rr_AverageAnnualReturnYear10	8.09%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 08, 2012
Columbia Acorn Fund | Class C
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	1.00%	[5]
Management fees	rr_ManagementFeesOverAssets	0.67%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	1.00%
Other expenses	rr_OtherExpensesOverAssets	0.18%	[2]
Acquired fund fees and expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.08%
Total annual Fund operating expenses	rr_ExpensesOverAssets	1.93%	[3]
Less: Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.07%)	[4]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	1.86%
Expenses Deferred Charges [Text Block]	rr_ExpensesDeferredChargesTextBlock	This charge applies to redemptions within 12 months after purchase, with certain limited exceptions.
1 year	rr_ExpenseExampleYear01	$ 289
3 years	rr_ExpenseExampleYear03	599
5 years	rr_ExpenseExampleYear05	1,035
10 years	rr_ExpenseExampleYear10	2,248
1 year	rr_ExpenseExampleNoRedemptionYear01	189
3 years	rr_ExpenseExampleNoRedemptionYear03	599
5 years	rr_ExpenseExampleNoRedemptionYear05	1,035
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 2,248
1 Year	rr_AverageAnnualReturnYear01	23.10%
5 Years	rr_AverageAnnualReturnYear05	11.28%
10 Years	rr_AverageAnnualReturnYear10	7.00%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Oct. 16, 2000
Columbia Acorn Fund | Class Inst
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.67%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.18%	[2]
Acquired fund fees and expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.08%
Total annual Fund operating expenses	rr_ExpensesOverAssets	0.93%	[3]
Less: Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.07%)	[4]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	0.86%
1 year	rr_ExpenseExampleYear01	$ 88
3 years	rr_ExpenseExampleYear03	289
5 years	rr_ExpenseExampleYear05	508
10 years	rr_ExpenseExampleYear10	1,137
1 year	rr_ExpenseExampleNoRedemptionYear01	88
3 years	rr_ExpenseExampleNoRedemptionYear03	289
5 years	rr_ExpenseExampleNoRedemptionYear05	508
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 1,137
2008	rr_AnnualReturn2008	(38.55%)	[6]
2009	rr_AnnualReturn2009	39.65%	[6]
2010	rr_AnnualReturn2010	26.00%	[6]
2011	rr_AnnualReturn2011	(4.61%)	[6]
2012	rr_AnnualReturn2012	17.93%	[6]
2013	rr_AnnualReturn2013	30.90%	[6]
2014	rr_AnnualReturn2014	0.82%	[6]
2015	rr_AnnualReturn2015	(1.57%)	[6]
2016	rr_AnnualReturn2016	10.39%	[6]
2017	rr_AnnualReturn2017	25.24%	[6]
Year to Date Return, Label	rr_YearToDateReturnLabel	Year to Date return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Mar. 31, 2018
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	3.57%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Jun. 30, 2009
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	20.90%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Dec. 31, 2008
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(25.11%)
1 Year	rr_AverageAnnualReturnYear01	25.24%
5 Years	rr_AverageAnnualReturnYear05	12.42%
10 Years	rr_AverageAnnualReturnYear10	8.13%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Jun. 10, 1970
Columbia Acorn Fund | Class Inst2
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.67%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.16%	[2]
Acquired fund fees and expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.08%
Total annual Fund operating expenses	rr_ExpensesOverAssets	0.91%	[3]
Less: Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.07%)	[4]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	0.84%
1 year	rr_ExpenseExampleYear01	$ 86
3 years	rr_ExpenseExampleYear03	283
5 years	rr_ExpenseExampleYear05	497
10 years	rr_ExpenseExampleYear10	1,113
1 year	rr_ExpenseExampleNoRedemptionYear01	86
3 years	rr_ExpenseExampleNoRedemptionYear03	283
5 years	rr_ExpenseExampleNoRedemptionYear05	497
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 1,113
1 Year	rr_AverageAnnualReturnYear01	25.21%
5 Years	rr_AverageAnnualReturnYear05	12.44%
10 Years	rr_AverageAnnualReturnYear10	8.14%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 08, 2012
Columbia Acorn Fund | Class Inst3
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.67%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.09%	[2]
Acquired fund fees and expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.08%
Total annual Fund operating expenses	rr_ExpensesOverAssets	0.84%	[3]
Less: Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.05%)	[4]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	0.79%
1 year	rr_ExpenseExampleYear01	$ 81
3 years	rr_ExpenseExampleYear03	263
5 years	rr_ExpenseExampleYear05	461
10 years	rr_ExpenseExampleYear10	1,033
1 year	rr_ExpenseExampleNoRedemptionYear01	81
3 years	rr_ExpenseExampleNoRedemptionYear03	263
5 years	rr_ExpenseExampleNoRedemptionYear05	461
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 1,033
1 Year	rr_AverageAnnualReturnYear01	25.31%
5 Years	rr_AverageAnnualReturnYear05	12.51%
10 Years	rr_AverageAnnualReturnYear10	8.17%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 08, 2012
Columbia Acorn Fund | returns after taxes on distributions | Class Inst
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	18.84%
5 Years	rr_AverageAnnualReturnYear05	6.33%
10 Years	rr_AverageAnnualReturnYear10	4.90%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Jun. 10, 1970
Columbia Acorn Fund | returns after taxes on distributions and sale of Fund shares | Class Inst
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	18.52%
5 Years	rr_AverageAnnualReturnYear05	8.87%
10 Years	rr_AverageAnnualReturnYear10	6.09%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Jun. 10, 1970
Columbia Acorn Fund | Russell 2500 Growth Index (reflects no deductions for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	24.46%
5 Years	rr_AverageAnnualReturnYear05	15.47%
10 Years	rr_AverageAnnualReturnYear10	9.62%

[1] This charge is imposed on certain investments of between $1 million and $50 million redeemed within 18 months after purchase, as follows: 1.00% if redeemed within 12 months after purchase, and 0.50% if redeemed more than 12, but less than 18, months after purchase, with certain limited exceptions.

[2] Other expenses have been restated to reflect current transfer agency fees paid by the Fund.

[3] “Total annual Fund operating expenses” include acquired fund fees and expenses (expenses the Fund incurs indirectly through its investments in other investment companies) and may be higher than “Total Net Expenses” shown in the Financial Highlights section of this prospectus because “Total Net Expenses” do not include acquired fund fees and expenses.

[4] Columbia Wanger Asset Management, LLC (the Investment Manager) has contractually agreed to waive fees and reimburse certain expenses of the Fund, through April 30, 2019, so that ordinary operating expenses (excluding transaction costs and certain other investment-related expenses, interest and fees on borrowings, if any) do not exceed the annual rates of 1.11% for Class A shares, 0.86% for Class Adv shares, 1.86% for Class C shares, 0.86% for Class Inst shares, 0.84% for Class Inst2 shares and 0.79% for Class Inst3 shares. This arrangement may only be amended or terminated with approval from the Fund’s Board of Trustees and the Investment Manager. The fee waivers and/or expense reimbursements shown in the table also reflect the contractual agreement of the Fund’s transfer agent, Columbia Management Investment Services Corp. (the Transfer Agent), to waive a portion of its fees through June 30, 2019, such that the Fund’s transfer agency fees do not exceed the annual rate of 0.07% of the average daily net assets of each share class other than Class Inst2 and Class Inst3. With respect to Class Inst2 and Class Inst3 shares, the Transfer Agent has contractually agreed to waive a portion of its fees such that the Fund's transfer agency fees do not exceed the annual rates of 0.05% of the average daily net assets of Class Inst2 shares and 0.00% of the average daily net assets of Class Inst3 shares.

[5] This charge applies to redemptions within 12 months after purchase, with certain limited exceptions.

[6] Year to Date return as of March 31, 2018: 3.57%

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Columbia Acorn International

SUMMARY OF THE FUND

Investment Objective

Columbia Acorn International^® (the Fund) seeks long-term capital appreciation.

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. An investor transacting in a class of Fund shares without any front-end sales charge, contingent deferred sales charge, or other asset-based fee for sales or distribution may be required to pay a commission to the financial intermediary for effecting such transactions. Such commission rates are set by the financial intermediary and are not reflected in the tables or the example below. You may qualify for sales charge discounts if you and members of your immediate family invest, or agree to invest in the future, at least $50,000 in certain classes of shares of eligible funds distributed by Columbia Management Investment Distributors, Inc. (the Distributor). More information is available about these and other sales charge discounts and waivers from your financial intermediary, and can be found in the Choosing a Share Class section beginning on page 24 of the Fund’s prospectus, in Appendix A to the prospectus beginning on page A-1 and in Appendix S to the Statement of Additional Information (SAI) under Sales Charge Waivers beginning on page S-1.

Shareholder Fees (fees paid directly from your investment)

Shareholder Fees - Columbia Acorn International	Class A		Class C		Class Adv	Class Inst	Class Inst2	Class Inst3	Class R
Maximum sales charge (load) imposed on purchases (as a % of offering price)	5.75%		none		none	none	none	none	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	1.00%	[1]	1.00%	[2]	none	none	none	none	none

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)

Annual Fund Operating Expenses - Columbia Acorn International		Class A	Class Adv	Class C	Class Inst	Class Inst2	Class Inst3	Class R
Management fees		0.77%	0.77%	0.77%	0.77%	0.77%	0.77%	0.77%
Distribution and/or service (12b-1) fees		0.25%	none	1.00%	none	none	none	0.50%
Other expenses	[1]	0.22%	0.22%	0.22%	0.22%	0.18%	0.11%	0.22%
Total annual Fund operating expenses		1.24%	0.99%	1.99%	0.99%	0.95%	0.88%	1.49%
Less: Fee waivers and/or expense reimbursements	[2]	none	none	none	none	(0.02%)	none	none
Total annual Fund operating expenses after fee waivers and/or expense reimbursements		1.24%	0.99%	1.99%	0.99%	0.93%	0.88%	1.49%

Example

The following example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example illustrates the hypothetical expenses that you would incur over the time periods indicated, and assumes that:

• you invest $10,000 in the applicable class of Fund shares for the periods indicated,

• your investment has a 5% return each year, and

• the Fund’s total annual operating expenses remain the same as shown in the Annual Fund Operating Expenses table above.

Since the waivers and/or reimbursements shown in the Annual Fund Operating Expenses table above expire as indicated in the preceding table, they are only reflected in the 1 year example and the first year of the other examples. Although your actual costs may be higher or lower, based on the assumptions listed above, your costs would be:

Expense Example - Columbia Acorn International - USD ($)	1 year	3 years	5 years	10 years
Class A	694	946	1,217	1,989
Class Adv	101	315	547	1,213
Class C	302	624	1,073	2,317
Class Inst	101	315	547	1,213
Class Inst2	95	301	524	1,165
Class Inst3	90	281	488	1,084
Class R	152	471	813	1,779

Expense Example, No Redemption - Columbia Acorn International - USD ($)	1 year	3 years	5 years	10 years
Class A	694	946	1,217	1,989
Class Adv	101	315	547	1,213
Class C	202	624	1,073	2,317
Class Inst	101	315	547	1,213
Class Inst2	95	301	524	1,165
Class Inst3	90	281	488	1,084
Class R	152	471	813	1,779

Portfolio Turnover

The Fund may pay transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 37% of the average value of its portfolio.

Principal Investment Strategies

Under normal circumstances, the Fund invests at least 75% of its net assets in foreign companies in developed markets (for example, Japan, Canada and the United Kingdom) and in emerging markets (for example, China, India and Brazil).

Under normal circumstances, the Fund (i) invests a majority of its net assets in the common stock of small- and mid-sized companies with market capitalizations under $10 billion at the time of initial investment (“Focus Stocks”) and (ii) may also invest in companies with market capitalizations above $10 billion, provided that immediately after that investment a majority of the Fund’s net assets would be invested in Focus Stocks. The Fund may continue to hold, and make additional investments in, Focus Stocks whose market capitalizations have grown to exceed $10 billion, regardless of whether the Fund’s investments in Focus Stocks are a majority of the Fund’s net assets. The Fund may from time to time emphasize one or more sectors in selecting its investments.

Columbia Wanger Asset Management, LLC, the Fund's investment adviser (the Investment Manager), believes that stocks of small- and mid-sized companies, which generally are not as well known by financial analysts as larger companies, may offer higher return potential than stocks of larger companies. The Fund also may invest in larger-sized companies.

The Investment Manager typically seeks companies with:

• A strong business franchise that offers growth potential.

• Products and services in which the company has a competitive advantage.

• A stock price the Investment Manager believes is reasonable relative to the assets and earning power of the company.

The Investment Manager may sell a portfolio holding if the security reaches the Investment Manager's price target, if the company has a deterioration of fundamentals, such as failing to meet key operating benchmarks, or if the Investment Manager believes other securities are more attractive. The Investment Manager also may sell a portfolio holding to fund redemptions.

Principal Risks

An investment in the Fund involves risks, including those described below. There is no assurance that the Fund will achieve its investment objective and you may lose money. The value of the Fund’s holdings may decline, and the Fund’s net asset value (NAV) and share price may go down. An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.

Active Management Risk. The Investment Manager’s active management of the Fund could cause the Fund to underperform its benchmark index and/or other funds with similar investment objectives and/or strategies.

Small- and Mid-Cap Company Securities Risk. Investments in small- and mid-capitalization companies (small- and mid-cap companies) often involve greater risks than investments in larger, more established companies (larger companies) because small- and mid-cap companies tend to have less predictable earnings and may lack the management experience, financial resources, product diversification and competitive strengths of larger companies. Securities of small- and mid-cap companies may be less liquid and more volatile than the securities of larger companies.

Market Risk. Market risk refers to the possibility that the market values of securities or other investments that the Fund holds will fall, sometimes rapidly or unpredictably, or fail to rise. An investment in the Fund could lose money over short or long periods.

Foreign Securities Risk. Investments in or exposure to foreign securities involve certain risks not associated with investments in or exposure to securities of U.S. companies. Foreign securities subject the Fund to the risks associated with investing in the particular country of an issuer, including political, regulatory, economic, social, diplomatic and other conditions or events (including, for example, military confrontations, war and terrorism), occurring in the country or region, as well as risks associated with less developed custody and settlement practices. Foreign securities may be more volatile and less liquid than securities of U.S. companies, and are subject to the risks associated with potential imposition of economic and other sanctions against a particular foreign country, its nationals or industries or businesses within the country. In addition, foreign governments may impose withholding or other taxes on the Fund’s income, capital gains or proceeds from the disposition of foreign securities, which could reduce the Fund’s return on such securities.

Operational and Settlement Risks of Foreign Securities. The Fund’s foreign securities are generally held outside the United States in the primary market for the securities in the custody of foreign sub-custodians. Some countries have limited governmental oversight and regulation, which increases the risk of corruption and fraud and the possibility of losses to the Fund. In particular, under certain circumstances, foreign securities may settle on a delayed delivery basis, meaning that the Fund may be required to make payment for securities before the Fund has actually received delivery of the securities or deliver securities prior to the receipt of payment. As a result, there is a risk that the security will not be delivered to the Fund or that payment will not be received. Losses can also result from lost, stolen or counterfeit securities; defaults by brokers and banks; failures or defects of the settlement system; or poor and improper record keeping by registrars and issuers.

Share Blocking. In certain non-U.S. markets, an issuer’s securities are blocked from trading for a specified number of days before and, in certain instances, after a shareholder meeting. The blocking period can last up to several weeks. Share blocking may prevent the Fund from buying or selling securities during this period. As a consequence of these restrictions, the Investment Manager, on behalf of the Fund, may abstain from voting proxies in markets that require share blocking.

Emerging Market Securities Risk. Securities issued by foreign governments or companies in emerging market countries, such as China, Russia and certain countries in Eastern Europe, the Middle East, Asia, Latin America or Africa, are more likely to have greater exposure to the risks of investing in foreign securities that are described in Foreign Securities Risk. In addition, emerging market countries are more likely to experience instability resulting, for example, from rapid changes or developments in social, political, economic or other conditions. Their economies are usually less mature and their securities markets are typically less developed with more limited trading activity (i.e., lower trading volumes and less liquidity) than more developed countries. Emerging market securities tend to be more volatile than securities in more developed markets. Many emerging market countries are heavily dependent on international trade and have fewer trading partners, which makes them more sensitive to world commodity prices and economic downturns in other countries, and some have a higher risk of currency devaluations.

Operational and Settlement Risks of Securities in Emerging Markets. Foreign sub-custodians in emerging markets may be recently organized, lack extensive operating experience or lack effective government oversight or regulation. In addition, there may be legal restrictions or limitations on the ability of the Fund to recover assets held in custody by a foreign sub-custodian in the event of the bankruptcy of the sub-custodian. There may also be a greater risk that settlement may be delayed and that cash or securities of the Fund may be lost because of failures of or defects in the system, including fraud or corruption. Settlement systems in emerging markets also have a higher risk of failed trades.

Risks Related to Currencies and Corporate Actions in Emerging Markets. Risks related to currencies and corporate actions are also greater in emerging market countries than in developed countries. Emerging market currencies may not have an active trading market and are subject to a higher risk of currency devaluations.

Risks Related to Corporate and Securities Laws in Emerging Markets. Securities laws in emerging markets may be relatively new and unsettled and, consequently, there is a risk of rapid and unpredictable change in laws regarding foreign investment, securities regulation, title to securities and shareholder rights.

Liquidity and Trading Volume Risk. Due to market conditions, including uncertainty regarding the price of a security, it may be difficult for the Fund to buy or sell portfolio securities at a desirable time or price, which could result in investment losses. This risk of portfolio illiquidity is heightened with respect to small- and mid-cap securities, generally, and foreign small- and mid-cap securities in particular. The Fund may have to lower the selling price, liquidate other investments, or forego another, more appealing investment opportunity as a result of illiquidity in the markets. As a result of significant and sustained reductions in emerging and developed international market trading volumes in the wake of the 2007-2009 financial crisis, it may take longer to buy or sell securities, which can exacerbate the Fund’s exposure to volatile markets. The Fund may also be limited in its ability to execute favorable trades in portfolio securities in response to changes in share prices and fundamentals, and may be forced to dispose of securities under disadvantageous circumstances and at a loss. As the Fund grows in size or, conversely, if it faces significant redemption pressure, these considerations take on increasing significance and may adversely impact performance.

Sector Risk. At times, the Fund may have a significant portion of its assets invested in securities of companies conducting business in a related group of industries within a sector. Companies in the same sector may be similarly affected by economic, regulatory, political or market events or conditions, which may make the Fund more vulnerable to unfavorable developments in that sector than funds that invest more broadly. Generally, the more the Fund diversifies its investments, the more it spreads risk and potentially reduces the risks of loss and volatility.

Foreign Currency Risk. The performance of the Fund may be materially affected positively or negatively by foreign currency strength or weakness relative to the U.S. dollar, particularly if the Fund invests a significant percentage of its assets in foreign securities or other assets denominated in currencies other than the U.S. dollar.

Issuer Risk. An issuer in which the Fund invests may perform poorly, and the value of its securities may therefore decline, which would negatively affect the Fund’s performance. Poor performance may be caused by poor management decisions, competitive pressures, breakthroughs in technology, reliance on suppliers, labor problems or shortages, corporate restructurings, fraudulent disclosures, natural disasters or other events, conditions or factors.

Geographic Focus Risk. The Fund may be particularly susceptible to economic, political, regulatory or other events or conditions affecting issuers and countries within the specific geographic regions in which the Fund invests. The Fund’s NAV may be more volatile than the NAV of a more geographically diversified fund.

Asia Pacific Region. Many of the countries in the Asia Pacific region are considered underdeveloped or developing, including from a political, economic and/or social perspective, and may have relatively unstable governments and economies based on limited business, industries and/or natural resources or commodities. Events in any one country within the region may impact other countries in the region or the region as a whole. As a result, events in the region will generally have a greater effect on the Fund than if the Fund were more geographically diversified. This could result in increased volatility in the value of the Fund’s investments and losses for the Fund. Also, securities of some companies in the region can be less liquid than U.S. or other foreign securities, potentially making it difficult for the Fund to sell such securities at a desirable time and price.

Europe. The Fund is particularly susceptible to economic, political, regulatory or other events or conditions affecting issuers and countries in Europe. In addition, the private and public sectors’ debt problems of a single European Union (EU) country can pose significant economic risks to the EU as a whole. As a result, the Fund’s NAV may be more volatile than the NAV of a more geographically diversified fund. If securities of issuers in Europe fall out of favor, it may cause the Fund to underperform other funds that do not focus their investments in this region of the world. The impact of any partial or complete dissolution of the EU on European economies could be significant, resulting in negative impacts on currency and financial markets generally, such as increased volatility and illiquidity, and potentially lower economic growth in markets in Europe, which may adversely affect the value of your investment in the Fund.

Growth Securities Risk. Growth securities typically trade at a higher multiple of earnings than other types of equity securities. Accordingly, the market values of growth securities may never reach their expected market value and may decline in price. In addition, growth securities, at times, may not perform as well as value securities or the stock market in general, and may be out of favor with investors for varying periods of time.

Performance Information

The following bar chart and table show you how the Fund has performed in the past, and can help you understand the risks of investing in the Fund. The bar chart shows how the Fund's Class Inst share performance has varied for each full calendar year shown. The table below the bar chart compares the Fund's returns for the periods shown with those of the MSCI ACWI ex USA SMID Cap Index (Net), the Fund's primary benchmark, and those of the MSCI ACWI ex USA SMID Cap Growth Index (Net), the Fund's secondary benchmark.  The MSCI ACWI ex USA SMID Cap Index (Net) captures mid- and small-cap representation across 22 of 23 developed market countries (excluding the United States) and 24 emerging market countries.  It has 5,290 constituents (as of March 31, 2018) and covers approximately 28% of the free float adjusted market capitalization in each country.  The MSCI ACWI ex USA SMID Cap Growth Index (Net) captures mid- and small-cap representation across 22 developed markets countries and 24 emerging markets countries.  The Investment Manager believes that the secondary benchmark provides a meaningful additional basis for comparing the Fund's performance, because the Fund's portfolio will in general be as closely aligned with the additional growth benchmark as with the current core benchmark from an investment style perspective.

The performance of one or more share classes shown in the Average Annual Total Returns table below includes the Fund's Class Inst share returns (adjusted to reflect the higher class-related operating expenses of such classes, where applicable) for periods prior to the indicated inception date of such share classes. Except for differences in fees and expenses, all share classes of the Fund would have substantially similar annual returns because all share classes of the Fund invest in the same portfolio of securities.

The after-tax returns shown in the Average Annual Total Returns table below are calculated using the highest historical individual U.S. federal marginal income tax rates in effect during the period indicated in the table and do not reflect the impact of state, local or foreign taxes. Your actual after-tax returns will depend on your personal tax situation and may differ from those shown in the table. In addition, the after-tax returns shown in the table do not apply to shares held in tax-advantaged accounts such as 401(k) plans or Individual Retirement Accounts (IRAs). The after-tax returns are shown only for Class Inst shares and will vary for other share classes.

The Fund's past performance (before and after taxes) is no guarantee of how the Fund will perform in the future. Updated performance information can be obtained by calling toll-free 800.345.6611 or visiting columbiathreadneedle.com/us.

Year by Year Total Return (%)
as of December 31 Each Year

Best and Worst Quarterly Returns
During the Period Shown in the Bar Chart

Best                                                          2nd Quarter 2009                                                         33.30%

Worst                                                       3rd Quarter 2008                                                         -23.68%

Average Annual Total Returns After Applicable Sales Charges (for periods ended December 31, 2017)

Average Annual Total Returns - Columbia Acorn International	Share Class Inception Date	1 Year	5 Years	10 Years
Class Inst	Sep. 23, 1992	32.24%	8.35%	4.57%
Class Inst | returns after taxes on distributions	Sep. 23, 1992	30.27%	7.09%	3.78%
Class Inst | returns after taxes on distributions and sale of Fund shares	Sep. 23, 1992	20.07%	6.50%	3.59%
Class A	Oct. 16, 2000	24.33%	6.78%	3.63%
Class Adv	Nov. 08, 2012	32.21%	8.29%	4.54%
Class C	Oct. 16, 2000	29.93%	7.25%	3.45%
Class Inst2	Aug. 02, 2011	32.27%	8.39%	4.59%
Class Inst3	Nov. 08, 2012	32.36%	8.44%	4.62%
Class R	Aug. 02, 2011	31.58%	7.68%	3.92%
MSCI ACWI ex USA SMID Cap Index (Net) (reflects reinvested dividends net of withholding taxes but reflects no deductions for fees, expenses or other taxes)		30.35%	9.14%	3.81%
MSCI ACWI ex USA SMID Cap Growth Index (Net) (reflects reinvested dividends net of withholding taxes but reflects no deductions for fees, expenses or other taxes)		32.40%	9.35%	3.17%

[1] This charge is imposed on certain investments of between $1 million and $50 million redeemed within 18 months after purchase, as follows: 1.00% if redeemed within 12 months after purchase, and 0.50% if redeemed more than 12, but less than 18, months after purchase, with certain limited exceptions.

[2] This charge applies to redemptions within 12 months after purchase, with certain limited exceptions.

[1] Other expenses have been restated to reflect current transfer agency fees paid by the Fund.

[2] The Fund’s transfer agent, Columbia Management Investment Services Corp. (the Transfer Agent), has contractually agreed to waive a portion of its fees through June 30, 2019, such that the Fund’s transfer agency fees do not exceed the annual rate of 0.11% of the average daily net assets of each share class other than Class Inst2 and Class Inst3. With respect to Class Inst2 and Class Inst3 shares, the Transfer Agent has contractually agreed to waive a portion of its fees such that the Fund's transfer agency fees do not exceed the annual rates of 0.05% of the average daily net assets of Class Inst2 shares and 0.00% of the average daily net assets of Class Inst3 shares.

