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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 ASSET FUND  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading Baron Asset Fund
Objective [Heading] rr_ObjectiveHeading Investment Goal
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The investment goal of Baron Asset Fund (the “Fund”) is capital appreciation through long-term investments primarily in securities of mid-sized companies with undervalued assets or favorable growth prospects.
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 12.54% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 12.54%
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 for the long term primarily in equity securities in the form of common stock of mid-sized growth companies. BAMCO, Inc. (“BAMCO” or the “Adviser”) defines mid-sized companies as those, at the time of purchase, with market capitalizations above $2.5 billion or the smallest market cap stock in the Russell Midcap Growth Index at reconstitution, whichever is larger, and below the largest market cap stock in the Russell Midcap Growth Index at reconstitution. At the last reconstitution by Russell on June 30, 2016, the market cap range of stocks in the Russell Midcap Growth Index was $1.59 billion and $28.58 billion. The Adviser seeks to invest in businesses it believes have significant opportunities for growth, sustainable competitive advantages, exceptional management, and an attractive valuation. Because of its long-term approach, the Fund could have a significant percentage of its assets invested in securities that have appreciated beyond their market capitalizations at the time of the Fund’s investment.
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.

Mid-Sized Companies.  The Adviser believes there is more potential for capital appreciation in mid-sized companies, but there also may be more risk. Securities of 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 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. Mid-sized companies rely more on the skills of management and on their continued tenure. Investing in 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:  20.73%
Worst Quarter:  12/31/08: (27.51)%
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 Midcap® Growth Index is an unmanaged index of mid-cap growth 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 ASSET 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 0.06%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 1.31%
1 rr_ExpenseExampleYear01 $ 133
3 rr_ExpenseExampleYear03 415
5 rr_ExpenseExampleYear05 718
10 rr_ExpenseExampleYear10 $ 1,579
2007 rr_AnnualReturn2007 10.13%
2008 rr_AnnualReturn2008 (40.75%)
2009 rr_AnnualReturn2009 31.70%
2010 rr_AnnualReturn2010 21.29%
2011 rr_AnnualReturn2011 (2.94%)
2012 rr_AnnualReturn2012 15.31%
2013 rr_AnnualReturn2013 38.88%
2014 rr_AnnualReturn2014 9.48%
2015 rr_AnnualReturn2015 (0.08%)
2016 rr_AnnualReturn2016 6.23%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Best Quarter:
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2009
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 20.73%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Worst Quarter:
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Dec. 31, 2008
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (27.51%)
1 year rr_AverageAnnualReturnYear01 6.23%
5 years rr_AverageAnnualReturnYear05 13.23%
10 years rr_AverageAnnualReturnYear10 6.53%
Since Inception rr_AverageAnnualReturnSinceInception 10.97%
Inception date rr_AverageAnnualReturnInceptionDate Jun. 12, 1987
BARON ASSET FUND | Institutional Shares  
Risk/Return: rr_RiskReturnAbstract  
Management Fee rr_ManagementFeesOverAssets 1.00%
Distribution (12b-1) Fee rr_DistributionAndService12b1FeesOverAssets none
Other Expenses rr_OtherExpensesOverAssets 0.04%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 1.04%
1 rr_ExpenseExampleYear01 $ 106
3 rr_ExpenseExampleYear03 331
5 rr_ExpenseExampleYear05 574
10 rr_ExpenseExampleYear10 $ 1,271
1 year rr_AverageAnnualReturnYear01 6.51% [1]
5 years rr_AverageAnnualReturnYear05 13.53% [1]
10 years rr_AverageAnnualReturnYear10 6.75% [1]
Since Inception rr_AverageAnnualReturnSinceInception 11.05% [1]
Inception date rr_AverageAnnualReturnInceptionDate May 29, 2009 [1]
BARON ASSET FUND | R6 Shares  
Risk/Return: rr_RiskReturnAbstract  
Management Fee rr_ManagementFeesOverAssets 1.00%
Distribution (12b-1) Fee rr_DistributionAndService12b1FeesOverAssets none
Other Expenses rr_OtherExpensesOverAssets 0.04%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 1.04%
1 rr_ExpenseExampleYear01 $ 106
3 rr_ExpenseExampleYear03 331
5 rr_ExpenseExampleYear05 574
10 rr_ExpenseExampleYear10 $ 1,271
1 year rr_AverageAnnualReturnYear01 6.51% [2]
5 years rr_AverageAnnualReturnYear05 13.53% [2]
10 years rr_AverageAnnualReturnYear10 6.75% [2]
Since Inception rr_AverageAnnualReturnSinceInception 11.05% [2]
Inception date rr_AverageAnnualReturnInceptionDate Jan. 29, 2016 [2]
BARON ASSET FUND | Return after taxes on distributions | Retail Shares  
Risk/Return: rr_RiskReturnAbstract  
1 year rr_AverageAnnualReturnYear01 5.06%
5 years rr_AverageAnnualReturnYear05 11.38%
10 years rr_AverageAnnualReturnYear10 5.24%
Since Inception rr_AverageAnnualReturnSinceInception 9.98%
Inception date rr_AverageAnnualReturnInceptionDate Jun. 12, 1987
BARON ASSET FUND | Return after taxes on distributions and sale of Fund shares | Retail Shares  
Risk/Return: rr_RiskReturnAbstract  
1 year rr_AverageAnnualReturnYear01 4.50%
5 years rr_AverageAnnualReturnYear05 10.53%
10 years rr_AverageAnnualReturnYear10 5.19%
Since Inception rr_AverageAnnualReturnSinceInception 9.61%
Inception date rr_AverageAnnualReturnInceptionDate Jun. 12, 1987
BARON ASSET FUND | Russell Midcap® Growth Index (reflects no deduction for fees, expenses or taxes)  
Risk/Return: rr_RiskReturnAbstract  
1 year rr_AverageAnnualReturnYear01 7.33%
5 years rr_AverageAnnualReturnYear05 13.51%
10 years rr_AverageAnnualReturnYear10 7.83%
Since Inception rr_AverageAnnualReturnSinceInception 9.66% [3]
BARON ASSET 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 9.45%
[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 January 29, 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.
[3] For the period June 30, 1987 to December 31, 2016.