Leases - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Leases [Abstract]
Rent expense for operating leases	$ 463,000	$ 391,000
Right of use lease asset	$ 5,200,000
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	banf:AccruedInterestReceivableAndOtherAssetsMember
Operating lease liability	$ 5,045,000
Operating Lease, Liability, Statement of Financial Position [Extensible List]	banf:AccruedInterestPayableAndOtherLiabilitiesMember
Operating lease weighted-average remaining lease term	3 years 4 months 24 days
Operating lease weighted-average discount rate	3.40%
Operating lease revenue	$ 1,400,000	$ 1,500,000