[1] Year to Date return as of March 31, 2018: 2.19%

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	COLUMBIA ACORN TRUST
Prospectus Date	rr_ProspectusDate	May 01, 2018
Columbia Acorn International
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	SUMMARY OF THE FUND
Objective [Heading]	rr_ObjectiveHeading	Investment Objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	Columbia Acorn International® (the Fund) seeks long-term capital appreciation.
Expense [Heading]	rr_ExpenseHeading	Fees and Expenses of the Fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. An investor transacting in a class of Fund shares without any front-end sales charge, contingent deferred sales charge, or other asset-based fee for sales or distribution may be required to pay a commission to the financial intermediary for effecting such transactions. Such commission rates are set by the financial intermediary and are not reflected in the tables or the example below. You may qualify for sales charge discounts if you and members of your immediate family invest, or agree to invest in the future, at least $50,000 in certain classes of shares of eligible funds distributed by Columbia Management Investment Distributors, Inc. (the Distributor). More information is available about these and other sales charge discounts and waivers from your financial intermediary, and can be found in the Choosing a Share Class section beginning on page 24 of the Fund’s prospectus, in Appendix A to the prospectus beginning on page A-1 and in Appendix S to the Statement of Additional Information (SAI) under Sales Charge Waivers beginning on page S-1.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder Fees (fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	June 30, 2019
Portfolio Turnover [Heading]	rr_PortfolioTurnoverHeading	Portfolio Turnover
Portfolio Turnover [Text Block]	rr_PortfolioTurnoverTextBlock	The Fund may pay transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 37% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	37.00%
Expense Breakpoint Discounts [Text]	rr_ExpenseBreakpointDiscounts	You may qualify for sales charge discounts if you and members of your immediate family invest, or agree to invest in the future, at least $50,000 in certain classes of shares of eligible funds distributed by Columbia Management Investment Distributors, Inc. (the Distributor). More information is available about these and other sales charge discounts and waivers from your financial intermediary, and can be found in the Choosing a Share Class section beginning on page 24 of the Fund’s prospectus, in Appendix A to the prospectus beginning on page A-1 and in Appendix S to the Statement of Additional Information (SAI) under Sales Charge Waivers beginning on page S-1.
Expense Breakpoint, Minimum Investment Required [Amount]	rr_ExpenseBreakpointMinimumInvestmentRequiredAmount	$ 50,000
Expenses Restated to Reflect Current [Text]	rr_ExpensesRestatedToReflectCurrent	Other expenses have been restated to reflect current transfer agency fees paid by the Fund.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	The following example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example illustrates the hypothetical expenses that you would incur over the time periods indicated, and assumes that: you invest $10,000 in the applicable class of Fund shares for the periods indicated, your investment has a 5% return each year, and the Fund’s total annual operating expenses remain the same as shown in the Annual Fund Operating Expenses table above. Since the waivers and/or reimbursements shown in the Annual Fund Operating Expenses table above expire as indicated in the preceding table, they are only reflected in the 1 year example and the first year of the other examples. Although your actual costs may be higher or lower, based on the assumptions listed above, your costs would be:
Strategy [Heading]	rr_StrategyHeading	Principal Investment Strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	Under normal circumstances, the Fund invests at least 75% of its net assets in foreign companies in developed markets (for example, Japan, Canada and the United Kingdom) and in emerging markets (for example, China, India and Brazil). Under normal circumstances, the Fund (i) invests a majority of its net assets in the common stock of small- and mid-sized companies with market capitalizations under $10 billion at the time of initial investment (“Focus Stocks”) and (ii) may also invest in companies with market capitalizations above $10 billion, provided that immediately after that investment a majority of the Fund’s net assets would be invested in Focus Stocks. The Fund may continue to hold, and make additional investments in, Focus Stocks whose market capitalizations have grown to exceed $10 billion, regardless of whether the Fund’s investments in Focus Stocks are a majority of the Fund’s net assets. The Fund may from time to time emphasize one or more sectors in selecting its investments. Columbia Wanger Asset Management, LLC, the Fund's investment adviser (the Investment Manager), believes that stocks of small- and mid-sized companies, which generally are not as well known by financial analysts as larger companies, may offer higher return potential than stocks of larger companies. The Fund also may invest in larger-sized companies. The Investment Manager typically seeks companies with: A strong business franchise that offers growth potential. Products and services in which the company has a competitive advantage. A stock price the Investment Manager believes is reasonable relative to the assets and earning power of the company. The Investment Manager may sell a portfolio holding if the security reaches the Investment Manager's price target, if the company has a deterioration of fundamentals, such as failing to meet key operating benchmarks, or if the Investment Manager believes other securities are more attractive. The Investment Manager also may sell a portfolio holding to fund redemptions.
Risk [Heading]	rr_RiskHeading	Principal Risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the Fund involves risks, including those described below. There is no assurance that the Fund will achieve its investment objective and you may lose money. The value of the Fund’s holdings may decline, and the Fund’s net asset value (NAV) and share price may go down. An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Active Management Risk. The Investment Manager’s active management of the Fund could cause the Fund to underperform its benchmark index and/or other funds with similar investment objectives and/or strategies. Small- and Mid-Cap Company Securities Risk. Investments in small- and mid-capitalization companies (small- and mid-cap companies) often involve greater risks than investments in larger, more established companies (larger companies) because small- and mid-cap companies tend to have less predictable earnings and may lack the management experience, financial resources, product diversification and competitive strengths of larger companies. Securities of small- and mid-cap companies may be less liquid and more volatile than the securities of larger companies. Market Risk. Market risk refers to the possibility that the market values of securities or other investments that the Fund holds will fall, sometimes rapidly or unpredictably, or fail to rise. An investment in the Fund could lose money over short or long periods. Foreign Securities Risk. Investments in or exposure to foreign securities involve certain risks not associated with investments in or exposure to securities of U.S. companies. Foreign securities subject the Fund to the risks associated with investing in the particular country of an issuer, including political, regulatory, economic, social, diplomatic and other conditions or events (including, for example, military confrontations, war and terrorism), occurring in the country or region, as well as risks associated with less developed custody and settlement practices. Foreign securities may be more volatile and less liquid than securities of U.S. companies, and are subject to the risks associated with potential imposition of economic and other sanctions against a particular foreign country, its nationals or industries or businesses within the country. In addition, foreign governments may impose withholding or other taxes on the Fund’s income, capital gains or proceeds from the disposition of foreign securities, which could reduce the Fund’s return on such securities. Operational and Settlement Risks of Foreign Securities. The Fund’s foreign securities are generally held outside the United States in the primary market for the securities in the custody of foreign sub-custodians. Some countries have limited governmental oversight and regulation, which increases the risk of corruption and fraud and the possibility of losses to the Fund. In particular, under certain circumstances, foreign securities may settle on a delayed delivery basis, meaning that the Fund may be required to make payment for securities before the Fund has actually received delivery of the securities or deliver securities prior to the receipt of payment. As a result, there is a risk that the security will not be delivered to the Fund or that payment will not be received. Losses can also result from lost, stolen or counterfeit securities; defaults by brokers and banks; failures or defects of the settlement system; or poor and improper record keeping by registrars and issuers. Share Blocking. In certain non-U.S. markets, an issuer’s securities are blocked from trading for a specified number of days before and, in certain instances, after a shareholder meeting. The blocking period can last up to several weeks. Share blocking may prevent the Fund from buying or selling securities during this period. As a consequence of these restrictions, the Investment Manager, on behalf of the Fund, may abstain from voting proxies in markets that require share blocking. Emerging Market Securities Risk. Securities issued by foreign governments or companies in emerging market countries, such as China, Russia and certain countries in Eastern Europe, the Middle East, Asia, Latin America or Africa, are more likely to have greater exposure to the risks of investing in foreign securities that are described in Foreign Securities Risk. In addition, emerging market countries are more likely to experience instability resulting, for example, from rapid changes or developments in social, political, economic or other conditions. Their economies are usually less mature and their securities markets are typically less developed with more limited trading activity (i.e., lower trading volumes and less liquidity) than more developed countries. Emerging market securities tend to be more volatile than securities in more developed markets. Many emerging market countries are heavily dependent on international trade and have fewer trading partners, which makes them more sensitive to world commodity prices and economic downturns in other countries, and some have a higher risk of currency devaluations. Operational and Settlement Risks of Securities in Emerging Markets. Foreign sub-custodians in emerging markets may be recently organized, lack extensive operating experience or lack effective government oversight or regulation. In addition, there may be legal restrictions or limitations on the ability of the Fund to recover assets held in custody by a foreign sub-custodian in the event of the bankruptcy of the sub-custodian. There may also be a greater risk that settlement may be delayed and that cash or securities of the Fund may be lost because of failures of or defects in the system, including fraud or corruption. Settlement systems in emerging markets also have a higher risk of failed trades. Risks Related to Currencies and Corporate Actions in Emerging Markets. Risks related to currencies and corporate actions are also greater in emerging market countries than in developed countries. Emerging market currencies may not have an active trading market and are subject to a higher risk of currency devaluations. Risks Related to Corporate and Securities Laws in Emerging Markets. Securities laws in emerging markets may be relatively new and unsettled and, consequently, there is a risk of rapid and unpredictable change in laws regarding foreign investment, securities regulation, title to securities and shareholder rights. Liquidity and Trading Volume Risk. Due to market conditions, including uncertainty regarding the price of a security, it may be difficult for the Fund to buy or sell portfolio securities at a desirable time or price, which could result in investment losses. This risk of portfolio illiquidity is heightened with respect to small- and mid-cap securities, generally, and foreign small- and mid-cap securities in particular. The Fund may have to lower the selling price, liquidate other investments, or forego another, more appealing investment opportunity as a result of illiquidity in the markets. As a result of significant and sustained reductions in emerging and developed international market trading volumes in the wake of the 2007-2009 financial crisis, it may take longer to buy or sell securities, which can exacerbate the Fund’s exposure to volatile markets. The Fund may also be limited in its ability to execute favorable trades in portfolio securities in response to changes in share prices and fundamentals, and may be forced to dispose of securities under disadvantageous circumstances and at a loss. As the Fund grows in size or, conversely, if it faces significant redemption pressure, these considerations take on increasing significance and may adversely impact performance. Sector Risk. At times, the Fund may have a significant portion of its assets invested in securities of companies conducting business in a related group of industries within a sector. Companies in the same sector may be similarly affected by economic, regulatory, political or market events or conditions, which may make the Fund more vulnerable to unfavorable developments in that sector than funds that invest more broadly. Generally, the more the Fund diversifies its investments, the more it spreads risk and potentially reduces the risks of loss and volatility. Foreign Currency Risk. The performance of the Fund may be materially affected positively or negatively by foreign currency strength or weakness relative to the U.S. dollar, particularly if the Fund invests a significant percentage of its assets in foreign securities or other assets denominated in currencies other than the U.S. dollar. Issuer Risk. An issuer in which the Fund invests may perform poorly, and the value of its securities may therefore decline, which would negatively affect the Fund’s performance. Poor performance may be caused by poor management decisions, competitive pressures, breakthroughs in technology, reliance on suppliers, labor problems or shortages, corporate restructurings, fraudulent disclosures, natural disasters or other events, conditions or factors. Geographic Focus Risk. The Fund may be particularly susceptible to economic, political, regulatory or other events or conditions affecting issuers and countries within the specific geographic regions in which the Fund invests. The Fund’s NAV may be more volatile than the NAV of a more geographically diversified fund. Asia Pacific Region. Many of the countries in the Asia Pacific region are considered underdeveloped or developing, including from a political, economic and/or social perspective, and may have relatively unstable governments and economies based on limited business, industries and/or natural resources or commodities. Events in any one country within the region may impact other countries in the region or the region as a whole. As a result, events in the region will generally have a greater effect on the Fund than if the Fund were more geographically diversified. This could result in increased volatility in the value of the Fund’s investments and losses for the Fund. Also, securities of some companies in the region can be less liquid than U.S. or other foreign securities, potentially making it difficult for the Fund to sell such securities at a desirable time and price. Europe. The Fund is particularly susceptible to economic, political, regulatory or other events or conditions affecting issuers and countries in Europe. In addition, the private and public sectors’ debt problems of a single European Union (EU) country can pose significant economic risks to the EU as a whole. As a result, the Fund’s NAV may be more volatile than the NAV of a more geographically diversified fund. If securities of issuers in Europe fall out of favor, it may cause the Fund to underperform other funds that do not focus their investments in this region of the world. The impact of any partial or complete dissolution of the EU on European economies could be significant, resulting in negative impacts on currency and financial markets generally, such as increased volatility and illiquidity, and potentially lower economic growth in markets in Europe, which may adversely affect the value of your investment in the Fund. Growth Securities Risk. Growth securities typically trade at a higher multiple of earnings than other types of equity securities. Accordingly, the market values of growth securities may never reach their expected market value and may decline in price. In addition, growth securities, at times, may not perform as well as value securities or the stock market in general, and may be out of favor with investors for varying periods of time.
Risk Lose Money [Text]	rr_RiskLoseMoney	There is no assurance that the Fund will achieve its investment objective and you may lose money.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance Information
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The following bar chart and table show you how the Fund has performed in the past, and can help you understand the risks of investing in the Fund. The bar chart shows how the Fund's Class Inst share performance has varied for each full calendar year shown. The table below the bar chart compares the Fund's returns for the periods shown with those of the MSCI ACWI ex USA SMID Cap Index (Net), the Fund's primary benchmark, and those of the MSCI ACWI ex USA SMID Cap Growth Index (Net), the Fund's secondary benchmark.  The MSCI ACWI ex USA SMID Cap Index (Net) captures mid- and small-cap representation across 22 of 23 developed market countries (excluding the United States) and 24 emerging market countries.  It has 5,290 constituents (as of March 31, 2018) and covers approximately 28% of the free float adjusted market capitalization in each country.  The MSCI ACWI ex USA SMID Cap Growth Index (Net) captures mid- and small-cap representation across 22 developed markets countries and 24 emerging markets countries.  The Investment Manager believes that the secondary benchmark provides a meaningful additional basis for comparing the Fund's performance, because the Fund's portfolio will in general be as closely aligned with the additional growth benchmark as with the current core benchmark from an investment style perspective. The performance of one or more share classes shown in the Average Annual Total Returns table below includes the Fund's Class Inst share returns (adjusted to reflect the higher class-related operating expenses of such classes, where applicable) for periods prior to the indicated inception date of such share classes. Except for differences in fees and expenses, all share classes of the Fund would have substantially similar annual returns because all share classes of the Fund invest in the same portfolio of securities. The after-tax returns shown in the Average Annual Total Returns table below are calculated using the highest historical individual U.S. federal marginal income tax rates in effect during the period indicated in the table and do not reflect the impact of state, local or foreign taxes. Your actual after-tax returns will depend on your personal tax situation and may differ from those shown in the table. In addition, the after-tax returns shown in the table do not apply to shares held in tax-advantaged accounts such as 401(k) plans or Individual Retirement Accounts (IRAs). The after-tax returns are shown only for Class Inst shares and will vary for other share classes. The Fund's past performance (before and after taxes) is no guarantee of how the Fund will perform in the future. Updated performance information can be obtained by calling toll-free 800.345.6611 or visiting columbiathreadneedle.com/us.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The following bar chart and table show you how the Fund has performed in the past, and can help you understand the risks of investing in the Fund. The bar chart shows how the Fund’s Class Inst share performance has varied for each full calendar year shown. The table below the bar chart compares the Fund’s returns for the periods shown with those of the MSCI ACWI ex USA SMID Cap Index (Net), the Fund's primary benchmark, and those of the MSCI ACWI ex USA SMID Cap Growth Index (Net), the Fund's secondary benchmark.
Performance Additional Market Index [Text]	rr_PerformanceAdditionalMarketIndex	The MSCI ACWI ex USA SMID Cap Growth Index (Net) captures mid- and small-cap representation across 22 developed markets countries and 24 emerging markets countries.  The Investment Manager believes that the secondary benchmark provides a meaningful additional basis for comparing the Fund's performance, because the Fund’s portfolio will in general be as closely aligned with the additional growth benchmark as with the current core benchmark from an investment style perspective.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	800.345.6611
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	columbiathreadneedle.com/us
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The Fund’s past performance (before and after taxes) is no guarantee of how the Fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Year by Year Total Return (%) as of December 31 Each Year
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Best and Worst Quarterly Returns During the Period Shown in the Bar Chart Best                                                          2nd Quarter 2009                                                         33.30% Worst                                                       3rd Quarter 2008                                                         -23.68%
Performance Table Heading	rr_PerformanceTableHeading	Average Annual Total Returns After Applicable Sales Charges (for periods ended December 31, 2017)
Performance Table Uses Highest Federal Rate	rr_PerformanceTableUsesHighestFederalRate	The after-tax returns shown in the Average Annual Total Returns table below are calculated using the highest historical individual U.S. federal marginal income tax rates in effect during the period indicated in the table and do not reflect the impact of state, local or foreign taxes.
Performance Table Not Relevant to Tax Deferred	rr_PerformanceTableNotRelevantToTaxDeferred	In addition, the after-tax returns shown in the table do not apply to shares held in tax-advantaged accounts such as 401(k) plans or Individual Retirement Accounts (IRAs).
Performance Table One Class of after Tax Shown [Text]	rr_PerformanceTableOneClassOfAfterTaxShown	The after-tax returns are shown only for Class Inst shares and will vary for other share classes.
Columbia Acorn International | Class A
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	5.75%
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	1.00%	[1]
Management fees	rr_ManagementFeesOverAssets	0.77%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Other expenses	rr_OtherExpensesOverAssets	0.22%	[2]
Total annual Fund operating expenses	rr_ExpensesOverAssets	1.24%
Less: Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	none	[3]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	1.24%
Expenses Deferred Charges [Text Block]	rr_ExpensesDeferredChargesTextBlock	This charge is imposed on certain investments of between $1 million and $50 million redeemed within 18 months after purchase, as follows: 1.00% if redeemed within 12 months after purchase, and 0.50% if redeemed more than 12, but less than 18, months after purchase, with certain limited exceptions.
1 year	rr_ExpenseExampleYear01	$ 694
3 years	rr_ExpenseExampleYear03	946
5 years	rr_ExpenseExampleYear05	1,217
10 years	rr_ExpenseExampleYear10	1,989
1 year	rr_ExpenseExampleNoRedemptionYear01	694
3 years	rr_ExpenseExampleNoRedemptionYear03	946
5 years	rr_ExpenseExampleNoRedemptionYear05	1,217
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 1,989
1 Year	rr_AverageAnnualReturnYear01	24.33%
5 Years	rr_AverageAnnualReturnYear05	6.78%
10 Years	rr_AverageAnnualReturnYear10	3.63%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Oct. 16, 2000
Columbia Acorn International | Class Adv
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.77%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.22%	[2]
Total annual Fund operating expenses	rr_ExpensesOverAssets	0.99%
Less: Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	none	[3]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	0.99%
1 year	rr_ExpenseExampleYear01	$ 101
3 years	rr_ExpenseExampleYear03	315
5 years	rr_ExpenseExampleYear05	547
10 years	rr_ExpenseExampleYear10	1,213
1 year	rr_ExpenseExampleNoRedemptionYear01	101
3 years	rr_ExpenseExampleNoRedemptionYear03	315
5 years	rr_ExpenseExampleNoRedemptionYear05	547
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 1,213
1 Year	rr_AverageAnnualReturnYear01	32.21%
5 Years	rr_AverageAnnualReturnYear05	8.29%
10 Years	rr_AverageAnnualReturnYear10	4.54%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 08, 2012
Columbia Acorn International | Class C
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	1.00%	[4]
Management fees	rr_ManagementFeesOverAssets	0.77%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	1.00%
Other expenses	rr_OtherExpensesOverAssets	0.22%	[2]
Total annual Fund operating expenses	rr_ExpensesOverAssets	1.99%
Less: Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	none	[3]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	1.99%
Expenses Deferred Charges [Text Block]	rr_ExpensesDeferredChargesTextBlock	This charge applies to redemptions within 12 months after purchase, with certain limited exceptions.
1 year	rr_ExpenseExampleYear01	$ 302
3 years	rr_ExpenseExampleYear03	624
5 years	rr_ExpenseExampleYear05	1,073
10 years	rr_ExpenseExampleYear10	2,317
1 year	rr_ExpenseExampleNoRedemptionYear01	202
3 years	rr_ExpenseExampleNoRedemptionYear03	624
5 years	rr_ExpenseExampleNoRedemptionYear05	1,073
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 2,317
1 Year	rr_AverageAnnualReturnYear01	29.93%
5 Years	rr_AverageAnnualReturnYear05	7.25%
10 Years	rr_AverageAnnualReturnYear10	3.45%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Oct. 16, 2000
Columbia Acorn International | Class Inst
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.77%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.22%	[2]
Total annual Fund operating expenses	rr_ExpensesOverAssets	0.99%
Less: Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	none	[3]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	0.99%
1 year	rr_ExpenseExampleYear01	$ 101
3 years	rr_ExpenseExampleYear03	315
5 years	rr_ExpenseExampleYear05	547
10 years	rr_ExpenseExampleYear10	1,213
1 year	rr_ExpenseExampleNoRedemptionYear01	101
3 years	rr_ExpenseExampleNoRedemptionYear03	315
5 years	rr_ExpenseExampleNoRedemptionYear05	547
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 1,213
2008	rr_AnnualReturn2008	(45.89%)	[5]
2009	rr_AnnualReturn2009	50.97%	[5]
2010	rr_AnnualReturn2010	22.70%	[5]
2011	rr_AnnualReturn2011	(14.06%)	[5]
2012	rr_AnnualReturn2012	21.60%	[5]
2013	rr_AnnualReturn2013	22.33%	[5]
2014	rr_AnnualReturn2014	(4.28%)	[5]
2015	rr_AnnualReturn2015	(1.33%)	[5]
2016	rr_AnnualReturn2016	(2.28%)	[5]
2017	rr_AnnualReturn2017	32.24%	[5]
Year to Date Return, Label	rr_YearToDateReturnLabel	Year to Date return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Mar. 31, 2018
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	2.19%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Jun. 30, 2009
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	33.30%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Sep. 30, 2008
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(23.68%)
1 Year	rr_AverageAnnualReturnYear01	32.24%
5 Years	rr_AverageAnnualReturnYear05	8.35%
10 Years	rr_AverageAnnualReturnYear10	4.57%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Sep. 23, 1992
Columbia Acorn International | Class Inst2
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.77%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.18%	[2]
Total annual Fund operating expenses	rr_ExpensesOverAssets	0.95%
Less: Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.02%)	[3]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	0.93%
1 year	rr_ExpenseExampleYear01	$ 95
3 years	rr_ExpenseExampleYear03	301
5 years	rr_ExpenseExampleYear05	524
10 years	rr_ExpenseExampleYear10	1,165
1 year	rr_ExpenseExampleNoRedemptionYear01	95
3 years	rr_ExpenseExampleNoRedemptionYear03	301
5 years	rr_ExpenseExampleNoRedemptionYear05	524
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 1,165
1 Year	rr_AverageAnnualReturnYear01	32.27%
5 Years	rr_AverageAnnualReturnYear05	8.39%
10 Years	rr_AverageAnnualReturnYear10	4.59%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Aug. 02, 2011
Columbia Acorn International | Class Inst3
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.77%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.11%	[2]
Total annual Fund operating expenses	rr_ExpensesOverAssets	0.88%
Less: Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	none	[3]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	0.88%
1 year	rr_ExpenseExampleYear01	$ 90
3 years	rr_ExpenseExampleYear03	281
5 years	rr_ExpenseExampleYear05	488
10 years	rr_ExpenseExampleYear10	1,084
1 year	rr_ExpenseExampleNoRedemptionYear01	90
3 years	rr_ExpenseExampleNoRedemptionYear03	281
5 years	rr_ExpenseExampleNoRedemptionYear05	488
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 1,084
1 Year	rr_AverageAnnualReturnYear01	32.36%
5 Years	rr_AverageAnnualReturnYear05	8.44%
10 Years	rr_AverageAnnualReturnYear10	4.62%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 08, 2012
Columbia Acorn International | Class R
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.77%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.50%
Other expenses	rr_OtherExpensesOverAssets	0.22%	[2]
Total annual Fund operating expenses	rr_ExpensesOverAssets	1.49%
Less: Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	none	[3]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	1.49%
1 year	rr_ExpenseExampleYear01	$ 152
3 years	rr_ExpenseExampleYear03	471
5 years	rr_ExpenseExampleYear05	813
10 years	rr_ExpenseExampleYear10	1,779
1 year	rr_ExpenseExampleNoRedemptionYear01	152
3 years	rr_ExpenseExampleNoRedemptionYear03	471
5 years	rr_ExpenseExampleNoRedemptionYear05	813
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 1,779
1 Year	rr_AverageAnnualReturnYear01	31.58%
5 Years	rr_AverageAnnualReturnYear05	7.68%
10 Years	rr_AverageAnnualReturnYear10	3.92%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Aug. 02, 2011
Columbia Acorn International | returns after taxes on distributions | Class Inst
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	30.27%
5 Years	rr_AverageAnnualReturnYear05	7.09%
10 Years	rr_AverageAnnualReturnYear10	3.78%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Sep. 23, 1992
Columbia Acorn International | returns after taxes on distributions and sale of Fund shares | Class Inst
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	20.07%
5 Years	rr_AverageAnnualReturnYear05	6.50%
10 Years	rr_AverageAnnualReturnYear10	3.59%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Sep. 23, 1992
Columbia Acorn International | MSCI ACWI ex USA SMID Cap Index (Net) (reflects reinvested dividends net of withholding taxes but reflects no deductions for fees, expenses or other taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	30.35%
5 Years	rr_AverageAnnualReturnYear05	9.14%
10 Years	rr_AverageAnnualReturnYear10	3.81%
Columbia Acorn International | MSCI ACWI ex USA SMID Cap Growth Index (Net) (reflects reinvested dividends net of withholding taxes but reflects no deductions for fees, expenses or other taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	32.40%
5 Years	rr_AverageAnnualReturnYear05	9.35%
10 Years	rr_AverageAnnualReturnYear10	3.17%

[1] This charge is imposed on certain investments of between $1 million and $50 million redeemed within 18 months after purchase, as follows: 1.00% if redeemed within 12 months after purchase, and 0.50% if redeemed more than 12, but less than 18, months after purchase, with certain limited exceptions.

[2] Other expenses have been restated to reflect current transfer agency fees paid by the Fund.

[3] The Fund’s transfer agent, Columbia Management Investment Services Corp. (the Transfer Agent), has contractually agreed to waive a portion of its fees through June 30, 2019, such that the Fund’s transfer agency fees do not exceed the annual rate of 0.11% of the average daily net assets of each share class other than Class Inst2 and Class Inst3. With respect to Class Inst2 and Class Inst3 shares, the Transfer Agent has contractually agreed to waive a portion of its fees such that the Fund's transfer agency fees do not exceed the annual rates of 0.05% of the average daily net assets of Class Inst2 shares and 0.00% of the average daily net assets of Class Inst3 shares.

[4] This charge applies to redemptions within 12 months after purchase, with certain limited exceptions.

[5] Year to Date return as of March 31, 2018: 2.19%

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Columbia Acorn International Select

SUMMARY OF THE FUND

Investment Objective

Columbia Acorn International Select^SM (the Fund) seeks long-term capital appreciation.

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. An investor transacting in a class of Fund shares without any front-end sales charge, contingent deferred sales charge, or other asset-based fee for sales or distribution may be required to pay a commission to the financial intermediary for effecting such transactions. Such commission rates are set by the financial intermediary and are not reflected in the tables or the example below. You may qualify for sales charge discounts if you and members of your immediate family invest, or agree to invest in the future, at least $50,000 in certain classes of shares of eligible funds distributed by Columbia Management Investment Distributors, Inc. (the Distributor). More information is available about these and other sales charge discounts and waivers from your financial intermediary, and can be found in the Choosing a Share Class section beginning on page 24 of the Fund’s prospectus, in Appendix A to the prospectus beginning on page A-1 and in Appendix S to the Statement of Additional Information (SAI) under Sales Charge Waivers beginning on page S-1.

Shareholder Fees (fees paid directly from your investment)

Shareholder Fees - Columbia Acorn International Select	Class A		Class C		Class Adv	Class Inst	Class Inst2	Class Inst3
Maximum sales charge (load) imposed on purchases (as a % of offering price)	5.75%		none		none	none	none	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	1.00%	[1]	1.00%	[2]	none	none	none	none

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)

Annual Fund Operating Expenses - Columbia Acorn International Select		Class A	Class Adv	Class C	Class Inst	Class Inst2	Class Inst3
Management fees		0.89%	0.89%	0.89%	0.89%	0.89%	0.89%
Distribution and/or service (12b-1) fees		0.25%	none	1.00%	none	none	none
Other expenses	[1]	0.40%	0.40%	0.40%	0.40%	0.34%	0.27%
Total annual Fund operating expenses		1.54%	1.29%	2.29%	1.29%	1.23%	1.16%
Less: Fee waivers and/or expense reimbursements	[2]	(0.14%)	(0.14%)	(0.14%)	(0.14%)	(0.17%)	(0.15%)
Total annual Fund operating expenses after fee waivers and/or expense reimbursements		1.40%	1.15%	2.15%	1.15%	1.06%	1.01%

Example

The following example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example illustrates the hypothetical expenses that you would incur over the time periods indicated, and assumes that:

• you invest $10,000 in the applicable class of Fund shares for the periods indicated,

• your investment has a 5% return each year, and

• the Fund’s total annual operating expenses remain the same as shown in the Annual Fund Operating Expenses table above.

Since the waivers and/or reimbursements shown in the Annual Fund Operating Expenses table above expire as indicated in the preceding table, they are only reflected in the 1 year example and the first year of the other examples. Although your actual costs may be higher or lower, based on the assumptions listed above, your costs would be:

Expense Example - Columbia Acorn International Select - USD ($)	1 year	3 years	5 years	10 years
Class A	709	1,021	1,354	2,293
Class Adv	117	395	694	1,544
Class C	318	702	1,212	2,615
Class Inst	117	395	694	1,544
Class Inst2	108	374	659	1,474
Class Inst3	103	354	624	1,396

Expense Example, No Redemption - Columbia Acorn International Select - USD ($)	1 year	3 years	5 years	10 years
Class A	709	1,021	1,354	2,293
Class Adv	117	395	694	1,544
Class C	218	702	1,212	2,615
Class Inst	117	395	694	1,544
Class Inst2	108	374	659	1,474
Class Inst3	103	354	624	1,396

Portfolio Turnover

The Fund may pay transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 49% of the average value of its portfolio.

Principal Investment Strategies

Under normal circumstances, the Fund invests at least 65% of its net assets in foreign companies in developed markets (for example, Japan, Canada and the United Kingdom).

The Fund also may invest up to 35% of its total assets in companies in emerging markets (for example, China, India and Brazil). The Fund generally invests in at least three countries other than the United States but may invest up to 25% of its total assets in securities of U.S. issuers.

Under normal circumstances, the Fund (i) invests a majority of its net assets in the common stock of small- and mid-sized companies with market capitalizations under $25 billion at the time of initial investment (“Focus Stocks”) and (ii) may also invest in companies with market capitalizations above $25 billion, provided that immediately after that investment a majority of the Fund’s net assets would be invested in Focus Stocks. The Fund may continue to hold, and make additional investments in, Focus Stocks whose market capitalizations have grown to exceed $25 billion, regardless of whether the Fund’s investments in Focus Stocks are a majority of the Fund’s net assets.

Columbia Wanger Asset Management, LLC, the Fund's investment adviser (the Investment Manager), believes that stocks of small- and mid-sized companies, which generally are not as well known by financial analysts as larger companies, may offer higher return potential than stocks of larger companies. The Fund also may invest in larger-sized companies. The Fund may from time to time emphasize one or more sectors in selecting its investments.

The Fund invests in a limited number of foreign companies (generally between 30-60), offering the potential to provide above-average growth over time. In pursuit of the Fund’s objective, the portfolio managers will take advantage of the research and stock-picking capabilities of the Investment Manager and will generally concentrate the Fund’s investments in those sectors, companies, geographic regions or industries that the portfolio managers believe offer the best investment return potential.

The Investment Manager typically seeks companies with:

• A strong business franchise that offers growth potential.

• Products and services in which the company has a competitive advantage.

• A stock price the Investment Manager believes is reasonable relative to the assets and earning power of the company.

The Investment Manager may sell a portfolio holding if the security reaches the Investment Manager's price target, if the company has a deterioration of fundamentals, such as failing to meet key operating benchmarks, or if the Investment Manager believes other securities are more attractive. The Investment Manager also may sell a portfolio holding to fund redemptions.

Principal Risks

An investment in the Fund involves risks, including those described below. There is no assurance that the Fund will achieve its investment objective and you may lose money. The value of the Fund’s holdings may decline, and the Fund’s net asset value (NAV) and share price may go down. An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.

Active Management Risk. The Investment Manager’s active management of the Fund could cause the Fund to underperform its benchmark index and/or other funds with similar investment objectives and/or strategies.

Sector Risk. At times, the Fund may have a significant portion of its assets invested in securities of companies conducting business in a related group of industries within a sector. Companies in the same sector may be similarly affected by economic, regulatory, political or market events or conditions, which may make the Fund more vulnerable to unfavorable developments in that sector than funds that invest more broadly. Generally, the more the Fund diversifies its investments, the more it spreads risk and potentially reduces the risks of loss and volatility.

Select Portfolio Risk. Because the Fund may invest in a limited number of companies, the Fund as a whole is subject to greater risk of loss if any of its portfolio securities decline in price. In addition, the Fund’s holdings and weightings will diverge significantly from its primary benchmark’s holdings and weightings and the Fund may therefore experience greater risk and volatility relative to the benchmark. Because the Fund may invest in more than one company concentrated in a similar industry, sector or geographic region, the Fund may be even more concentrated than the number of companies it may hold would suggest.

Liquidity and Trading Volume Risk. Due to market conditions, including uncertainty regarding the price of a security, it may be difficult for the Fund to buy or sell portfolio securities at a desirable time or price, which could result in investment losses. This risk of portfolio illiquidity is heightened with respect to small- and mid-cap securities, generally, and foreign small- and mid-cap securities in particular. The Fund may have to lower the selling price, liquidate other investments, or forego another, more appealing investment opportunity as a result of illiquidity in the markets. As a result of significant and sustained reductions in emerging and developed international market trading volumes in the wake of the 2007-2009 financial crisis, it may take longer to buy or sell securities, which can exacerbate the Fund’s exposure to volatile markets. The Fund may also be limited in its ability to execute favorable trades in portfolio securities in response to changes in share prices and fundamentals, and may be forced to dispose of securities under disadvantageous circumstances and at a loss. As the Fund grows in size or, conversely, if it faces significant redemption pressure, these considerations take on increasing significance and may adversely impact performance.

Foreign Securities Risk. Investments in or exposure to foreign securities involve certain risks not associated with investments in or exposure to securities of U.S. companies. Foreign securities subject the Fund to the risks associated with investing in the particular country of an issuer, including political, regulatory, economic, social, diplomatic and other conditions or events (including, for example, military confrontations, war and terrorism), occurring in the country or region, as well as risks associated with less developed custody and settlement practices. Foreign securities may be more volatile and less liquid than securities of U.S. companies, and are subject to the risks associated with potential imposition of economic and other sanctions against a particular foreign country, its nationals or industries or businesses within the country. In addition, foreign governments may impose withholding or other taxes on the Fund’s income, capital gains or proceeds from the disposition of foreign securities, which could reduce the Fund’s return on such securities.

Operational and Settlement Risks of Foreign Securities. The Fund’s foreign securities are generally held outside the United States in the primary market for the securities in the custody of foreign sub-custodians. Some countries have limited governmental oversight and regulation, which increases the risk of corruption and fraud and the possibility of losses to the Fund. In particular, under certain circumstances, foreign securities may settle on a delayed delivery basis, meaning that the Fund may be required to make payment for securities before the Fund has actually received delivery of the securities or deliver securities prior to the receipt of payment. As a result, there is a risk that the security will not be delivered to the Fund or that payment will not be received. Losses can also result from lost, stolen or counterfeit securities; defaults by brokers and banks; failures or defects of the settlement system; or poor and improper record keeping by registrars and issuers.

Share Blocking. In certain non-U.S. markets, an issuer’s securities are blocked from trading for a specified number of days before and, in certain instances, after a shareholder meeting. The blocking period can last up to several weeks. Share blocking may prevent the Fund from buying or selling securities during this period. As a consequence of these restrictions, the Investment Manager, on behalf of the Fund, may abstain from voting proxies in markets that require share blocking.

Emerging Market Securities Risk. Securities issued by foreign governments or companies in emerging market countries, such as China, Russia and certain countries in Eastern Europe, the Middle East, Asia, Latin America or Africa, are more likely to have greater exposure to the risks of investing in foreign securities that are described in Foreign Securities Risk. In addition, emerging market countries are more likely to experience instability resulting, for example, from rapid changes or developments in social, political, economic or other conditions. Their economies are usually less mature and their securities markets are typically less developed with more limited trading activity (i.e., lower trading volumes and less liquidity) than more developed countries. Emerging market securities tend to be more volatile than securities in more developed markets. Many emerging market countries are heavily dependent on international trade and have fewer trading partners, which makes them more sensitive to world commodity prices and economic downturns in other countries, and some have a higher risk of currency devaluations.

Operational and Settlement Risks of Securities in Emerging Markets. Foreign sub-custodians in emerging markets may be recently organized, lack extensive operating experience or lack effective government oversight or regulation. In addition, there may be legal restrictions or limitations on the ability of the Fund to recover assets held in custody by a foreign sub-custodian in the event of the bankruptcy of the sub-custodian. There may also be a greater risk that settlement may be delayed and that cash or securities of the Fund may be lost because of failures of or defects in the system, including fraud or corruption. Settlement systems in emerging markets also have a higher risk of failed trades.

Risks Related to Currencies and Corporate Actions in Emerging Markets. Risks related to currencies and corporate actions are also greater in emerging market countries than in developed countries. Emerging market currencies may not have an active trading market and are subject to a higher risk of currency devaluations.

Risks Related to Corporate and Securities Laws in Emerging Markets. Securities laws in emerging markets may be relatively new and unsettled and, consequently, there is a risk of rapid and unpredictable change in laws regarding foreign investment, securities regulation, title to securities and shareholder rights.

Small- and Mid-Cap Company Securities Risk. Investments in small- and mid-capitalization companies (small- and mid-cap companies) often involve greater risks than investments in larger, more established companies (larger companies) because small- and mid-cap companies tend to have less predictable earnings and may lack the management experience, financial resources, product diversification and competitive strengths of larger companies. Securities of small- and mid-cap companies may be less liquid and more volatile than the securities of larger companies.

Large-Cap Company Securities Risk. Larger, more established companies may be unable to respond quickly to new competitive challenges, such as changes in consumer tastes or innovative smaller competitors. Also, large-cap companies are sometimes unable to attain the high growth rates of successful, smaller companies, especially during extended periods of economic expansion.

Issuer Risk. An issuer in which the Fund invests may perform poorly, and the value of its securities may therefore decline, which would negatively affect the Fund’s performance. Poor performance may be caused by poor management decisions, competitive pressures, breakthroughs in technology, reliance on suppliers, labor problems or shortages, corporate restructurings, fraudulent disclosures, natural disasters or other events, conditions or factors.

Market Risk. Market risk refers to the possibility that the market values of securities or other investments that the Fund holds will fall, sometimes rapidly or unpredictably, or fail to rise. An investment in the Fund could lose money over short or long periods.

Foreign Currency Risk. The performance of the Fund may be materially affected positively or negatively by foreign currency strength or weakness relative to the U.S. dollar, particularly if the Fund invests a significant percentage of its assets in foreign securities or other assets denominated in currencies other than the U.S. dollar.

Geographic Focus Risk. The Fund may be particularly susceptible to economic, political, regulatory or other events or conditions affecting issuers and countries within the specific geographic regions in which the Fund invests. The Fund’s NAV may be more volatile than the NAV of a more geographically diversified fund.

Asia Pacific Region. Many of the countries in the Asia Pacific region are considered underdeveloped or developing, including from a political, economic and/or social perspective, and may have relatively unstable governments and economies based on limited business, industries and/or natural resources or commodities. Events in any one country within the region may impact other countries in the region or the region as a whole. As a result, events in the region will generally have a greater effect on the Fund than if the Fund were more geographically diversified. This could result in increased volatility in the value of the Fund’s investments and losses for the Fund. Also, securities of some companies in the region can be less liquid than U.S. or other foreign securities, potentially making it difficult for the Fund to sell such securities at a desirable time and price.

Europe. The Fund is particularly susceptible to economic, political, regulatory or other events or conditions affecting issuers and countries in Europe. In addition, the private and public sectors’ debt problems of a single European Union (EU) country can pose significant economic risks to the EU as a whole. As a result, the Fund’s NAV may be more volatile than the NAV of a more geographically diversified fund. If securities of issuers in Europe fall out of favor, it may cause the Fund to underperform other funds that do not focus their investments in this region of the world. The impact of any partial or complete dissolution of the EU on European economies could be significant, resulting in negative impacts on currency and financial markets generally, such as increased volatility and illiquidity, and potentially lower economic growth in markets in Europe, which may adversely affect the value of your investment in the Fund.

Growth Securities Risk. Growth securities typically trade at a higher multiple of earnings than other types of equity securities. Accordingly, the market values of growth securities may never reach their expected market value and may decline in price. In addition, growth securities, at times, may not perform as well as value securities or the stock market in general, and may be out of favor with investors for varying periods of time.

Performance Information

The following bar chart and table show you how the Fund has performed in the past, and can help you understand the risks of investing in the Fund. The bar chart shows how the Fund's Class Inst share performance has varied for each full calendar year shown. The table below the bar chart compares the Fund's returns for the periods shown with those of the MSCI ACWI ex USA Index (Net), the Fund's primary benchmark, and those of the MSCI ACWI ex USA Growth Index (Net), the Fund's secondary benchmark.  The MSCI ACWI ex USA Index (Net) captures large- and mid-cap representation across 22 of 23 developed market countries (excluding the United States) and 24 emerging markets countries.  It has 1,864 constituents (as of March 31, 2018) and covers approximately 85% of the global equity opportunity set outside the United States.  The MSCI ACWI ex USA Growth Index (Net) captures large-and mid-cap representation across 22 developed markets countries and 24 emerging markets countries.  The Investment Manager believes that the secondary benchmark provides a meaningful additional basis for comparing the Fund's performance, because the Fund's portfolio will in general be as closely aligned with the additional growth benchmark as with the current core benchmark from an investment style perspective.

The performance of one or more share classes shown in the Average Annual Total Returns table below includes the Fund's Class Inst share returns (adjusted to reflect the higher class-related operating expenses of such classes, where applicable) for periods prior to the indicated inception date of such share classes. Except for differences in fees and expenses, all share classes of the Fund would have substantially similar annual returns because all share classes of the Fund invest in the same portfolio of securities.

The after-tax returns shown in the Average Annual Total Returns table below are calculated using the highest historical individual U.S. federal marginal income tax rates in effect during the period indicated in the table and do not reflect the impact of state, local or foreign taxes. Your actual after-tax returns will depend on your personal tax situation and may differ from those shown in the table. In addition, the after-tax returns shown in the table do not apply to shares held in tax-advantaged accounts such as 401(k) plans or Individual Retirement Accounts (IRAs). The after-tax returns are shown only for Class Inst shares and will vary for other share classes.

The Fund's past performance (before and after taxes) is no guarantee of how the Fund will perform in the future. Updated performance information can be obtained by calling toll-free 800.345.6611 or visiting columbiathreadneedle.com/us.

Year by Year Total Return (%)
as of December 31 Each Year

Best and Worst Quarterly Returns
During the Period Shown in the Bar Chart

Best                                                          2nd Quarter 2009                                                         23.44%

Worst                                                       3rd Quarter 2008                                                         -24.76%

Average Annual Total Returns After Applicable Sales Charges (for periods ended December 31, 2017)

Average Annual Total Returns - Columbia Acorn International Select	Share Class Inception Date	1 Year	5 Years	10 Years
Class Inst	Nov. 23, 1998	35.70%	7.77%	4.07%
Class Inst | returns after taxes on distributions	Nov. 23, 1998	35.76%	6.62%	3.09%
Class Inst | returns after taxes on distributions and sale of Fund shares	Nov. 23, 1998	20.42%	5.96%	3.12%
Class A	Oct. 16, 2000	27.54%	6.20%	3.12%
Class Adv	Nov. 08, 2012	35.67%	7.74%	4.06%
Class C	Oct. 16, 2000	33.32%	6.64%	2.93%
Class Inst2	Nov. 08, 2012	35.72%	7.82%	4.09%
Class Inst3	Nov. 08, 2012	35.81%	7.87%	4.12%
MSCI ACWI ex USA Index (Net) (reflects reinvested dividends net of withholding taxes but reflects no deductions for fees, expenses or other taxes)		27.19%	6.80%	1.84%
MSCI ACWI ex USA Growth Index (Net) (reflects reinvested dividends net of withholding taxes but reflects no deductions for fees, expenses or other taxes)		32.01%	7.97%	2.40%

[1] This charge is imposed on certain investments of between $1 million and $50 million redeemed within 18 months after purchase, as follows: 1.00% if redeemed within 12 months after purchase, and 0.50% if redeemed more than 12, but less than 18, months after purchase, with certain limited exceptions.

[2] This charge applies to redemptions within 12 months after purchase, with certain limited exceptions.

[1] Other expenses have been restated to reflect current transfer agency fees paid by the Fund.

[2] Columbia Wanger Asset Management, LLC (the Investment Manager) has contractually agreed to waive fees and reimburse certain expenses of the Fund, through April 30, 2019, so that ordinary operating expenses (excluding transaction costs and certain other investment-related expenses, interest and fees on borrowings and expenses associated with the Fund’s investment in other investment companies, if any) do not exceed the annual rates of 1.40% for Class A shares, 1.15% for Class Adv shares, 2.15% for Class C shares, 1.15% for Class Inst shares, 1.06% for Class Inst2 shares and 1.01% for Class Inst3 shares. This arrangement may only be amended or terminated with approval from the Fund’s Board of Trustees and the Investment Manager. The fee waivers and/or expense reimbursements shown in the table also reflect the contractual agreement of the Fund’s transfer agent, Columbia Management Investment Services Corp. (the Transfer Agent), to waive a portion of its fees through June 30, 2019, such that the Fund’s transfer agency fees do not exceed the annual rates of 0.05% of the average daily net assets of Class Inst2 shares and 0.00% of the average daily net assets of Class Inst3 shares.

