S000004424 [Member] Shareholder Fees - Ultra-Small Company Market Fund - Class N	Jun. 30, 2025
Prospectus [Line Items]
Maximum Sales Charge Imposed on Purchases (as a percentage of Offering Price)	0.00%
Maximum Sales Charge on Reinvested Dividends and Distributions (as a percentage)	0.00%
Redemption Fee (as a percentage of Amount Redeemed)	(2.00%)
Exchange Fee (as a percentage of Amount Redeemed)	0.00%

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Form N-1A
-Section 3
-Subsection instruction
-Paragraph 2

Name:	oef_ExchangeFeeOverRedemption
Namespace Prefix:	oef_
Data Type:	oef:nonNegativePure4ItemType
Balance Type:	na
Period Type:	duration

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Form N-1A
-Section 3
-Subsection instruction
-Paragraph 2

Name:	oef_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice
Namespace Prefix:	oef_
Data Type:	oef:nonNegativePure4ItemType
Balance Type:	na
Period Type:	duration

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Form N-1A
-Section 3
-Subsection instruction
-Paragraph 2

Name:	oef_MaximumSalesChargeOnReinvestedDividendsAndDistributionsOverOther
Namespace Prefix:	oef_
Data Type:	oef:nonNegativePure4ItemType
Balance Type:	na
Period Type:	duration

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Form N-1A
-Section 1
-Subsection a

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Form N-1A
-Section 3
-Subsection instruction
-Paragraph 2

Name:	oef_RedemptionFeeOverRedemption
Namespace Prefix:	oef_
Data Type:	oef:nonPositivePure4ItemType
Balance Type:	na
Period Type:	duration

Name:	dei_LegalEntityAxis=bfi_S000004424Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: