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Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName BARON INVESTMENT FUNDS TRUST (f/k/a BARON ASSET FUND)
Prospectus Date rr_ProspectusDate Jan. 27, 2017
BARON OPPORTUNITY FUND  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading Baron Opportunity Fund
Objective [Heading] rr_ObjectiveHeading Investment Goal
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The investment goal of Baron Opportunity Fund (the “Fund”) is capital appreciation through investments primarily in growth companies that benefit from technology advances.
Expense [Heading] rr_ExpenseHeading Fees and Expenses of the Fund
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock The table below describes the fees and expenses that you would pay if you bought and held shares of the Fund.
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)
Portfolio Turnover [Heading] rr_PortfolioTurnoverHeading Portfolio Turnover.
Portfolio Turnover [Text Block] rr_PortfolioTurnoverTextBlock The Fund pays 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 for Fund shareholders. These costs, which are not reflected in Total Annual Fund Operating Expenses or in the example, affect the Fund’s performance. During the most recent fiscal year ended September 30, 2016, the Fund’s portfolio turnover rate was 32.38% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 32.38%
Expense Example [Heading] rr_ExpenseExampleHeading Example
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock This 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 assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading] rr_StrategyHeading Investments, Risks, and Performance

Principal Investment Strategies of the Fund
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock The Fund invests primarily in equity securities in the form of common stock of high growth businesses of any market capitalization selected for their capital appreciation potential. BAMCO, Inc. (“BAMCO” or the “Adviser”) may invest in companies in any sector or industry that it believes will benefit from innovations and advances in technology. The Adviser seeks to invest in businesses it believes have significant opportunities for growth, sustainable competitive advantages, exceptional management, and an attractive valuation.
Risk [Heading] rr_RiskHeading Principal Risks of Investing in the Fund
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock General Stock Market.  Investing in the stock market is risky because securities fluctuate in value. These fluctuations may be due to political, economic or general market circumstances. Other factors may affect a single company or industry but not the broader market. Because the values of securities fluctuate, when you sell your investment in the Fund, you may lose money. Current and future portfolio holdings in the Fund are subject to risk.

Technology.  Technology companies, including internet-related and information technology companies, as well as companies propelled by new technologies, may present the risk of rapid change and product obsolescence, and their successes may be difficult to predict for the long term. Some technology companies may be newly formed and have limited operating history and experience. Technology companies may also be adversely affected by changes in governmental policies, competitive pressures and changing demand. The securities of these companies may also experience significant price movements caused by disproportionate investor optimism or pessimism, with little or no basis in the companies’ fundamentals or economic conditions.

Small- and Mid-Sized Companies.  The Adviser believes there is more potential for capital appreciation in small- and mid-sized companies, but there also may be more risk. Securities of small- and mid-sized companies may not be well known to most investors, and the securities may be less actively traded than those of large businesses. The securities of small- and mid-sized companies may fluctuate in price more widely than the stock market generally, and they may be more difficult to sell during market downturns. Small- and mid-sized companies rely more on the skills of management and on their continued tenure. Investing in small- and mid-sized companies requires a long-term outlook and may require shareholders to assume more risk and to have more patience than investing in the securities of larger, more established companies.