[1] Year to Date return as of March 31, 2018: 0.51%

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	COLUMBIA ACORN TRUST
Prospectus Date	rr_ProspectusDate	May 01, 2018
Columbia Acorn International Select
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	SUMMARY OF THE FUND
Objective [Heading]	rr_ObjectiveHeading	Investment Objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	Columbia Acorn International SelectSM (the Fund) seeks long-term capital appreciation.
Expense [Heading]	rr_ExpenseHeading	Fees and Expenses of the Fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. An investor transacting in a class of Fund shares without any front-end sales charge, contingent deferred sales charge, or other asset-based fee for sales or distribution may be required to pay a commission to the financial intermediary for effecting such transactions. Such commission rates are set by the financial intermediary and are not reflected in the tables or the example below. You may qualify for sales charge discounts if you and members of your immediate family invest, or agree to invest in the future, at least $50,000 in certain classes of shares of eligible funds distributed by Columbia Management Investment Distributors, Inc. (the Distributor). More information is available about these and other sales charge discounts and waivers from your financial intermediary, and can be found in the Choosing a Share Class section beginning on page 24 of the Fund’s prospectus, in Appendix A to the prospectus beginning on page A-1 and in Appendix S to the Statement of Additional Information (SAI) under Sales Charge Waivers beginning on page S-1.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder Fees (fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	April 30, 2019 June 30, 2019
Portfolio Turnover [Heading]	rr_PortfolioTurnoverHeading	Portfolio Turnover
Portfolio Turnover [Text Block]	rr_PortfolioTurnoverTextBlock	The Fund may pay transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 49% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	49.00%
Expense Breakpoint Discounts [Text]	rr_ExpenseBreakpointDiscounts	You may qualify for sales charge discounts if you and members of your immediate family invest, or agree to invest in the future, at least $50,000 in certain classes of shares of eligible funds distributed by Columbia Management Investment Distributors, Inc. (the Distributor). More information is available about these and other sales charge discounts and waivers from your financial intermediary, and can be found in the Choosing a Share Class section beginning on page 24 of the Fund’s prospectus, in Appendix A to the prospectus beginning on page A-1 and in Appendix S to the Statement of Additional Information (SAI) under Sales Charge Waivers beginning on page S-1.
Expense Breakpoint, Minimum Investment Required [Amount]	rr_ExpenseBreakpointMinimumInvestmentRequiredAmount	$ 50,000
Expenses Restated to Reflect Current [Text]	rr_ExpensesRestatedToReflectCurrent	Other expenses have been restated to reflect current transfer agency fees paid by the Fund.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	The following example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example illustrates the hypothetical expenses that you would incur over the time periods indicated, and assumes that: you invest $10,000 in the applicable class of Fund shares for the periods indicated, your investment has a 5% return each year, and the Fund’s total annual operating expenses remain the same as shown in the Annual Fund Operating Expenses table above. Since the waivers and/or reimbursements shown in the Annual Fund Operating Expenses table above expire as indicated in the preceding table, they are only reflected in the 1 year example and the first year of the other examples. Although your actual costs may be higher or lower, based on the assumptions listed above, your costs would be:
Strategy [Heading]	rr_StrategyHeading	Principal Investment Strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	Under normal circumstances, the Fund invests at least 65% of its net assets in foreign companies in developed markets (for example, Japan, Canada and the United Kingdom). The Fund also may invest up to 35% of its total assets in companies in emerging markets (for example, China, India and Brazil). The Fund generally invests in at least three countries other than the United States but may invest up to 25% of its total assets in securities of U.S. issuers. Under normal circumstances, the Fund (i) invests a majority of its net assets in the common stock of small- and mid-sized companies with market capitalizations under $25 billion at the time of initial investment (“Focus Stocks”) and (ii) may also invest in companies with market capitalizations above $25 billion, provided that immediately after that investment a majority of the Fund’s net assets would be invested in Focus Stocks. The Fund may continue to hold, and make additional investments in, Focus Stocks whose market capitalizations have grown to exceed $25 billion, regardless of whether the Fund’s investments in Focus Stocks are a majority of the Fund’s net assets. Columbia Wanger Asset Management, LLC, the Fund's investment adviser (the Investment Manager), believes that stocks of small- and mid-sized companies, which generally are not as well known by financial analysts as larger companies, may offer higher return potential than stocks of larger companies. The Fund also may invest in larger-sized companies. The Fund may from time to time emphasize one or more sectors in selecting its investments. The Fund invests in a limited number of foreign companies (generally between 30-60), offering the potential to provide above-average growth over time. In pursuit of the Fund’s objective, the portfolio managers will take advantage of the research and stock-picking capabilities of the Investment Manager and will generally concentrate the Fund’s investments in those sectors, companies, geographic regions or industries that the portfolio managers believe offer the best investment return potential. The Investment Manager typically seeks companies with: A strong business franchise that offers growth potential. Products and services in which the company has a competitive advantage. A stock price the Investment Manager believes is reasonable relative to the assets and earning power of the company. The Investment Manager may sell a portfolio holding if the security reaches the Investment Manager's price target, if the company has a deterioration of fundamentals, such as failing to meet key operating benchmarks, or if the Investment Manager believes other securities are more attractive. The Investment Manager also may sell a portfolio holding to fund redemptions.
Risk [Heading]	rr_RiskHeading	Principal Risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the Fund involves risks, including those described below. There is no assurance that the Fund will achieve its investment objective and you may lose money. The value of the Fund’s holdings may decline, and the Fund’s net asset value (NAV) and share price may go down. An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Active Management Risk. The Investment Manager’s active management of the Fund could cause the Fund to underperform its benchmark index and/or other funds with similar investment objectives and/or strategies. Sector Risk. At times, the Fund may have a significant portion of its assets invested in securities of companies conducting business in a related group of industries within a sector. Companies in the same sector may be similarly affected by economic, regulatory, political or market events or conditions, which may make the Fund more vulnerable to unfavorable developments in that sector than funds that invest more broadly. Generally, the more the Fund diversifies its investments, the more it spreads risk and potentially reduces the risks of loss and volatility. Select Portfolio Risk. Because the Fund may invest in a limited number of companies, the Fund as a whole is subject to greater risk of loss if any of its portfolio securities decline in price. In addition, the Fund’s holdings and weightings will diverge significantly from its primary benchmark’s holdings and weightings and the Fund may therefore experience greater risk and volatility relative to the benchmark. Because the Fund may invest in more than one company concentrated in a similar industry, sector or geographic region, the Fund may be even more concentrated than the number of companies it may hold would suggest. Liquidity and Trading Volume Risk. Due to market conditions, including uncertainty regarding the price of a security, it may be difficult for the Fund to buy or sell portfolio securities at a desirable time or price, which could result in investment losses. This risk of portfolio illiquidity is heightened with respect to small- and mid-cap securities, generally, and foreign small- and mid-cap securities in particular. The Fund may have to lower the selling price, liquidate other investments, or forego another, more appealing investment opportunity as a result of illiquidity in the markets. As a result of significant and sustained reductions in emerging and developed international market trading volumes in the wake of the 2007-2009 financial crisis, it may take longer to buy or sell securities, which can exacerbate the Fund’s exposure to volatile markets. The Fund may also be limited in its ability to execute favorable trades in portfolio securities in response to changes in share prices and fundamentals, and may be forced to dispose of securities under disadvantageous circumstances and at a loss. As the Fund grows in size or, conversely, if it faces significant redemption pressure, these considerations take on increasing significance and may adversely impact performance. Foreign Securities Risk. Investments in or exposure to foreign securities involve certain risks not associated with investments in or exposure to securities of U.S. companies. Foreign securities subject the Fund to the risks associated with investing in the particular country of an issuer, including political, regulatory, economic, social, diplomatic and other conditions or events (including, for example, military confrontations, war and terrorism), occurring in the country or region, as well as risks associated with less developed custody and settlement practices. Foreign securities may be more volatile and less liquid than securities of U.S. companies, and are subject to the risks associated with potential imposition of economic and other sanctions against a particular foreign country, its nationals or industries or businesses within the country. In addition, foreign governments may impose withholding or other taxes on the Fund’s income, capital gains or proceeds from the disposition of foreign securities, which could reduce the Fund’s return on such securities. Operational and Settlement Risks of Foreign Securities. The Fund’s foreign securities are generally held outside the United States in the primary market for the securities in the custody of foreign sub-custodians. Some countries have limited governmental oversight and regulation, which increases the risk of corruption and fraud and the possibility of losses to the Fund. In particular, under certain circumstances, foreign securities may settle on a delayed delivery basis, meaning that the Fund may be required to make payment for securities before the Fund has actually received delivery of the securities or deliver securities prior to the receipt of payment. As a result, there is a risk that the security will not be delivered to the Fund or that payment will not be received. Losses can also result from lost, stolen or counterfeit securities; defaults by brokers and banks; failures or defects of the settlement system; or poor and improper record keeping by registrars and issuers. Share Blocking. In certain non-U.S. markets, an issuer’s securities are blocked from trading for a specified number of days before and, in certain instances, after a shareholder meeting. The blocking period can last up to several weeks. Share blocking may prevent the Fund from buying or selling securities during this period. As a consequence of these restrictions, the Investment Manager, on behalf of the Fund, may abstain from voting proxies in markets that require share blocking. Emerging Market Securities Risk. Securities issued by foreign governments or companies in emerging market countries, such as China, Russia and certain countries in Eastern Europe, the Middle East, Asia, Latin America or Africa, are more likely to have greater exposure to the risks of investing in foreign securities that are described in Foreign Securities Risk. In addition, emerging market countries are more likely to experience instability resulting, for example, from rapid changes or developments in social, political, economic or other conditions. Their economies are usually less mature and their securities markets are typically less developed with more limited trading activity (i.e., lower trading volumes and less liquidity) than more developed countries. Emerging market securities tend to be more volatile than securities in more developed markets. Many emerging market countries are heavily dependent on international trade and have fewer trading partners, which makes them more sensitive to world commodity prices and economic downturns in other countries, and some have a higher risk of currency devaluations. Operational and Settlement Risks of Securities in Emerging Markets. Foreign sub-custodians in emerging markets may be recently organized, lack extensive operating experience or lack effective government oversight or regulation. In addition, there may be legal restrictions or limitations on the ability of the Fund to recover assets held in custody by a foreign sub-custodian in the event of the bankruptcy of the sub-custodian. There may also be a greater risk that settlement may be delayed and that cash or securities of the Fund may be lost because of failures of or defects in the system, including fraud or corruption. Settlement systems in emerging markets also have a higher risk of failed trades. Risks Related to Currencies and Corporate Actions in Emerging Markets. Risks related to currencies and corporate actions are also greater in emerging market countries than in developed countries. Emerging market currencies may not have an active trading market and are subject to a higher risk of currency devaluations. Risks Related to Corporate and Securities Laws in Emerging Markets. Securities laws in emerging markets may be relatively new and unsettled and, consequently, there is a risk of rapid and unpredictable change in laws regarding foreign investment, securities regulation, title to securities and shareholder rights. Small- and Mid-Cap Company Securities Risk. Investments in small- and mid-capitalization companies (small- and mid-cap companies) often involve greater risks than investments in larger, more established companies (larger companies) because small- and mid-cap companies tend to have less predictable earnings and may lack the management experience, financial resources, product diversification and competitive strengths of larger companies. Securities of small- and mid-cap companies may be less liquid and more volatile than the securities of larger companies. Large-Cap Company Securities Risk. Larger, more established companies may be unable to respond quickly to new competitive challenges, such as changes in consumer tastes or innovative smaller competitors. Also, large-cap companies are sometimes unable to attain the high growth rates of successful, smaller companies, especially during extended periods of economic expansion. Issuer Risk. An issuer in which the Fund invests may perform poorly, and the value of its securities may therefore decline, which would negatively affect the Fund’s performance. Poor performance may be caused by poor management decisions, competitive pressures, breakthroughs in technology, reliance on suppliers, labor problems or shortages, corporate restructurings, fraudulent disclosures, natural disasters or other events, conditions or factors. Market Risk. Market risk refers to the possibility that the market values of securities or other investments that the Fund holds will fall, sometimes rapidly or unpredictably, or fail to rise. An investment in the Fund could lose money over short or long periods. Foreign Currency Risk. The performance of the Fund may be materially affected positively or negatively by foreign currency strength or weakness relative to the U.S. dollar, particularly if the Fund invests a significant percentage of its assets in foreign securities or other assets denominated in currencies other than the U.S. dollar. Geographic Focus Risk. The Fund may be particularly susceptible to economic, political, regulatory or other events or conditions affecting issuers and countries within the specific geographic regions in which the Fund invests. The Fund’s NAV may be more volatile than the NAV of a more geographically diversified fund. Asia Pacific Region. Many of the countries in the Asia Pacific region are considered underdeveloped or developing, including from a political, economic and/or social perspective, and may have relatively unstable governments and economies based on limited business, industries and/or natural resources or commodities. Events in any one country within the region may impact other countries in the region or the region as a whole. As a result, events in the region will generally have a greater effect on the Fund than if the Fund were more geographically diversified. This could result in increased volatility in the value of the Fund’s investments and losses for the Fund. Also, securities of some companies in the region can be less liquid than U.S. or other foreign securities, potentially making it difficult for the Fund to sell such securities at a desirable time and price. Europe. The Fund is particularly susceptible to economic, political, regulatory or other events or conditions affecting issuers and countries in Europe. In addition, the private and public sectors’ debt problems of a single European Union (EU) country can pose significant economic risks to the EU as a whole. As a result, the Fund’s NAV may be more volatile than the NAV of a more geographically diversified fund. If securities of issuers in Europe fall out of favor, it may cause the Fund to underperform other funds that do not focus their investments in this region of the world. The impact of any partial or complete dissolution of the EU on European economies could be significant, resulting in negative impacts on currency and financial markets generally, such as increased volatility and illiquidity, and potentially lower economic growth in markets in Europe, which may adversely affect the value of your investment in the Fund. Growth Securities Risk. Growth securities typically trade at a higher multiple of earnings than other types of equity securities. Accordingly, the market values of growth securities may never reach their expected market value and may decline in price. In addition, growth securities, at times, may not perform as well as value securities or the stock market in general, and may be out of favor with investors for varying periods of time.
Risk Lose Money [Text]	rr_RiskLoseMoney	There is no assurance that the Fund will achieve its investment objective and you may lose money.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance Information
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The following bar chart and table show you how the Fund has performed in the past, and can help you understand the risks of investing in the Fund. The bar chart shows how the Fund's Class Inst share performance has varied for each full calendar year shown. The table below the bar chart compares the Fund's returns for the periods shown with those of the MSCI ACWI ex USA Index (Net), the Fund's primary benchmark, and those of the MSCI ACWI ex USA Growth Index (Net), the Fund's secondary benchmark.  The MSCI ACWI ex USA Index (Net) captures large- and mid-cap representation across 22 of 23 developed market countries (excluding the United States) and 24 emerging markets countries.  It has 1,864 constituents (as of March 31, 2018) and covers approximately 85% of the global equity opportunity set outside the United States.  The MSCI ACWI ex USA Growth Index (Net) captures large-and mid-cap representation across 22 developed markets countries and 24 emerging markets countries.  The Investment Manager believes that the secondary benchmark provides a meaningful additional basis for comparing the Fund's performance, because the Fund's portfolio will in general be as closely aligned with the additional growth benchmark as with the current core benchmark from an investment style perspective. The performance of one or more share classes shown in the Average Annual Total Returns table below includes the Fund's Class Inst share returns (adjusted to reflect the higher class-related operating expenses of such classes, where applicable) for periods prior to the indicated inception date of such share classes. Except for differences in fees and expenses, all share classes of the Fund would have substantially similar annual returns because all share classes of the Fund invest in the same portfolio of securities. The after-tax returns shown in the Average Annual Total Returns table below are calculated using the highest historical individual U.S. federal marginal income tax rates in effect during the period indicated in the table and do not reflect the impact of state, local or foreign taxes. Your actual after-tax returns will depend on your personal tax situation and may differ from those shown in the table. In addition, the after-tax returns shown in the table do not apply to shares held in tax-advantaged accounts such as 401(k) plans or Individual Retirement Accounts (IRAs). The after-tax returns are shown only for Class Inst shares and will vary for other share classes. The Fund's past performance (before and after taxes) is no guarantee of how the Fund will perform in the future. Updated performance information can be obtained by calling toll-free 800.345.6611 or visiting columbiathreadneedle.com/us.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The following bar chart and table show you how the Fund has performed in the past, and can help you understand the risks of investing in the Fund. The bar chart shows how the Fund’s Class Inst share performance has varied for each full calendar year shown. The table below the bar chart compares the Fund’s returns for the periods shown with those of the MSCI ACWI ex USA Index (Net), the Fund's primary benchmark, and those of the MSCI ACWI ex USA Growth Index (Net), the Fund's secondary benchmark.
Performance Additional Market Index [Text]	rr_PerformanceAdditionalMarketIndex	The MSCI ACWI ex USA Growth Index (Net) captures large-and mid-cap representation across 22 developed markets countries and 24 emerging markets countries.  The Investment Manager believes that the secondary benchmark provides a meaningful additional basis for comparing the Fund's performance, because the Fund’s portfolio will in general be as closely aligned with the additional growth benchmark as with the current core benchmark from an investment style perspective.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	800.345.6611
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	columbiathreadneedle.com/us
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The Fund’s past performance (before and after taxes) is no guarantee of how the Fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Year by Year Total Return (%) as of December 31 Each Year
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Best and Worst Quarterly Returns During the Period Shown in the Bar Chart Best                                                          2nd Quarter 2009                                                         23.44% Worst                                                       3rd Quarter 2008                                                         -24.76%
Performance Table Heading	rr_PerformanceTableHeading	Average Annual Total Returns After Applicable Sales Charges (for periods ended December 31, 2017)
Performance Table Uses Highest Federal Rate	rr_PerformanceTableUsesHighestFederalRate	The after-tax returns shown in the Average Annual Total Returns table below are calculated using the highest historical individual U.S. federal marginal income tax rates in effect during the period indicated in the table and do not reflect the impact of state, local or foreign taxes.
Performance Table Not Relevant to Tax Deferred	rr_PerformanceTableNotRelevantToTaxDeferred	In addition, the after-tax returns shown in the table do not apply to shares held in tax-advantaged accounts such as 401(k) plans or Individual Retirement Accounts (IRAs).
Performance Table One Class of after Tax Shown [Text]	rr_PerformanceTableOneClassOfAfterTaxShown	The after-tax returns are shown only for Class Inst shares and will vary for other share classes.
Columbia Acorn International Select | Class A
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	5.75%
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	1.00%	[1]
Management fees	rr_ManagementFeesOverAssets	0.89%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Other expenses	rr_OtherExpensesOverAssets	0.40%	[2]
Total annual Fund operating expenses	rr_ExpensesOverAssets	1.54%
Less: Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.14%)	[3]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	1.40%
Expenses Deferred Charges [Text Block]	rr_ExpensesDeferredChargesTextBlock	This charge is imposed on certain investments of between $1 million and $50 million redeemed within 18 months after purchase, as follows: 1.00% if redeemed within 12 months after purchase, and 0.50% if redeemed more than 12, but less than 18, months after purchase, with certain limited exceptions.
1 year	rr_ExpenseExampleYear01	$ 709
3 years	rr_ExpenseExampleYear03	1,021
5 years	rr_ExpenseExampleYear05	1,354
10 years	rr_ExpenseExampleYear10	2,293
1 year	rr_ExpenseExampleNoRedemptionYear01	709
3 years	rr_ExpenseExampleNoRedemptionYear03	1,021
5 years	rr_ExpenseExampleNoRedemptionYear05	1,354
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 2,293
1 Year	rr_AverageAnnualReturnYear01	27.54%
5 Years	rr_AverageAnnualReturnYear05	6.20%
10 Years	rr_AverageAnnualReturnYear10	3.12%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Oct. 16, 2000
Columbia Acorn International Select | Class Adv
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.89%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.40%	[2]
Total annual Fund operating expenses	rr_ExpensesOverAssets	1.29%
Less: Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.14%)	[3]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	1.15%
1 year	rr_ExpenseExampleYear01	$ 117
3 years	rr_ExpenseExampleYear03	395
5 years	rr_ExpenseExampleYear05	694
10 years	rr_ExpenseExampleYear10	1,544
1 year	rr_ExpenseExampleNoRedemptionYear01	117
3 years	rr_ExpenseExampleNoRedemptionYear03	395
5 years	rr_ExpenseExampleNoRedemptionYear05	694
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 1,544
1 Year	rr_AverageAnnualReturnYear01	35.67%
5 Years	rr_AverageAnnualReturnYear05	7.74%
10 Years	rr_AverageAnnualReturnYear10	4.06%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 08, 2012
Columbia Acorn International Select | Class C
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	1.00%	[4]
Management fees	rr_ManagementFeesOverAssets	0.89%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	1.00%
Other expenses	rr_OtherExpensesOverAssets	0.40%	[2]
Total annual Fund operating expenses	rr_ExpensesOverAssets	2.29%
Less: Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.14%)	[3]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	2.15%
Expenses Deferred Charges [Text Block]	rr_ExpensesDeferredChargesTextBlock	This charge applies to redemptions within 12 months after purchase, with certain limited exceptions.
1 year	rr_ExpenseExampleYear01	$ 318
3 years	rr_ExpenseExampleYear03	702
5 years	rr_ExpenseExampleYear05	1,212
10 years	rr_ExpenseExampleYear10	2,615
1 year	rr_ExpenseExampleNoRedemptionYear01	218
3 years	rr_ExpenseExampleNoRedemptionYear03	702
5 years	rr_ExpenseExampleNoRedemptionYear05	1,212
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 2,615
1 Year	rr_AverageAnnualReturnYear01	33.32%
5 Years	rr_AverageAnnualReturnYear05	6.64%
10 Years	rr_AverageAnnualReturnYear10	2.93%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Oct. 16, 2000
Columbia Acorn International Select | Class Inst
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.89%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.40%	[2]
Total annual Fund operating expenses	rr_ExpensesOverAssets	1.29%
Less: Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.14%)	[3]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	1.15%
1 year	rr_ExpenseExampleYear01	$ 117
3 years	rr_ExpenseExampleYear03	395
5 years	rr_ExpenseExampleYear05	694
10 years	rr_ExpenseExampleYear10	1,544
1 year	rr_ExpenseExampleNoRedemptionYear01	117
3 years	rr_ExpenseExampleNoRedemptionYear03	395
5 years	rr_ExpenseExampleNoRedemptionYear05	694
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 1,544
2008	rr_AnnualReturn2008	(42.10%)	[5]
2009	rr_AnnualReturn2009	31.52%	[5]
2010	rr_AnnualReturn2010	21.89%	[5]
2011	rr_AnnualReturn2011	(9.76%)	[5]
2012	rr_AnnualReturn2012	22.42%	[5]
2013	rr_AnnualReturn2013	14.75%	[5]
2014	rr_AnnualReturn2014	(6.79%)	[5]
2015	rr_AnnualReturn2015	(1.03%)	[5]
2016	rr_AnnualReturn2016	1.18%	[5]
2017	rr_AnnualReturn2017	35.70%	[5]
Year to Date Return, Label	rr_YearToDateReturnLabel	Year to Date return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Mar. 31, 2018
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	0.51%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Jun. 30, 2009
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	23.44%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Sep. 30, 2008
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(24.76%)
1 Year	rr_AverageAnnualReturnYear01	35.70%
5 Years	rr_AverageAnnualReturnYear05	7.77%
10 Years	rr_AverageAnnualReturnYear10	4.07%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 23, 1998
Columbia Acorn International Select | Class Inst2
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.89%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.34%	[2]
Total annual Fund operating expenses	rr_ExpensesOverAssets	1.23%
Less: Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.17%)	[3]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	1.06%
1 year	rr_ExpenseExampleYear01	$ 108
3 years	rr_ExpenseExampleYear03	374
5 years	rr_ExpenseExampleYear05	659
10 years	rr_ExpenseExampleYear10	1,474
1 year	rr_ExpenseExampleNoRedemptionYear01	108
3 years	rr_ExpenseExampleNoRedemptionYear03	374
5 years	rr_ExpenseExampleNoRedemptionYear05	659
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 1,474
1 Year	rr_AverageAnnualReturnYear01	35.72%
5 Years	rr_AverageAnnualReturnYear05	7.82%
10 Years	rr_AverageAnnualReturnYear10	4.09%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 08, 2012
Columbia Acorn International Select | Class Inst3
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.89%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.27%	[2]
Total annual Fund operating expenses	rr_ExpensesOverAssets	1.16%
Less: Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.15%)	[3]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	1.01%
1 year	rr_ExpenseExampleYear01	$ 103
3 years	rr_ExpenseExampleYear03	354
5 years	rr_ExpenseExampleYear05	624
10 years	rr_ExpenseExampleYear10	1,396
1 year	rr_ExpenseExampleNoRedemptionYear01	103
3 years	rr_ExpenseExampleNoRedemptionYear03	354
5 years	rr_ExpenseExampleNoRedemptionYear05	624
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 1,396
1 Year	rr_AverageAnnualReturnYear01	35.81%
5 Years	rr_AverageAnnualReturnYear05	7.87%
10 Years	rr_AverageAnnualReturnYear10	4.12%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 08, 2012
Columbia Acorn International Select | returns after taxes on distributions | Class Inst
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	35.76%
5 Years	rr_AverageAnnualReturnYear05	6.62%
10 Years	rr_AverageAnnualReturnYear10	3.09%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 23, 1998
Columbia Acorn International Select | returns after taxes on distributions and sale of Fund shares | Class Inst
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	20.42%
5 Years	rr_AverageAnnualReturnYear05	5.96%
10 Years	rr_AverageAnnualReturnYear10	3.12%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 23, 1998
Columbia Acorn International Select | MSCI ACWI ex USA Index (Net) (reflects reinvested dividends net of withholding taxes but reflects no deductions for fees, expenses or other taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	27.19%
5 Years	rr_AverageAnnualReturnYear05	6.80%
10 Years	rr_AverageAnnualReturnYear10	1.84%
Columbia Acorn International Select | MSCI ACWI ex USA Growth Index (Net) (reflects reinvested dividends net of withholding taxes but reflects no deductions for fees, expenses or other taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	32.01%
5 Years	rr_AverageAnnualReturnYear05	7.97%
10 Years	rr_AverageAnnualReturnYear10	2.40%

[1] This charge is imposed on certain investments of between $1 million and $50 million redeemed within 18 months after purchase, as follows: 1.00% if redeemed within 12 months after purchase, and 0.50% if redeemed more than 12, but less than 18, months after purchase, with certain limited exceptions.

[2] Other expenses have been restated to reflect current transfer agency fees paid by the Fund.

[3] Columbia Wanger Asset Management, LLC (the Investment Manager) has contractually agreed to waive fees and reimburse certain expenses of the Fund, through April 30, 2019, so that ordinary operating expenses (excluding transaction costs and certain other investment-related expenses, interest and fees on borrowings and expenses associated with the Fund’s investment in other investment companies, if any) do not exceed the annual rates of 1.40% for Class A shares, 1.15% for Class Adv shares, 2.15% for Class C shares, 1.15% for Class Inst shares, 1.06% for Class Inst2 shares and 1.01% for Class Inst3 shares. This arrangement may only be amended or terminated with approval from the Fund’s Board of Trustees and the Investment Manager. The fee waivers and/or expense reimbursements shown in the table also reflect the contractual agreement of the Fund’s transfer agent, Columbia Management Investment Services Corp. (the Transfer Agent), to waive a portion of its fees through June 30, 2019, such that the Fund’s transfer agency fees do not exceed the annual rates of 0.05% of the average daily net assets of Class Inst2 shares and 0.00% of the average daily net assets of Class Inst3 shares.

[4] This charge applies to redemptions within 12 months after purchase, with certain limited exceptions.

[5] Year to Date return as of March 31, 2018: 0.51%

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Columbia Acorn Select

SUMMARY OF THE FUND

Investment Objective

Columbia Acorn Select^SM (the Fund) seeks long-term capital appreciation.

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. An investor transacting in a class of Fund shares without any front-end sales charge, contingent deferred sales charge, or other asset-based fee for sales or distribution may be required to pay a commission to the financial intermediary for effecting such transactions. Such commission rates are set by the financial intermediary and are not reflected in the tables or the example below. You may qualify for sales charge discounts if you and members of your immediate family invest, or agree to invest in the future, at least $50,000 in certain classes of shares of eligible funds distributed by Columbia Management Investment Distributors, Inc. (the Distributor). More information is available about these and other sales charge discounts and waivers from your financial intermediary, and can be found in the Choosing a Share Class section beginning on page 23 of the Fund’s prospectus, in Appendix A to the prospectus beginning on page A-1 and in Appendix S to the Statement of Additional Information (SAI) under Sales Charge Waivers beginning on page S-1.

Shareholder Fees (fees paid directly from your investment)

Shareholder Fees - Columbia Acorn Select	Class A		Class C		Class Adv	Class Inst	Class Inst2	Class Inst3
Maximum sales charge (load) imposed on purchases (as a % of offering price)	5.75%		none		none	none	none	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	1.00%	[1]	1.00%	[2]	none	none	none	none

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)

Annual Fund Operating Expenses - Columbia Acorn Select		Class A	Class Adv	Class C	Class Inst	Class Inst2	Class Inst3
Management fees		0.85%	0.85%	0.85%	0.85%	0.85%	0.85%
Distribution and/or service (12b-1) fees		0.25%	none	1.00%	none	none	none
Other expenses	[1]	0.29%	0.29%	0.29%	0.29%	0.22%	0.15%
Acquired fund fees and expenses		0.01%	0.01%	0.01%	0.01%	0.01%	0.01%
Total annual Fund operating expenses	[2]	1.40%	1.15%	2.15%	1.15%	1.08%	1.01%
Less: Fee waivers and/or expense reimbursements	[3]	(0.23%)	(0.23%)	(0.23%)	(0.23%)	(0.22%)	(0.20%)
Total annual Fund operating expenses after fee waivers and/or expense reimbursements		1.17%	0.92%	1.92%	0.92%	0.86%	0.81%

Example

The following example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example illustrates the hypothetical expenses that you would incur over the time periods indicated, and assumes that:

• you invest $10,000 in the applicable class of Fund shares for the periods indicated,

• your investment has a 5% return each year, and

• the Fund’s total annual operating expenses remain the same as shown in the Annual Fund Operating Expenses table above.

Since the waivers and/or reimbursements shown in the Annual Fund Operating Expenses table above expire as indicated in the preceding table, they are only reflected in the 1 year example and the first year of the other examples. Although your actual costs may be higher or lower, based on the assumptions listed above, your costs would be:

Expense Example - Columbia Acorn Select - USD ($)	1 year	3 years	5 years	10 years
Class A	687	971	1,276	2,139
Class Adv	94	343	611	1,377
Class C	295	651	1,133	2,465
Class Inst	94	343	611	1,377
Class Inst2	88	322	574	1,297
Class Inst3	83	302	538	1,218

Expense Example, No Redemption - Columbia Acorn Select - USD ($)	1 year	3 years	5 years	10 years
Class A	687	971	1,276	2,139
Class Adv	94	343	611	1,377
Class C	195	651	1,133	2,465
Class Inst	94	343	611	1,377
Class Inst2	88	322	574	1,297
Class Inst3	83	302	538	1,218

Portfolio Turnover

The Fund may pay transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 41% of the average value of its portfolio.

Principal Investment Strategies

Under normal circumstances, the Fund (i) invests a majority of its net assets in the common stock of small- and mid-sized companies with market capitalizations under $20 billion at the time of initial investment (“Focus Stocks”) and (ii) may also invest in companies with market capitalizations above $20 billion, provided that immediately after that investment a majority of the Fund’s net assets would be invested in Focus Stocks. The Fund may continue to hold, and make additional investments in, Focus Stocks whose market capitalizations have grown to exceed $20 billion, regardless of whether the Fund’s investments in Focus Stocks are a majority of the Fund’s net assets.

Columbia Wanger Asset Management, LLC, the Fund's investment adviser (the Investment Manager), believes that stocks of small- and mid-sized companies, which generally are not as well known by financial analysts as larger companies, may offer higher return potential than stocks of larger companies.

The Fund invests the majority of its assets in U.S. companies, but also may invest up to 20% of its total assets in foreign companies in developed markets (for example, Japan, Canada and the United Kingdom). A portion of the Fund’s foreign exposure may also include companies in emerging markets (for example, China, India and Colombia).

The Fund also may invest up to 20% of its net assets in real estate investment trusts.

The Fund invests in a limited number of companies (generally between 20-40), offering the potential to provide above-average growth over time. In pursuit of the Fund's objective, the portfolio manager takes advantage of the research and stock-picking capabilities of the Investment Manager. The Investment Manager seeks to select investments which offer the best combination of risk and return. The Investment Manager’s efforts are generally focused on sectors, industries and companies which provide higher expected growth than the overall U.S. economy.

The Investment Manager typically seeks companies with:

• A strong business franchise that offers growth potential.

• Products and services in which the company has a competitive advantage.

• A stock price the Investment Manager believes is reasonable relative to the assets and earning power of the company.

The Investment Manager may sell a portfolio holding if the security reaches the Investment Manager's price target, if the company has a deterioration of fundamentals, such as failing to meet key operating benchmarks, or if the Investment Manager believes other securities are more attractive. The Investment Manager also may sell a portfolio holding to fund redemptions.

Principal Risks

An investment in the Fund involves risks, including those described below. There is no assurance that the Fund will achieve its investment objective and you may lose money. The value of the Fund’s holdings may decline, and the Fund’s net asset value (NAV) and share price may go down. An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.

Select Portfolio Risk. Because the Fund may invest in a limited number of companies, the Fund as a whole is subject to greater risk of loss if any of its portfolio securities decline in price. In addition, the Fund’s holdings and weightings will diverge significantly from its primary benchmark’s holdings and weightings and the Fund may therefore experience greater risk and volatility relative to the benchmark. Because the Fund may invest in more than one company concentrated in a similar industry, sector or geographic region, the Fund may be even more concentrated than the number of companies it may hold would suggest.

Active Management Risk. The Investment Manager’s active management of the Fund could cause the Fund to underperform its benchmark index and/or other funds with similar investment objectives and/or strategies.

Market Risk. Market risk refers to the possibility that the market values of securities or other investments that the Fund holds will fall, sometimes rapidly or unpredictably, or fail to rise. An investment in the Fund could lose money over short or long periods.

Small- and Mid-Cap Company Securities Risk. Investments in small- and mid-capitalization companies (small- and mid-cap companies) often involve greater risks than investments in larger, more established companies (larger companies) because small- and mid-cap companies tend to have less predictable earnings and may lack the management experience, financial resources, product diversification and competitive strengths of larger companies. Securities of small- and mid-cap companies may be less liquid and more volatile than the securities of larger companies.

Growth Securities Risk. Growth securities typically trade at a higher multiple of earnings than other types of equity securities. Accordingly, the market values of growth securities may never reach their expected market value and may decline in price. In addition, growth securities, at times, may not perform as well as value securities or the stock market in general, and may be out of favor with investors for varying periods of time.

Issuer Risk. An issuer in which the Fund invests may perform poorly, and the value of its securities may therefore decline, which would negatively affect the Fund’s performance. Poor performance may be caused by poor management decisions, competitive pressures, breakthroughs in technology, reliance on suppliers, labor problems or shortages, corporate restructurings, fraudulent disclosures, natural disasters or other events, conditions or factors.

Sector Risk. At times, the Fund may have a significant portion of its assets invested in securities of companies conducting business in a related group of industries within a sector. Companies in the same sector may be similarly affected by economic, regulatory, political or market events or conditions, which may make the Fund more vulnerable to unfavorable developments in that sector than funds that invest more broadly. Generally, the more the Fund diversifies its investments, the more it spreads risk and potentially reduces the risks of loss and volatility.

Foreign Securities Risk. Investments in or exposure to foreign securities involve certain risks not associated with investments in or exposure to securities of U.S. companies. Foreign securities subject the Fund to the risks associated with investing in the particular country of an issuer, including political, regulatory, economic, social, diplomatic and other conditions or events (including, for example, military confrontations, war and terrorism), occurring in the country or region, as well as risks associated with less developed custody and settlement practices. Foreign securities may be more volatile and less liquid than securities of U.S. companies, and are subject to the risks associated with potential imposition of economic and other sanctions against a particular foreign country, its nationals or industries or businesses within the country. In addition, foreign governments may impose withholding or other taxes on the Fund’s income, capital gains or proceeds from the disposition of foreign securities, which could reduce the Fund’s return on such securities.

Operational and Settlement Risks of Foreign Securities. The Fund’s foreign securities are generally held outside the United States in the primary market for the securities in the custody of foreign sub-custodians. Some countries have limited governmental oversight and regulation, which increases the risk of corruption and fraud and the possibility of losses to the Fund. In particular, under certain circumstances, foreign securities may settle on a delayed delivery basis, meaning that the Fund may be required to make payment for securities before the Fund has actually received delivery of the securities or deliver securities prior to the receipt of payment. As a result, there is a risk that the security will not be delivered to the Fund or that payment will not be received. Losses can also result from lost, stolen or counterfeit securities; defaults by brokers and banks; failures or defects of the settlement system; or poor and improper record keeping by registrars and issuers.

Share Blocking. In certain non-U.S. markets, an issuer’s securities are blocked from trading for a specified number of days before and, in certain instances, after a shareholder meeting. The blocking period can last up to several weeks. Share blocking may prevent the Fund from buying or selling securities during this period. As a consequence of these restrictions, the Investment Manager, on behalf of the Fund, may abstain from voting proxies in markets that require share blocking.

Emerging Market Securities Risk. Securities issued by foreign governments or companies in emerging market countries, such as China, Russia and certain countries in Eastern Europe, the Middle East, Asia, Latin America or Africa, are more likely to have greater exposure to the risks of investing in foreign securities that are described in Foreign Securities Risk. In addition, emerging market countries are more likely to experience instability resulting, for example, from rapid changes or developments in social, political, economic or other conditions. Their economies are usually less mature and their securities markets are typically less developed with more limited trading activity (i.e., lower trading volumes and less liquidity) than more developed countries. Emerging market securities tend to be more volatile than securities in more developed markets. Many emerging market countries are heavily dependent on international trade and have fewer trading partners, which makes them more sensitive to world commodity prices and economic downturns in other countries, and some have a higher risk of currency devaluations.

Operational and Settlement Risks of Securities in Emerging Markets. Foreign sub-custodians in emerging markets may be recently organized, lack extensive operating experience or lack effective government oversight or regulation. In addition, there may be legal restrictions or limitations on the ability of the Fund to recover assets held in custody by a foreign sub-custodian in the event of the bankruptcy of the sub-custodian. There may also be a greater risk that settlement may be delayed and that cash or securities of the Fund may be lost because of failures of or defects in the system, including fraud or corruption. Settlement systems in emerging markets also have a higher risk of failed trades.

Risks Related to Currencies and Corporate Actions in Emerging Markets. Risks related to currencies and corporate actions are also greater in emerging market countries than in developed countries. Emerging market currencies may not have an active trading market and are subject to a higher risk of currency devaluations.

Risks Related to Corporate and Securities Laws in Emerging Markets. Securities laws in emerging markets may be relatively new and unsettled and, consequently, there is a risk of rapid and unpredictable change in laws regarding foreign investment, securities regulation, title to securities and shareholder rights.

