Leases - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Leases [Abstract]
Right of use lease asset	$ 4,400
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	banf:AccruedInterestReceivableAndOtherAssetsMember
Operating lease liability	$ 4,330
Operating Lease, Liability, Statement of Financial Position [Extensible List]	banf:AccruedInterestPayableAndOtherLiabilitiesMember
Operating lease weighted-average remaining lease term	3 years 3 months 18 days
Operating lease weighted-average discount rate	3.10%
Operating lease revenue	$ 5,300	$ 6,100	$ 2,500