Leases - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Leases [Abstract]
Rent expense for operating leases	$ 504,000	$ 392,000	$ 1,400,000	$ 1,200,000
Right of use lease asset	$ 4,300,000		$ 4,300,000
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	banf:AccruedInterestReceivableAndOtherAssetsMember		banf:AccruedInterestReceivableAndOtherAssetsMember
Operating lease liability	$ 4,191,000		$ 4,191,000
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	3.30%		3.30%
Operating lease revenue	$ 1,300,000	$ 1,600,000	$ 4,100,000	$ 4,700,000