Liquidity and Trading Volume Risk. Because the Fund may invest a percentage of its assets in foreign securities, it may be subject to the liquidity and trading volume risks associated with international investing. Due to market conditions, including uncertainty regarding the price of a security, it may be difficult for the Fund to buy or sell foreign portfolio securities at a desirable time or price, which could result in investment losses and may affect the value of your investment in the Fund. This risk of portfolio illiquidity is heightened with respect to small- and mid-cap securities, generally, and foreign small- and mid-cap securities in particular. The Fund may have to lower the selling price, liquidate other investments, or forego another, more appealing investment opportunity as a result of illiquidity in the markets. As a result of significant and sustained reductions in emerging and developed international market trading volumes in the wake of the 2007-2009 financial crisis, it may take longer to buy or sell these securities, which can exacerbate the Fund's exposure to volatile markets. The Fund may also be limited in its ability to execute favorable trades in foreign portfolio securities in response to changes in share prices and fundamentals, and may be forced to dispose of securities under disadvantageous circumstances and at a loss. As the Fund grows in size or, conversely, if it faces significant redemption pressure, these considerations take on increasing significance and may adversely impact performance.

Foreign Currency Risk. The performance of the Fund may be materially affected positively or negatively by foreign currency strength or weakness relative to the U.S. dollar, particularly if the Fund invests a significant percentage of its assets in foreign securities or other assets denominated in currencies other than the U.S. dollar.

Real Estate-Related Investment Risk. Investments in real estate investment trusts (REITs) and in securities of other companies (wherever organized) principally engaged in the real estate industry subject the Fund to, among other things, risks similar to those of direct investments in real estate and the real estate industry in general. These include risks related to general and local economic conditions, possible lack of availability of financing and changes in interest rates or property values. REITs are entities that either own properties or make construction or mortgage loans, and also may include operating or finance companies. The value of interests in a REIT may be affected by, among other factors, changes in the value of the underlying properties owned by the REIT, changes in the prospect for earnings and/or cash flow growth of the REIT itself, defaults by borrowers or tenants, market saturation, decreases in market rates for rents, and other economic, political, or regulatory matters affecting the real estate industry, including REITs. REITs and similar non-U.S. entities depend upon specialized management skills, may have limited financial resources, may have less trading volume in their securities, and may be subject to more abrupt or erratic price movements than the overall securities markets. REITs are also subject to the risk of failing to qualify for favorable tax treatment under the Internal Revenue Code of 1986, as amended. Some REITs (especially mortgage REITs) are affected by risks similar to those associated with investments in debt securities including changes in interest rates and the quality of credit extended.

Performance Information

The following bar chart and table show you how the Fund has performed in the past, and can help you understand the risks of investing in the Fund. The bar chart shows how the Fund’s Class Inst share performance has varied for each full calendar year shown. The table below the bar chart compares the Fund’s returns for the periods shown with the Russell 2500 Growth Index.

The performance of one or more share classes shown in the Average Annual Total Returns table below includes the Fund’s Class Inst share returns (adjusted to reflect the higher class-related operating expenses of such classes, where applicable) for periods prior to the indicated inception date of such share classes. Except for differences in fees and expenses, all share classes of the Fund would have substantially similar annual returns because all share classes of the Fund invest in the same portfolio of securities.

The after-tax returns shown in the Average Annual Total Returns table below are calculated using the highest historical individual U.S. federal marginal income tax rates in effect during the period indicated in the table and do not reflect the impact of state, local or foreign taxes. Your actual after-tax returns will depend on your personal tax situation and may differ from those shown in the table. In addition, the after-tax returns shown in the table do not apply to shares held in tax-advantaged accounts such as 401(k) plans or Individual Retirement Accounts (IRAs). The after-tax returns are shown only for Class Inst shares and will vary for other share classes.

The Fund’s past performance (before and after taxes) is no guarantee of how the Fund will perform in the future. Updated performance information can be obtained by calling toll-free 800.345.6611 or visiting columbiathreadneedle.com/us.

Year by Year Total Return (%)
as of December 31 Each Year

Best and Worst Quarterly Returns
During the Period Shown in the Bar Chart

Best                                                     2nd Quarter 2009                                                      28.11%

Worst                                                   4th Quarter 2008                                                     -30.14%

Average Annual Total Returns After Applicable Sales Charges (for periods ended December 31, 2017)

Average Annual Total Returns - Columbia Acorn Select	Share Class Inception Date	1 Year	5 Years	10 Years
Class Inst	Nov. 23, 1998	26.59%	14.16%	7.02%
Class Inst | returns after taxes on distributions	Nov. 23, 1998	23.31%	8.55%	4.14%
Class Inst | returns after taxes on distributions and sale of Fund shares	Nov. 23, 1998	17.32%	10.07%	5.02%
Class A	Oct. 16, 2000	19.16%	12.51%	6.09%
Class Adv	Nov. 08, 2012	26.71%	14.13%	7.01%
Class C	Oct. 16, 2000	24.40%	13.01%	5.90%
Class Inst2	Nov. 08, 2012	26.72%	14.21%	7.05%
Class Inst3	Nov. 08, 2012	26.79%	14.27%	7.07%
Russell 2500 Growth Index (reflects no deductions for fees, expenses or taxes)		24.46%	15.47%	9.62%

[1] This charge is imposed on certain investments of between $1 million and $50 million redeemed within 18 months after purchase, as follows: 1.00% if redeemed within 12 months after purchase, and 0.50% if redeemed more than 12, but less than 18, months after purchase, with certain limited exceptions.

[2] This charge applies to redemptions within 12 months after purchase, with certain limited exceptions.

[1] Other expenses have been restated to reflect current transfer agency fees paid by the Fund.

[2] "Total annual Fund operating expenses" include acquired fund fees and expenses (expenses the Fund incurs indirectly through its investments in other investment companies) and may be higher than "Total Net Expenses" shown in the Financial Highlights section of this prospectus because "Total Net Expenses" do not include acquired fund fees and expenses.

[3] Columbia Wanger Asset Management, LLC (the Investment Manager) has contractually agreed to waive 0.20% of the advisory fee otherwise payable to it by the Fund through April 30, 2019. This arrangement may only be amended or terminated with approval from the Fund's Board of Trustees and the Investment Manager. The fee waivers and/or expense reimbursements shown in the table also reflect the contractual agreement of the Fund’s transfer agent, Columbia Management Investment Services Corp. (the Transfer Agent), to waive a portion of its fees through June 30, 2019, such that the Fund’s transfer agency fees do not exceed the annual rate of 0.11% of the average daily net assets of each share class other than Class Inst2 and Class Inst3. With respect to Class Inst2 and Class Inst3 shares, the Transfer Agent has contractually agreed to waive a portion of its fees such that the Fund's transfer agency fees do not exceed the annual rates of 0.05% of the average daily net assets of Class Inst2 shares and 0.00% of the average daily net assets of Class Inst3 shares.

[1] Year to Date return as of March 31, 2018: -0.79%

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	COLUMBIA ACORN TRUST
Prospectus Date	rr_ProspectusDate	May 01, 2018
Columbia Acorn Select
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	SUMMARY OF THE FUND
Objective [Heading]	rr_ObjectiveHeading	Investment Objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	Columbia Acorn SelectSM (the Fund) seeks long-term capital appreciation.
Expense [Heading]	rr_ExpenseHeading	Fees and Expenses of the Fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. An investor transacting in a class of Fund shares without any front-end sales charge, contingent deferred sales charge, or other asset-based fee for sales or distribution may be required to pay a commission to the financial intermediary for effecting such transactions. Such commission rates are set by the financial intermediary and are not reflected in the tables or the example below. You may qualify for sales charge discounts if you and members of your immediate family invest, or agree to invest in the future, at least $50,000 in certain classes of shares of eligible funds distributed by Columbia Management Investment Distributors, Inc. (the Distributor). More information is available about these and other sales charge discounts and waivers from your financial intermediary, and can be found in the Choosing a Share Class section beginning on page 23 of the Fund’s prospectus, in Appendix A to the prospectus beginning on page A-1 and in Appendix S to the Statement of Additional Information (SAI) under Sales Charge Waivers beginning on page S-1.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder Fees (fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	April 30, 2019 June 30, 2019
Portfolio Turnover [Heading]	rr_PortfolioTurnoverHeading	Portfolio Turnover
Portfolio Turnover [Text Block]	rr_PortfolioTurnoverTextBlock	The Fund may pay transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 41% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	41.00%
Expense Breakpoint Discounts [Text]	rr_ExpenseBreakpointDiscounts	You may qualify for sales charge discounts if you and members of your immediate family invest, or agree to invest in the future, at least $50,000 in certain classes of shares of eligible funds distributed by Columbia Management Investment Distributors, Inc. (the Distributor). More information is available about these and other sales charge discounts and waivers from your financial intermediary, and can be found in the Choosing a Share Class section beginning on page 23 of the Fund’s prospectus, in Appendix A to the prospectus beginning on page A-1 and in Appendix S to the Statement of Additional Information (SAI) under Sales Charge Waivers beginning on page S-1.
Expense Breakpoint, Minimum Investment Required [Amount]	rr_ExpenseBreakpointMinimumInvestmentRequiredAmount	$ 50,000
Expenses Restated to Reflect Current [Text]	rr_ExpensesRestatedToReflectCurrent	Other expenses have been restated to reflect current transfer agency fees paid by the Fund.
Expenses Not Correlated to Ratio Due to Acquired Fund Fees [Text]	rr_ExpensesNotCorrelatedToRatioDueToAcquiredFundFees	"Total annual Fund operating expenses" include acquired fund fees and expenses (expenses the Fund incurs indirectly through its investments in other investment companies) and may be higher than "Total Net Expenses" shown in the Financial Highlights section of this prospectus because "Total Net Expenses" do not include acquired fund fees and expenses.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	The following example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example illustrates the hypothetical expenses that you would incur over the time periods indicated, and assumes that: you invest $10,000 in the applicable class of Fund shares for the periods indicated, your investment has a 5% return each year, and the Fund’s total annual operating expenses remain the same as shown in the Annual Fund Operating Expenses table above. Since the waivers and/or reimbursements shown in the Annual Fund Operating Expenses table above expire as indicated in the preceding table, they are only reflected in the 1 year example and the first year of the other examples. Although your actual costs may be higher or lower, based on the assumptions listed above, your costs would be:
Strategy [Heading]	rr_StrategyHeading	Principal Investment Strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	Under normal circumstances, the Fund (i) invests a majority of its net assets in the common stock of small- and mid-sized companies with market capitalizations under $20 billion at the time of initial investment (“Focus Stocks”) and (ii) may also invest in companies with market capitalizations above $20 billion, provided that immediately after that investment a majority of the Fund’s net assets would be invested in Focus Stocks. The Fund may continue to hold, and make additional investments in, Focus Stocks whose market capitalizations have grown to exceed $20 billion, regardless of whether the Fund’s investments in Focus Stocks are a majority of the Fund’s net assets. Columbia Wanger Asset Management, LLC, the Fund's investment adviser (the Investment Manager), believes that stocks of small- and mid-sized companies, which generally are not as well known by financial analysts as larger companies, may offer higher return potential than stocks of larger companies. The Fund invests the majority of its assets in U.S. companies, but also may invest up to 20% of its total assets in foreign companies in developed markets (for example, Japan, Canada and the United Kingdom). A portion of the Fund’s foreign exposure may also include companies in emerging markets (for example, China, India and Colombia). The Fund also may invest up to 20% of its net assets in real estate investment trusts. The Fund invests in a limited number of companies (generally between 20-40), offering the potential to provide above-average growth over time. In pursuit of the Fund's objective, the portfolio manager takes advantage of the research and stock-picking capabilities of the Investment Manager. The Investment Manager seeks to select investments which offer the best combination of risk and return. The Investment Manager’s efforts are generally focused on sectors, industries and companies which provide higher expected growth than the overall U.S. economy. The Investment Manager typically seeks companies with: A strong business franchise that offers growth potential. Products and services in which the company has a competitive advantage. A stock price the Investment Manager believes is reasonable relative to the assets and earning power of the company. The Investment Manager may sell a portfolio holding if the security reaches the Investment Manager's price target, if the company has a deterioration of fundamentals, such as failing to meet key operating benchmarks, or if the Investment Manager believes other securities are more attractive. The Investment Manager also may sell a portfolio holding to fund redemptions.
Risk [Heading]	rr_RiskHeading	Principal Risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the Fund involves risks, including those described below. There is no assurance that the Fund will achieve its investment objective and you may lose money. The value of the Fund’s holdings may decline, and the Fund’s net asset value (NAV) and share price may go down. An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Select Portfolio Risk. Because the Fund may invest in a limited number of companies, the Fund as a whole is subject to greater risk of loss if any of its portfolio securities decline in price. In addition, the Fund’s holdings and weightings will diverge significantly from its primary benchmark’s holdings and weightings and the Fund may therefore experience greater risk and volatility relative to the benchmark. Because the Fund may invest in more than one company concentrated in a similar industry, sector or geographic region, the Fund may be even more concentrated than the number of companies it may hold would suggest. Active Management Risk. The Investment Manager’s active management of the Fund could cause the Fund to underperform its benchmark index and/or other funds with similar investment objectives and/or strategies. Market Risk. Market risk refers to the possibility that the market values of securities or other investments that the Fund holds will fall, sometimes rapidly or unpredictably, or fail to rise. An investment in the Fund could lose money over short or long periods. Small- and Mid-Cap Company Securities Risk. Investments in small- and mid-capitalization companies (small- and mid-cap companies) often involve greater risks than investments in larger, more established companies (larger companies) because small- and mid-cap companies tend to have less predictable earnings and may lack the management experience, financial resources, product diversification and competitive strengths of larger companies. Securities of small- and mid-cap companies may be less liquid and more volatile than the securities of larger companies. Growth Securities Risk. Growth securities typically trade at a higher multiple of earnings than other types of equity securities. Accordingly, the market values of growth securities may never reach their expected market value and may decline in price. In addition, growth securities, at times, may not perform as well as value securities or the stock market in general, and may be out of favor with investors for varying periods of time. Issuer Risk. An issuer in which the Fund invests may perform poorly, and the value of its securities may therefore decline, which would negatively affect the Fund’s performance. Poor performance may be caused by poor management decisions, competitive pressures, breakthroughs in technology, reliance on suppliers, labor problems or shortages, corporate restructurings, fraudulent disclosures, natural disasters or other events, conditions or factors. Sector Risk. At times, the Fund may have a significant portion of its assets invested in securities of companies conducting business in a related group of industries within a sector. Companies in the same sector may be similarly affected by economic, regulatory, political or market events or conditions, which may make the Fund more vulnerable to unfavorable developments in that sector than funds that invest more broadly. Generally, the more the Fund diversifies its investments, the more it spreads risk and potentially reduces the risks of loss and volatility. Foreign Securities Risk. Investments in or exposure to foreign securities involve certain risks not associated with investments in or exposure to securities of U.S. companies. Foreign securities subject the Fund to the risks associated with investing in the particular country of an issuer, including political, regulatory, economic, social, diplomatic and other conditions or events (including, for example, military confrontations, war and terrorism), occurring in the country or region, as well as risks associated with less developed custody and settlement practices. Foreign securities may be more volatile and less liquid than securities of U.S. companies, and are subject to the risks associated with potential imposition of economic and other sanctions against a particular foreign country, its nationals or industries or businesses within the country. In addition, foreign governments may impose withholding or other taxes on the Fund’s income, capital gains or proceeds from the disposition of foreign securities, which could reduce the Fund’s return on such securities. Operational and Settlement Risks of Foreign Securities. The Fund’s foreign securities are generally held outside the United States in the primary market for the securities in the custody of foreign sub-custodians. Some countries have limited governmental oversight and regulation, which increases the risk of corruption and fraud and the possibility of losses to the Fund. In particular, under certain circumstances, foreign securities may settle on a delayed delivery basis, meaning that the Fund may be required to make payment for securities before the Fund has actually received delivery of the securities or deliver securities prior to the receipt of payment. As a result, there is a risk that the security will not be delivered to the Fund or that payment will not be received. Losses can also result from lost, stolen or counterfeit securities; defaults by brokers and banks; failures or defects of the settlement system; or poor and improper record keeping by registrars and issuers. Share Blocking. In certain non-U.S. markets, an issuer’s securities are blocked from trading for a specified number of days before and, in certain instances, after a shareholder meeting. The blocking period can last up to several weeks. Share blocking may prevent the Fund from buying or selling securities during this period. As a consequence of these restrictions, the Investment Manager, on behalf of the Fund, may abstain from voting proxies in markets that require share blocking. Emerging Market Securities Risk. Securities issued by foreign governments or companies in emerging market countries, such as China, Russia and certain countries in Eastern Europe, the Middle East, Asia, Latin America or Africa, are more likely to have greater exposure to the risks of investing in foreign securities that are described in Foreign Securities Risk. In addition, emerging market countries are more likely to experience instability resulting, for example, from rapid changes or developments in social, political, economic or other conditions. Their economies are usually less mature and their securities markets are typically less developed with more limited trading activity (i.e., lower trading volumes and less liquidity) than more developed countries. Emerging market securities tend to be more volatile than securities in more developed markets. Many emerging market countries are heavily dependent on international trade and have fewer trading partners, which makes them more sensitive to world commodity prices and economic downturns in other countries, and some have a higher risk of currency devaluations. Operational and Settlement Risks of Securities in Emerging Markets. Foreign sub-custodians in emerging markets may be recently organized, lack extensive operating experience or lack effective government oversight or regulation. In addition, there may be legal restrictions or limitations on the ability of the Fund to recover assets held in custody by a foreign sub-custodian in the event of the bankruptcy of the sub-custodian. There may also be a greater risk that settlement may be delayed and that cash or securities of the Fund may be lost because of failures of or defects in the system, including fraud or corruption. Settlement systems in emerging markets also have a higher risk of failed trades. Risks Related to Currencies and Corporate Actions in Emerging Markets. Risks related to currencies and corporate actions are also greater in emerging market countries than in developed countries. Emerging market currencies may not have an active trading market and are subject to a higher risk of currency devaluations. Risks Related to Corporate and Securities Laws in Emerging Markets. Securities laws in emerging markets may be relatively new and unsettled and, consequently, there is a risk of rapid and unpredictable change in laws regarding foreign investment, securities regulation, title to securities and shareholder rights. Liquidity and Trading Volume Risk. Because the Fund may invest a percentage of its assets in foreign securities, it may be subject to the liquidity and trading volume risks associated with international investing. Due to market conditions, including uncertainty regarding the price of a security, it may be difficult for the Fund to buy or sell foreign portfolio securities at a desirable time or price, which could result in investment losses and may affect the value of your investment in the Fund. This risk of portfolio illiquidity is heightened with respect to small- and mid-cap securities, generally, and foreign small- and mid-cap securities in particular. The Fund may have to lower the selling price, liquidate other investments, or forego another, more appealing investment opportunity as a result of illiquidity in the markets. As a result of significant and sustained reductions in emerging and developed international market trading volumes in the wake of the 2007-2009 financial crisis, it may take longer to buy or sell these securities, which can exacerbate the Fund's exposure to volatile markets. The Fund may also be limited in its ability to execute favorable trades in foreign portfolio securities in response to changes in share prices and fundamentals, and may be forced to dispose of securities under disadvantageous circumstances and at a loss. As the Fund grows in size or, conversely, if it faces significant redemption pressure, these considerations take on increasing significance and may adversely impact performance. Foreign Currency Risk. The performance of the Fund may be materially affected positively or negatively by foreign currency strength or weakness relative to the U.S. dollar, particularly if the Fund invests a significant percentage of its assets in foreign securities or other assets denominated in currencies other than the U.S. dollar. Real Estate-Related Investment Risk. Investments in real estate investment trusts (REITs) and in securities of other companies (wherever organized) principally engaged in the real estate industry subject the Fund to, among other things, risks similar to those of direct investments in real estate and the real estate industry in general. These include risks related to general and local economic conditions, possible lack of availability of financing and changes in interest rates or property values. REITs are entities that either own properties or make construction or mortgage loans, and also may include operating or finance companies. The value of interests in a REIT may be affected by, among other factors, changes in the value of the underlying properties owned by the REIT, changes in the prospect for earnings and/or cash flow growth of the REIT itself, defaults by borrowers or tenants, market saturation, decreases in market rates for rents, and other economic, political, or regulatory matters affecting the real estate industry, including REITs. REITs and similar non-U.S. entities depend upon specialized management skills, may have limited financial resources, may have less trading volume in their securities, and may be subject to more abrupt or erratic price movements than the overall securities markets. REITs are also subject to the risk of failing to qualify for favorable tax treatment under the Internal Revenue Code of 1986, as amended. Some REITs (especially mortgage REITs) are affected by risks similar to those associated with investments in debt securities including changes in interest rates and the quality of credit extended.
Risk Lose Money [Text]	rr_RiskLoseMoney	There is no assurance that the Fund will achieve its investment objective and you may lose money.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance Information
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The following bar chart and table show you how the Fund has performed in the past, and can help you understand the risks of investing in the Fund. The bar chart shows how the Fund’s Class Inst share performance has varied for each full calendar year shown. The table below the bar chart compares the Fund’s returns for the periods shown with the Russell 2500 Growth Index. The performance of one or more share classes shown in the Average Annual Total Returns table below includes the Fund’s Class Inst share returns (adjusted to reflect the higher class-related operating expenses of such classes, where applicable) for periods prior to the indicated inception date of such share classes. Except for differences in fees and expenses, all share classes of the Fund would have substantially similar annual returns because all share classes of the Fund invest in the same portfolio of securities. The after-tax returns shown in the Average Annual Total Returns table below are calculated using the highest historical individual U.S. federal marginal income tax rates in effect during the period indicated in the table and do not reflect the impact of state, local or foreign taxes. Your actual after-tax returns will depend on your personal tax situation and may differ from those shown in the table. In addition, the after-tax returns shown in the table do not apply to shares held in tax-advantaged accounts such as 401(k) plans or Individual Retirement Accounts (IRAs). The after-tax returns are shown only for Class Inst shares and will vary for other share classes. The Fund’s past performance (before and after taxes) is no guarantee of how the Fund will perform in the future. Updated performance information can be obtained by calling toll-free 800.345.6611 or visiting columbiathreadneedle.com/us.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The following bar chart and table show you how the Fund has performed in the past, and can help you understand the risks of investing in the Fund. The bar chart shows how the Fund’s Class Inst share performance has varied for each full calendar year shown. The table below the bar chart compares the Fund’s returns for the periods shown with the Russell 2500 Growth Index.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	800.345.6611
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	columbiathreadneedle.com/us
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The Fund’s past performance (before and after taxes) is no guarantee of how the Fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Year by Year Total Return (%) as of December 31 Each Year
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Best and Worst Quarterly Returns During the Period Shown in the Bar Chart Best                                                     2nd Quarter 2009                                                      28.11% Worst                                                   4th Quarter 2008                                                     -30.14%
Performance Table Heading	rr_PerformanceTableHeading	Average Annual Total Returns After Applicable Sales Charges (for periods ended December 31, 2017)
Performance Table Uses Highest Federal Rate	rr_PerformanceTableUsesHighestFederalRate	The after-tax returns shown in the Average Annual Total Returns table below are calculated using the highest historical individual U.S. federal marginal income tax rates in effect during the period indicated in the table and do not reflect the impact of state, local or foreign taxes.
Performance Table Not Relevant to Tax Deferred	rr_PerformanceTableNotRelevantToTaxDeferred	In addition, the after-tax returns shown in the table do not apply to shares held in tax-advantaged accounts such as 401(k) plans or Individual Retirement Accounts (IRAs).
Performance Table One Class of after Tax Shown [Text]	rr_PerformanceTableOneClassOfAfterTaxShown	The after-tax returns are shown only for Class Inst shares and will vary for other share classes.
Columbia Acorn Select | Class A
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	5.75%
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	1.00%	[1]
Management fees	rr_ManagementFeesOverAssets	0.85%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Other expenses	rr_OtherExpensesOverAssets	0.29%	[2]
Acquired fund fees and expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.01%
Total annual Fund operating expenses	rr_ExpensesOverAssets	1.40%	[3]
Less: Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.23%)	[4]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	1.17%
Expenses Deferred Charges [Text Block]	rr_ExpensesDeferredChargesTextBlock	This charge is imposed on certain investments of between $1 million and $50 million redeemed within 18 months after purchase, as follows: 1.00% if redeemed within 12 months after purchase, and 0.50% if redeemed more than 12, but less than 18, months after purchase, with certain limited exceptions.
1 year	rr_ExpenseExampleYear01	$ 687
3 years	rr_ExpenseExampleYear03	971
5 years	rr_ExpenseExampleYear05	1,276
10 years	rr_ExpenseExampleYear10	2,139
1 year	rr_ExpenseExampleNoRedemptionYear01	687
3 years	rr_ExpenseExampleNoRedemptionYear03	971
5 years	rr_ExpenseExampleNoRedemptionYear05	1,276
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 2,139
1 Year	rr_AverageAnnualReturnYear01	19.16%
5 Years	rr_AverageAnnualReturnYear05	12.51%
10 Years	rr_AverageAnnualReturnYear10	6.09%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Oct. 16, 2000
Columbia Acorn Select | Class Adv
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.85%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.29%	[2]
Acquired fund fees and expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.01%
Total annual Fund operating expenses	rr_ExpensesOverAssets	1.15%	[3]
Less: Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.23%)	[4]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	0.92%
1 year	rr_ExpenseExampleYear01	$ 94
3 years	rr_ExpenseExampleYear03	343
5 years	rr_ExpenseExampleYear05	611
10 years	rr_ExpenseExampleYear10	1,377
1 year	rr_ExpenseExampleNoRedemptionYear01	94
3 years	rr_ExpenseExampleNoRedemptionYear03	343
5 years	rr_ExpenseExampleNoRedemptionYear05	611
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 1,377
1 Year	rr_AverageAnnualReturnYear01	26.71%
5 Years	rr_AverageAnnualReturnYear05	14.13%
10 Years	rr_AverageAnnualReturnYear10	7.01%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 08, 2012
Columbia Acorn Select | Class C
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	1.00%	[5]
Management fees	rr_ManagementFeesOverAssets	0.85%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	1.00%
Other expenses	rr_OtherExpensesOverAssets	0.29%	[2]
Acquired fund fees and expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.01%
Total annual Fund operating expenses	rr_ExpensesOverAssets	2.15%	[3]
Less: Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.23%)	[4]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	1.92%
Expenses Deferred Charges [Text Block]	rr_ExpensesDeferredChargesTextBlock	This charge applies to redemptions within 12 months after purchase, with certain limited exceptions.
1 year	rr_ExpenseExampleYear01	$ 295
3 years	rr_ExpenseExampleYear03	651
5 years	rr_ExpenseExampleYear05	1,133
10 years	rr_ExpenseExampleYear10	2,465
1 year	rr_ExpenseExampleNoRedemptionYear01	195
3 years	rr_ExpenseExampleNoRedemptionYear03	651
5 years	rr_ExpenseExampleNoRedemptionYear05	1,133
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 2,465
1 Year	rr_AverageAnnualReturnYear01	24.40%
5 Years	rr_AverageAnnualReturnYear05	13.01%
10 Years	rr_AverageAnnualReturnYear10	5.90%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Oct. 16, 2000
Columbia Acorn Select | Class Inst
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.85%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.29%	[2]
Acquired fund fees and expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.01%
Total annual Fund operating expenses	rr_ExpensesOverAssets	1.15%	[3]
Less: Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.23%)	[4]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	0.92%
1 year	rr_ExpenseExampleYear01	$ 94
3 years	rr_ExpenseExampleYear03	343
5 years	rr_ExpenseExampleYear05	611
10 years	rr_ExpenseExampleYear10	1,377
1 year	rr_ExpenseExampleNoRedemptionYear01	94
3 years	rr_ExpenseExampleNoRedemptionYear03	343
5 years	rr_ExpenseExampleNoRedemptionYear05	611
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 1,377
2008	rr_AnnualReturn2008	(49.18%)	[6]
2009	rr_AnnualReturn2009	66.17%	[6]
2010	rr_AnnualReturn2010	22.88%	[6]
2011	rr_AnnualReturn2011	(16.37%)	[6]
2012	rr_AnnualReturn2012	17.15%	[6]
2013	rr_AnnualReturn2013	34.16%	[6]
2014	rr_AnnualReturn2014	2.47%	[6]
2015	rr_AnnualReturn2015	(0.44%)	[6]
2016	rr_AnnualReturn2016	11.88%	[6]
2017	rr_AnnualReturn2017	26.59%	[6]
Year to Date Return, Label	rr_YearToDateReturnLabel	Year to Date return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Mar. 31, 2018
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	(0.79%)
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Jun. 30, 2009
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	28.11%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Dec. 31, 2008
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(30.14%)
1 Year	rr_AverageAnnualReturnYear01	26.59%
5 Years	rr_AverageAnnualReturnYear05	14.16%
10 Years	rr_AverageAnnualReturnYear10	7.02%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 23, 1998
Columbia Acorn Select | Class Inst2
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.85%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.22%	[2]
Acquired fund fees and expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.01%
Total annual Fund operating expenses	rr_ExpensesOverAssets	1.08%	[3]
Less: Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.22%)	[4]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	0.86%
1 year	rr_ExpenseExampleYear01	$ 88
3 years	rr_ExpenseExampleYear03	322
5 years	rr_ExpenseExampleYear05	574
10 years	rr_ExpenseExampleYear10	1,297
1 year	rr_ExpenseExampleNoRedemptionYear01	88
3 years	rr_ExpenseExampleNoRedemptionYear03	322
5 years	rr_ExpenseExampleNoRedemptionYear05	574
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 1,297
1 Year	rr_AverageAnnualReturnYear01	26.72%
5 Years	rr_AverageAnnualReturnYear05	14.21%
10 Years	rr_AverageAnnualReturnYear10	7.05%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 08, 2012
Columbia Acorn Select | Class Inst3
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.85%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.15%	[2]
Acquired fund fees and expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.01%
Total annual Fund operating expenses	rr_ExpensesOverAssets	1.01%	[3]
Less: Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.20%)	[4]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	0.81%
1 year	rr_ExpenseExampleYear01	$ 83
3 years	rr_ExpenseExampleYear03	302
5 years	rr_ExpenseExampleYear05	538
10 years	rr_ExpenseExampleYear10	1,218
1 year	rr_ExpenseExampleNoRedemptionYear01	83
3 years	rr_ExpenseExampleNoRedemptionYear03	302
5 years	rr_ExpenseExampleNoRedemptionYear05	538
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 1,218
1 Year	rr_AverageAnnualReturnYear01	26.79%
5 Years	rr_AverageAnnualReturnYear05	14.27%
10 Years	rr_AverageAnnualReturnYear10	7.07%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 08, 2012
Columbia Acorn Select | returns after taxes on distributions | Class Inst
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	23.31%
5 Years	rr_AverageAnnualReturnYear05	8.55%
10 Years	rr_AverageAnnualReturnYear10	4.14%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 23, 1998
Columbia Acorn Select | returns after taxes on distributions and sale of Fund shares | Class Inst
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	17.32%
5 Years	rr_AverageAnnualReturnYear05	10.07%
10 Years	rr_AverageAnnualReturnYear10	5.02%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 23, 1998
Columbia Acorn Select | Russell 2500 Growth Index (reflects no deductions for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	24.46%
5 Years	rr_AverageAnnualReturnYear05	15.47%
10 Years	rr_AverageAnnualReturnYear10	9.62%

[1] This charge is imposed on certain investments of between $1 million and $50 million redeemed within 18 months after purchase, as follows: 1.00% if redeemed within 12 months after purchase, and 0.50% if redeemed more than 12, but less than 18, months after purchase, with certain limited exceptions.

[2] Other expenses have been restated to reflect current transfer agency fees paid by the Fund.

[3] "Total annual Fund operating expenses" include acquired fund fees and expenses (expenses the Fund incurs indirectly through its investments in other investment companies) and may be higher than "Total Net Expenses" shown in the Financial Highlights section of this prospectus because "Total Net Expenses" do not include acquired fund fees and expenses.

[4] Columbia Wanger Asset Management, LLC (the Investment Manager) has contractually agreed to waive 0.20% of the advisory fee otherwise payable to it by the Fund through April 30, 2019. This arrangement may only be amended or terminated with approval from the Fund's Board of Trustees and the Investment Manager. The fee waivers and/or expense reimbursements shown in the table also reflect the contractual agreement of the Fund’s transfer agent, Columbia Management Investment Services Corp. (the Transfer Agent), to waive a portion of its fees through June 30, 2019, such that the Fund’s transfer agency fees do not exceed the annual rate of 0.11% of the average daily net assets of each share class other than Class Inst2 and Class Inst3. With respect to Class Inst2 and Class Inst3 shares, the Transfer Agent has contractually agreed to waive a portion of its fees such that the Fund's transfer agency fees do not exceed the annual rates of 0.05% of the average daily net assets of Class Inst2 shares and 0.00% of the average daily net assets of Class Inst3 shares.

[5] This charge applies to redemptions within 12 months after purchase, with certain limited exceptions.

[6] Year to Date return as of March 31, 2018: -0.79%

XML

Columbia Acorn USA

SUMMARY OF THE FUND

Investment Objective

Columbia Acorn USA^® (the Fund) seeks long-term capital appreciation.

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. An investor transacting in a class of Fund shares without any front-end sales charge, contingent deferred sales charge, or other asset-based fee for sales or distribution may be required to pay a commission to the financial intermediary for effecting such transactions. Such commission rates are set by the financial intermediary and are not reflected in the tables or the example below. You may qualify for sales charge discounts if you and members of your immediate family invest, or agree to invest in the future, at least $50,000 in certain classes of shares of eligible funds distributed by Columbia Management Investment Distributors, Inc. (the Distributor). More information is available about these and other sales charge discounts and waivers from your financial intermediary, and can be found in the Choosing a Share Class section beginning on page 18 of the Fund’s prospectus, in Appendix A to the prospectus beginning on page A-1 and in Appendix S to the Statement of Additional Information (SAI) under Sales Charge Waivers beginning on page S-1.

Shareholder Fees (fees paid directly from your investment)

Shareholder Fees - Columbia Acorn USA	Class A		Class C		Class Adv	Class Inst	Class Inst2	Class Inst3
Maximum sales charge (load) imposed on purchases (as a % of offering price)	5.75%		none		none	none	none	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	1.00%	[1]	1.00%	[2]	none	none	none	none

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)

Annual Fund Operating Expenses - Columbia Acorn USA		Class A	Class Adv	Class C	Class Inst	Class Inst2	Class Inst3
Management fees		0.90%	0.90%	0.90%	0.90%	0.90%	0.90%
Distribution and/or service (12b-1) fees		0.25%	none	1.00%	none	none	none
Other expenses	[1]	0.28%	0.28%	0.28%	0.28%	0.20%	0.13%
Acquired fund fees and expenses		0.01%	0.01%	0.01%	0.01%	0.01%	0.01%
Total annual Fund operating expenses	[2]	1.44%	1.19%	2.19%	1.19%	1.11%	1.04%
Less: Fee waivers and/or expense reimbursements	[3]	(0.02%)	(0.02%)	(0.02%)	(0.02%)	(0.03%)	(0.01%)
Total annual Fund operating expenses after fee waivers and/or expense reimbursements		1.42%	1.17%	2.17%	1.17%	1.08%	1.03%

Example

The following example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example illustrates the hypothetical expenses that you would incur over the time periods indicated, and assumes that:

• you invest $10,000 in the applicable class of Fund shares for the periods indicated,

• your investment has a 5% return each year, and

• the Fund’s total annual operating expenses remain the same as shown in the Annual Fund Operating Expenses table above.

Since the waivers and/or reimbursements shown in the Annual Fund Operating Expenses table above expire as indicated in the preceding table, they are only reflected in the 1 year example and the first year of the other examples. Although your actual costs may be higher or lower, based on the assumptions listed above, your costs would be:

Expense Example - Columbia Acorn USA - USD ($)	1 year	3 years	5 years	10 years
Class A	711	1,002	1,315	2,198
Class Adv	119	376	652	1,441
Class C	320	683	1,173	2,522
Class Inst	119	376	652	1,441
Class Inst2	110	350	609	1,349
Class Inst3	105	330	573	1,270

Expense Example, No Redemption - Columbia Acorn USA - USD ($)	1 year	3 years	5 years	10 years
Class A	711	1,002	1,315	2,198
Class Adv	119	376	652	1,441
Class C	220	683	1,173	2,522
Class Inst	119	376	652	1,441
Class Inst2	110	350	609	1,349
Class Inst3	105	330	573	1,270

Portfolio Turnover

The Fund may pay transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 84% of the average value of its portfolio.

Principal Investment Strategies

Under normal circumstances, the Fund invests at least 80% of its net assets (including the amount any borrowings for investment purposes) in U.S. companies.

Under normal circumstances, the Fund (i) invests a majority of its net assets in the common stock of small- and mid-sized companies with market capitalizations under $5 billion at the time of initial investment (“Focus Stocks”) and (ii) may also invest in companies with market capitalizations above $5 billion, provided that immediately after that investment a majority of the Fund’s net assets would be invested in Focus Stocks. The Fund may continue to hold, and make additional investments in, Focus Stocks whose market capitalizations have grown to exceed $5 billion, regardless of whether the Fund’s investments in Focus Stocks are a majority of the Fund’s net assets.

