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Leases - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Leases [Abstract]
Rent expense for operating leases	$ 357,000	$ 381,000
Right of use lease asset	$ 4,300,000
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	banf:AccruedInterestReceivableAndOtherAssetsMember
Operating lease liability	$ 4,300,000
Operating Lease, Liability, Statement of Financial Position [Extensible List]	banf:AccruedInterestPayableAndOtherLiabilitiesMember
Operating lease weighted-average remaining lease term	4 years 4 months 24 days
Operating lease weighted-average discount rate	3.60%
Operating lease revenue	$ 1,500,000

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Amount of operating lease expense. Excludes sublease income.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 20
-Section 45
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Amount of operating lease income from lease payments and variable lease payments paid and payable to lessor. Includes, but is not limited to, variable lease payments not included in measurement of lease receivable.

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Present value of lessee's discounted obligation for lease payments from operating lease.

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Indicates line item in statement of financial position that includes operating lease liability.

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Amount of lessee's right to use underlying asset under operating lease.

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Indicates line item in statement of financial position that includes operating lease right-of-use asset.

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Weighted average discount rate for operating lease calculated at point in time.

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Weighted average remaining lease term for operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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