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
Supplement [Text Block]	bift_SupplementTextBlock	BARON FUNDS^® Baron Fifth Avenue Growth Fund Baron Durable Advantage Fund Supplement dated June 7, 2018 to Prospectus dated January 26, 2018 IMPORTANT NOTICE REGARDING CHANGE IN THE INVESTMENT STRATEGY OF BARON FIFTH AVENUE GROWTH FUND AND BARON DURABLE ADVANTAGE FUND Effective Monday, August 6, 2018, in connection with the principal investment strategy of Baron Fifth Avenue Growth Fund and Baron Durable Advantage Fund (the "Funds"), the Prospectus of the Funds is modified as follows: On page 32 of the Prospectus, the following sentence under "Principal Investment Strategies of the Fund," which reads as follows: "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." is deleted in its entirety and replaced with the following: "The Adviser defines large-sized companies as those, at the time of purchase, with market capitalizations no smaller than the top 85^th percentile by total market capitalization of the Russell 1000 Growth Index at June 30, or companies with market capitalizations above $10 billion, whichever is smaller." On page 46 of the Prospectus, the following sentence under "Principal Investment Strategies of the Fund," which reads as follows: "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." is deleted in its entirety and replaced with the following: "The Adviser defines large-sized companies as those, at the time of purchase, with market capitalizations no smaller than the top 90^th percentile by total market capitalization of the S&P 500 Index at June 30, or companies with market capitalizations above $10 billion, whichever is smaller." This information supplements the Prospectus dated January 26, 2018. This Supplement and the Prospectus constitute a current prospectus. To request another copy of the Prospectus, please call 1-800-992-2766 or visit our website at www.BaronFunds.com.
BARON FIFTH AVENUE GROWTH FUND
Risk/Return:	rr_RiskReturnAbstract
Supplement [Text Block]	bift_SupplementTextBlock	BARON FUNDS^® Baron Fifth Avenue Growth Fund Supplement dated June 7, 2018 to Prospectus dated January 26, 2018 IMPORTANT NOTICE REGARDING CHANGE IN THE INVESTMENT STRATEGY OF BARON FIFTH AVENUE GROWTH FUND AND BARON DURABLE ADVANTAGE FUND Effective Monday, August 6, 2018, in connection with the principal investment strategy of Baron Fifth Avenue Growth Fund and Baron Durable Advantage Fund (the "Funds"), the Prospectus of the Funds is modified as follows: On page 32 of the Prospectus, the following sentence under "Principal Investment Strategies of the Fund," which reads as follows: "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." is deleted in its entirety and replaced with the following: "The Adviser defines large-sized companies as those, at the time of purchase, with market capitalizations no smaller than the top 85^th percentile by total market capitalization of the Russell 1000 Growth Index at June 30, or companies with market capitalizations above $10 billion, whichever is smaller." This information supplements the Prospectus dated January 26, 2018. This Supplement and the Prospectus constitute a current prospectus. To request another copy of the Prospectus, please call 1-800-992-2766 or visit our website at www.BaronFunds.com.
Baron Durable Advantage Fund
Risk/Return:	rr_RiskReturnAbstract
Supplement [Text Block]	bift_SupplementTextBlock	BARON FUNDS^® Baron Durable Advantage Fund Supplement dated June 7, 2018 to Prospectus dated January 26, 2018 IMPORTANT NOTICE REGARDING CHANGE IN THE INVESTMENT STRATEGY OF BARON FIFTH AVENUE GROWTH FUND AND BARON DURABLE ADVANTAGE FUND Effective Monday, August 6, 2018, in connection with the principal investment strategy of Baron Fifth Avenue Growth Fund and Baron Durable Advantage Fund (the "Funds"), the Prospectus of the Funds is modified as follows: On page 46 of the Prospectus, the following sentence under "Principal Investment Strategies of the Fund," which reads as follows: "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." is deleted in its entirety and replaced with the following: "The Adviser defines large-sized companies as those, at the time of purchase, with market capitalizations no smaller than the top 90^th percentile by total market capitalization of the S&P 500 Index at June 30, or companies with market capitalizations above $10 billion, whichever is smaller." This information supplements the Prospectus dated January 26, 2018. This Supplement and the Prospectus constitute a current prospectus. To request another copy of the Prospectus, please call 1-800-992-2766 or visit our website at www.BaronFunds.com.

dei_EntityRegistrantName

BARON INVESTMENT FUNDS TRUST (f/k/a BARON ASSET FUND)

bift_SupplementTextBlock

BARON

FUNDS^®

Baron Fifth Avenue Growth Fund

Baron Durable Advantage Fund

Supplement dated June 7, 2018

to Prospectus dated January 26, 2018

IMPORTANT NOTICE REGARDING CHANGE IN THE INVESTMENT STRATEGY OF BARON FIFTH AVENUE GROWTH FUND AND BARON DURABLE ADVANTAGE FUND

Effective Monday, August 6, 2018, in connection with the principal investment strategy of Baron Fifth Avenue Growth Fund and Baron Durable Advantage Fund (the "Funds"), the Prospectus of the Funds is modified as follows:

On page 32 of the Prospectus, the following sentence under "Principal Investment Strategies of the Fund," which reads as follows: "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." is deleted in its entirety and replaced with the following: "The Adviser defines large-sized companies as those, at the time of purchase, with market capitalizations no smaller than the top 85^th percentile by total market capitalization of the Russell 1000 Growth Index at June 30, or companies with market capitalizations above $10 billion, whichever is smaller."

On page 46 of the Prospectus, the following sentence under "Principal Investment Strategies of the Fund," which reads as follows: "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." is deleted in its entirety and replaced with the following: "The Adviser defines large-sized companies as those, at the time of purchase, with market capitalizations no smaller than the top 90^th percentile by total market capitalization of the S&P 500 Index at June 30, or companies with market capitalizations above $10 billion, whichever is smaller."

This information supplements the Prospectus dated January 26, 2018. This Supplement and the Prospectus constitute a current prospectus. To request another copy of the Prospectus, please call 1-800-992-2766 or visit our website at www.BaronFunds.com.

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

bift_SupplementTextBlock

BARON

FUNDS^®

Baron Durable Advantage Fund

Supplement dated June 7, 2018

to Prospectus dated January 26, 2018

IMPORTANT NOTICE REGARDING CHANGE IN THE INVESTMENT STRATEGY OF BARON FIFTH AVENUE GROWTH FUND AND BARON DURABLE ADVANTAGE FUND

Effective Monday, August 6, 2018, in connection with the principal investment strategy of Baron Fifth Avenue Growth Fund and Baron Durable Advantage Fund (the "Funds"), the Prospectus of the Funds is modified as follows:

On page 46 of the Prospectus, the following sentence under "Principal Investment Strategies of the Fund," which reads as follows: "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." is deleted in its entirety and replaced with the following: "The Adviser defines large-sized companies as those, at the time of purchase, with market capitalizations no smaller than the top 90^th percentile by total market capitalization of the S&P 500 Index at June 30, or companies with market capitalizations above $10 billion, whichever is smaller."

This information supplements the Prospectus dated January 26, 2018. This Supplement and the Prospectus constitute a current prospectus. To request another copy of the Prospectus, please call 1-800-992-2766 or visit our website at www.BaronFunds.com.