Leases - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jan. 01, 2019
Leases [Abstract]
Rent expense for operating leases	$ 343,000	$ 395,000	$ 700,000	$ 776,000
Right of use lease asset	$ 5,300,000		$ 5,300,000		$ 4,300,000
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	banf:AccruedInterestReceivableAndOtherAssetsMember		banf:AccruedInterestReceivableAndOtherAssetsMember
Operating lease liability	$ 5,200,000		$ 5,200,000		$ 4,300,000
Operating Lease, Liability, Statement of Financial Position [Extensible List]	banf:AccruedInterestPayableAndOtherLiabilitiesMember		banf:AccruedInterestPayableAndOtherLiabilitiesMember
Operating lease weighted-average remaining lease term	4 years 6 months		4 years 6 months
Operating lease weighted-average discount rate	3.60%		3.60%
Operating lease revenue	$ 1,500,000		$ 3,100,000