Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	BARON INVESTMENT FUNDS TRUST (f/k/a BARON ASSET FUND)
Prospectus Date	rr_ProspectusDate	Jan. 26, 2018
Baron Durable Advantage Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Baron Durable Advantage Fund
Objective [Heading]	rr_ObjectiveHeading	Investment Goal
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The investment goal of the Baron Durable Advantage Fund (the “Fund”) is capital appreciation through investments primarily in securities of large-sized companies.
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. As Baron Durable Advantage Fund is a newly formed mutual fund, it has no prior turnover rate.
Other Expenses, New Fund, Based on Estimates [Text]	rr_OtherExpensesNewFundBasedOnEstimates	Because Baron Durable Advantage Fund is a new fund, “Other Expenses” are based on estimated amounts for the current fiscal year.
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 U.S. large-sized companies. The Adviser defines large-sized companies as those, at the time of purchase, with market capitalizations above the smallest market cap stock in the top 85% of the Russell 1000 Growth Index at reconstitution, or companies with market capitalizations above $10 billion, whichever is smaller. The Adviser seeks to invest primarily in what it believes are unique, well-managed, competitively advantaged businesses that generate significant excess free cash flow that is consistently returned to shareholders in the form of stock buybacks and/or dividends.
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. Large-Cap Companies. Because the Fund invests primarily in large-cap company securities, it may underperform other funds during periods when the Fund’s securities are out of favor.
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	As Baron Durable Advantage Fund is a newly formed mutual fund, it has no prior performance.
Performance One Year or Less [Text]	rr_PerformanceOneYearOrLess	As Baron Durable Advantage Fund is a newly formed mutual fund, it has no prior performance.
Baron Durable Advantage Fund | Retail Shares
Risk/Return:	rr_RiskReturnAbstract
Management Fee	rr_ManagementFeesOverAssets	0.65%
Distribution (12b-1) Fee	rr_DistributionAndService12b1FeesOverAssets	0.25%
Other Expenses	rr_OtherExpensesOverAssets	0.96%	[1]
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.86%
Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.91%)
Total Annual Fund Operating Expenses After Expense Reimbursements	rr_NetExpensesOverAssets	0.95%	[2]
1	rr_ExpenseExampleYear01	$ 97
3	rr_ExpenseExampleYear03	$ 303
Baron Durable Advantage Fund | Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
Management Fee	rr_ManagementFeesOverAssets	0.65%
Distribution (12b-1) Fee	rr_DistributionAndService12b1FeesOverAssets
Other Expenses	rr_OtherExpensesOverAssets	0.96%	[1]
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.61%
Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.91%)
Total Annual Fund Operating Expenses After Expense Reimbursements	rr_NetExpensesOverAssets	0.70%	[2]
1	rr_ExpenseExampleYear01	$ 72
3	rr_ExpenseExampleYear03	$ 224
Baron Durable Advantage Fund | R6 Shares
Risk/Return:	rr_RiskReturnAbstract
Management Fee	rr_ManagementFeesOverAssets	0.65%
Distribution (12b-1) Fee	rr_DistributionAndService12b1FeesOverAssets
Other Expenses	rr_OtherExpensesOverAssets	0.96%	[1]
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.61%
Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.91%)
Total Annual Fund Operating Expenses After Expense Reimbursements	rr_NetExpensesOverAssets	0.70%	[2]
1	rr_ExpenseExampleYear01	$ 72
3	rr_ExpenseExampleYear03	$ 224

[1] Because Baron Durable Advantage Fund is a new fund, "Other Expenses" are based on estimated amounts for the current fiscal year.

[2] BAMCO ("BAMCO" or the "Adviser") has agreed that for so long as it serves as the Adviser to the Fund, it will reimburse certain expenses of the Fund, limiting net annual operating expenses (portfolio transaction costs, interest, dividend and extraordinary expenses are not subject to the operating expense limitation) to 0.95% of average daily net assets of Retail Shares, 0.70% of average daily net assets of Institutional Shares and 0.70% of average daily net assets of R6 shares.