Growth Investing.  Growth stocks can react differently to issuer, political, market and economic developments than the market as a whole and other types of stocks. Growth stocks tend to be more expensive relative to their earnings or assets compared to other types of stocks. As a result, growth stocks tend to be sensitive to changes in their earnings and more volatile than other types of stocks.
Risk Lose Money [Text] rr_RiskLoseMoney Because the values of securities fluctuate, when you sell your investment in the Fund, you may lose money.
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading Performance
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock The following bar chart and table provide some indication of the risks of investing in the Fund (Retail Shares) by showing changes in the Fund’s performance from year to year and by showing how the Fund’s average annual returns for 1, 5 and 10 years compare with those of a broad measure of market performance. The Fund’s past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. Updated performance information is available online at www.BaronFunds.com/performance or by calling 1-800-99BARON (1-800-992-2766).
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns The following bar chart and table provide some indication of the risks of investing in the Fund (Retail Shares) by showing changes in the Fund’s performance from year to year and by showing how the Fund’s average annual returns for 1, 5 and 10 years compare with those of a broad measure of market performance.
Performance Availability Phone [Text] rr_PerformanceAvailabilityPhone 1-800-99BARON (1-800-992-2766)
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress www.BaronFunds.com/performance
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture The Fund’s past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future.
Bar Chart [Heading] rr_BarChartHeading Year by Year Total Return (%) as of December 31 of Each Year (Retail Shares)
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock
Best Quarter:  06/30/09:  23.97%
Worst Quarter:  12/31/08: (23.08)%
Performance Table Heading rr_PerformanceTableHeading Average Annual Total Returns (for periods ended 12/31/16)
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate After-tax returns are calculated using the highest individual federal marginal income tax rate in effect at the time of each distribution and assumed sale, but they do not include the impact of state and local taxes.
Performance Table Not Relevant to Tax Deferred rr_PerformanceTableNotRelevantToTaxDeferred After-tax returns are not relevant to investors who hold their Fund’s shares in a tax-deferred account (including a 401(k) or IRA or Coverdell account), or to investors that are tax-exempt.
Performance Table Narrative rr_PerformanceTableNarrativeTextBlock The table below shows the Fund’s Retail Shares’ annual returns and long-term performance (before and after taxes) and the change in value of broad-based market indexes over various periods ended December 31, 2016. The table also shows the average annual returns of the Fund’s Institutional Shares, but it does not show after-tax returns.

After-tax returns are calculated using the highest individual federal marginal income tax rate in effect at the time of each distribution and assumed sale, but they do not include the impact of state and local taxes.

Your actual after-tax returns depend on your own tax situation and may differ from those shown. After-tax returns reflect past tax effects and are not predictive of future tax effects. After-tax returns are not relevant to investors who hold their Fund’s shares in a tax-deferred account (including a 401(k) or IRA or Coverdell account), or to investors that are tax-exempt.
Performance Table Closing [Text Block] rr_PerformanceTableClosingTextBlock The Russell 3000® Growth Index is an unmanaged index of companies classified as growth among the largest 3,000 U.S. companies. The S&P 500 Index is an unmanaged index of larger-cap companies.
Caption rr_AverageAnnualReturnCaption Average Annual Total Returns for the periods ended December 31, 2016
BARON OPPORTUNITY FUND | Retail Shares  
Risk/Return: rr_RiskReturnAbstract  
Management Fee rr_ManagementFeesOverAssets 1.00%
Distribution (12b-1) Fee rr_DistributionAndService12b1FeesOverAssets 0.25%
Other Expenses rr_OtherExpensesOverAssets 0.16%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 1.41%
1 rr_ExpenseExampleYear01 $ 144
3 rr_ExpenseExampleYear03 446
5 rr_ExpenseExampleYear05 771
10 rr_ExpenseExampleYear10 $ 1,691
2007 rr_AnnualReturn2007 21.10%
2008 rr_AnnualReturn2008 (46.06%)
2009 rr_AnnualReturn2009 61.80%
2010 rr_AnnualReturn2010 25.43%
2011 rr_AnnualReturn2011 (6.16%)
2012 rr_AnnualReturn2012 13.69%
2013 rr_AnnualReturn2013 37.55%
2014 rr_AnnualReturn2014 (1.67%)
2015 rr_AnnualReturn2015 1.20%
2016 rr_AnnualReturn2016 (4.84%)
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Best Quarter:
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2009
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 23.97%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Worst Quarter:
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Dec. 31, 2008
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (23.08%)
1 year rr_AverageAnnualReturnYear01 (4.84%)
5 years rr_AverageAnnualReturnYear05 8.17%
10 years rr_AverageAnnualReturnYear10 6.30%
Since Inception rr_AverageAnnualReturnSinceInception 4.25%
Inception date rr_AverageAnnualReturnInceptionDate Feb. 29, 2000
BARON OPPORTUNITY FUND | Institutional Shares  
Risk/Return: rr_RiskReturnAbstract  
Management Fee rr_ManagementFeesOverAssets 1.00%
Distribution (12b-1) Fee rr_DistributionAndService12b1FeesOverAssets none
Other Expenses rr_OtherExpensesOverAssets 0.13%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 1.13%
1 rr_ExpenseExampleYear01 $ 115
3 rr_ExpenseExampleYear03 359
5 rr_ExpenseExampleYear05 622
10 rr_ExpenseExampleYear10 $ 1,375
1 year rr_AverageAnnualReturnYear01 (4.56%) [1]
5 years rr_AverageAnnualReturnYear05 8.47% [1]
10 years rr_AverageAnnualReturnYear10 6.52% [1]
Since Inception rr_AverageAnnualReturnSinceInception 4.38% [1]
Inception date rr_AverageAnnualReturnInceptionDate May 29, 2009 [1]
BARON OPPORTUNITY FUND | R6 Shares  
Risk/Return: rr_RiskReturnAbstract  
Management Fee rr_ManagementFeesOverAssets 1.00%
Distribution (12b-1) Fee rr_DistributionAndService12b1FeesOverAssets none
Other Expenses rr_OtherExpensesOverAssets 0.12%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 1.12%
1 rr_ExpenseExampleYear01 $ 114
3 rr_ExpenseExampleYear03 356
5 rr_ExpenseExampleYear05 617
10 rr_ExpenseExampleYear10 $ 1,363
1 year rr_AverageAnnualReturnYear01 (4.56%) [2]
5 years rr_AverageAnnualReturnYear05 8.47% [2]
10 years rr_AverageAnnualReturnYear10 6.52% [2]
Since Inception rr_AverageAnnualReturnSinceInception 4.38% [2]
Inception date rr_AverageAnnualReturnInceptionDate Aug. 31, 2016 [2]
BARON OPPORTUNITY FUND | Return after taxes on distributions | Retail Shares  
Risk/Return: rr_RiskReturnAbstract  
1 year rr_AverageAnnualReturnYear01 (7.48%)
5 years rr_AverageAnnualReturnYear05 6.30%
10 years rr_AverageAnnualReturnYear10 5.38%
Since Inception rr_AverageAnnualReturnSinceInception 3.70%
Inception date rr_AverageAnnualReturnInceptionDate Feb. 29, 2000
BARON OPPORTUNITY FUND | Return after taxes on distributions and sale of Fund shares | Retail Shares  
Risk/Return: rr_RiskReturnAbstract  
1 year rr_AverageAnnualReturnYear01 (0.48%)
5 years rr_AverageAnnualReturnYear05 6.38%
10 years rr_AverageAnnualReturnYear10 5.05%
Since Inception rr_AverageAnnualReturnSinceInception 3.44%
Inception date rr_AverageAnnualReturnInceptionDate Feb. 29, 2000
BARON OPPORTUNITY FUND | Russell 3000® Growth Index (reflects no deduction for fees, expenses or taxes)  
Risk/Return: rr_RiskReturnAbstract  
1 year rr_AverageAnnualReturnYear01 7.39%
5 years rr_AverageAnnualReturnYear05 14.44%
10 years rr_AverageAnnualReturnYear10 8.28%
Since Inception rr_AverageAnnualReturnSinceInception 2.75%
BARON OPPORTUNITY FUND | S&P 500 Index (reflects no deduction for fees, expenses or taxes)  
Risk/Return: rr_RiskReturnAbstract  
1 year rr_AverageAnnualReturnYear01 11.96%
5 years rr_AverageAnnualReturnYear05 14.66%
10 years rr_AverageAnnualReturnYear10 6.95%
Since Inception rr_AverageAnnualReturnSinceInception 5.00%
[1] Performance for the Institutional Shares prior to May 29, 2009 is based on the performance of the Retail Shares, which have a distribution fee. The Institutional Shares do not have a distribution fee. If the annual returns for the Institutional Shares prior to May 29, 2009 did not reflect this fee, the returns would be higher.
[2] Performance for the R6 Shares prior to August 31, 2016 is based on the performance of the Institutional Shares, and prior to May 29, 2009 is based on the Retail Shares, which have a distribution fee. The Institutional and R6 Shares do not have a distribution fee. If the annual returns for the R6 Shares prior to May 29, 2009 did not reflect this fee, the returns would be higher.