Leases - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Leases [Abstract]
Right of use lease asset	$ 3,500		$ 3,500
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	banf:AccruedInterestReceivableAndOtherAssetsMember		banf:AccruedInterestReceivableAndOtherAssetsMember
Operating lease liability	$ 3,427		$ 3,427
Operating Lease, Liability, Statement of Financial Position [Extensible List]	banf:AccruedInterestPayableAndOtherLiabilitiesMember		banf:AccruedInterestPayableAndOtherLiabilitiesMember
Operating lease weighted-average remaining lease term	3 years 4 months 24 days		3 years 4 months 24 days
Operating lease weighted-average discount rate	2.70%		2.70%
Operating lease revenue	$ 1,100	$ 1,300	$ 3,700	$ 4,100