Note 5 - Foreclosed Assets - Activity in the Valuation Allowance (Details) - USD ($)	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Beginning balance	$ 0	$ 0	$ 0	$ 0
Additions charged to expense	281,000	0	281,000	0
Reductions from sales	0	0	0	0
Ending balance	$ 281,000	$ 0	$ 281,000	$ 0

Amount of the valuation allowance for foreclosed assets that was applied in the period against the carrying amount of foreclosed assets that was sold, written down or written off.

No definition available.

Name:	bfin_ForeclosedAssetsValuationAllowanceAmountsApplied
Namespace Prefix:	bfin_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration