rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

COLUMBIA ACORN TRUST

Prospectus Date

rr_ProspectusDate

May 01, 2016

Supplement [Text Block]

cat7_SupplementTextBlock

Supplement dated June 27, 2016

to the Prospectus and Summary Prospectus of the following Fund:

Fund	Prospectus and Summary Prospectus Dated
Columbia Acorn Trust
Columbia Thermostat Fund^SM	5/1/2016

1.	The table under the caption “Fees and Expenses of the Fund” in the “Summary of the Fund” section of the Prospectus and the Summary Prospectus is hereby superseded and replaced with the following:

Shareholder Fees (fees paid directly from your investment)
	Class A	Class C	Classes R4, R5, Y and Z
Maximum sales charge (load) imposed on purchases (as a % of offering price)	5.75%	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% ^(a)	1.00% ^(b)	None

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
	Class A	Class C	Class R4	Class R5	Class Y	Class Z
Management fees	0.10%	0.10%	0.10%	0.10%	0.10%	0.10%
Distribution and/or service (12b-1) fees	0.25%	1.00%	0.00%	0.00%	0.00%	0.00%
Other expenses^(c)	0.17%	0.17%	0.17%	0.15%	0.10%	0.17%
Acquired fund fees and expenses	0.52%	0.52%	0.52%	0.52%	0.52%	0.52%
Total annual Fund operating expenses^(d)	1.04%	1.79%	0.79%	0.77%	0.72%	0.79%
Fee waivers and/or expense reimbursements^(e)	(0.02%)	(0.02%)	(0.02%)	(0.01%)	(0.01%)	(0.02%)
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	1.02%	1.77%	0.77%	0.76%	0.71%	0.77%

(a)	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.
(b)	This charge applies to redemptions within 12 months after purchase, with certain limited exceptions.
(c)	Other expenses have been restated and are based on estimated amounts for the Fund's current fiscal year, taking into consideration changes in the method of allocating transfer agent expenses among Class A, Class C, Class R4 and Class Z shares of the Fund.
(d)	“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.
(e)	Columbia Wanger Asset Management, LLC (the Investment Manager) has contractually agreed to waive fees and reimburse certain expenses of the Fund 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 the Portfolio Funds (acquired funds)), do not exceed the annual rates of 0.50% for Class A shares, 1.25% for Class C shares, 0.25% for Class R4 shares, 0.24% for Class R5 shares, 0.19% for Class Y shares and 0.25% for Class Z shares, through April 30, 2017. This arrangement may only be modified or amended with approval from the Fund and the Investment Manager.
2.	The expense example table that appears under the caption “Fees and Expenses of the Fund – Example” in the “Summary of the Fund” section of the Prospectus and the Summary Prospectus is hereby superseded and replaced with the following:

	1 year	3 years	5 years	10 years
Class A (whether or not shares are redeemed)	$673	$885	$1,114	$1,771
Class C (assuming redemption of all shares at the end of the period)	$280	$561	$ 968	$2,104
Class C (assuming no redemption of shares)	$180	$561	$ 968	$2,104
Class R4 (whether or not shares are redeemed)	$ 79	$250	$ 437	$ 976
Class R5 (whether or not shares are redeemed)	$ 78	$245	$ 427	$ 953
Class Y (whether or not shares are redeemed)	$ 73	$229	$ 400	$ 894
Class Z (whether or not shares are redeemed)	$ 79	$250	$ 437	$ 976

Columbia Thermostat Fund

Shareholder Fees Caption [Text]

rr_RiskReturnAbstract

Supplement [Text Block]

cat7_SupplementTextBlock

Supplement dated June 27, 2016

to the Prospectus and Summary Prospectus of the following Fund:

Fund	Prospectus and Summary Prospectus Dated
Columbia Acorn Trust
Columbia Thermostat Fund^SM	5/1/2016

1.	The table under the caption “Fees and Expenses of the Fund” in the “Summary of the Fund” section of the Prospectus and the Summary Prospectus is hereby superseded and replaced with the following:

Shareholder Fees (fees paid directly from your investment)
	Class A	Class C	Classes R4, R5, Y and Z
Maximum sales charge (load) imposed on purchases (as a % of offering price)	5.75%	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% ^(a)	1.00% ^(b)	None

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
	Class A	Class C	Class R4	Class R5	Class Y	Class Z
Management fees	0.10%	0.10%	0.10%	0.10%	0.10%	0.10%
Distribution and/or service (12b-1) fees	0.25%	1.00%	0.00%	0.00%	0.00%	0.00%
Other expenses^(c)	0.17%	0.17%	0.17%	0.15%	0.10%	0.17%
Acquired fund fees and expenses	0.52%	0.52%	0.52%	0.52%	0.52%	0.52%
Total annual Fund operating expenses^(d)	1.04%	1.79%	0.79%	0.77%	0.72%	0.79%
Fee waivers and/or expense reimbursements^(e)	(0.02%)	(0.02%)	(0.02%)	(0.01%)	(0.01%)	(0.02%)
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	1.02%	1.77%	0.77%	0.76%	0.71%	0.77%

(a)	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.
(b)	This charge applies to redemptions within 12 months after purchase, with certain limited exceptions.
(c)	Other expenses have been restated and are based on estimated amounts for the Fund's current fiscal year, taking into consideration changes in the method of allocating transfer agent expenses among Class A, Class C, Class R4 and Class Z shares of the Fund.
(d)	“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.
(e)	Columbia Wanger Asset Management, LLC (the Investment Manager) has contractually agreed to waive fees and reimburse certain expenses of the Fund 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 the Portfolio Funds (acquired funds)), do not exceed the annual rates of 0.50% for Class A shares, 1.25% for Class C shares, 0.25% for Class R4 shares, 0.24% for Class R5 shares, 0.19% for Class Y shares and 0.25% for Class Z shares, through April 30, 2017. This arrangement may only be modified or amended with approval from the Fund and the Investment Manager.
2.	The expense example table that appears under the caption “Fees and Expenses of the Fund – Example” in the “Summary of the Fund” section of the Prospectus and the Summary Prospectus is hereby superseded and replaced with the following:

	1 year	3 years	5 years	10 years
Class A (whether or not shares are redeemed)	$673	$885	$1,114	$1,771
Class C (assuming redemption of all shares at the end of the period)	$280	$561	$ 968	$2,104
Class C (assuming no redemption of shares)	$180	$561	$ 968	$2,104
Class R4 (whether or not shares are redeemed)	$ 79	$250	$ 437	$ 976
Class R5 (whether or not shares are redeemed)	$ 78	$245	$ 427	$ 953
Class Y (whether or not shares are redeemed)	$ 73	$229	$ 400	$ 894
Class Z (whether or not shares are redeemed)	$ 79	$250	$ 437	$ 976

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, 2017

Expenses Restated to Reflect Current [Text]

rr_ExpensesRestatedToReflectCurrent

Other expenses have been restated and are based on estimated amounts for the Fund's current fiscal year, taking into consideration changes in the method of allocating transfer agent expenses among Class A, Class C, Class R4 and Class Z shares of the Fund.

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.

Columbia Thermostat Fund | Class A

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

rr_RiskReturnAbstract

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]

rr_ManagementFeesOverAssets

0.10%

rr_DistributionAndService12b1FeesOverAssets

0.25%

rr_OtherExpensesOverAssets

0.17%

^[2]

rr_AcquiredFundFeesAndExpensesOverAssets

0.52%

rr_ExpensesOverAssets

1.04%

^[3]

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

rr_FeeWaiverOrReimbursementOverAssets

(0.02%)

^[4]

rr_NetExpensesOverAssets

1.02%

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.

rr_ExpenseExampleYear01

$ 673

rr_ExpenseExampleYear03

885

rr_ExpenseExampleYear05

1,114

rr_ExpenseExampleYear10

1,771

rr_ExpenseExampleNoRedemptionYear01

673

rr_ExpenseExampleNoRedemptionYear03

885

rr_ExpenseExampleNoRedemptionYear05

1,114

Columbia Thermostat Fund | Class C

rr_ExpenseExampleNoRedemptionYear10

$ 1,771

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

rr_RiskReturnAbstract

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]

rr_ManagementFeesOverAssets

0.10%

rr_DistributionAndService12b1FeesOverAssets

1.00%

rr_OtherExpensesOverAssets

0.17%

^[2]

rr_AcquiredFundFeesAndExpensesOverAssets

0.52%

rr_ExpensesOverAssets

1.79%

^[3]

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

rr_FeeWaiverOrReimbursementOverAssets

(0.02%)

