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. 28, 2013
Supplement [Text Block]	bift_SupplementTextBlock	B A R O N F U N D S^® Baron Fifth Avenue Growth Fund Supplement dated August 22, 2013 to Prospectus dated January 28, 2013 IMPORTANT NOTICE REGARDING CHANGE IN INVESTMENT POLICY Effective October 21, 2013, in connection with the principal investment strategy of Baron Fifth Avenue Growth Fund, the Prospectus of the Baron Funds^® (the "Funds") is modified as follows: On page 28 of the Prospectus, the first sentence of the paragraph under "Principal Investment Strategies of the Fund" that reads: "The Fund invests primarily in equity securities in the form of common stock of large-sized growth companies with market capitalizations of more than $5 billion at the time of purchase selected for their capital appreciation potential." is deleted in its entirety and replaced with "The Fund invests primarily in equity securities in the form of common stock of large-sized growth companies with market capitalizations of more than $10 billion at the time of purchase selected for their capital appreciation potential." This information supplements the Prospectus dated January 28, 2013. 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	B A R O N F U N D S^® Baron Fifth Avenue Growth Fund Supplement dated August 22, 2013 to Prospectus dated January 28, 2013 IMPORTANT NOTICE REGARDING CHANGE IN INVESTMENT POLICY Effective October 21, 2013, in connection with the principal investment strategy of Baron Fifth Avenue Growth Fund, the Prospectus of the Baron Funds^® (the "Funds") is modified as follows: On page 28 of the Prospectus, the first sentence of the paragraph under "Principal Investment Strategies of the Fund" that reads: "The Fund invests primarily in equity securities in the form of common stock of large-sized growth companies with market capitalizations of more than $5 billion at the time of purchase selected for their capital appreciation potential." is deleted in its entirety and replaced with "The Fund invests primarily in equity securities in the form of common stock of large-sized growth companies with market capitalizations of more than $10 billion at the time of purchase selected for their capital appreciation potential." This information supplements the Prospectus dated January 28, 2013. 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.

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. 28, 2013

Supplement [Text Block]

bift_SupplementTextBlock

B A R O N

F U N D S^®

Baron Fifth Avenue Growth Fund

Supplement dated August 22, 2013 to Prospectus dated January 28, 2013

IMPORTANT NOTICE REGARDING CHANGE IN INVESTMENT POLICY

Effective October 21, 2013, in connection with the principal investment strategy of Baron Fifth Avenue Growth Fund, the Prospectus of the Baron Funds^® (the "Funds") is modified as follows:

On page 28 of the Prospectus, the first sentence of the paragraph under "Principal Investment Strategies of the Fund" that reads: "The Fund invests primarily in equity securities in the form of common stock of large-sized growth companies with market capitalizations of more than $5 billion at the time of purchase selected for their capital appreciation potential." is deleted in its entirety and replaced with "The Fund invests primarily in equity securities in the form of common stock of large-sized growth companies with market capitalizations of more than $10 billion at the time of purchase selected for their capital appreciation potential."

This information supplements the Prospectus dated January 28, 2013. 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