Columbia Wanger Asset Management, LLC, the Fund's investment adviser (the Investment Manager), believes that stocks of small- and mid-sized companies, which generally are not as well known by financial analysts as larger companies, may offer higher return potential than stocks of larger companies.

The Investment Manager typically seeks companies with:

• A strong business franchise that offers growth potential.

• Products and services in which the company has a competitive advantage.

• A stock price the Investment Manager believes is reasonable relative to the assets and earning power of the company.

The Investment Manager may sell a portfolio holding if the security reaches the Investment Manager's price target, if the company has a deterioration of fundamentals, such as failing to meet key operating benchmarks, or if the Investment Manager believes other securities are more attractive. The Investment Manager also may sell a portfolio holding to fund redemptions.

Principal Risks

An investment in the Fund involves risks, including those described below. There is no assurance that the Fund will achieve its investment objective and you may lose money. The value of the Fund’s holdings may decline, and the Fund’s net asset value (NAV) and share price may go down. An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.

Active Management Risk. The Investment Manager’s active management of the Fund could cause the Fund to underperform its benchmark index and/or other funds with similar investment objectives and/or strategies.

Market Risk. Market risk refers to the possibility that the market values of securities or other investments that the Fund holds will fall, sometimes rapidly or unpredictably, or fail to rise. An investment in the Fund could lose money over short or long periods.

Small- and Mid-Cap Company Securities Risk. Investments in small- and mid-capitalization companies (small- and mid-cap companies) often involve greater risks than investments in larger, more established companies (larger companies) because small- and mid-cap companies tend to have less predictable earnings and may lack the management experience, financial resources, product diversification and competitive strengths of larger companies. Securities of small- and mid-cap companies may be less liquid and more volatile than the securities of larger companies.

Growth Securities Risk. Growth securities typically trade at a higher multiple of earnings than other types of equity securities. Accordingly, the market values of growth securities may never reach their expected market value and may decline in price. In addition, growth securities, at times, may not perform as well as value securities or the stock market in general, and may be out of favor with investors for varying periods of time.

Issuer Risk. An issuer in which the Fund invests may perform poorly, and the value of its securities may therefore decline, which would negatively affect the Fund’s performance. Poor performance may be caused by poor management decisions, competitive pressures, breakthroughs in technology, reliance on suppliers, labor problems or shortages, corporate restructurings, fraudulent disclosures, natural disasters or other events, conditions or factors.

Sector Risk. At times, the Fund may have a significant portion of its assets invested in securities of companies conducting business in a related group of industries within a sector. Companies in the same sector may be similarly affected by economic, regulatory, political or market events or conditions, which may make the Fund more vulnerable to unfavorable developments in that sector than funds that invest more broadly. Generally, the more the Fund diversifies its investments, the more it spreads risk and potentially reduces the risks of loss and volatility.

Performance Information

The following bar chart and table show you how the Fund has performed in the past, and can help you understand the risks of investing in the Fund. The bar chart shows how the Fund’s Class Inst share performance has varied for each full calendar year shown. The table below the bar chart compares the Fund’s returns for the periods shown with the Russell 2000 Growth Index.

The performance of one or more share classes shown in the Average Annual Total Returns table below includes the Fund’s Class Inst share returns (adjusted to reflect the higher class-related operating expenses of such classes, where applicable) for periods prior to the indicated inception date of such share classes. Except for differences in fees and expenses, all share classes of the Fund would have substantially similar annual returns because all share classes of the Fund invest in the same portfolio of securities.

The after-tax returns shown in the Average Annual Total Returns table below are calculated using the highest historical individual U.S. federal marginal income tax rates in effect during the period indicated in the table and do not reflect the impact of state, local or foreign taxes. Your actual after-tax returns will depend on your personal tax situation and may differ from those shown in the table. In addition, the after-tax returns shown in the table do not apply to shares held in tax-advantaged accounts such as 401(k) plans or Individual Retirement Accounts (IRAs). The after-tax returns are shown only for Class Inst shares and will vary for other share classes.

The Fund’s past performance (before and after taxes) is no guarantee of how the Fund will perform in the future. Updated performance information can be obtained by calling toll-free 800.345.6611 or visiting columbiathreadneedle.com/us.

Year by Year Total Return (%)
as of December 31 Each Year

Best and Worst Quarterly Returns
During the Period Shown in the Bar Chart

Best                                                     3rd Quarter 2009                                                      23.55%

Worst                                                   4th Quarter 2008                                                     -27.96%

Average Annual Total Returns After Applicable Sales Charges (for periods ended December 31, 2017)

Average Annual Total Returns - Columbia Acorn USA	Share Class Inception Date	1 Year	5 Years	10 Years
Class Inst	Sep. 04, 1996	19.44%	12.85%	8.17%
Class Inst | returns after taxes on distributions	Sep. 04, 1996	11.14%	6.62%	4.94%
Class Inst | returns after taxes on distributions and sale of Fund shares	Sep. 04, 1996	16.22%	9.29%	6.16%
Class A	Oct. 16, 2000	12.30%	11.24%	7.24%
Class Adv	Nov. 08, 2012	19.42%	12.85%	8.16%
Class C	Oct. 16, 2000	17.65%	11.80%	7.10%
Class Inst2	Nov. 08, 2012	19.56%	12.96%	8.22%
Class Inst3	Nov. 08, 2012	19.60%	13.02%	8.24%
Russell 2000 Growth Index (reflects no deductions for fees, expenses or taxes)		22.17%	15.21%	9.19%

[1] This charge is imposed on certain investments of between $1 million and $50 million redeemed within 18 months after purchase, as follows: 1.00% if redeemed within 12 months after purchase, and 0.50% if redeemed more than 12, but less than 18, months after purchase, with certain limited exceptions.

[2] This charge applies to redemptions within 12 months after purchase, with certain limited exceptions.

[1] Other expenses have been restated to reflect current transfer agency fees paid by the Fund.

[2] "Total annual Fund operating expenses" include acquired fund fees and expenses (expenses the Fund incurs indirectly through its investments in other investment companies) and may be higher than "Total Net Expenses" shown in the Financial Highlights section of this prospectus because "Total Net Expenses" do not include acquired fund fees and expenses.

[3] Columbia Wanger Asset Management, LLC (the Investment Manager) has contractually agreed to waive fees and reimburse certain expenses of the Fund, through April 30, 2019, so that ordinary operating expenses (excluding transaction costs and certain other investment-related expenses, interest and fees on borrowings, if any) do not exceed the annual rates of 1.42% for Class A shares, 1.17% for Class Adv shares, 2.17% for Class C shares, 1.17% for Class Inst shares, 1.08% for Class Inst2 shares and 1.03% for Class Inst3 shares. This arrangement may only be amended or terminated with approval from the Fund’s Board of Trustees and the Investment Manager. The fee waivers and/or expense reimbursements shown in the table also reflect the contractual agreement of the Fund’s transfer agent, Columbia Management Investment Services Corp. (the Transfer Agent), to waive a portion of its fees through June 30, 2019, such that the Fund’s transfer agency fees do not exceed the annual rate of 0.14% of the average daily net assets of each share class other than Class Inst2 and Class Inst3. With respect to Class Inst2 and Class Inst3 shares, the Transfer Agent has contractually agreed to waive a portion of its fees such that the Fund's transfer agency fees do not exceed the annual rates of 0.05% of the average daily net assets of Class Inst2 shares and 0.00% of the average daily net assets of Class Inst3 shares.

[1] Year to Date return as of March 31, 2018: 3.81%

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	COLUMBIA ACORN TRUST
Prospectus Date	rr_ProspectusDate	May 01, 2018
Columbia Acorn USA
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	SUMMARY OF THE FUND
Objective [Heading]	rr_ObjectiveHeading	Investment Objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	Columbia Acorn USA® (the Fund) seeks long-term capital appreciation.
Expense [Heading]	rr_ExpenseHeading	Fees and Expenses of the Fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. An investor transacting in a class of Fund shares without any front-end sales charge, contingent deferred sales charge, or other asset-based fee for sales or distribution may be required to pay a commission to the financial intermediary for effecting such transactions. Such commission rates are set by the financial intermediary and are not reflected in the tables or the example below. You may qualify for sales charge discounts if you and members of your immediate family invest, or agree to invest in the future, at least $50,000 in certain classes of shares of eligible funds distributed by Columbia Management Investment Distributors, Inc. (the Distributor). More information is available about these and other sales charge discounts and waivers from your financial intermediary, and can be found in the Choosing a Share Class section beginning on page 18 of the Fund’s prospectus, in Appendix A to the prospectus beginning on page A-1 and in Appendix S to the Statement of Additional Information (SAI) under Sales Charge Waivers beginning on page S-1.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder Fees (fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	April 30, 2019 June 30, 2019
Portfolio Turnover [Heading]	rr_PortfolioTurnoverHeading	Portfolio Turnover
Portfolio Turnover [Text Block]	rr_PortfolioTurnoverTextBlock	The Fund may pay transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 84% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	84.00%
Expense Breakpoint Discounts [Text]	rr_ExpenseBreakpointDiscounts	You may qualify for sales charge discounts if you and members of your immediate family invest, or agree to invest in the future, at least $50,000 in certain classes of shares of eligible funds distributed by Columbia Management Investment Distributors, Inc. (the Distributor). More information is available about these and other sales charge discounts and waivers from your financial intermediary, and can be found in the Choosing a Share Class section beginning on page 18 of the Fund’s prospectus, in Appendix A to the prospectus beginning on page A-1 and in Appendix S to the Statement of Additional Information (SAI) under Sales Charge Waivers beginning on page S-1.
Expense Breakpoint, Minimum Investment Required [Amount]	rr_ExpenseBreakpointMinimumInvestmentRequiredAmount	$ 50,000
Expenses Restated to Reflect Current [Text]	rr_ExpensesRestatedToReflectCurrent	Other expenses have been restated to reflect current transfer agency fees paid by the Fund.
Expenses Not Correlated to Ratio Due to Acquired Fund Fees [Text]	rr_ExpensesNotCorrelatedToRatioDueToAcquiredFundFees	"Total annual Fund operating expenses" include acquired fund fees and expenses (expenses the Fund incurs indirectly through its investments in other investment companies) and may be higher than "Total Net Expenses" shown in the Financial Highlights section of this prospectus because "Total Net Expenses" do not include acquired fund fees and expenses.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	The following example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example illustrates the hypothetical expenses that you would incur over the time periods indicated, and assumes that: you invest $10,000 in the applicable class of Fund shares for the periods indicated, your investment has a 5% return each year, and the Fund’s total annual operating expenses remain the same as shown in the Annual Fund Operating Expenses table above. Since the waivers and/or reimbursements shown in the Annual Fund Operating Expenses table above expire as indicated in the preceding table, they are only reflected in the 1 year example and the first year of the other examples. Although your actual costs may be higher or lower, based on the assumptions listed above, your costs would be:
Strategy [Heading]	rr_StrategyHeading	Principal Investment Strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	Under normal circumstances, the Fund invests at least 80% of its net assets (including the amount any borrowings for investment purposes) in U.S. companies. Under normal circumstances, the Fund (i) invests a majority of its net assets in the common stock of small- and mid-sized companies with market capitalizations under $5 billion at the time of initial investment (“Focus Stocks”) and (ii) may also invest in companies with market capitalizations above $5 billion, provided that immediately after that investment a majority of the Fund’s net assets would be invested in Focus Stocks. The Fund may continue to hold, and make additional investments in, Focus Stocks whose market capitalizations have grown to exceed $5 billion, regardless of whether the Fund’s investments in Focus Stocks are a majority of the Fund’s net assets. Columbia Wanger Asset Management, LLC, the Fund's investment adviser (the Investment Manager), believes that stocks of small- and mid-sized companies, which generally are not as well known by financial analysts as larger companies, may offer higher return potential than stocks of larger companies. The Investment Manager typically seeks companies with: A strong business franchise that offers growth potential. Products and services in which the company has a competitive advantage. A stock price the Investment Manager believes is reasonable relative to the assets and earning power of the company. The Investment Manager may sell a portfolio holding if the security reaches the Investment Manager's price target, if the company has a deterioration of fundamentals, such as failing to meet key operating benchmarks, or if the Investment Manager believes other securities are more attractive. The Investment Manager also may sell a portfolio holding to fund redemptions.
Risk [Heading]	rr_RiskHeading	Principal Risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	An investment in the Fund involves risks, including those described below. There is no assurance that the Fund will achieve its investment objective and you may lose money. The value of the Fund’s holdings may decline, and the Fund’s net asset value (NAV) and share price may go down. An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Active Management Risk. The Investment Manager’s active management of the Fund could cause the Fund to underperform its benchmark index and/or other funds with similar investment objectives and/or strategies. Market Risk. Market risk refers to the possibility that the market values of securities or other investments that the Fund holds will fall, sometimes rapidly or unpredictably, or fail to rise. An investment in the Fund could lose money over short or long periods. Small- and Mid-Cap Company Securities Risk. Investments in small- and mid-capitalization companies (small- and mid-cap companies) often involve greater risks than investments in larger, more established companies (larger companies) because small- and mid-cap companies tend to have less predictable earnings and may lack the management experience, financial resources, product diversification and competitive strengths of larger companies. Securities of small- and mid-cap companies may be less liquid and more volatile than the securities of larger companies. Growth Securities Risk. Growth securities typically trade at a higher multiple of earnings than other types of equity securities. Accordingly, the market values of growth securities may never reach their expected market value and may decline in price. In addition, growth securities, at times, may not perform as well as value securities or the stock market in general, and may be out of favor with investors for varying periods of time. Issuer Risk. An issuer in which the Fund invests may perform poorly, and the value of its securities may therefore decline, which would negatively affect the Fund’s performance. Poor performance may be caused by poor management decisions, competitive pressures, breakthroughs in technology, reliance on suppliers, labor problems or shortages, corporate restructurings, fraudulent disclosures, natural disasters or other events, conditions or factors. Sector Risk. At times, the Fund may have a significant portion of its assets invested in securities of companies conducting business in a related group of industries within a sector. Companies in the same sector may be similarly affected by economic, regulatory, political or market events or conditions, which may make the Fund more vulnerable to unfavorable developments in that sector than funds that invest more broadly. Generally, the more the Fund diversifies its investments, the more it spreads risk and potentially reduces the risks of loss and volatility.
Risk Lose Money [Text]	rr_RiskLoseMoney	There is no assurance that the Fund will achieve its investment objective and you may lose money.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance Information
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The following bar chart and table show you how the Fund has performed in the past, and can help you understand the risks of investing in the Fund. The bar chart shows how the Fund’s Class Inst share performance has varied for each full calendar year shown. The table below the bar chart compares the Fund’s returns for the periods shown with the Russell 2000 Growth Index. The performance of one or more share classes shown in the Average Annual Total Returns table below includes the Fund’s Class Inst share returns (adjusted to reflect the higher class-related operating expenses of such classes, where applicable) for periods prior to the indicated inception date of such share classes. Except for differences in fees and expenses, all share classes of the Fund would have substantially similar annual returns because all share classes of the Fund invest in the same portfolio of securities. The after-tax returns shown in the Average Annual Total Returns table below are calculated using the highest historical individual U.S. federal marginal income tax rates in effect during the period indicated in the table and do not reflect the impact of state, local or foreign taxes. Your actual after-tax returns will depend on your personal tax situation and may differ from those shown in the table. In addition, the after-tax returns shown in the table do not apply to shares held in tax-advantaged accounts such as 401(k) plans or Individual Retirement Accounts (IRAs). The after-tax returns are shown only for Class Inst shares and will vary for other share classes. The Fund’s past performance (before and after taxes) is no guarantee of how the Fund will perform in the future. Updated performance information can be obtained by calling toll-free 800.345.6611 or visiting columbiathreadneedle.com/us.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The following bar chart and table show you how the Fund has performed in the past, and can help you understand the risks of investing in the Fund. The bar chart shows how the Fund’s Class Inst share performance has varied for each full calendar year shown. The table below the bar chart compares the Fund’s returns for the periods shown with the Russell 2000 Growth Index.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	800.345.6611
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	columbiathreadneedle.com/us
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The Fund’s past performance (before and after taxes) is no guarantee of how the Fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Year by Year Total Return (%) as of December 31 Each Year
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Best and Worst Quarterly Returns During the Period Shown in the Bar Chart Best                                                     3rd Quarter 2009                                                      23.55% Worst                                                   4th Quarter 2008                                                     -27.96%
Performance Table Heading	rr_PerformanceTableHeading	Average Annual Total Returns After Applicable Sales Charges (for periods ended December 31, 2017)
Performance Table Uses Highest Federal Rate	rr_PerformanceTableUsesHighestFederalRate	The after-tax returns shown in the Average Annual Total Returns table below are calculated using the highest historical individual U.S. federal marginal income tax rates in effect during the period indicated in the table and do not reflect the impact of state, local or foreign taxes.
Performance Table Not Relevant to Tax Deferred	rr_PerformanceTableNotRelevantToTaxDeferred	In addition, the after-tax returns shown in the table do not apply to shares held in tax-advantaged accounts such as 401(k) plans or Individual Retirement Accounts (IRAs).
Performance Table One Class of after Tax Shown [Text]	rr_PerformanceTableOneClassOfAfterTaxShown	The after-tax returns are shown only for Class Inst shares and will vary for other share classes.
Columbia Acorn USA | Class A
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	5.75%
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	1.00%	[1]
Management fees	rr_ManagementFeesOverAssets	0.90%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Other expenses	rr_OtherExpensesOverAssets	0.28%	[2]
Acquired fund fees and expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.01%
Total annual Fund operating expenses	rr_ExpensesOverAssets	1.44%	[3]
Less: Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.02%)	[4]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	1.42%
Expenses Deferred Charges [Text Block]	rr_ExpensesDeferredChargesTextBlock	This charge is imposed on certain investments of between $1 million and $50 million redeemed within 18 months after purchase, as follows: 1.00% if redeemed within 12 months after purchase, and 0.50% if redeemed more than 12, but less than 18, months after purchase, with certain limited exceptions.
1 year	rr_ExpenseExampleYear01	$ 711
3 years	rr_ExpenseExampleYear03	1,002
5 years	rr_ExpenseExampleYear05	1,315
10 years	rr_ExpenseExampleYear10	2,198
1 year	rr_ExpenseExampleNoRedemptionYear01	711
3 years	rr_ExpenseExampleNoRedemptionYear03	1,002
5 years	rr_ExpenseExampleNoRedemptionYear05	1,315
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 2,198
1 Year	rr_AverageAnnualReturnYear01	12.30%
5 Years	rr_AverageAnnualReturnYear05	11.24%
10 Years	rr_AverageAnnualReturnYear10	7.24%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Oct. 16, 2000
Columbia Acorn USA | Class Adv
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.90%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.28%	[2]
Acquired fund fees and expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.01%
Total annual Fund operating expenses	rr_ExpensesOverAssets	1.19%	[3]
Less: Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.02%)	[4]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	1.17%
1 year	rr_ExpenseExampleYear01	$ 119
3 years	rr_ExpenseExampleYear03	376
5 years	rr_ExpenseExampleYear05	652
10 years	rr_ExpenseExampleYear10	1,441
1 year	rr_ExpenseExampleNoRedemptionYear01	119
3 years	rr_ExpenseExampleNoRedemptionYear03	376
5 years	rr_ExpenseExampleNoRedemptionYear05	652
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 1,441
1 Year	rr_AverageAnnualReturnYear01	19.42%
5 Years	rr_AverageAnnualReturnYear05	12.85%
10 Years	rr_AverageAnnualReturnYear10	8.16%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 08, 2012
Columbia Acorn USA | Class C
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	1.00%	[5]
Management fees	rr_ManagementFeesOverAssets	0.90%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	1.00%
Other expenses	rr_OtherExpensesOverAssets	0.28%	[2]
Acquired fund fees and expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.01%
Total annual Fund operating expenses	rr_ExpensesOverAssets	2.19%	[3]
Less: Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.02%)	[4]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	2.17%
Expenses Deferred Charges [Text Block]	rr_ExpensesDeferredChargesTextBlock	This charge applies to redemptions within 12 months after purchase, with certain limited exceptions.
1 year	rr_ExpenseExampleYear01	$ 320
3 years	rr_ExpenseExampleYear03	683
5 years	rr_ExpenseExampleYear05	1,173
10 years	rr_ExpenseExampleYear10	2,522
1 year	rr_ExpenseExampleNoRedemptionYear01	220
3 years	rr_ExpenseExampleNoRedemptionYear03	683
5 years	rr_ExpenseExampleNoRedemptionYear05	1,173
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 2,522
1 Year	rr_AverageAnnualReturnYear01	17.65%
5 Years	rr_AverageAnnualReturnYear05	11.80%
10 Years	rr_AverageAnnualReturnYear10	7.10%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Oct. 16, 2000
Columbia Acorn USA | Class Inst
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.90%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.28%	[2]
Acquired fund fees and expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.01%
Total annual Fund operating expenses	rr_ExpensesOverAssets	1.19%	[3]
Less: Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.02%)	[4]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	1.17%
1 year	rr_ExpenseExampleYear01	$ 119
3 years	rr_ExpenseExampleYear03	376
5 years	rr_ExpenseExampleYear05	652
10 years	rr_ExpenseExampleYear10	1,441
1 year	rr_ExpenseExampleNoRedemptionYear01	119
3 years	rr_ExpenseExampleNoRedemptionYear03	376
5 years	rr_ExpenseExampleNoRedemptionYear05	652
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 1,441
2008	rr_AnnualReturn2008	(39.22%)	[6]
2009	rr_AnnualReturn2009	41.49%	[6]
2010	rr_AnnualReturn2010	23.16%	[6]
2011	rr_AnnualReturn2011	(4.95%)	[6]
2012	rr_AnnualReturn2012	18.98%	[6]
2013	rr_AnnualReturn2013	32.72%	[6]
2014	rr_AnnualReturn2014	3.61%	[6]
2015	rr_AnnualReturn2015	(1.36%)	[6]
2016	rr_AnnualReturn2016	13.00%	[6]
2017	rr_AnnualReturn2017	19.44%	[6]
Year to Date Return, Label	rr_YearToDateReturnLabel	Year to Date return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Mar. 31, 2018
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	3.81%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Sep. 30, 2009
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	23.55%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Dec. 31, 2008
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(27.96%)
1 Year	rr_AverageAnnualReturnYear01	19.44%
5 Years	rr_AverageAnnualReturnYear05	12.85%
10 Years	rr_AverageAnnualReturnYear10	8.17%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Sep. 04, 1996
Columbia Acorn USA | Class Inst2
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.90%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.20%	[2]
Acquired fund fees and expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.01%
Total annual Fund operating expenses	rr_ExpensesOverAssets	1.11%	[3]
Less: Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.03%)	[4]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	1.08%
1 year	rr_ExpenseExampleYear01	$ 110
3 years	rr_ExpenseExampleYear03	350
5 years	rr_ExpenseExampleYear05	609
10 years	rr_ExpenseExampleYear10	1,349
1 year	rr_ExpenseExampleNoRedemptionYear01	110
3 years	rr_ExpenseExampleNoRedemptionYear03	350
5 years	rr_ExpenseExampleNoRedemptionYear05	609
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 1,349
1 Year	rr_AverageAnnualReturnYear01	19.56%
5 Years	rr_AverageAnnualReturnYear05	12.96%
10 Years	rr_AverageAnnualReturnYear10	8.22%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 08, 2012
Columbia Acorn USA | Class Inst3
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases (as a % of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	rr_MaximumDeferredSalesChargeOverOther	none
Management fees	rr_ManagementFeesOverAssets	0.90%
Distribution and/or service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.13%	[2]
Acquired fund fees and expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.01%
Total annual Fund operating expenses	rr_ExpensesOverAssets	1.04%	[3]
Less: Fee waivers and/or expense reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.01%)	[4]
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	rr_NetExpensesOverAssets	1.03%
1 year	rr_ExpenseExampleYear01	$ 105
3 years	rr_ExpenseExampleYear03	330
5 years	rr_ExpenseExampleYear05	573
10 years	rr_ExpenseExampleYear10	1,270
1 year	rr_ExpenseExampleNoRedemptionYear01	105
3 years	rr_ExpenseExampleNoRedemptionYear03	330
5 years	rr_ExpenseExampleNoRedemptionYear05	573
10 years	rr_ExpenseExampleNoRedemptionYear10	$ 1,270
1 Year	rr_AverageAnnualReturnYear01	19.60%
5 Years	rr_AverageAnnualReturnYear05	13.02%
10 Years	rr_AverageAnnualReturnYear10	8.24%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 08, 2012
Columbia Acorn USA | returns after taxes on distributions | Class Inst
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	11.14%
5 Years	rr_AverageAnnualReturnYear05	6.62%
10 Years	rr_AverageAnnualReturnYear10	4.94%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Sep. 04, 1996
Columbia Acorn USA | returns after taxes on distributions and sale of Fund shares | Class Inst
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	16.22%
5 Years	rr_AverageAnnualReturnYear05	9.29%
10 Years	rr_AverageAnnualReturnYear10	6.16%
Share Class Inception Date	rr_AverageAnnualReturnInceptionDate	Sep. 04, 1996
Columbia Acorn USA | Russell 2000 Growth Index (reflects no deductions for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	22.17%
5 Years	rr_AverageAnnualReturnYear05	15.21%
10 Years	rr_AverageAnnualReturnYear10	9.19%

[1] This charge is imposed on certain investments of between $1 million and $50 million redeemed within 18 months after purchase, as follows: 1.00% if redeemed within 12 months after purchase, and 0.50% if redeemed more than 12, but less than 18, months after purchase, with certain limited exceptions.

[2] Other expenses have been restated to reflect current transfer agency fees paid by the Fund.

[3] "Total annual Fund operating expenses" include acquired fund fees and expenses (expenses the Fund incurs indirectly through its investments in other investment companies) and may be higher than "Total Net Expenses" shown in the Financial Highlights section of this prospectus because "Total Net Expenses" do not include acquired fund fees and expenses.

[4] Columbia Wanger Asset Management, LLC (the Investment Manager) has contractually agreed to waive fees and reimburse certain expenses of the Fund, through April 30, 2019, so that ordinary operating expenses (excluding transaction costs and certain other investment-related expenses, interest and fees on borrowings, if any) do not exceed the annual rates of 1.42% for Class A shares, 1.17% for Class Adv shares, 2.17% for Class C shares, 1.17% for Class Inst shares, 1.08% for Class Inst2 shares and 1.03% for Class Inst3 shares. This arrangement may only be amended or terminated with approval from the Fund’s Board of Trustees and the Investment Manager. The fee waivers and/or expense reimbursements shown in the table also reflect the contractual agreement of the Fund’s transfer agent, Columbia Management Investment Services Corp. (the Transfer Agent), to waive a portion of its fees through June 30, 2019, such that the Fund’s transfer agency fees do not exceed the annual rate of 0.14% of the average daily net assets of each share class other than Class Inst2 and Class Inst3. With respect to Class Inst2 and Class Inst3 shares, the Transfer Agent has contractually agreed to waive a portion of its fees such that the Fund's transfer agency fees do not exceed the annual rates of 0.05% of the average daily net assets of Class Inst2 shares and 0.00% of the average daily net assets of Class Inst3 shares.

[5] This charge applies to redemptions within 12 months after purchase, with certain limited exceptions.

[6] Year to Date return as of March 31, 2018: 3.81%

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Columbia Thermostat Fund

SUMMARY OF THE FUND

Investment Objective

Columbia Thermostat Fund^SM (the Fund) seeks long-term capital appreciation.

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. An investor transacting in a class of Fund shares without any front-end sales charge, contingent deferred sales charge, or other asset-based fee for sales or distribution may be required to pay a commission to the financial intermediary for effecting such transactions. Such commission rates are set by the financial intermediary and are not reflected in the tables or the example below. You may qualify for sales charge discounts if you and members of your immediate family invest, or agree to invest in the future, at least $50,000 in certain classes of shares of eligible funds distributed by Columbia Management Investment Distributors, Inc. (the Distributor). More information is available about these and other sales charge discounts and waivers from your financial intermediary, and can be found in the Choosing a Share Class section beginning on page 51 of the Fund’s prospectus, in Appendix A to the prospectus beginning on page A-1 and in Appendix S to the Statement of Additional Information (SAI) under Sales Charge Waivers beginning on page S-1.

Shareholder Fees (fees paid directly from your investment)

Shareholder Fees - Columbia Thermostat Fund	Class A		Class C		Class Adv	Class Inst	Class Inst2	Class Inst3
Maximum sales charge (load) imposed on purchases (as a % of offering price)	5.75%		none		none	none	none	none
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	1.00%	[1]	1.00%	[2]	none	none	none	none

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)

Annual Fund Operating Expenses - Columbia Thermostat Fund		Class A	Class Adv	Class C	Class Inst	Class Inst2	Class Inst3
Management fees		0.10%	0.10%	0.10%	0.10%	0.10%	0.10%
Distribution and/or service (12b-1) fees		0.25%	none	1.00%	none	none	none
Other expenses	[1]	0.19%	0.19%	0.19%	0.19%	0.18%	0.12%
Acquired fund fees and expenses	[2]	0.35%	0.35%	0.35%	0.35%	0.35%	0.35%
Total annual Fund operating expenses	[3]	0.89%	0.64%	1.64%	0.64%	0.63%	0.57%
Fee waivers and/or expense reimbursements	[4]	(0.04%)	(0.04%)	(0.04%)	(0.04%)	(0.08%)	(0.07%)
Total annual Fund operating expenses after fee waivers and/or expense reimbursements		0.85%	0.60%	1.60%	0.60%	0.55%	0.50%

Example

The following example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example illustrates the hypothetical expenses that you would incur over the time periods indicated, and assumes that:

• you invest $10,000 in the applicable class of Fund shares for the periods indicated,

• your investment has a 5% return each year, and

• the Fund’s total annual operating expenses remain the same as shown in the Annual Fund Operating Expenses table above.

Since the waivers and/or reimbursements shown in the Annual Fund Operating Expenses table above expire as indicated in the preceding table, they are only reflected in the 1 year example and the first year of the other examples. Although your actual costs may be higher or lower, based on the assumptions listed above, your costs would be:

Expense Example - Columbia Thermostat Fund - USD ($)	1 year	3 years	5 years	10 years
Class A	657	839	1,036	1,605
Class Adv	61	201	353	795
Class C	263	513	888	1,940
Class Inst	61	201	353	795
Class Inst2	56	194	343	779
Class Inst3	51	176	311	707

Expense Example, No Redemption - Columbia Thermostat Fund - USD ($)	1 year	3 years	5 years	10 years
Class A	657	839	1,036	1,605
Class Adv	61	201	353	795
Class C	163	513	888	1,940
Class Inst	61	201	353	795
Class Inst2	56	194	343	779
Class Inst3	51	176	311	707

Portfolio Turnover

The Fund will indirectly bear the expenses associated with portfolio turnover of the underlying Portfolio Funds in which the Fund invests. Each Portfolio Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A Portfolio Fund’s higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when its shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 33% of the average value of its portfolio.

Principal Investment Strategies

The Fund is primarily managed as a fund that invests in other funds (i.e., a "fund-of-funds") that seeks to achieve its investment objective by investing its assets among a selected group of underlying stock and bond mutual funds and exchanged-traded funds (ETFs) for which Columbia Wanger Asset Management, LLC, the Fund's investment adviser (the Investment Manager) or its affiliates, including Columbia Management Investment Advisers, LLC (Columbia Management), serves as investment adviser or principal underwriter (the Portfolio Funds). Under normal circumstances, the Fund allocates at least 95% of its net assets (stock/bond assets) among the Portfolio Funds according to an asset allocation table based on the current level of Standard & Poor's (S&P) 500^® Index.

Generally, the Fund's allocation to stock funds increases as the S&P 500^® Index declines and decreases as the S&P 500^® Index rises. When the S&P 500^® Index goes up in relation to trading range bands that are predetermined by the Investment Manager, the Fund sells a portion of its stock Portfolio Funds and invests more in the bond Portfolio Funds, and when the S&P 500^® Index goes down in relation to the predetermined bands, the Fund increases its investment in the stock Portfolio Funds. Under normal circumstances, the Fund may invest up to 5% of net assets plus any cash received that day in cash, high quality short-term paper and government securities.

Although many asset allocation funds follow a basic approach of moving assets from stocks to bonds when the equity market goes up, and from bonds to stocks when the equity market goes down, some are run by investment managers who allocate fund assets by making subjective decisions based on complicated economic and financial models and complex graphs of market behavior. By contrast, the day-to-day investment decisions for the Fund are made according to a single predetermined allocation table set forth below. The temperature in your house is run by a single rule: your thermostat turns on the furnace if your house is too cold or turns on the air conditioner if your house is too warm. This Fund works the same way, so it is named Columbia Thermostat Fund.

Just as a thermostat may be set at different ranges for different seasons, the structure and allocation ranges of the Fund's asset allocation table may be changed from time to time between two forms, based on the Investment Manager's determination of whether it expects the equity market to be moving over the next year in a side-ways or non-directional pattern, which the Investment Manager terms an "expensive" market, or to be trending upward, which the Investment Manager terms a "normal" market. The Investment Manager's process and methodology for determining whether the current market is "expensive" or "normal" and the structure of each form of allocation table are described in more detail below under Determination of Current Market State. Such determination will be made on at least an annual basis and will be reflected in the asset allocation table disclosed in this prospectus. In general, the two different forms of allocation table are intended to maximize the capture of value under the two different sets of market conditions.

The Fund's current asset allocation table, which is set forth below, reflects the Investment Manager's determination that the equity market is currently "expensive." The structure of the Fund's current allocation table reflects the same form that has been in place since the Fund's inception in 2002.

Stock/Bond Allocation Table
	How the Fund will Invest the Stock/Bond Assets
Level of the S&P 500® Index	Stock Percentage	Bond Percentage
over 2890	10%	90%
over 2779 – 2890	15%	85%
over 2672 – 2779	20%	80%
over 2569 – 2672	25%	75%
over 2470 – 2569	30%	70%
over 2375 – 2470	35%	65%
over 2284 – 2375	40%	60%
over 2196 – 2284	45%	55%
over 2110 – 2196	50%	50%
over 2026 – 2110	55%	45%
over 1945 – 2026	60%	40%
over 1867 – 1945	65%	35%
over 1792 – 1867	70%	30%
over 1720 – 1792	75%	25%
over 1651 – 1720	80%	20%
over 1585 – 1651	85%	15%
1585 and under	90%	10%

When the S&P 500^® Index moves into a new band on the table, the Fund will rebalance the stock/bond mix to reflect the new S&P 500^® Index price level by redeeming shares of some Portfolio Funds and purchasing shares of other Portfolio Funds. Any such rebalancing typically will be implemented promptly. However, there are two circumstances when a rebalancing may be implemented over a longer timeline. First, when a rebalancing or allocation table change would trigger a 10% point or greater change in stock and bond allocations or individual Portfolio Funds, the rebalancing may be implemented over a period of up to two weeks, if deemed by the Investment Manager to be in the best interest of shareholders. The second exception is a "31-day Rule;" in order to reduce taxable events and minimize short-term trading if the S&P 500^® Index price moves back and forth across a band in the allocation table, after the Fund has increased its percentage allocation to either stock funds or bond funds, it will not decrease that allocation for at least 31 days. Following a change in the Fund's stock/bond mix, if the S&P 500^® Index remains within the same band for a while, normal market fluctuations will change the values of the Fund's holdings of stock Portfolio Funds and bond Portfolio Funds. The Investment Manager will invest cash flows from sales (or redemptions) of Fund shares to bring the stock/bond mix back toward the allocation percentages for that S&P 500^® Index band. For example, if the S&P 500^® Index is in the 1792 band, and the value of the holdings of the stock Portfolio Funds has dropped to 68% of the value of the holdings of all Portfolio Funds, the Investment Manager would invest new cash in the stock Portfolio Funds (or cash for redemptions would come from the bond Portfolio Funds). If the 31-day Rule is in effect, the Investment Manager will invest new cash at the stock/bond percentage allocation as of the latest rebalancing.

As an illustrative example, suppose the following:

Date	Level of the S&P 500® Index	How the Fund will invest the Stock/ Bond Assets
Nov. 1	We begin when the market is 2176	50% stocks, 50% bonds
Dec. 1	The S&P 500® goes to 2198	rebalance 45% stocks, 55% bonds
Dec. 6	The S&P 500® drops back to 2186	no reversal for 31 days
Jan. 2	The S&P 500® is at 2150	rebalance 50% stocks, 50% bonds
Jan. 20	The S&P 500® drops to 2056	rebalance 55% stocks, 45% bonds
	The market has made a continuation move by going through a second action level, not a reversal move, so the 31-day Rule does not apply in this case.
Jan. 30	The S&P 500® goes up to 2115	no reversal for 31 days
Feb. 20	The S&P 500® is at 2123	rebalance 50% stocks, 50% bonds

The Investment Manager chooses the Portfolio Funds to be generally consistent with the composition of the Fund's primary stock and bond benchmarks and to allow the Fund to participate in strategies the Investment Manager believes can provide additional return due to active management.