^[4]

rr_NetExpensesOverAssets

1.77%

Expenses Deferred Charges [Text Block]

rr_ExpensesDeferredChargesTextBlock

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

rr_ExpenseExampleYear01

$ 280

rr_ExpenseExampleYear03

561

rr_ExpenseExampleYear05

968

rr_ExpenseExampleYear10

2,104

rr_ExpenseExampleNoRedemptionYear01

180

rr_ExpenseExampleNoRedemptionYear03

561

rr_ExpenseExampleNoRedemptionYear05

968

Columbia Thermostat Fund | Class R4

rr_ExpenseExampleNoRedemptionYear10

$ 2,104

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

rr_RiskReturnAbstract

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

rr_ManagementFeesOverAssets

0.10%

rr_DistributionAndService12b1FeesOverAssets

none

rr_OtherExpensesOverAssets

0.17%

^[2]

rr_AcquiredFundFeesAndExpensesOverAssets

0.52%

rr_ExpensesOverAssets

0.79%

^[3]

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

rr_FeeWaiverOrReimbursementOverAssets

(0.02%)

^[4]

rr_NetExpensesOverAssets

0.77%

rr_ExpenseExampleYear01

$ 79

rr_ExpenseExampleYear03

250

rr_ExpenseExampleYear05

437

rr_ExpenseExampleYear10

976

rr_ExpenseExampleNoRedemptionYear01

rr_ExpenseExampleNoRedemptionYear03

250

rr_ExpenseExampleNoRedemptionYear05

437

Columbia Thermostat Fund | Class R5

rr_ExpenseExampleNoRedemptionYear10

$ 976

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

rr_RiskReturnAbstract

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

rr_ManagementFeesOverAssets

0.10%

rr_DistributionAndService12b1FeesOverAssets

none

rr_OtherExpensesOverAssets

0.15%

^[2]

rr_AcquiredFundFeesAndExpensesOverAssets

0.52%

rr_ExpensesOverAssets

0.77%

^[3]

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

rr_FeeWaiverOrReimbursementOverAssets

(0.01%)

^[4]

rr_NetExpensesOverAssets

0.76%

rr_ExpenseExampleYear01

$ 78

rr_ExpenseExampleYear03

245

rr_ExpenseExampleYear05

427

rr_ExpenseExampleYear10

953

rr_ExpenseExampleNoRedemptionYear01

rr_ExpenseExampleNoRedemptionYear03

245

rr_ExpenseExampleNoRedemptionYear05

427

Columbia Thermostat Fund | Class Y

rr_ExpenseExampleNoRedemptionYear10

$ 953

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

rr_RiskReturnAbstract

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

rr_ManagementFeesOverAssets

0.10%

rr_DistributionAndService12b1FeesOverAssets

none

rr_OtherExpensesOverAssets

0.10%

^[2]

rr_AcquiredFundFeesAndExpensesOverAssets

0.52%

rr_ExpensesOverAssets

0.72%

^[3]

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

rr_FeeWaiverOrReimbursementOverAssets

(0.01%)

^[4]

rr_NetExpensesOverAssets

0.71%

rr_ExpenseExampleYear01

$ 73

rr_ExpenseExampleYear03

229

rr_ExpenseExampleYear05

400

rr_ExpenseExampleYear10

894

rr_ExpenseExampleNoRedemptionYear01

rr_ExpenseExampleNoRedemptionYear03

229

rr_ExpenseExampleNoRedemptionYear05

400

Columbia Thermostat Fund | Class Z

rr_ExpenseExampleNoRedemptionYear10

$ 894

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

rr_RiskReturnAbstract

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

rr_ManagementFeesOverAssets

0.10%

rr_DistributionAndService12b1FeesOverAssets

none

rr_OtherExpensesOverAssets

0.17%

^[2]

rr_AcquiredFundFeesAndExpensesOverAssets

0.52%

rr_ExpensesOverAssets

0.79%

^[3]

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

rr_FeeWaiverOrReimbursementOverAssets

(0.02%)

^[4]

rr_NetExpensesOverAssets

0.77%

rr_ExpenseExampleYear01

$ 79

rr_ExpenseExampleYear03

250

rr_ExpenseExampleYear05

437

rr_ExpenseExampleYear10

976

rr_ExpenseExampleNoRedemptionYear01

rr_ExpenseExampleNoRedemptionYear03

250

rr_ExpenseExampleNoRedemptionYear05

437