The following table shows the stock and bond Portfolio Funds that the Fund currently uses in its "fund-of-funds" structure and the current target percentage for each Portfolio Fund within the stock or bond asset class. As described more fully below, the Investment Manager may substitute or add additional Portfolio Funds at any time, including funds introduced after the date of this prospectus. The target percentage within each asset class category is achieved by rebalancing the investments within the asset class whenever the S&P 500^® Index moves into a new band on the allocation table, subject to the 31-day Rule described above. The Fund will not liquidate its investment in one Portfolio Fund in order to invest in another Portfolio Fund except in connection with a rebalancing due to a move of the S&P 500^® Index into a new band (or due to a change by the Investment Manager in the Portfolio Funds or to the relative target percentages among them). Until a subsequent rebalancing, the Fund's cash flows are invested in, or redeemed from, the Portfolio Funds in a manner that will reduce any deviation of the relative values of the Fund's holdings of the Portfolio Funds from the percentages shown below.

Allocation of Stock/Bond Asset Within Asset Classes
	Type of Fund	Allocation
Stock Funds
Columbia Acorn® Fund	Small/Mid-cap growth	10%
Columbia Acorn International®	Small/Mid-cap international growth	10%
Columbia Acorn SelectSM	Mid-cap growth	10%
Columbia Contrarian Core Fund	Large-cap blend	10%
Columbia Dividend Income Fund	Large-cap value	10%
Columbia Large Cap Enhanced Core Fund	Large-cap blend	10%
Columbia Large Cap Index Fund	Large-cap blend	40%
Total		100%
Bond Funds	Type of Fund	Allocation
Columbia Corporate Income Fund	Corporate bond	10%
Columbia Diversified Fixed Income Allocation ETF	Beta advantage multi-sector bond	10%
Columbia Quality Income Fund	Government bond	20%
Columbia Short Term Bond Fund	Short term bond	25%
Columbia U.S. Treasury Index Fund	U.S. Treasury notes/bonds	35%
Total		100%

See the Portfolio Funds Summary section of this prospectus for information about the Portfolio Funds' investment objectives and principal investment strategies. Each of the Portfolio Funds is managed by the Investment Manager or its affiliates. The Fund does not pay any sales load on its purchases of shares of the Portfolio Funds.

The Investment Manager has the authority to review the Portfolio Funds and to change the relative percentages among them or to add or eliminate funds. Such review will occur on at least an annual basis and will be reflected in the Portfolio Funds table disclosed in this prospectus.

Determination of Market State

Just as a thermostat may be set at different ranges for different seasons, the Fund’s allocation of assets between stock and bond funds is expected to be set at different range bands depending principally on the Investment Manager’s view of how expensive the equity market is compared to historical levels.

At least annually, the Investment Manager will conduct a review of the current equity market price levels compared to historical price levels, and will determine if the equity market is in an “expensive” state, or a “normal” state. This review will begin with a comparison of the S&P 500^® Index’s cyclically adjusted price-to-earnings ratio for the prior seven years against a 40-year history, but the Investment Manager will also take into account additional quantitative or qualitative market factors, including for example, trends in equity price measures and comparisons of equity prices against prices for other asset classes. Based on its determination of the current market state, the Investment Manager will select the form of allocation table to be effective in the Fund’s current prospectus. Also as part of this review, the Investment Manager will update the median historical information used in the table, and the resulting S&P 500^® Index levels used to create the bands around the median, regardless of the market state.

As noted below, the form of table used for an “expensive” market is expected to be different from that used for a “normal” market.

Generally, the Investment Manager will determine the current state of the equity market to be “expensive” when its analysis of the S&P 500^® Index’s cyclically adjusted price-to-earnings ratio for the prior seven years is within the top quartile of ratio observations over a 40-year history. The “expensive” equity market form of allocation table for the Fund is structured around an implied S&P 500^® Index trading range median of 50% stocks/50% bonds (the 50/50 Band), with allocations to bonds decreasing through stratified range bands below the 50/50 Band and allocations to bonds increasing through stratified range bands above the 50/50 Band, as shown by the following example:

“Expensive” Equity Market Form of Stock/Bond Allocation Table
Level of the S&P 500® Index	Stock Percentage	Bond Percentage
above top of trading range below	10%	90%
4% trading range above top of range below	15%	85%
4% trading range above top of range below	20%	80%
4% trading range above top of range below	25%	75%
4% trading range above top of range below	30%	70%
4% trading range above top of range below	35%	65%
4% trading range above top of range below	40%	60%
4% trading range above top of range below	45%	55%
4% trading range centered on implied median (50/50 Band)*	50%	50%
4% trading range below bottom of range above	55%	45%
4% trading range below bottom of range above	60%	40%
4% trading range below bottom of range above	65%	35%
4% trading range below bottom of range above	70%	30%
4% trading range below bottom of range above	75%	25%
4% trading range below bottom of range above	80%	20%
4% trading range below bottom of range above	85%	15%
below bottom of trading range above	90%	10%

*Implied median price level as calculated by the Investment Manager.

The “expensive” form of table has been in place since the Fund’s inception in 2002.

Generally, the Investment Manager will determine the current state of the equity market to be “normal” when its analysis of the S&P 500^® Index’s cyclically adjusted price-to-earnings ratio for the prior seven years is outside the top quartile of ratio observations over a 40-year history. The “normal” equity market form allocation table for the Fund is also structured around the 50/50 Band, with allocations to bonds decreasing through stratified range bands below the 50/50 Band, and allocations to stocks and bonds set at 50% stocks/50% bonds for all trading levels above the 50/50 Band, as shown by the following example:

“Normal” Equity Market Form of Stock/Bond Allocation Table
Level of the S&P 500® Index	Stock Percentage	Bond Percentage
above trading range that is 2% below implied median (50/50 Band)*	50%	50%
4% trading range below bottom of range above	55%	45%
4% trading range below bottom of range above	60%	40%
4% trading range below bottom of range above	65%	35%
4% trading range below bottom of range above	70%	30%
4% trading range below bottom of range above	75%	25%
4% trading range below bottom of range above	80%	20%
4% trading range below bottom of range above	85%	15%
below bottom of trading range above	90%	10%

*Implied median price level as calculated by the Investment Manager.

The Investment Manager will conduct the market state review at least annually prior to the annual updating of this prospectus, and may, in its discretion, assess the market state on an "emergency" basis and make any changes deemed necessary to reflect, for example, a very significant move in market levels or a structural change affecting the markets.

Any such "emergency" changes by the Investment Manager, which are expected to be infrequent, would be disclosed in this prospectus.

Principal Risks

An investment in the Fund involves risks, including specific risks relating to the investment in the Fund based on its investment process and its "fund-of-funds" structure, as well as specific risks related to the individual Portfolio Funds in which it invests that in the aggregate are principal risks to the Fund, including among others, those described below. More information about the Portfolio Funds, including their principal risks, is available in their prospectuses. This prospectus is not an offer for any of the Portfolio Funds. There is no assurance that the Fund will achieve its investment objective and you may lose money. The value of the Fund's holdings may decline, and the Fund's net asset value (NAV) and share price may go down. An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.

Allocation Risk. The Investment Manager uses an asset allocation strategy in pursuit of its investment objective, there is a risk that the Fund's allocation among asset classes or investments will cause the Fund's shares to lose value or cause the Fund to underperform other funds with similar investment objectives and/or strategies, or that the investments themselves will not produce the returns expected.

Fund-of-Funds Risk. Determinations regarding asset classes or underlying funds and the Fund’s allocations thereto may not successfully achieve the Fund’s investment objective, in whole or in part. The ability of the Fund to realize its investment objective will depend, in large part, on the extent to which the underlying funds realize their investment objective. There is no guarantee that the underlying funds will achieve their respective investment objectives. The Fund is exposed to the same risks as the underlying funds in direct proportion to the allocation of its assets among the underlying funds. By investing in a combination of underlying funds, the Fund has exposure to the risks of many areas of the market. The performance of underlying funds could be adversely affected if other entities that invest in the same underlying funds make relatively large investments or redemptions in such underlying funds. The Fund, and its shareholders, indirectly bear a portion of the expenses of any funds in which the Fund invests. Because the expenses and costs of each underlying fund are shared by its investors, redemptions by other investors in an underlying fund could result in decreased economies of scale and increased operating expenses for such fund. The Investment Manager may have potential conflicts of interest in selecting affiliated funds over unaffiliated funds for investment by the Fund, and may also face potential conflicts of interest in selecting affiliated funds, because the fees the Investment Manager receives from some underlying funds may be higher than the fees paid by other underlying funds. Further, because of the Investment Manager’s confidence in its own strategies, investment philosophy and capacities, it will, in selecting underlying funds, at times prefer a fund in the Columbia Acorn Family of Funds over alternative investments. There can be no assurance, however, that a fund in the Columbia Acorn Family of Funds selected for inclusion in Columbia Thermostat Fund’s portfolio will, in fact, outperform similar funds managed by the Investment Manager’s affiliates.

The Fund is subject indirectly to the risks of the Portfolio Funds described below. Please see the Portfolio Funds Summary for information about which risks apply to which Portfolio Funds.

Market Risk. Market risk refers to the possibility that the market values of securities or other investments that the Fund holds will fall, sometimes rapidly or unpredictably, or fail to rise. An investment in the Fund could lose money over short or long periods.

Interest Rate Risk. Interest rate risk is the risk of losses attributable to changes in interest rates. In general, if prevailing interest rates rise, the values of loans and other debt instruments tend to fall, and if interest rates fall, the values of loans and other debt instruments tend to rise. Changes in the value of a debt instrument usually will not affect the amount of income the Fund receives from it but will generally affect the value of your investment in the Fund. Changes in interest rates may also affect the liquidity of the Fund's investments in debt instruments. In general, the longer the maturity or duration of a debt instrument, the greater its sensitivity to changes in interest rates. Interest rate declines also may increase prepayments of debt obligations, which, in turn, would increase prepayment risk. Similarly, a period of rising interest rates may negatively impact the Fund's performance. Actions by governments and central banking authorities can result in increases in interest rates. Such actions may negatively affect the value of debt instruments held by the Fund, resulting in a negative impact on the Fund's performance and NAV. Securities with floating interest rates are typically less sensitive to interest rate changes, but may decline in value if their interest rates do not rise as much as interest rates in general. Because rates on certain floating rate loans and other debt securities reset only periodically, changes in prevailing interest rates (and particularly sudden and significant changes) can be expected to cause fluctuations in the Fund's NAV. Any interest rate increases could cause the value of the Fund's investments in debt instruments to decrease. Rising interest rates may prompt redemptions from the Fund, which may force the Fund to sell investments at a time when it is not advantageous to do so, which could result in losses.

Credit Risk. Credit risk is the risk that the value of loans or other debt instruments may decline if the issuer thereof defaults or otherwise becomes unable or unwilling, or is perceived to be unable or unwilling, to honor its financial obligations, such as making payments to the Fund when due. Rating agencies assign credit ratings to certain debt instruments to indicate their credit risk. Unless otherwise provided in the Fund's Principal Investment Strategies, investment grade debt instruments are those rated at or above BBB- by Standard and Poor's Ratings Services, or equivalently rated by Moody's Investors Service, Inc. or Fitch, Inc., or, if unrated, determined by the management team to be of comparable quality. Conversely, below investment grade (commonly called "high-yield” or "junk”) debt instruments are those rated below BBB- by Standard and Poor's Ratings Services, or equivalently rated by Moody's Investors Service, Inc. or Fitch, Inc., or, if unrated, determined by the management team to be of comparable quality. A rating downgrade by such agencies can negatively impact the value of such instruments. Lower quality or unrated instruments held by the Fund may present increased credit risk as compared to higher-rated instruments. Non-investment grade debt instruments may be subject to greater price fluctuations and are more likely to experience a default than investment grade debt instruments and therefore may expose the Fund to increased credit risk. If the Fund purchases unrated instruments, or if the ratings of instruments held by the Fund are lowered after purchase, the Fund will depend on analysis of credit risk more heavily than usual.

High-Yield Investments Risk. Securities and other debt instruments held by the Fund that are rated below investment grade (commonly called “high-yield” or “junk” bonds) and unrated debt instruments of comparable quality expose the Fund to a greater risk of loss of principal and income than a fund that invests solely or primarily in investment grade debt instruments. In addition, these investments have greater price fluctuations, are less liquid and are more likely to experience a default than higher-rated debt instruments. High-yield debt instruments are considered to be predominantly speculative with respect to the issuer’s capacity to pay interest and repay principal.

Value Securities Risk. Value securities are securities of companies that may have experienced, for example, adverse business, industry or other developments or may be subject to special risks that have caused the securities to be out of favor and, in turn, potentially undervalued. The market value of a portfolio security may not meet the Investment Manager’s perceived value assessment of that security, or may decline in price, even though the Investment Manager believes the securities are already undervalued. There is also a risk that it may take longer than expected for the value of these investments to rise to the portfolio manager’s perceived value. In addition, value securities, at times, may not perform as well as growth securities or the stock market in general, and may be out of favor with investors for varying periods of time.

Growth Securities Risk. Growth securities typically trade at a higher multiple of earnings than other types of equity securities. Accordingly, the market values of growth securities may never reach their expected market value and may decline in price. In addition, growth securities, at times, may not perform as well as value securities or the stock market in general, and may be out of favor with investors for varying periods of time.

Sector Risk. At times, the Fund may have a significant portion of its assets invested in securities of companies conducting business in a related group of industries within a sector. Companies in the same sector may be similarly affected by economic, regulatory, political or market events or conditions, which may make the Fund more vulnerable to unfavorable developments in that sector than funds that invest more broadly. Generally, the more the Fund diversifies its investments, the more it spreads risk and potentially reduces the risks of loss and volatility.

Foreign Securities Risk. Investments in or exposure to foreign securities involve certain risks not associated with investments in or exposure to securities of U.S. companies. Foreign securities subject the Fund to the risks associated with investing in the particular country of an issuer, including political, regulatory, economic, social, diplomatic and other conditions or events (including, for example, military confrontations, war and terrorism), occurring in the country or region, as well as risks associated with less developed custody and settlement practices. Foreign securities may be more volatile and less liquid than securities of U.S. companies, and are subject to the risks associated with potential imposition of economic and other sanctions against a particular foreign country, its nationals or industries or businesses within the country. In addition, foreign governments may impose withholding or other taxes on the Fund’s income, capital gains or proceeds from the disposition of foreign securities, which could reduce the Fund’s return on such securities.

Operational and Settlement Risks of Foreign Securities. The Fund’s foreign securities are generally held outside the United States in the primary market for the securities in the custody of foreign sub-custodians. Some countries have limited governmental oversight and regulation, which increases the risk of corruption and fraud and the possibility of losses to the Fund. In particular, under certain circumstances, foreign securities may settle on a delayed delivery basis, meaning that the Fund may be required to make payment for securities before the Fund has actually received delivery of the securities or deliver securities prior to the receipt of payment. As a result, there is a risk that the security will not be delivered to the Fund or that payment will not be received. Losses can also result from lost, stolen or counterfeit securities; defaults by brokers and banks; failures or defects of the settlement system; or poor and improper record keeping by registrars and issuers.

Share Blocking. In certain non-U.S. markets, an issuer’s securities are blocked from trading for a specified number of days before and, in certain instances, after a shareholder meeting. The blocking period can last up to several weeks. Share blocking may prevent the Fund from buying or selling securities during this period. As a consequence of these restrictions, the Investment Manager, on behalf of the Fund, may abstain from voting proxies in markets that require share blocking.

Emerging Market Securities Risk. Securities issued by foreign governments or companies in emerging market countries, such as China, Russia and certain countries in Eastern Europe, the Middle East, Asia, Latin America or Africa, are more likely to have greater exposure to the risks of investing in foreign securities that are described in Foreign Securities Risk. In addition, emerging market countries are more likely to experience instability resulting, for example, from rapid changes or developments in social, political, economic or other conditions. Their economies are usually less mature and their securities markets are typically less developed with more limited trading activity (i.e., lower trading volumes and less liquidity) than more developed countries. Emerging market securities tend to be more volatile than securities in more developed markets. Many emerging market countries are heavily dependent on international trade and have fewer trading partners, which makes them more sensitive to world commodity prices and economic downturns in other countries, and some have a higher risk of currency devaluations.

Operational and Settlement Risks of Securities in Emerging Markets. Foreign sub-custodians in emerging markets may be recently organized, lack extensive operating experience or lack effective government oversight or regulation. In addition, there may be legal restrictions or limitations on the ability of the Fund to recover assets held in custody by a foreign sub-custodian in the event of the bankruptcy of the sub-custodian. There may also be a greater risk that settlement may be delayed and that cash or securities of the Fund may be lost because of failures of or defects in the system, including fraud or corruption. Settlement systems in emerging markets also have a higher risk of failed trades.

Risks Related to Currencies and Corporate Actions in Emerging Markets. Risks related to currencies and corporate actions are also greater in emerging market countries than in developed countries. Emerging market currencies may not have an active trading market and are subject to a higher risk of currency devaluations.

Risks Related to Corporate and Securities Laws in Emerging Markets. Securities laws in emerging markets may be relatively new and unsettled and, consequently, there is a risk of rapid and unpredictable change in laws regarding foreign investment, securities regulation, title to securities and shareholder rights.

Liquidity and Trading Volume Risk. Due to market conditions, including uncertainty regarding the price of a security, it may be difficult for the Fund to buy or sell portfolio securities at a desirable time or price, which could result in investment losses. This risk of portfolio illiquidity is heightened with respect to small- and mid-cap securities, generally, and foreign small- and mid-cap securities in particular. The Fund may have to lower the selling price, liquidate other investments, or forego another, more appealing investment opportunity as a result of illiquidity in the markets. As a result of significant and sustained reductions in emerging and developed international market trading volumes in the wake of the 2007-2009 financial crisis, it may take longer to buy or sell securities, which can exacerbate the Fund’s exposure to volatile markets. The Fund may also be limited in its ability to execute favorable trades in portfolio securities in response to changes in share prices and fundamentals, and may be forced to dispose of securities under disadvantageous circumstances and at a loss. As the Fund grows in size or, conversely, if it faces significant redemption pressure, these considerations take on increasing significance and may adversely impact performance.

Foreign Currency Risk. The performance of the Fund may be materially affected positively or negatively by foreign currency strength or weakness relative to the U.S. dollar, particularly if the Fund invests a significant percentage of its assets in foreign securities or other assets denominated in currencies other than the U.S. dollar.

Small- and Mid-Cap Company Securities Risk. Investments in small- and mid-capitalization companies (small- and mid-cap companies) often involve greater risks than investments in larger, more established companies (larger companies) because small- and mid-cap companies tend to have less predictable earnings and may lack the management experience, financial resources, product diversification and competitive strengths of larger companies. Securities of small- and mid-cap companies may be less liquid and more volatile than the securities of larger companies.

U.S. Government Obligations Risk. While U.S. Treasury obligations are backed by the “full faith and credit” of the U.S. Government, such securities are nonetheless subject to credit risk (i.e., the risk that the U.S. Government may be, or be perceived to be, unable or unwilling to honor its financial obligations, such as making payments). Securities issued or guaranteed by federal agencies or authorities and U.S. Government-sponsored instrumentalities or enterprises may or may not be backed by the full faith and credit of the U.S. Government.

Derivatives Risk. Derivatives may involve significant risks. Derivatives are financial instruments with a value in relation to, or derived from, the value of an underlying asset(s) or other reference, such as an index, rate or other economic indicator (each an underlying reference). Derivatives may include those that are privately placed or otherwise exempt from SEC registration, including certain Rule 144A eligible securities. Derivatives could result in Fund losses if the underlying reference does not perform as anticipated. Use of derivatives is a highly specialized activity that can involve investment techniques, risks, and tax planning different from those associated with more traditional investment instruments. The Fund’s derivatives strategy may not be successful and use of certain derivatives could result in substantial, potentially unlimited, losses to the Fund regardless of the Fund’s actual investment. A relatively small movement in the price, rate or other economic indicator associated with the underlying reference may result in substantial loss for the Fund. Derivatives may be more volatile than other types of investments. The value of derivatives may be influenced by a variety of factors, including national and international political and economic developments. Potential changes to the regulation of the derivatives markets may make derivatives more costly, may limit the market for derivatives, or may otherwise adversely affect the value or performance of derivatives. Derivatives can increase the Fund’s risk exposure to underlying references and their attendant risks, such as credit risk, market risk, foreign currency risk and interest rate risk, while exposing the Fund to correlation risk, counterparty risk, hedging risk, leverage risk, liquidity risk, pricing risk and volatility risk.

Convertible Securities Risk. Convertible securities are subject to the usual risks associated with debt instruments, such as interest rate risk and credit risk. Convertible securities also react to changes in the value of the common stock into which they convert, and are thus subject to market risk. The Fund may also be forced to convert a convertible security at an inopportune time, which may decrease the Fund’s return.

Forward Commitments on Mortgage-Backed Securities (including Dollar Rolls) Risk. When purchasing mortgage-backed securities in the “to be announced” (TBA) market (MBS TBAs), the seller agrees to deliver mortgage-backed securities for an agreed upon price on an agreed upon date, but may make no guarantee as to the specific securities to be delivered. In lieu of taking delivery of mortgage-backed securities, the Fund could enter into dollar rolls, which are transactions in which the Fund sells securities to a counterparty and simultaneously agrees to purchase those or similar securities in the future at a predetermined price. Dollar rolls involve the risk that the market value of the securities the Fund is obligated to repurchase may decline below the repurchase price, or that the counterparty may default on its obligations. These transactions may also increase the Fund’s portfolio turnover rate. If the Fund reinvests the proceeds of the security sold, the Fund will also be subject to the risk that the investments purchased with such proceeds will decline in value (a form of leverage risk). MBS TBAs and dollar rolls are subject to the risk that the counterparty to the transaction may not perform or be unable to perform in accordance with the terms of the instrument.

Mortgage- and Other Asset-Backed Securities Risk. The value of any mortgage-backed and other asset-backed securities held by the Fund may be affected by, among other things, changes or perceived changes in: interest rates; factors concerning the interests in and structure of the issuer or the originator of the mortgages or other assets; the creditworthiness of the entities that provide any supporting letters of credit, surety bonds or other credit enhancements; or the market's assessment of the quality of underlying assets. Payment of principal and interest on some mortgage-backed securities (but not the market value of the securities themselves) may be guaranteed by the full faith and credit of a particular U.S. Government agency, authority, enterprise or instrumentality, and some, but not all, are also insured or guaranteed by the U.S. Government. Mortgage-backed securities issued by non-governmental issuers (such as commercial banks, savings and loan institutions, private mortgage insurance companies, mortgage bankers and other secondary market issuers) may entail greater risk than obligations guaranteed by the U.S. Government. Mortgage- and other asset-backed securities are subject to prepayment risk, which is the possibility that the underlying mortgage or other asset may be refinanced or prepaid prior to maturity during periods of declining or low interest rates, causing the Fund to have to reinvest the money received in securities that have lower yields. Rising or high interest rates tend to extend the duration of mortgage- and other asset-backed securities, making their prices more volatile and more sensitive to changes in interest rates.

Stripped Securities Risk. Stripped securities are the separate income or principal components of debt securities. These securities are particularly sensitive to changes in interest rates, and therefore subject to greater fluctuations in price than typical interest bearing debt securities.

Prepayment and Extension Risk. Prepayment and extension risk is the risk that a bond or other security or investment might, in the case of prepayment risk, be called or otherwise converted, prepaid or redeemed before maturity and, in the case of extension risk, that the investment might not be called as expected. In the case of prepayment risk, if the investment is converted, prepaid or redeemed before maturity, the portfolio manager may not be able to invest the proceeds in other investments providing as high a level of income, resulting in a reduced yield to the Fund. In the case of mortgage- or asset-backed securities, as interest rates decrease or spreads narrow, the likelihood of prepayment increases. Conversely, extension risk is the risk that an unexpected rise in interest rates will extend the life of a mortgage- or asset-backed security beyond the prepayment time. If the Fund’s investments are locked in at a lower interest rate for a longer period of time, the portfolio manager may be unable to capitalize on securities with higher interest rates or wider spreads.

Reinvestment Risk. Reinvestment risk is the risk that the Fund will not be able to reinvest income or principal at the same return it is currently earning.

Depositary Receipts Risk. Depositary receipts are receipts issued by a bank or trust company reflecting ownership of underlying securities issued by foreign companies. Some foreign securities are traded in the form of American Depositary Receipts and/or Global Depositary Receipts. Depositary receipts involve risks similar to the risks associated with investments in foreign securities, including those associated with investing in the particular country of an issuer, which may be related to the particular political, regulatory, economic, social and other conditions or events, including, for example, military confrontations, war and terrorism, occurring in the country and fluctuations in such country's currency, as well as market risk tied to the underlying foreign company. In addition, holders of depositary receipts may have limited voting rights, may not have the same rights afforded to stockholders of a typical domestic company in the event of a corporate action, such as an acquisition, merger or rights offering, and may experience difficulty in receiving company stockholder communications. There is no guarantee that a financial institution will continue to sponsor a depositary receipt, or that a depositary receipt will continue to trade on an exchange, either of which could adversely affect the liquidity, availability and pricing of the depositary receipt. Changes in foreign currency exchange rates will affect the value of depositary receipts and, therefore, may affect the value of your investment in the Fund.

Derivatives Risk – Futures Contracts Risk. A futures contract is an exchange-traded derivative transaction between two parties in which a buyer (holding the “long” position) agrees to pay a fixed price (or rate) at a specified future date for delivery of an underlying reference from a seller (holding the “short” position). The seller hopes that the market price on the delivery date is less than the agreed upon price, while the buyer hopes for the contrary. Certain futures contract markets are highly volatile, and futures contracts may be illiquid. Futures exchanges may limit fluctuations in futures contract prices by imposing a maximum permissible daily price movement. The Fund may be disadvantaged if it is prohibited from executing a trade outside the daily permissible price movement. At or prior to maturity of a futures contract, the Fund may enter into an offsetting contract and may incur a loss to the extent there has been adverse movement in futures contract prices. The liquidity of the futures markets depends on participants entering into offsetting transactions rather than making or taking delivery. To the extent participants make or take delivery, liquidity in the futures market could be reduced. Because of the low margin deposits normally required in futures trading, it is possible that the Fund may employ a high degree of leverage in the portfolio. As a result, a relatively small price movement in a futures contract may result in substantial losses to the Fund, exceeding the amount of the margin paid. For certain types of futures contracts, losses are potentially unlimited. Futures markets are highly volatile and the use of futures may increase the volatility of the Fund’s NAV. Futures contracts executed (if any) on foreign exchanges may not provide the same protection as U.S. exchanges. Futures contracts can increase the Fund’s risk exposure to underlying references and their attendant risks, such as credit risk, market risk, foreign currency risk and interest rate risk, while also exposing the Fund to correlation risk, counterparty risk, hedging risk, leverage risk, liquidity risk, pricing risk and volatility risk.

Changing Distribution Level Risk. The Fund will normally receive income which may include interest, dividends and/or capital gains, depending upon its investments. The amount of the distributions the Fund pays will vary and generally depends on the amount of income the Fund earns (less expenses) on its portfolio holdings, and capital gains or losses it recognizes. A decline in the Fund's income or net capital gains from its investments may reduce its distribution level.

Rule 144A and Other Exempted Securities Risk. The Fund may invest in privately placed and other securities or instruments exempt from SEC registration (collectively “private placements”), subject to liquidity and other regulatory restrictions. In the U.S. market, private placements are typically sold only to qualified institutional buyers, or qualified purchasers, as applicable. An insufficient number of buyers interested in purchasing private placements at a particular time could adversely affect the marketability of such investments and the Fund might be unable to dispose of them promptly or at reasonable prices, subjecting the Fund to liquidity risk. The Fund may invest in private placements determined to be liquid as well as those determined to be illiquid. Even if determined to be liquid, the Fund’s holdings of private placements may increase the level of Fund illiquidity if eligible buyers are unable or unwilling to purchase them at a particular time. Issuers of Rule 144A eligible securities are required to furnish information to potential investors upon request. However, the required disclosure is much less extensive than that required of public companies and is not publicly available since the offering is not filed with the SEC. Further, issuers of Rule 144A eligible securities can require recipients of the offering information (such as the Fund) to agree contractually to keep the information confidential, which could also adversely affect the Fund’s ability to dispose of the security.

Select Portfolio Risk. Because the Fund may invest in a limited number of companies, the Fund as a whole is subject to greater risk of loss if any of its portfolio securities decline in price. In addition, the Fund’s holdings and weightings will diverge significantly from its primary benchmark’s holdings and weightings and the Fund may therefore experience greater risk and volatility relative to the benchmark. Because the Fund may invest in more than one company concentrated in a similar industry, sector or geographic region, the Fund may be even more concentrated than the number of companies it may hold would suggest.

Derivatives Risk – Swaps Risk. In a typical swap transaction, two parties agree to exchange the return earned on a specified underlying reference for a fixed return or the return from another underlying reference during a specified period of time. Swaps may be difficult to value and may be illiquid. Swaps could result in Fund losses if the underlying asset or reference does not perform as anticipated. Swaps create significant investment leverage such that a relatively small price movement in a swap may result in immediate and substantial losses to the Fund. The Fund may only close out a swap with its particular counterparty, and may only transfer a position with the consent of that counterparty. Certain swaps, such as short swap transactions and total return swaps, have the potential for unlimited losses, regardless of the size of the initial investment. Swaps can increase the Fund’s risk exposure to underlying references and their attendant risks, such as credit risk, market risk, foreign currency risk and interest rate risk, while also exposing the Fund to correlation risk, counterparty risk, hedging risk, inflation risk, leverage risk, liquidity risk, pricing risk and volatility risk.

Highly Leveraged Transactions Risk. The loans or other debt instruments in which the Fund invests may include highly leveraged transactions whereby the borrower assumes large amounts of debt in order to have the financial resources to attempt to achieve its business objectives. Loans or other debt instruments that are part of highly leveraged transactions involve a greater risk (including default and bankruptcy) than other investments.

Issuer Risk. An issuer in which the Fund invests or to which it has exposure may perform poorly, and the value of its securities may therefore decline, which would negatively affect the Fund's performance. Poor performance may be caused by poor management decisions, competitive pressures, breakthroughs in technology, reliance on suppliers, labor problems or shortages, corporate restructurings, fraudulent disclosures, natural disasters or other events, conditions or factors. The market capitalization of an issuer may also impact its risk profile. Investments in larger, more established companies may involve certain risks associated with their larger size. For instance, larger, more established companies may be less able to respond quickly to new competitive challenges, such as changes in consumer tastes or innovation from smaller competitors. Also, larger companies are sometimes less able to attain the high growth rates of successful smaller companies, especially during extended periods of economic expansion.

Counterparty Risk. Counterparty risk is the risk that a counterparty to a transaction in a financial instrument held by the Fund or by a special purpose or structured vehicle invested in by the Fund may become insolvent or otherwise fail to perform its obligations. As a result, the Fund may obtain no or limited recovery of its investment, and any recovery may be significantly delayed.

Exchange-Traded Fund (ETF) Risk. ETFs are subject to, among other risks, tracking risk and passive and, in some cases, active investment risk. An ETF's share price may not track its specified market index (if any) and may trade below its NAV. Certain ETFs use a "passive" investment strategy and do not take defensive positions in volatile or declining markets. Other ETFs are actively managed ETFs (i.e., they do not track a particular benchmark), which are subject to active management risk. An active secondary market in ETF shares may not develop or be maintained and may be halted or interrupted due to actions by its listing exchange, unusual market conditions or other reasons. There can be no assurance that an ETF's shares will continue to be listed on an active exchange. In addition, shareholders bear expenses incurred through ownership of the ETF. There is a risk that ETFs may terminate due to extraordinary events. For example, any of the service providers to ETFs, such as the trustee or sponsor, may close or otherwise fail to perform their obligations to the ETF, and the ETF may not be able to find a substitute service provider. Also, certain ETFs may be dependent upon licenses to use various indexes as a basis for determining their compositions and/or otherwise to use certain trade names. If these licenses are terminated, the ETFs may also terminate. In addition, an ETF may terminate if its net assets fall below a certain amount.

Liquidity Risk. Liquidity risk is the risk associated with any event, circumstance, or characteristic of an investment or market that negatively impacts the Fund's ability to sell, or realize the proceeds from the sale of, an investment at a desirable time or price. Liquidity risk may arise because of, for example, a lack of marketability of the investment, which means that when seeking to sell its portfolio investments, the Fund could find that selling is more difficult than anticipated, especially during times of high market volatility. Decreases in the number of financial institutions, including banks and broker-dealers, willing to make markets (match up sellers and buyers) in the Fund's investments or decreases in their capacity or willingness to trade such investments may increase the Fund's exposure to this risk. The debt market has experienced considerable growth, and financial institutions making markets in instruments purchased and sold by the Fund (e.g., bond dealers) have been subject to increased regulation. The impact of that growth and regulation on the ability and willingness of financial institutions to engage in trading or "making a market" in such instruments remains unsettled. Certain types of investments, such as lower-rated securities or those that are purchased and sold in over-the-counter markets, may be especially subject to liquidity risk. Securities or other assets in which the Fund invests may be traded in the over-the-counter market rather than on an exchange and therefore may be more difficult to purchase or sell at a fair price, which may have a negative impact on the Fund's performance. Market participants attempting to sell the same or a similar instrument at the same time as the Fund could exacerbate the Fund's exposure to liquidity risk. The Fund may have to accept a lower selling price for the holding, sell other liquid or more liquid investments that it might otherwise prefer to hold (thereby increasing the proportion of the Fund's investments in less liquid or illiquid securities), or forego another more appealing investment opportunity. Certain investments that were liquid when purchased by the Fund may later become illiquid, particularly in times of overall economic distress. Changing regulatory, market or other conditions or environments (for example, the interest rate or credit environments) may also adversely affect the liquidity and the price of the Fund's investments. Judgment plays a larger role in valuing illiquid or less liquid investments as compared to valuing liquid or more liquid investments. Price volatility may be higher for illiquid or less liquid investments as a result of, for example, the relatively less frequent pricing of such securities (as compared to liquid or more liquid investments). Generally, the less liquid the market at the time the Fund sells a portfolio investment, the greater the risk of loss or decline of value to the Fund. Overall market liquidity and other factors can lead to an increase in redemptions, which may negatively impact Fund performance and NAV, including, for example, if the Fund is forced to sell investments in a down market.

The Fund is subject indirectly to the following risks of Portfolio Funds seeking returns that correspond to a stated market index (the Index).

Active Management Risk. Due to its active management, the Fund could underperform its benchmark index and/or other funds with similar investment objectives and/or strategies.

Index Fund Risk. An index fund seeks to track the performance of the Index by using indexing strategies and, therefore, would not necessarily sell a security because the security's issuer was in financial trouble or defaulted, or whose credit rating was downgraded, unless that security was removed from the Index. The decision of whether to remove a security from an index is made by an independent index provider who is not affiliated with the Fund or the Investment Manager or its affiliates.

Index Methodology Risk. An index fund seeks performance that corresponds to the performance of the Index. There is no guarantee or assurance that the Index will achieve high, or even positive, returns. The Index may underperform more traditional indices. The Fund could lose value while other indices or measures of market performance increase in value or performance. In addition, the Fund may be subject to the risk that the index provider may not follow its stated methodology for construction of the Index and/or achieve the index provider's intended performance objective. Errors may result in a negative performance impact to the Fund and its shareholders.

Correlation/Tracking Error Risk. An index fund's value will generally decline when the performance of the Index declines. A number of factors may affect the Fund's ability to achieve a high degree of correlation with the Index, and there is no guarantee that the Fund will achieve a high degree of correlation. Failure to achieve such degree of correlation may prevent the Fund from achieving its investment objective. The factors that may adversely affect the Fund's correlation with the Index include the size of the Fund's portfolio, fees, expenses, transaction costs, income items, valuation methodology, accounting standards, the effectiveness of sampling techniques (if applicable), changes in the Index and disruptions or illiquidity in the markets for the securities in which the Fund invests. While the Fund typically attempts to track the performance of the Index by investing all, or substantially all, of its assets in the securities that make up the Index in approximately the same proportion as their weighting in the Index, at times, the Fund may not have investment exposure to all securities in the Index, or its weighting of investment exposure to securities may be different from that of the Index. In addition, the Fund may invest in securities not included in the Index. The Fund may take or refrain from taking investment positions for various reasons, such as tax efficiency purposes, or to comply with regulatory restrictions, which may negatively affect the Fund's correlation with the Index. The Fund may also be subject to large movements of assets into and out of the Fund, potentially resulting in the Fund being over- or under-exposed to certain securities comprising the Index and may be impacted by Index reconstitutions and Index rebalancing events. Holding cash balances may detract from the Fund's ability to track the Index. In addition, the Fund's NAV may deviate from the Index if the Fund fair values a portfolio security at a price other than the price used by the Index for that security. The Fund also bears management and other expenses and transaction costs in trading securities, which the Index does not bear. Accordingly, the Fund's performance will likely fail to match the performance of the Index, after taking expenses into account. Any of these factors could decrease correlation between the performance of the Fund and the Index and may hinder the Fund's ability to meet its investment objective. It is not possible to invest directly in an index.

New Fund Risk. Columbia Diversified Fixed Income Allocation ETF is a newly-formed Fund. Accordingly, investors in the Fund bear the risk that the Fund may not be successful in implementing its investment strategy of replicating the Index, which could result in the Fund being liquidated at any time without shareholder approval and/or at a time that may not be favorable for shareholders. Such a liquidation could have negative tax consequences for shareholders.

Performance Information

The following bar chart and table show you how the Fund has performed in the past, and can help you understand the risks of investing in the Fund. The bar chart shows how the Fund’s Class Inst share performance has varied for each full calendar year shown. The table below the bar chart compares the Fund’s returns for the periods shown with those of two broad-based indexes: the S&P 500^® Index, the Fund’s primary benchmark for equity securities, and the Bloomberg Barclays U.S. Aggregate Bond Index, the Fund’s primary benchmark for debt securities. The table below also compares the Fund's returns with a secondary, custom index, the Blended Benchmark. The S&P 500^® Index tracks the performance of 500 widely held, large-capitalization U.S. stocks. The Bloomberg Barclays U.S. Aggregate Bond Index is a market value-weighted index that tracks the daily price, coupon, pay-downs, and total return performance of fixed-rate, publicly placed, dollar-denominated, and non-convertible investment grade debt issues with at least $250 million par amount outstanding and with at least one year to final maturity.  The Blended Benchmark was established by the Investment Manager to show how the Fund’s performance compares to an equally weighted custom composite of the Fund’s primary equity and primary debt benchmarks, the S&P 500^® Index and the Bloomberg Barclays U.S. Aggregate Bond Index, respectively. The percentage of the Fund’s assets allocated to underlying stock and bond Portfolio Funds will vary, and accordingly the composition of the Fund’s portfolio will not always reflect the composition of the Blended Benchmark.

The performance of one or more share classes shown in the Average Annual Total Returns table below includes the Fund’s Class Inst share returns (adjusted to reflect the higher class-related operating expenses of such classes, where applicable) for periods prior to the indicated inception date of such share classes. Except for differences in fees and expenses, all share classes of the Fund would have substantially similar annual returns because all share classes of the Fund invest in the same portfolio of securities.

The after-tax returns shown in the Average Annual Total Returns table below are calculated using the highest historical individual U.S. federal marginal income tax rates in effect during the period indicated in the table and do not reflect the impact of state, local or foreign taxes. Your actual after-tax returns will depend on your personal tax situation and may differ from those shown in the table. In addition, the after-tax returns shown in the table do not apply to shares held in tax-advantaged accounts such as 401(k) plans or Individual Retirement Accounts (IRAs). The after-tax returns are shown only for Class Inst shares and will vary for other share classes.

The Fund’s performance prior to May 2018 reflects returns achieved following a principal investment strategy with a single form of allocation table. While the Fund now follows a principal investment strategy with two potential forms of allocation table, as described above in the Principal Investment Strategies section, the form of the Fund’s currently effective allocation table has been in place since the Fund’s inception in 2002. The Fund's performance prior to May 2018 reflects the current form of allocation table.

The Fund’s past performance (before and after taxes) is no guarantee of how the Fund will perform in the future. Updated performance information can be obtained by calling toll-free 800.345.6611 or visiting columbiathreadneedle.com/us.

Year by Year Total Return (%)
as of December 31 Each Year

Best and Worst Quarterly Returns
During the Period Shown in the Bar Chart

Best                                                     2nd Quarter 2009                                                      19.33%

Worst                                                   4th Quarter 2008                                                     -19.26%

Average Annual Total Returns After Applicable Sales Charges (for periods ended December 31, 2017)

Average Annual Total Returns - Columbia Thermostat Fund	Share Class Inception Date	1 Year	5 Years	10 Years
Class Inst	Sep. 25, 2002	5.52%	5.05%	5.12%
Class Inst | returns after taxes on distributions	Sep. 25, 2002	3.60%	3.48%	4.07%
Class Inst | returns after taxes on distributions and sale of Fund shares	Sep. 25, 2002	3.52%	3.39%	3.73%
Class A	Mar. 03, 2003	(0.86%)	3.55%	4.24%
Class Adv	Nov. 08, 2012	5.49%	5.04%	5.12%
Class C	Mar. 03, 2003	3.48%	4.00%	4.08%
Class Inst2	Nov. 08, 2012	5.51%	5.08%	5.13%
Class Inst3	Nov. 08, 2012	5.64%	5.13%	5.16%
Blended Benchmark (Secondary Benchmark; an equally weighted custom composite of the Fund's primary benchmarks for equity and debt securities, established by the Investment Manager; reflects no deductions for fees, expenses or taxes)		12.37%	8.88%	6.54%
S&P 500® Index (Primary Equity Benchmark; reflects no deductions for fees, expenses or taxes)		21.83%	15.79%	8.50%
Bloomberg Barclays U.S. Aggregate Bond Index (Primary Debt Benchmark; reflects no deductions for fees, expenses or taxes)		3.54%	2.10%	4.01%

[1] This charge is imposed on certain investments of between $1 million and $50 million redeemed within 18 months after purchase, as follows: 1.00% if redeemed within 12 months after purchase, and 0.50% if redeemed more than 12, but less than 18, months after purchase, with certain limited exceptions.

[2] This charge applies to redemptions within 12 months after purchase, with certain limited exceptions.

[1] Other expenses have been restated to reflect current transfer agency fees paid by the Fund.

[2] Acquired fund (Portfolio Fund) fees and expenses (expenses the Fund incurs indirectly through its investments in other funds) have been restated to reflect the fees and expenses of the Fund’s current Portfolio Funds.

[3] "Total annual Fund operating expenses" include acquired fund (Portfolio Fund) fees and expenses (expenses the Fund incurs indirectly through its investments in other funds) and may be higher than "Total Net Expenses" shown in the Financial Highlights section of this prospectus because "Total Net Expenses" do not include Portfolio Fund (acquired fund) fees and expenses.

[4] Columbia Wanger Asset Management, LLC (the Investment Manager) has contractually agreed to waive fees and reimburse certain expenses of the Fund, through April 30, 2019, so that ordinary operating expenses (excluding transaction costs and certain other investment-related expenses, interest and fees on borrowings and expenses associated with the Fund's investment in other investment companies, including the Portfolio Funds) do not exceed the annual rates of 0.50% for Class A shares, 0.25% for Class Adv shares, 1.25% for Class C shares, 0.25% for Class Inst shares, 0.20% for Class Inst2 shares and 0.15% for Class Inst3 shares. This arrangement may only be amended or terminated with approval from the Fund's Board of Trustees and the Investment Manager. The fee waivers and/or expense reimbursements shown in the table also reflect the contractual agreement of the Fund's transfer agent, Columbia Management Investment Services Corp. (the Transfer Agent), to waive a portion of its fees through June 30, 2019, such that the Fund's transfer agency fees do not exceed the annual rates of 0.05% of the average daily net assets of Class Inst2 shares and 0.00% of the average daily net assets of Class Inst3 shares.

[1] Year to Date return as of March 31, 2018: -0.21%

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

COLUMBIA ACORN TRUST

Prospectus Date

rr_ProspectusDate

May 01, 2018

Columbia Thermostat Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

SUMMARY OF THE FUND

Objective [Heading]

rr_ObjectiveHeading

Investment Objective

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

Columbia Thermostat Fund^SM (the Fund) seeks long-term capital appreciation.

Expense [Heading]

rr_ExpenseHeading

Fees and Expenses of the Fund

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. An investor transacting in a class of Fund shares without any front-end sales charge, contingent deferred sales charge, or other asset-based fee for sales or distribution may be required to pay a commission to the financial intermediary for effecting such transactions. Such commission rates are set by the financial intermediary and are not reflected in the tables or the example below. You may qualify for sales charge discounts if you and members of your immediate family invest, or agree to invest in the future, at least $50,000 in certain classes of shares of eligible funds distributed by Columbia Management Investment Distributors, Inc. (the Distributor). More information is available about these and other sales charge discounts and waivers from your financial intermediary, and can be found in the Choosing a Share Class section beginning on page 51 of the Fund’s prospectus, in Appendix A to the prospectus beginning on page A-1 and in Appendix S to the Statement of Additional Information (SAI) under Sales Charge Waivers beginning on page S-1.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

Shareholder Fees (fees paid directly from your investment)

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

April 30, 2019

June 30, 2019

Portfolio Turnover [Heading]

rr_PortfolioTurnoverHeading

Portfolio Turnover

Portfolio Turnover [Text Block]

rr_PortfolioTurnoverTextBlock

The Fund will indirectly bear the expenses associated with portfolio turnover of the underlying Portfolio Funds in which the Fund invests. Each Portfolio Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A Portfolio Fund’s higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when its shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 33% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

33.00%

Expense Breakpoint Discounts [Text]

rr_ExpenseBreakpointDiscounts

You may qualify for sales charge discounts if you and members of your immediate family invest, or agree to invest in the future, at least $50,000 in certain classes of shares of eligible funds distributed by Columbia Management Investment Distributors, Inc. (the Distributor). More information is available about these and other sales charge discounts and waivers from your financial intermediary, and can be found in the Choosing a Share Class section beginning on page 51 of the Fund’s prospectus, in Appendix A to the prospectus beginning on page A-1 and in Appendix S to the Statement of Additional Information (SAI) under Sales Charge Waivers beginning on page S-1.

Expense Breakpoint, Minimum Investment Required [Amount]

rr_ExpenseBreakpointMinimumInvestmentRequiredAmount

$ 50,000

Expenses Restated to Reflect Current [Text]

rr_ExpensesRestatedToReflectCurrent

Other expenses have been restated to reflect current transfer agency fees paid by the Fund.

Acquired fund (Portfolio Fund) fees and expenses (expenses the Fund incurs indirectly through its investments in other funds) have been restated to reflect the fees and expenses of the Fund’s current Portfolio Funds.

Expenses Not Correlated to Ratio Due to Acquired Fund Fees [Text]

rr_ExpensesNotCorrelatedToRatioDueToAcquiredFundFees

"Total annual Fund operating expenses" include acquired fund (Portfolio Fund) fees and expenses (expenses the Fund incurs indirectly through its investments in other funds) and may be higher than "Total Net Expenses" shown in the Financial Highlights section of this prospectus because "Total Net Expenses" do not include Portfolio Fund (acquired fund) fees and expenses.

Expense Example [Heading]

rr_ExpenseExampleHeading

Example

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

The following example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example illustrates the hypothetical expenses that you would incur over the time periods indicated, and assumes that:

• you invest $10,000 in the applicable class of Fund shares for the periods indicated,

• your investment has a 5% return each year, and

• the Fund’s total annual operating expenses remain the same as shown in the Annual Fund Operating Expenses table above.

Since the waivers and/or reimbursements shown in the Annual Fund Operating Expenses table above expire as indicated in the preceding table, they are only reflected in the 1 year example and the first year of the other examples. Although your actual costs may be higher or lower, based on the assumptions listed above, your costs would be:

Strategy [Heading]

rr_StrategyHeading

Principal Investment Strategies

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

The Fund is primarily managed as a fund that invests in other funds (i.e., a "fund-of-funds") that seeks to achieve its investment objective by investing its assets among a selected group of underlying stock and bond mutual funds and exchanged-traded funds (ETFs) for which Columbia Wanger Asset Management, LLC, the Fund's investment adviser (the Investment Manager) or its affiliates, including Columbia Management Investment Advisers, LLC (Columbia Management), serves as investment adviser or principal underwriter (the Portfolio Funds). Under normal circumstances, the Fund allocates at least 95% of its net assets (stock/bond assets) among the Portfolio Funds according to an asset allocation table based on the current level of Standard & Poor's (S&P) 500^® Index.

Generally, the Fund's allocation to stock funds increases as the S&P 500^® Index declines and decreases as the S&P 500^® Index rises. When the S&P 500^® Index goes up in relation to trading range bands that are predetermined by the Investment Manager, the Fund sells a portion of its stock Portfolio Funds and invests more in the bond Portfolio Funds, and when the S&P 500^® Index goes down in relation to the predetermined bands, the Fund increases its investment in the stock Portfolio Funds. Under normal circumstances, the Fund may invest up to 5% of net assets plus any cash received that day in cash, high quality short-term paper and government securities.

Although many asset allocation funds follow a basic approach of moving assets from stocks to bonds when the equity market goes up, and from bonds to stocks when the equity market goes down, some are run by investment managers who allocate fund assets by making subjective decisions based on complicated economic and financial models and complex graphs of market behavior. By contrast, the day-to-day investment decisions for the Fund are made according to a single predetermined allocation table set forth below. The temperature in your house is run by a single rule: your thermostat turns on the furnace if your house is too cold or turns on the air conditioner if your house is too warm. This Fund works the same way, so it is named Columbia Thermostat Fund.

Just as a thermostat may be set at different ranges for different seasons, the structure and allocation ranges of the Fund's asset allocation table may be changed from time to time between two forms, based on the Investment Manager's determination of whether it expects the equity market to be moving over the next year in a side-ways or non-directional pattern, which the Investment Manager terms an "expensive" market, or to be trending upward, which the Investment Manager terms a "normal" market. The Investment Manager's process and methodology for determining whether the current market is "expensive" or "normal" and the structure of each form of allocation table are described in more detail below under Determination of Current Market State. Such determination will be made on at least an annual basis and will be reflected in the asset allocation table disclosed in this prospectus. In general, the two different forms of allocation table are intended to maximize the capture of value under the two different sets of market conditions.

The Fund's current asset allocation table, which is set forth below, reflects the Investment Manager's determination that the equity market is currently "expensive." The structure of the Fund's current allocation table reflects the same form that has been in place since the Fund's inception in 2002.

Stock/Bond Allocation Table
	How the Fund will Invest the Stock/Bond Assets
Level of the S&P 500® Index	Stock Percentage	Bond Percentage
over 2890	10%	90%
over 2779 – 2890	15%	85%
over 2672 – 2779	20%	80%
over 2569 – 2672	25%	75%
over 2470 – 2569	30%	70%
over 2375 – 2470	35%	65%
over 2284 – 2375	40%	60%
over 2196 – 2284	45%	55%
over 2110 – 2196	50%	50%
over 2026 – 2110	55%	45%
over 1945 – 2026	60%	40%
over 1867 – 1945	65%	35%
over 1792 – 1867	70%	30%
over 1720 – 1792	75%	25%
over 1651 – 1720	80%	20%
over 1585 – 1651	85%	15%
1585 and under	90%	10%

When the S&P 500^® Index moves into a new band on the table, the Fund will rebalance the stock/bond mix to reflect the new S&P 500^® Index price level by redeeming shares of some Portfolio Funds and purchasing shares of other Portfolio Funds. Any such rebalancing typically will be implemented promptly. However, there are two circumstances when a rebalancing may be implemented over a longer timeline. First, when a rebalancing or allocation table change would trigger a 10% point or greater change in stock and bond allocations or individual Portfolio Funds, the rebalancing may be implemented over a period of up to two weeks, if deemed by the Investment Manager to be in the best interest of shareholders. The second exception is a "31-day Rule;" in order to reduce taxable events and minimize short-term trading if the S&P 500^® Index price moves back and forth across a band in the allocation table, after the Fund has increased its percentage allocation to either stock funds or bond funds, it will not decrease that allocation for at least 31 days. Following a change in the Fund's stock/bond mix, if the S&P 500^® Index remains within the same band for a while, normal market fluctuations will change the values of the Fund's holdings of stock Portfolio Funds and bond Portfolio Funds. The Investment Manager will invest cash flows from sales (or redemptions) of Fund shares to bring the stock/bond mix back toward the allocation percentages for that S&P 500^® Index band. For example, if the S&P 500^® Index is in the 1792 band, and the value of the holdings of the stock Portfolio Funds has dropped to 68% of the value of the holdings of all Portfolio Funds, the Investment Manager would invest new cash in the stock Portfolio Funds (or cash for redemptions would come from the bond Portfolio Funds). If the 31-day Rule is in effect, the Investment Manager will invest new cash at the stock/bond percentage allocation as of the latest rebalancing.

As an illustrative example, suppose the following:

Date	Level of the S&P 500® Index	How the Fund will invest the Stock/ Bond Assets
Nov. 1	We begin when the market is 2176	50% stocks, 50% bonds
Dec. 1	The S&P 500® goes to 2198	rebalance 45% stocks, 55% bonds
Dec. 6	The S&P 500® drops back to 2186	no reversal for 31 days
Jan. 2	The S&P 500® is at 2150	rebalance 50% stocks, 50% bonds
Jan. 20	The S&P 500® drops to 2056	rebalance 55% stocks, 45% bonds
	The market has made a continuation move by going through a second action level, not a reversal move, so the 31-day Rule does not apply in this case.
Jan. 30	The S&P 500® goes up to 2115	no reversal for 31 days
Feb. 20	The S&P 500® is at 2123	rebalance 50% stocks, 50% bonds

The Investment Manager chooses the Portfolio Funds to be generally consistent with the composition of the Fund's primary stock and bond benchmarks and to allow the Fund to participate in strategies the Investment Manager believes can provide additional return due to active management.

The following table shows the stock and bond Portfolio Funds that the Fund currently uses in its "fund-of-funds" structure and the current target percentage for each Portfolio Fund within the stock or bond asset class. As described more fully below, the Investment Manager may substitute or add additional Portfolio Funds at any time, including funds introduced after the date of this prospectus. The target percentage within each asset class category is achieved by rebalancing the investments within the asset class whenever the S&P 500^® Index moves into a new band on the allocation table, subject to the 31-day Rule described above. The Fund will not liquidate its investment in one Portfolio Fund in order to invest in another Portfolio Fund except in connection with a rebalancing due to a move of the S&P 500^® Index into a new band (or due to a change by the Investment Manager in the Portfolio Funds or to the relative target percentages among them). Until a subsequent rebalancing, the Fund's cash flows are invested in, or redeemed from, the Portfolio Funds in a manner that will reduce any deviation of the relative values of the Fund's holdings of the Portfolio Funds from the percentages shown below.

Allocation of Stock/Bond Asset Within Asset Classes
	Type of Fund	Allocation
Stock Funds
Columbia Acorn® Fund	Small/Mid-cap growth	10%
Columbia Acorn International®	Small/Mid-cap international growth	10%
Columbia Acorn SelectSM	Mid-cap growth	10%
Columbia Contrarian Core Fund	Large-cap blend	10%
Columbia Dividend Income Fund	Large-cap value	10%
Columbia Large Cap Enhanced Core Fund	Large-cap blend	10%
Columbia Large Cap Index Fund	Large-cap blend	40%
Total		100%
Bond Funds	Type of Fund	Allocation
Columbia Corporate Income Fund	Corporate bond	10%
Columbia Diversified Fixed Income Allocation ETF	Beta advantage multi-sector bond	10%
Columbia Quality Income Fund	Government bond	20%
Columbia Short Term Bond Fund	Short term bond	25%
Columbia U.S. Treasury Index Fund	U.S. Treasury notes/bonds	35%
Total		100%

See the Portfolio Funds Summary section of this prospectus for information about the Portfolio Funds' investment objectives and principal investment strategies. Each of the Portfolio Funds is managed by the Investment Manager or its affiliates. The Fund does not pay any sales load on its purchases of shares of the Portfolio Funds.

The Investment Manager has the authority to review the Portfolio Funds and to change the relative percentages among them or to add or eliminate funds. Such review will occur on at least an annual basis and will be reflected in the Portfolio Funds table disclosed in this prospectus.

Determination of Market State

Just as a thermostat may be set at different ranges for different seasons, the Fund’s allocation of assets between stock and bond funds is expected to be set at different range bands depending principally on the Investment Manager’s view of how expensive the equity market is compared to historical levels.

At least annually, the Investment Manager will conduct a review of the current equity market price levels compared to historical price levels, and will determine if the equity market is in an “expensive” state, or a “normal” state. This review will begin with a comparison of the S&P 500^® Index’s cyclically adjusted price-to-earnings ratio for the prior seven years against a 40-year history, but the Investment Manager will also take into account additional quantitative or qualitative market factors, including for example, trends in equity price measures and comparisons of equity prices against prices for other asset classes. Based on its determination of the current market state, the Investment Manager will select the form of allocation table to be effective in the Fund’s current prospectus. Also as part of this review, the Investment Manager will update the median historical information used in the table, and the resulting S&P 500^® Index levels used to create the bands around the median, regardless of the market state.

As noted below, the form of table used for an “expensive” market is expected to be different from that used for a “normal” market.

Generally, the Investment Manager will determine the current state of the equity market to be “expensive” when its analysis of the S&P 500^® Index’s cyclically adjusted price-to-earnings ratio for the prior seven years is within the top quartile of ratio observations over a 40-year history. The “expensive” equity market form of allocation table for the Fund is structured around an implied S&P 500^® Index trading range median of 50% stocks/50% bonds (the 50/50 Band), with allocations to bonds decreasing through stratified range bands below the 50/50 Band and allocations to bonds increasing through stratified range bands above the 50/50 Band, as shown by the following example:

“Expensive” Equity Market Form of Stock/Bond Allocation Table
Level of the S&P 500® Index	Stock Percentage	Bond Percentage
above top of trading range below	10%	90%
4% trading range above top of range below	15%	85%
4% trading range above top of range below	20%	80%
4% trading range above top of range below	25%	75%
4% trading range above top of range below	30%	70%
4% trading range above top of range below	35%	65%
4% trading range above top of range below	40%	60%
4% trading range above top of range below	45%	55%
4% trading range centered on implied median (50/50 Band)*	50%	50%
4% trading range below bottom of range above	55%	45%
4% trading range below bottom of range above	60%	40%
4% trading range below bottom of range above	65%	35%
4% trading range below bottom of range above	70%	30%
4% trading range below bottom of range above	75%	25%
4% trading range below bottom of range above	80%	20%
4% trading range below bottom of range above	85%	15%
below bottom of trading range above	90%	10%

*Implied median price level as calculated by the Investment Manager.

The “expensive” form of table has been in place since the Fund’s inception in 2002.

Generally, the Investment Manager will determine the current state of the equity market to be “normal” when its analysis of the S&P 500^® Index’s cyclically adjusted price-to-earnings ratio for the prior seven years is outside the top quartile of ratio observations over a 40-year history. The “normal” equity market form allocation table for the Fund is also structured around the 50/50 Band, with allocations to bonds decreasing through stratified range bands below the 50/50 Band, and allocations to stocks and bonds set at 50% stocks/50% bonds for all trading levels above the 50/50 Band, as shown by the following example:

“Normal” Equity Market Form of Stock/Bond Allocation Table
Level of the S&P 500® Index	Stock Percentage	Bond Percentage
above trading range that is 2% below implied median (50/50 Band)*	50%	50%
4% trading range below bottom of range above	55%	45%
4% trading range below bottom of range above	60%	40%
4% trading range below bottom of range above	65%	35%
4% trading range below bottom of range above	70%	30%
4% trading range below bottom of range above	75%	25%
4% trading range below bottom of range above	80%	20%
4% trading range below bottom of range above	85%	15%
below bottom of trading range above	90%	10%

*Implied median price level as calculated by the Investment Manager.

The Investment Manager will conduct the market state review at least annually prior to the annual updating of this prospectus, and may, in its discretion, assess the market state on an "emergency" basis and make any changes deemed necessary to reflect, for example, a very significant move in market levels or a structural change affecting the markets.

Any such "emergency" changes by the Investment Manager, which are expected to be infrequent, would be disclosed in this prospectus.

Risk [Heading]

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Principal Risks

Risk Narrative [Text Block]

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An investment in the Fund involves risks, including specific risks relating to the investment in the Fund based on its investment process and its "fund-of-funds" structure, as well as specific risks related to the individual Portfolio Funds in which it invests that in the aggregate are principal risks to the Fund, including among others, those described below. More information about the Portfolio Funds, including their principal risks, is available in their prospectuses. This prospectus is not an offer for any of the Portfolio Funds. There is no assurance that the Fund will achieve its investment objective and you may lose money. The value of the Fund's holdings may decline, and the Fund's net asset value (NAV) and share price may go down. An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.

Allocation Risk. The Investment Manager uses an asset allocation strategy in pursuit of its investment objective, there is a risk that the Fund's allocation among asset classes or investments will cause the Fund's shares to lose value or cause the Fund to underperform other funds with similar investment objectives and/or strategies, or that the investments themselves will not produce the returns expected.

Fund-of-Funds Risk. Determinations regarding asset classes or underlying funds and the Fund’s allocations thereto may not successfully achieve the Fund’s investment objective, in whole or in part. The ability of the Fund to realize its investment objective will depend, in large part, on the extent to which the underlying funds realize their investment objective. There is no guarantee that the underlying funds will achieve their respective investment objectives. The Fund is exposed to the same risks as the underlying funds in direct proportion to the allocation of its assets among the underlying funds. By investing in a combination of underlying funds, the Fund has exposure to the risks of many areas of the market. The performance of underlying funds could be adversely affected if other entities that invest in the same underlying funds make relatively large investments or redemptions in such underlying funds. The Fund, and its shareholders, indirectly bear a portion of the expenses of any funds in which the Fund invests. Because the expenses and costs of each underlying fund are shared by its investors, redemptions by other investors in an underlying fund could result in decreased economies of scale and increased operating expenses for such fund. The Investment Manager may have potential conflicts of interest in selecting affiliated funds over unaffiliated funds for investment by the Fund, and may also face potential conflicts of interest in selecting affiliated funds, because the fees the Investment Manager receives from some underlying funds may be higher than the fees paid by other underlying funds. Further, because of the Investment Manager’s confidence in its own strategies, investment philosophy and capacities, it will, in selecting underlying funds, at times prefer a fund in the Columbia Acorn Family of Funds over alternative investments. There can be no assurance, however, that a fund in the Columbia Acorn Family of Funds selected for inclusion in Columbia Thermostat Fund’s portfolio will, in fact, outperform similar funds managed by the Investment Manager’s affiliates.

The Fund is subject indirectly to the risks of the Portfolio Funds described below. Please see the Portfolio Funds Summary for information about which risks apply to which Portfolio Funds.

Market Risk. Market risk refers to the possibility that the market values of securities or other investments that the Fund holds will fall, sometimes rapidly or unpredictably, or fail to rise. An investment in the Fund could lose money over short or long periods.

Interest Rate Risk. Interest rate risk is the risk of losses attributable to changes in interest rates. In general, if prevailing interest rates rise, the values of loans and other debt instruments tend to fall, and if interest rates fall, the values of loans and other debt instruments tend to rise. Changes in the value of a debt instrument usually will not affect the amount of income the Fund receives from it but will generally affect the value of your investment in the Fund. Changes in interest rates may also affect the liquidity of the Fund's investments in debt instruments. In general, the longer the maturity or duration of a debt instrument, the greater its sensitivity to changes in interest rates. Interest rate declines also may increase prepayments of debt obligations, which, in turn, would increase prepayment risk. Similarly, a period of rising interest rates may negatively impact the Fund's performance. Actions by governments and central banking authorities can result in increases in interest rates. Such actions may negatively affect the value of debt instruments held by the Fund, resulting in a negative impact on the Fund's performance and NAV. Securities with floating interest rates are typically less sensitive to interest rate changes, but may decline in value if their interest rates do not rise as much as interest rates in general. Because rates on certain floating rate loans and other debt securities reset only periodically, changes in prevailing interest rates (and particularly sudden and significant changes) can be expected to cause fluctuations in the Fund's NAV. Any interest rate increases could cause the value of the Fund's investments in debt instruments to decrease. Rising interest rates may prompt redemptions from the Fund, which may force the Fund to sell investments at a time when it is not advantageous to do so, which could result in losses.

Credit Risk. Credit risk is the risk that the value of loans or other debt instruments may decline if the issuer thereof defaults or otherwise becomes unable or unwilling, or is perceived to be unable or unwilling, to honor its financial obligations, such as making payments to the Fund when due. Rating agencies assign credit ratings to certain debt instruments to indicate their credit risk. Unless otherwise provided in the Fund's Principal Investment Strategies, investment grade debt instruments are those rated at or above BBB- by Standard and Poor's Ratings Services, or equivalently rated by Moody's Investors Service, Inc. or Fitch, Inc., or, if unrated, determined by the management team to be of comparable quality. Conversely, below investment grade (commonly called "high-yield” or "junk”) debt instruments are those rated below BBB- by Standard and Poor's Ratings Services, or equivalently rated by Moody's Investors Service, Inc. or Fitch, Inc., or, if unrated, determined by the management team to be of comparable quality. A rating downgrade by such agencies can negatively impact the value of such instruments. Lower quality or unrated instruments held by the Fund may present increased credit risk as compared to higher-rated instruments. Non-investment grade debt instruments may be subject to greater price fluctuations and are more likely to experience a default than investment grade debt instruments and therefore may expose the Fund to increased credit risk. If the Fund purchases unrated instruments, or if the ratings of instruments held by the Fund are lowered after purchase, the Fund will depend on analysis of credit risk more heavily than usual.

High-Yield Investments Risk. Securities and other debt instruments held by the Fund that are rated below investment grade (commonly called “high-yield” or “junk” bonds) and unrated debt instruments of comparable quality expose the Fund to a greater risk of loss of principal and income than a fund that invests solely or primarily in investment grade debt instruments. In addition, these investments have greater price fluctuations, are less liquid and are more likely to experience a default than higher-rated debt instruments. High-yield debt instruments are considered to be predominantly speculative with respect to the issuer’s capacity to pay interest and repay principal.

Value Securities Risk. Value securities are securities of companies that may have experienced, for example, adverse business, industry or other developments or may be subject to special risks that have caused the securities to be out of favor and, in turn, potentially undervalued. The market value of a portfolio security may not meet the Investment Manager’s perceived value assessment of that security, or may decline in price, even though the Investment Manager believes the securities are already undervalued. There is also a risk that it may take longer than expected for the value of these investments to rise to the portfolio manager’s perceived value. In addition, value securities, at times, may not perform as well as growth securities or the stock market in general, and may be out of favor with investors for varying periods of time.

Growth Securities Risk. Growth securities typically trade at a higher multiple of earnings than other types of equity securities. Accordingly, the market values of growth securities may never reach their expected market value and may decline in price. In addition, growth securities, at times, may not perform as well as value securities or the stock market in general, and may be out of favor with investors for varying periods of time.

Sector Risk. At times, the Fund may have a significant portion of its assets invested in securities of companies conducting business in a related group of industries within a sector. Companies in the same sector may be similarly affected by economic, regulatory, political or market events or conditions, which may make the Fund more vulnerable to unfavorable developments in that sector than funds that invest more broadly. Generally, the more the Fund diversifies its investments, the more it spreads risk and potentially reduces the risks of loss and volatility.

Foreign Securities Risk. Investments in or exposure to foreign securities involve certain risks not associated with investments in or exposure to securities of U.S. companies. Foreign securities subject the Fund to the risks associated with investing in the particular country of an issuer, including political, regulatory, economic, social, diplomatic and other conditions or events (including, for example, military confrontations, war and terrorism), occurring in the country or region, as well as risks associated with less developed custody and settlement practices. Foreign securities may be more volatile and less liquid than securities of U.S. companies, and are subject to the risks associated with potential imposition of economic and other sanctions against a particular foreign country, its nationals or industries or businesses within the country. In addition, foreign governments may impose withholding or other taxes on the Fund’s income, capital gains or proceeds from the disposition of foreign securities, which could reduce the Fund’s return on such securities.

Operational and Settlement Risks of Foreign Securities. The Fund’s foreign securities are generally held outside the United States in the primary market for the securities in the custody of foreign sub-custodians. Some countries have limited governmental oversight and regulation, which increases the risk of corruption and fraud and the possibility of losses to the Fund. In particular, under certain circumstances, foreign securities may settle on a delayed delivery basis, meaning that the Fund may be required to make payment for securities before the Fund has actually received delivery of the securities or deliver securities prior to the receipt of payment. As a result, there is a risk that the security will not be delivered to the Fund or that payment will not be received. Losses can also result from lost, stolen or counterfeit securities; defaults by brokers and banks; failures or defects of the settlement system; or poor and improper record keeping by registrars and issuers.

Share Blocking. In certain non-U.S. markets, an issuer’s securities are blocked from trading for a specified number of days before and, in certain instances, after a shareholder meeting. The blocking period can last up to several weeks. Share blocking may prevent the Fund from buying or selling securities during this period. As a consequence of these restrictions, the Investment Manager, on behalf of the Fund, may abstain from voting proxies in markets that require share blocking.

Emerging Market Securities Risk. Securities issued by foreign governments or companies in emerging market countries, such as China, Russia and certain countries in Eastern Europe, the Middle East, Asia, Latin America or Africa, are more likely to have greater exposure to the risks of investing in foreign securities that are described in Foreign Securities Risk. In addition, emerging market countries are more likely to experience instability resulting, for example, from rapid changes or developments in social, political, economic or other conditions. Their economies are usually less mature and their securities markets are typically less developed with more limited trading activity (i.e., lower trading volumes and less liquidity) than more developed countries. Emerging market securities tend to be more volatile than securities in more developed markets. Many emerging market countries are heavily dependent on international trade and have fewer trading partners, which makes them more sensitive to world commodity prices and economic downturns in other countries, and some have a higher risk of currency devaluations.

Operational and Settlement Risks of Securities in Emerging Markets. Foreign sub-custodians in emerging markets may be recently organized, lack extensive operating experience or lack effective government oversight or regulation. In addition, there may be legal restrictions or limitations on the ability of the Fund to recover assets held in custody by a foreign sub-custodian in the event of the bankruptcy of the sub-custodian. There may also be a greater risk that settlement may be delayed and that cash or securities of the Fund may be lost because of failures of or defects in the system, including fraud or corruption. Settlement systems in emerging markets also have a higher risk of failed trades.

Risks Related to Currencies and Corporate Actions in Emerging Markets. Risks related to currencies and corporate actions are also greater in emerging market countries than in developed countries. Emerging market currencies may not have an active trading market and are subject to a higher risk of currency devaluations.

Risks Related to Corporate and Securities Laws in Emerging Markets. Securities laws in emerging markets may be relatively new and unsettled and, consequently, there is a risk of rapid and unpredictable change in laws regarding foreign investment, securities regulation, title to securities and shareholder rights.

Liquidity and Trading Volume Risk. Due to market conditions, including uncertainty regarding the price of a security, it may be difficult for the Fund to buy or sell portfolio securities at a desirable time or price, which could result in investment losses. This risk of portfolio illiquidity is heightened with respect to small- and mid-cap securities, generally, and foreign small- and mid-cap securities in particular. The Fund may have to lower the selling price, liquidate other investments, or forego another, more appealing investment opportunity as a result of illiquidity in the markets. As a result of significant and sustained reductions in emerging and developed international market trading volumes in the wake of the 2007-2009 financial crisis, it may take longer to buy or sell securities, which can exacerbate the Fund’s exposure to volatile markets. The Fund may also be limited in its ability to execute favorable trades in portfolio securities in response to changes in share prices and fundamentals, and may be forced to dispose of securities under disadvantageous circumstances and at a loss. As the Fund grows in size or, conversely, if it faces significant redemption pressure, these considerations take on increasing significance and may adversely impact performance.

Foreign Currency Risk. The performance of the Fund may be materially affected positively or negatively by foreign currency strength or weakness relative to the U.S. dollar, particularly if the Fund invests a significant percentage of its assets in foreign securities or other assets denominated in currencies other than the U.S. dollar.

Small- and Mid-Cap Company Securities Risk. Investments in small- and mid-capitalization companies (small- and mid-cap companies) often involve greater risks than investments in larger, more established companies (larger companies) because small- and mid-cap companies tend to have less predictable earnings and may lack the management experience, financial resources, product diversification and competitive strengths of larger companies. Securities of small- and mid-cap companies may be less liquid and more volatile than the securities of larger companies.

U.S. Government Obligations Risk. While U.S. Treasury obligations are backed by the “full faith and credit” of the U.S. Government, such securities are nonetheless subject to credit risk (i.e., the risk that the U.S. Government may be, or be perceived to be, unable or unwilling to honor its financial obligations, such as making payments). Securities issued or guaranteed by federal agencies or authorities and U.S. Government-sponsored instrumentalities or enterprises may or may not be backed by the full faith and credit of the U.S. Government.

Derivatives Risk. Derivatives may involve significant risks. Derivatives are financial instruments with a value in relation to, or derived from, the value of an underlying asset(s) or other reference, such as an index, rate or other economic indicator (each an underlying reference). Derivatives may include those that are privately placed or otherwise exempt from SEC registration, including certain Rule 144A eligible securities. Derivatives could result in Fund losses if the underlying reference does not perform as anticipated. Use of derivatives is a highly specialized activity that can involve investment techniques, risks, and tax planning different from those associated with more traditional investment instruments. The Fund’s derivatives strategy may not be successful and use of certain derivatives could result in substantial, potentially unlimited, losses to the Fund regardless of the Fund’s actual investment. A relatively small movement in the price, rate or other economic indicator associated with the underlying reference may result in substantial loss for the Fund. Derivatives may be more volatile than other types of investments. The value of derivatives may be influenced by a variety of factors, including national and international political and economic developments. Potential changes to the regulation of the derivatives markets may make derivatives more costly, may limit the market for derivatives, or may otherwise adversely affect the value or performance of derivatives. Derivatives can increase the Fund’s risk exposure to underlying references and their attendant risks, such as credit risk, market risk, foreign currency risk and interest rate risk, while exposing the Fund to correlation risk, counterparty risk, hedging risk, leverage risk, liquidity risk, pricing risk and volatility risk.

Convertible Securities Risk. Convertible securities are subject to the usual risks associated with debt instruments, such as interest rate risk and credit risk. Convertible securities also react to changes in the value of the common stock into which they convert, and are thus subject to market risk. The Fund may also be forced to convert a convertible security at an inopportune time, which may decrease the Fund’s return.

Forward Commitments on Mortgage-Backed Securities (including Dollar Rolls) Risk. When purchasing mortgage-backed securities in the “to be announced” (TBA) market (MBS TBAs), the seller agrees to deliver mortgage-backed securities for an agreed upon price on an agreed upon date, but may make no guarantee as to the specific securities to be delivered. In lieu of taking delivery of mortgage-backed securities, the Fund could enter into dollar rolls, which are transactions in which the Fund sells securities to a counterparty and simultaneously agrees to purchase those or similar securities in the future at a predetermined price. Dollar rolls involve the risk that the market value of the securities the Fund is obligated to repurchase may decline below the repurchase price, or that the counterparty may default on its obligations. These transactions may also increase the Fund’s portfolio turnover rate. If the Fund reinvests the proceeds of the security sold, the Fund will also be subject to the risk that the investments purchased with such proceeds will decline in value (a form of leverage risk). MBS TBAs and dollar rolls are subject to the risk that the counterparty to the transaction may not perform or be unable to perform in accordance with the terms of the instrument.

Mortgage- and Other Asset-Backed Securities Risk. The value of any mortgage-backed and other asset-backed securities held by the Fund may be affected by, among other things, changes or perceived changes in: interest rates; factors concerning the interests in and structure of the issuer or the originator of the mortgages or other assets; the creditworthiness of the entities that provide any supporting letters of credit, surety bonds or other credit enhancements; or the market's assessment of the quality of underlying assets. Payment of principal and interest on some mortgage-backed securities (but not the market value of the securities themselves) may be guaranteed by the full faith and credit of a particular U.S. Government agency, authority, enterprise or instrumentality, and some, but not all, are also insured or guaranteed by the U.S. Government. Mortgage-backed securities issued by non-governmental issuers (such as commercial banks, savings and loan institutions, private mortgage insurance companies, mortgage bankers and other secondary market issuers) may entail greater risk than obligations guaranteed by the U.S. Government. Mortgage- and other asset-backed securities are subject to prepayment risk, which is the possibility that the underlying mortgage or other asset may be refinanced or prepaid prior to maturity during periods of declining or low interest rates, causing the Fund to have to reinvest the money received in securities that have lower yields. Rising or high interest rates tend to extend the duration of mortgage- and other asset-backed securities, making their prices more volatile and more sensitive to changes in interest rates.

Stripped Securities Risk. Stripped securities are the separate income or principal components of debt securities. These securities are particularly sensitive to changes in interest rates, and therefore subject to greater fluctuations in price than typical interest bearing debt securities.

Prepayment and Extension Risk. Prepayment and extension risk is the risk that a bond or other security or investment might, in the case of prepayment risk, be called or otherwise converted, prepaid or redeemed before maturity and, in the case of extension risk, that the investment might not be called as expected. In the case of prepayment risk, if the investment is converted, prepaid or redeemed before maturity, the portfolio manager may not be able to invest the proceeds in other investments providing as high a level of income, resulting in a reduced yield to the Fund. In the case of mortgage- or asset-backed securities, as interest rates decrease or spreads narrow, the likelihood of prepayment increases. Conversely, extension risk is the risk that an unexpected rise in interest rates will extend the life of a mortgage- or asset-backed security beyond the prepayment time. If the Fund’s investments are locked in at a lower interest rate for a longer period of time, the portfolio manager may be unable to capitalize on securities with higher interest rates or wider spreads.

Reinvestment Risk. Reinvestment risk is the risk that the Fund will not be able to reinvest income or principal at the same return it is currently earning.

Depositary Receipts Risk. Depositary receipts are receipts issued by a bank or trust company reflecting ownership of underlying securities issued by foreign companies. Some foreign securities are traded in the form of American Depositary Receipts and/or Global Depositary Receipts. Depositary receipts involve risks similar to the risks associated with investments in foreign securities, including those associated with investing in the particular country of an issuer, which may be related to the particular political, regulatory, economic, social and other conditions or events, including, for example, military confrontations, war and terrorism, occurring in the country and fluctuations in such country's currency, as well as market risk tied to the underlying foreign company. In addition, holders of depositary receipts may have limited voting rights, may not have the same rights afforded to stockholders of a typical domestic company in the event of a corporate action, such as an acquisition, merger or rights offering, and may experience difficulty in receiving company stockholder communications. There is no guarantee that a financial institution will continue to sponsor a depositary receipt, or that a depositary receipt will continue to trade on an exchange, either of which could adversely affect the liquidity, availability and pricing of the depositary receipt. Changes in foreign currency exchange rates will affect the value of depositary receipts and, therefore, may affect the value of your investment in the Fund.

Derivatives Risk – Futures Contracts Risk. A futures contract is an exchange-traded derivative transaction between two parties in which a buyer (holding the “long” position) agrees to pay a fixed price (or rate) at a specified future date for delivery of an underlying reference from a seller (holding the “short” position). The seller hopes that the market price on the delivery date is less than the agreed upon price, while the buyer hopes for the contrary. Certain futures contract markets are highly volatile, and futures contracts may be illiquid. Futures exchanges may limit fluctuations in futures contract prices by imposing a maximum permissible daily price movement. The Fund may be disadvantaged if it is prohibited from executing a trade outside the daily permissible price movement. At or prior to maturity of a futures contract, the Fund may enter into an offsetting contract and may incur a loss to the extent there has been adverse movement in futures contract prices. The liquidity of the futures markets depends on participants entering into offsetting transactions rather than making or taking delivery. To the extent participants make or take delivery, liquidity in the futures market could be reduced. Because of the low margin deposits normally required in futures trading, it is possible that the Fund may employ a high degree of leverage in the portfolio. As a result, a relatively small price movement in a futures contract may result in substantial losses to the Fund, exceeding the amount of the margin paid. For certain types of futures contracts, losses are potentially unlimited. Futures markets are highly volatile and the use of futures may increase the volatility of the Fund’s NAV. Futures contracts executed (if any) on foreign exchanges may not provide the same protection as U.S. exchanges. Futures contracts can increase the Fund’s risk exposure to underlying references and their attendant risks, such as credit risk, market risk, foreign currency risk and interest rate risk, while also exposing the Fund to correlation risk, counterparty risk, hedging risk, leverage risk, liquidity risk, pricing risk and volatility risk.

Changing Distribution Level Risk. The Fund will normally receive income which may include interest, dividends and/or capital gains, depending upon its investments. The amount of the distributions the Fund pays will vary and generally depends on the amount of income the Fund earns (less expenses) on its portfolio holdings, and capital gains or losses it recognizes. A decline in the Fund's income or net capital gains from its investments may reduce its distribution level.

Rule 144A and Other Exempted Securities Risk. The Fund may invest in privately placed and other securities or instruments exempt from SEC registration (collectively “private placements”), subject to liquidity and other regulatory restrictions. In the U.S. market, private placements are typically sold only to qualified institutional buyers, or qualified purchasers, as applicable. An insufficient number of buyers interested in purchasing private placements at a particular time could adversely affect the marketability of such investments and the Fund might be unable to dispose of them promptly or at reasonable prices, subjecting the Fund to liquidity risk. The Fund may invest in private placements determined to be liquid as well as those determined to be illiquid. Even if determined to be liquid, the Fund’s holdings of private placements may increase the level of Fund illiquidity if eligible buyers are unable or unwilling to purchase them at a particular time. Issuers of Rule 144A eligible securities are required to furnish information to potential investors upon request. However, the required disclosure is much less extensive than that required of public companies and is not publicly available since the offering is not filed with the SEC. Further, issuers of Rule 144A eligible securities can require recipients of the offering information (such as the Fund) to agree contractually to keep the information confidential, which could also adversely affect the Fund’s ability to dispose of the security.

Select Portfolio Risk. Because the Fund may invest in a limited number of companies, the Fund as a whole is subject to greater risk of loss if any of its portfolio securities decline in price. In addition, the Fund’s holdings and weightings will diverge significantly from its primary benchmark’s holdings and weightings and the Fund may therefore experience greater risk and volatility relative to the benchmark. Because the Fund may invest in more than one company concentrated in a similar industry, sector or geographic region, the Fund may be even more concentrated than the number of companies it may hold would suggest.

Derivatives Risk – Swaps Risk. In a typical swap transaction, two parties agree to exchange the return earned on a specified underlying reference for a fixed return or the return from another underlying reference during a specified period of time. Swaps may be difficult to value and may be illiquid. Swaps could result in Fund losses if the underlying asset or reference does not perform as anticipated. Swaps create significant investment leverage such that a relatively small price movement in a swap may result in immediate and substantial losses to the Fund. The Fund may only close out a swap with its particular counterparty, and may only transfer a position with the consent of that counterparty. Certain swaps, such as short swap transactions and total return swaps, have the potential for unlimited losses, regardless of the size of the initial investment. Swaps can increase the Fund’s risk exposure to underlying references and their attendant risks, such as credit risk, market risk, foreign currency risk and interest rate risk, while also exposing the Fund to correlation risk, counterparty risk, hedging risk, inflation risk, leverage risk, liquidity risk, pricing risk and volatility risk.

Highly Leveraged Transactions Risk. The loans or other debt instruments in which the Fund invests may include highly leveraged transactions whereby the borrower assumes large amounts of debt in order to have the financial resources to attempt to achieve its business objectives. Loans or other debt instruments that are part of highly leveraged transactions involve a greater risk (including default and bankruptcy) than other investments.

Issuer Risk. An issuer in which the Fund invests or to which it has exposure may perform poorly, and the value of its securities may therefore decline, which would negatively affect the Fund's performance. Poor performance may be caused by poor management decisions, competitive pressures, breakthroughs in technology, reliance on suppliers, labor problems or shortages, corporate restructurings, fraudulent disclosures, natural disasters or other events, conditions or factors. The market capitalization of an issuer may also impact its risk profile. Investments in larger, more established companies may involve certain risks associated with their larger size. For instance, larger, more established companies may be less able to respond quickly to new competitive challenges, such as changes in consumer tastes or innovation from smaller competitors. Also, larger companies are sometimes less able to attain the high growth rates of successful smaller companies, especially during extended periods of economic expansion.

Counterparty Risk. Counterparty risk is the risk that a counterparty to a transaction in a financial instrument held by the Fund or by a special purpose or structured vehicle invested in by the Fund may become insolvent or otherwise fail to perform its obligations. As a result, the Fund may obtain no or limited recovery of its investment, and any recovery may be significantly delayed.

Exchange-Traded Fund (ETF) Risk. ETFs are subject to, among other risks, tracking risk and passive and, in some cases, active investment risk. An ETF's share price may not track its specified market index (if any) and may trade below its NAV. Certain ETFs use a "passive" investment strategy and do not take defensive positions in volatile or declining markets. Other ETFs are actively managed ETFs (i.e., they do not track a particular benchmark), which are subject to active management risk. An active secondary market in ETF shares may not develop or be maintained and may be halted or interrupted due to actions by its listing exchange, unusual market conditions or other reasons. There can be no assurance that an ETF's shares will continue to be listed on an active exchange. In addition, shareholders bear expenses incurred through ownership of the ETF. There is a risk that ETFs may terminate due to extraordinary events. For example, any of the service providers to ETFs, such as the trustee or sponsor, may close or otherwise fail to perform their obligations to the ETF, and the ETF may not be able to find a substitute service provider. Also, certain ETFs may be dependent upon licenses to use various indexes as a basis for determining their compositions and/or otherwise to use certain trade names. If these licenses are terminated, the ETFs may also terminate. In addition, an ETF may terminate if its net assets fall below a certain amount.

Liquidity Risk. Liquidity risk is the risk associated with any event, circumstance, or characteristic of an investment or market that negatively impacts the Fund's ability to sell, or realize the proceeds from the sale of, an investment at a desirable time or price. Liquidity risk may arise because of, for example, a lack of marketability of the investment, which means that when seeking to sell its portfolio investments, the Fund could find that selling is more difficult than anticipated, especially during times of high market volatility. Decreases in the number of financial institutions, including banks and broker-dealers, willing to make markets (match up sellers and buyers) in the Fund's investments or decreases in their capacity or willingness to trade such investments may increase the Fund's exposure to this risk. The debt market has experienced considerable growth, and financial institutions making markets in instruments purchased and sold by the Fund (e.g., bond dealers) have been subject to increased regulation. The impact of that growth and regulation on the ability and willingness of financial institutions to engage in trading or "making a market" in such instruments remains unsettled. Certain types of investments, such as lower-rated securities or those that are purchased and sold in over-the-counter markets, may be especially subject to liquidity risk. Securities or other assets in which the Fund invests may be traded in the over-the-counter market rather than on an exchange and therefore may be more difficult to purchase or sell at a fair price, which may have a negative impact on the Fund's performance. Market participants attempting to sell the same or a similar instrument at the same time as the Fund could exacerbate the Fund's exposure to liquidity risk. The Fund may have to accept a lower selling price for the holding, sell other liquid or more liquid investments that it might otherwise prefer to hold (thereby increasing the proportion of the Fund's investments in less liquid or illiquid securities), or forego another more appealing investment opportunity. Certain investments that were liquid when purchased by the Fund may later become illiquid, particularly in times of overall economic distress. Changing regulatory, market or other conditions or environments (for example, the interest rate or credit environments) may also adversely affect the liquidity and the price of the Fund's investments. Judgment plays a larger role in valuing illiquid or less liquid investments as compared to valuing liquid or more liquid investments. Price volatility may be higher for illiquid or less liquid investments as a result of, for example, the relatively less frequent pricing of such securities (as compared to liquid or more liquid investments). Generally, the less liquid the market at the time the Fund sells a portfolio investment, the greater the risk of loss or decline of value to the Fund. Overall market liquidity and other factors can lead to an increase in redemptions, which may negatively impact Fund performance and NAV, including, for example, if the Fund is forced to sell investments in a down market.

The Fund is subject indirectly to the following risks of Portfolio Funds seeking returns that correspond to a stated market index (the Index).

Active Management Risk. Due to its active management, the Fund could underperform its benchmark index and/or other funds with similar investment objectives and/or strategies.

Index Fund Risk. An index fund seeks to track the performance of the Index by using indexing strategies and, therefore, would not necessarily sell a security because the security's issuer was in financial trouble or defaulted, or whose credit rating was downgraded, unless that security was removed from the Index. The decision of whether to remove a security from an index is made by an independent index provider who is not affiliated with the Fund or the Investment Manager or its affiliates.

Index Methodology Risk. An index fund seeks performance that corresponds to the performance of the Index. There is no guarantee or assurance that the Index will achieve high, or even positive, returns. The Index may underperform more traditional indices. The Fund could lose value while other indices or measures of market performance increase in value or performance. In addition, the Fund may be subject to the risk that the index provider may not follow its stated methodology for construction of the Index and/or achieve the index provider's intended performance objective. Errors may result in a negative performance impact to the Fund and its shareholders.

Correlation/Tracking Error Risk. An index fund's value will generally decline when the performance of the Index declines. A number of factors may affect the Fund's ability to achieve a high degree of correlation with the Index, and there is no guarantee that the Fund will achieve a high degree of correlation. Failure to achieve such degree of correlation may prevent the Fund from achieving its investment objective. The factors that may adversely affect the Fund's correlation with the Index include the size of the Fund's portfolio, fees, expenses, transaction costs, income items, valuation methodology, accounting standards, the effectiveness of sampling techniques (if applicable), changes in the Index and disruptions or illiquidity in the markets for the securities in which the Fund invests. While the Fund typically attempts to track the performance of the Index by investing all, or substantially all, of its assets in the securities that make up the Index in approximately the same proportion as their weighting in the Index, at times, the Fund may not have investment exposure to all securities in the Index, or its weighting of investment exposure to securities may be different from that of the Index. In addition, the Fund may invest in securities not included in the Index. The Fund may take or refrain from taking investment positions for various reasons, such as tax efficiency purposes, or to comply with regulatory restrictions, which may negatively affect the Fund's correlation with the Index. The Fund may also be subject to large movements of assets into and out of the Fund, potentially resulting in the Fund being over- or under-exposed to certain securities comprising the Index and may be impacted by Index reconstitutions and Index rebalancing events. Holding cash balances may detract from the Fund's ability to track the Index. In addition, the Fund's NAV may deviate from the Index if the Fund fair values a portfolio security at a price other than the price used by the Index for that security. The Fund also bears management and other expenses and transaction costs in trading securities, which the Index does not bear. Accordingly, the Fund's performance will likely fail to match the performance of the Index, after taking expenses into account. Any of these factors could decrease correlation between the performance of the Fund and the Index and may hinder the Fund's ability to meet its investment objective. It is not possible to invest directly in an index.

New Fund Risk. Columbia Diversified Fixed Income Allocation ETF is a newly-formed Fund. Accordingly, investors in the Fund bear the risk that the Fund may not be successful in implementing its investment strategy of replicating the Index, which could result in the Fund being liquidated at any time without shareholder approval and/or at a time that may not be favorable for shareholders. Such a liquidation could have negative tax consequences for shareholders.

Risk Lose Money [Text]

rr_RiskLoseMoney

There is no assurance that the Fund will achieve its investment objective and you may lose money.

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.

Bar Chart and Performance Table [Heading]

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Performance Information

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following bar chart and table show you how the Fund has performed in the past, and can help you understand the risks of investing in the Fund. The bar chart shows how the Fund’s Class Inst share performance has varied for each full calendar year shown. The table below the bar chart compares the Fund’s returns for the periods shown with those of two broad-based indexes: the S&P 500^® Index, the Fund’s primary benchmark for equity securities, and the Bloomberg Barclays U.S. Aggregate Bond Index, the Fund’s primary benchmark for debt securities. The table below also compares the Fund's returns with a secondary, custom index, the Blended Benchmark. The S&P 500^® Index tracks the performance of 500 widely held, large-capitalization U.S. stocks. The Bloomberg Barclays U.S. Aggregate Bond Index is a market value-weighted index that tracks the daily price, coupon, pay-downs, and total return performance of fixed-rate, publicly placed, dollar-denominated, and non-convertible investment grade debt issues with at least $250 million par amount outstanding and with at least one year to final maturity.  The Blended Benchmark was established by the Investment Manager to show how the Fund’s performance compares to an equally weighted custom composite of the Fund’s primary equity and primary debt benchmarks, the S&P 500^® Index and the Bloomberg Barclays U.S. Aggregate Bond Index, respectively. The percentage of the Fund’s assets allocated to underlying stock and bond Portfolio Funds will vary, and accordingly the composition of the Fund’s portfolio will not always reflect the composition of the Blended Benchmark.

The performance of one or more share classes shown in the Average Annual Total Returns table below includes the Fund’s Class Inst share returns (adjusted to reflect the higher class-related operating expenses of such classes, where applicable) for periods prior to the indicated inception date of such share classes. Except for differences in fees and expenses, all share classes of the Fund would have substantially similar annual returns because all share classes of the Fund invest in the same portfolio of securities.

The after-tax returns shown in the Average Annual Total Returns table below are calculated using the highest historical individual U.S. federal marginal income tax rates in effect during the period indicated in the table and do not reflect the impact of state, local or foreign taxes. Your actual after-tax returns will depend on your personal tax situation and may differ from those shown in the table. In addition, the after-tax returns shown in the table do not apply to shares held in tax-advantaged accounts such as 401(k) plans or Individual Retirement Accounts (IRAs). The after-tax returns are shown only for Class Inst shares and will vary for other share classes.

The Fund’s performance prior to May 2018 reflects returns achieved following a principal investment strategy with a single form of allocation table. While the Fund now follows a principal investment strategy with two potential forms of allocation table, as described above in the Principal Investment Strategies section, the form of the Fund’s currently effective allocation table has been in place since the Fund’s inception in 2002. The Fund's performance prior to May 2018 reflects the current form of allocation table.

The Fund’s past performance (before and after taxes) is no guarantee of how the Fund will perform in the future. Updated performance information can be obtained by calling toll-free 800.345.6611 or visiting columbiathreadneedle.com/us.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

The following bar chart and table show you how the Fund has performed in the past, and can help you understand the risks of investing in the Fund. The bar chart shows how the Fund’s Class Inst share performance has varied for each full calendar year shown. The table below the bar chart compares the Fund’s returns for the periods shown with those of two broad-based indexes: the S&P 500^® Index, the Fund’s primary benchmark for equity securities, and the Bloomberg Barclays U.S. Aggregate Bond Index, the Fund’s primary benchmark for debt securities. The table below also compares the Fund's returns with a secondary, custom index, the Blended Benchmark.

Performance Additional Market Index [Text]

rr_PerformanceAdditionalMarketIndex

The Blended Benchmark was established by the Investment Manager to show how the Fund’s performance compares to an equally weighted custom composite of the Fund’s primary equity and primary debt benchmarks, the S&P 500^® Index and the Bloomberg Barclays U.S. Aggregate Bond Index, respectively. The percentage of the Fund’s assets allocated to underlying stock and bond Portfolio Funds will vary, and accordingly the composition of the Fund’s portfolio will not always reflect the composition of the Blended Benchmark.

Performance Availability Phone [Text]

rr_PerformanceAvailabilityPhone

800.345.6611

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

columbiathreadneedle.com/us

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

The Fund’s past performance (before and after taxes) is no guarantee of how the Fund will perform in the future.

Bar Chart [Heading]

rr_BarChartHeading

Year by Year Total Return (%)
as of December 31 Each Year

Bar Chart Closing [Text Block]

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Best and Worst Quarterly Returns
During the Period Shown in the Bar Chart

Best                                                     2nd Quarter 2009                                                      19.33%

Worst                                                   4th Quarter 2008                                                     -19.26%

Performance Table Heading

rr_PerformanceTableHeading

Average Annual Total Returns After Applicable Sales Charges (for periods ended December 31, 2017)

Performance Table Uses Highest Federal Rate

rr_PerformanceTableUsesHighestFederalRate

The after-tax returns shown in the Average Annual Total Returns table below are calculated using the highest historical individual U.S. federal marginal income tax rates in effect during the period indicated in the table and do not reflect the impact of state, local or foreign taxes.

Performance Table Not Relevant to Tax Deferred

rr_PerformanceTableNotRelevantToTaxDeferred

In addition, the after-tax returns shown in the table do not apply to shares held in tax-advantaged accounts such as 401(k) plans or Individual Retirement Accounts (IRAs).

Performance Table One Class of after Tax Shown [Text]

rr_PerformanceTableOneClassOfAfterTaxShown

The after-tax returns are shown only for Class Inst shares and will vary for other share classes.

Columbia Thermostat Fund | Class A

Risk/Return:

rr_RiskReturnAbstract

Maximum sales charge (load) imposed on purchases (as a % of offering price)

rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice

5.75%

Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)

rr_MaximumDeferredSalesChargeOverOther

1.00%

^[1]

Management fees

rr_ManagementFeesOverAssets

0.10%

Distribution and/or service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

0.25%

Other expenses

rr_OtherExpensesOverAssets

0.19%

^[2]

Acquired fund fees and expenses

rr_AcquiredFundFeesAndExpensesOverAssets

0.35%

^[3]

Total annual Fund operating expenses

rr_ExpensesOverAssets

0.89%

^[4]

Fee waivers and/or expense reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.04%)

^[5]

Total annual Fund operating expenses after fee waivers and/or expense reimbursements

rr_NetExpensesOverAssets

0.85%

Expenses Deferred Charges [Text Block]

rr_ExpensesDeferredChargesTextBlock

This charge is imposed on certain investments of between $1 million and $50 million redeemed within 18 months after purchase, as follows: 1.00% if redeemed within 12 months after purchase, and 0.50% if redeemed more than 12, but less than 18, months after purchase, with certain limited exceptions.

1 year

rr_ExpenseExampleYear01

$ 657

3 years

rr_ExpenseExampleYear03

839

5 years

rr_ExpenseExampleYear05

1,036

10 years

rr_ExpenseExampleYear10

1,605

1 year

rr_ExpenseExampleNoRedemptionYear01

657

3 years

rr_ExpenseExampleNoRedemptionYear03

839

5 years

rr_ExpenseExampleNoRedemptionYear05

1,036

10 years

rr_ExpenseExampleNoRedemptionYear10

$ 1,605

1 Year

rr_AverageAnnualReturnYear01

(0.86%)

5 Years

rr_AverageAnnualReturnYear05

3.55%

10 Years

rr_AverageAnnualReturnYear10

4.24%

Share Class Inception Date

rr_AverageAnnualReturnInceptionDate

Mar. 03, 2003

Columbia Thermostat Fund | Class Adv

Risk/Return:

rr_RiskReturnAbstract

Maximum sales charge (load) imposed on purchases (as a % of offering price)

rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice

none

Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)

rr_MaximumDeferredSalesChargeOverOther

none

Management fees

rr_ManagementFeesOverAssets

0.10%

Distribution and/or service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.19%

^[2]

Acquired fund fees and expenses

rr_AcquiredFundFeesAndExpensesOverAssets

0.35%

^[3]

Total annual Fund operating expenses

rr_ExpensesOverAssets

0.64%

^[4]

Fee waivers and/or expense reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.04%)

^[5]

Total annual Fund operating expenses after fee waivers and/or expense reimbursements

rr_NetExpensesOverAssets

0.60%

1 year

rr_ExpenseExampleYear01

$ 61

3 years

rr_ExpenseExampleYear03

201

5 years

rr_ExpenseExampleYear05

353

10 years

rr_ExpenseExampleYear10

795

1 year

rr_ExpenseExampleNoRedemptionYear01

61

3 years

rr_ExpenseExampleNoRedemptionYear03

201

5 years

rr_ExpenseExampleNoRedemptionYear05

353

10 years

rr_ExpenseExampleNoRedemptionYear10

$ 795

1 Year

rr_AverageAnnualReturnYear01

5.49%

5 Years

rr_AverageAnnualReturnYear05

5.04%

10 Years

rr_AverageAnnualReturnYear10

5.12%

Share Class Inception Date

rr_AverageAnnualReturnInceptionDate

Nov. 08, 2012

Columbia Thermostat Fund | Class C

Risk/Return:

rr_RiskReturnAbstract

Maximum sales charge (load) imposed on purchases (as a % of offering price)

rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice

none

Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)

rr_MaximumDeferredSalesChargeOverOther

1.00%

^[6]

Management fees

rr_ManagementFeesOverAssets

0.10%

Distribution and/or service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

1.00%

Other expenses

rr_OtherExpensesOverAssets

0.19%

^[2]

Acquired fund fees and expenses

rr_AcquiredFundFeesAndExpensesOverAssets

0.35%

^[3]

Total annual Fund operating expenses

rr_ExpensesOverAssets

1.64%

^[4]

Fee waivers and/or expense reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.04%)

^[5]

Total annual Fund operating expenses after fee waivers and/or expense reimbursements

rr_NetExpensesOverAssets

1.60%

Expenses Deferred Charges [Text Block]

rr_ExpensesDeferredChargesTextBlock

This charge applies to redemptions within 12 months after purchase, with certain limited exceptions.

1 year

rr_ExpenseExampleYear01

$ 263

3 years

rr_ExpenseExampleYear03

513

5 years

rr_ExpenseExampleYear05

888

10 years

rr_ExpenseExampleYear10

1,940

1 year

rr_ExpenseExampleNoRedemptionYear01

163

3 years

rr_ExpenseExampleNoRedemptionYear03

513

5 years

rr_ExpenseExampleNoRedemptionYear05

888

10 years

rr_ExpenseExampleNoRedemptionYear10

$ 1,940

1 Year

rr_AverageAnnualReturnYear01

3.48%

5 Years

rr_AverageAnnualReturnYear05

4.00%

10 Years

rr_AverageAnnualReturnYear10

4.08%

Share Class Inception Date

rr_AverageAnnualReturnInceptionDate

Mar. 03, 2003

Columbia Thermostat Fund | Class Inst

Risk/Return:

rr_RiskReturnAbstract

Maximum sales charge (load) imposed on purchases (as a % of offering price)

rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice

none

Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)

rr_MaximumDeferredSalesChargeOverOther

none

Management fees

rr_ManagementFeesOverAssets

0.10%

Distribution and/or service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.19%

^[2]

Acquired fund fees and expenses

rr_AcquiredFundFeesAndExpensesOverAssets

0.35%

^[3]

Total annual Fund operating expenses

rr_ExpensesOverAssets

0.64%

^[4]

Fee waivers and/or expense reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.04%)

^[5]

Total annual Fund operating expenses after fee waivers and/or expense reimbursements

rr_NetExpensesOverAssets

0.60%

1 year

rr_ExpenseExampleYear01

$ 61

3 years

rr_ExpenseExampleYear03

201

5 years

rr_ExpenseExampleYear05

353

10 years

rr_ExpenseExampleYear10

795

1 year

rr_ExpenseExampleNoRedemptionYear01

61

3 years

rr_ExpenseExampleNoRedemptionYear03

201

5 years

rr_ExpenseExampleNoRedemptionYear05

353

10 years

rr_ExpenseExampleNoRedemptionYear10

$ 795

2008

rr_AnnualReturn2008

(30.53%)

^[7]

2009

rr_AnnualReturn2009

32.29%

^[7]

2010

rr_AnnualReturn2010

17.58%

^[7]

2011

rr_AnnualReturn2011

4.85%

^[7]

2012

rr_AnnualReturn2012

13.69%

^[7]

2013

rr_AnnualReturn2013

9.30%

^[7]

2014

rr_AnnualReturn2014

5.57%

^[7]

2015

rr_AnnualReturn2015

0.33%

^[7]

2016

rr_AnnualReturn2016

4.72%

^[7]

2017

rr_AnnualReturn2017

5.52%

^[7]

Year to Date Return, Label

rr_YearToDateReturnLabel

Year to Date return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2018

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

(0.21%)

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Best

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Jun. 30, 2009

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

19.33%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Worst

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Dec. 31, 2008

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(19.26%)

1 Year

rr_AverageAnnualReturnYear01

5.52%

5 Years

rr_AverageAnnualReturnYear05

5.05%

10 Years

rr_AverageAnnualReturnYear10

5.12%

Share Class Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 25, 2002

Columbia Thermostat Fund | Class Inst2

Risk/Return:

rr_RiskReturnAbstract

Maximum sales charge (load) imposed on purchases (as a % of offering price)

rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice

none

Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)

rr_MaximumDeferredSalesChargeOverOther

none

Management fees

rr_ManagementFeesOverAssets

0.10%

Distribution and/or service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.18%

^[2]

Acquired fund fees and expenses

rr_AcquiredFundFeesAndExpensesOverAssets

0.35%

^[3]

Total annual Fund operating expenses

rr_ExpensesOverAssets

0.63%

^[4]

Fee waivers and/or expense reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.08%)

^[5]

Total annual Fund operating expenses after fee waivers and/or expense reimbursements

rr_NetExpensesOverAssets

0.55%

1 year

rr_ExpenseExampleYear01

$ 56

3 years

rr_ExpenseExampleYear03

194

5 years

rr_ExpenseExampleYear05

343

10 years

rr_ExpenseExampleYear10

779

1 year

rr_ExpenseExampleNoRedemptionYear01

56

3 years

rr_ExpenseExampleNoRedemptionYear03

194

5 years

rr_ExpenseExampleNoRedemptionYear05

343

10 years

rr_ExpenseExampleNoRedemptionYear10

$ 779

1 Year

rr_AverageAnnualReturnYear01

5.51%

5 Years

rr_AverageAnnualReturnYear05

5.08%

10 Years

rr_AverageAnnualReturnYear10

5.13%

Share Class Inception Date

rr_AverageAnnualReturnInceptionDate

Nov. 08, 2012

Columbia Thermostat Fund | Class Inst3

Risk/Return:

rr_RiskReturnAbstract

Maximum sales charge (load) imposed on purchases (as a % of offering price)

rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice

none

Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)

rr_MaximumDeferredSalesChargeOverOther

none

Management fees

rr_ManagementFeesOverAssets

0.10%

Distribution and/or service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.12%

^[2]

Acquired fund fees and expenses

rr_AcquiredFundFeesAndExpensesOverAssets

0.35%

^[3]

Total annual Fund operating expenses

rr_ExpensesOverAssets

0.57%

^[4]

Fee waivers and/or expense reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.07%)

^[5]

Total annual Fund operating expenses after fee waivers and/or expense reimbursements

rr_NetExpensesOverAssets

0.50%

1 year

rr_ExpenseExampleYear01

$ 51

3 years

rr_ExpenseExampleYear03

176

5 years

rr_ExpenseExampleYear05

311

10 years

rr_ExpenseExampleYear10

707

1 year

rr_ExpenseExampleNoRedemptionYear01

51

3 years

rr_ExpenseExampleNoRedemptionYear03

176

5 years

rr_ExpenseExampleNoRedemptionYear05

311

10 years

rr_ExpenseExampleNoRedemptionYear10

$ 707

1 Year

rr_AverageAnnualReturnYear01

5.64%

5 Years

rr_AverageAnnualReturnYear05

5.13%

10 Years

rr_AverageAnnualReturnYear10

5.16%

Share Class Inception Date

rr_AverageAnnualReturnInceptionDate

Nov. 08, 2012

Columbia Thermostat Fund | returns after taxes on distributions | Class Inst

Risk/Return:

rr_RiskReturnAbstract

1 Year

rr_AverageAnnualReturnYear01

3.60%

5 Years

rr_AverageAnnualReturnYear05

3.48%

10 Years

rr_AverageAnnualReturnYear10

4.07%

Share Class Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 25, 2002

Columbia Thermostat Fund | returns after taxes on distributions and sale of Fund shares | Class Inst

Risk/Return:

rr_RiskReturnAbstract

1 Year

rr_AverageAnnualReturnYear01

3.52%

5 Years

rr_AverageAnnualReturnYear05

3.39%

10 Years

rr_AverageAnnualReturnYear10

3.73%

Share Class Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 25, 2002

Columbia Thermostat Fund | Blended Benchmark (Secondary Benchmark; an equally weighted custom composite of the Fund's primary benchmarks for equity and debt securities, established by the Investment Manager; reflects no deductions for fees, expenses or taxes)

Risk/Return:

rr_RiskReturnAbstract

1 Year

rr_AverageAnnualReturnYear01

12.37%

5 Years

rr_AverageAnnualReturnYear05

8.88%

10 Years

rr_AverageAnnualReturnYear10

6.54%

Columbia Thermostat Fund | S&P 500® Index (Primary Equity Benchmark; reflects no deductions for fees, expenses or taxes)

Risk/Return:

rr_RiskReturnAbstract

1 Year

rr_AverageAnnualReturnYear01

21.83%

5 Years

rr_AverageAnnualReturnYear05

15.79%

10 Years

rr_AverageAnnualReturnYear10

8.50%

Columbia Thermostat Fund | Bloomberg Barclays U.S. Aggregate Bond Index (Primary Debt Benchmark; reflects no deductions for fees, expenses or taxes)

Risk/Return:

rr_RiskReturnAbstract

1 Year

rr_AverageAnnualReturnYear01

3.54%

5 Years

rr_AverageAnnualReturnYear05

2.10%

10 Years

rr_AverageAnnualReturnYear10

4.01%

[1] This charge is imposed on certain investments of between $1 million and $50 million redeemed within 18 months after purchase, as follows: 1.00% if redeemed within 12 months after purchase, and 0.50% if redeemed more than 12, but less than 18, months after purchase, with certain limited exceptions.

[2] Other expenses have been restated to reflect current transfer agency fees paid by the Fund.

[3] Acquired fund (Portfolio Fund) fees and expenses (expenses the Fund incurs indirectly through its investments in other funds) have been restated to reflect the fees and expenses of the Fund’s current Portfolio Funds.

[4] "Total annual Fund operating expenses" include acquired fund (Portfolio Fund) fees and expenses (expenses the Fund incurs indirectly through its investments in other funds) and may be higher than "Total Net Expenses" shown in the Financial Highlights section of this prospectus because "Total Net Expenses" do not include Portfolio Fund (acquired fund) fees and expenses.

[5] Columbia Wanger Asset Management, LLC (the Investment Manager) has contractually agreed to waive fees and reimburse certain expenses of the Fund, through April 30, 2019, so that ordinary operating expenses (excluding transaction costs and certain other investment-related expenses, interest and fees on borrowings and expenses associated with the Fund's investment in other investment companies, including the Portfolio Funds) do not exceed the annual rates of 0.50% for Class A shares, 0.25% for Class Adv shares, 1.25% for Class C shares, 0.25% for Class Inst shares, 0.20% for Class Inst2 shares and 0.15% for Class Inst3 shares. This arrangement may only be amended or terminated with approval from the Fund's Board of Trustees and the Investment Manager. The fee waivers and/or expense reimbursements shown in the table also reflect the contractual agreement of the Fund's transfer agent, Columbia Management Investment Services Corp. (the Transfer Agent), to waive a portion of its fees through June 30, 2019, such that the Fund's transfer agency fees do not exceed the annual rates of 0.05% of the average daily net assets of Class Inst2 shares and 0.00% of the average daily net assets of Class Inst3 shares.

[6] This charge applies to redemptions within 12 months after purchase, with certain limited exceptions.

[7] Year to Date return as of March 31, 2018: -0.21%

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	COLUMBIA ACORN TRUST
Prospectus Date	rr_ProspectusDate	May 01, 2018
Document Creation Date	dei_DocumentCreationDate	Apr. 30, 2018