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Fair Value Measurements - Additional Information (Detail) - USD ($)	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Debt Instrument Fair Value Disclosure [Abstract]
Days from origination after which mortgage loan are sold	30 days
Transfer of securities between levels of fair value hierarchy	$ 0	$ 0
Amount of non-financial assets (liabilities) measured at fair value on a recurring basis	$ 0		$ 0

X

- Definition

Amount of non-financial assets (liabilities) measured at fair value on a recurring basis.

+ References

No definition available.

+ Details

Name:	banf_NonFinancialAssetsLiabilitiesValue
Namespace Prefix:	banf_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Period for sale of mortgage loan from origination.

+ References

No definition available.

+ Details

Name:	banf_PeriodForSaleOfMortgageLoanFromOrigination
Namespace Prefix:	banf_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentFairValueDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Amount of transfers of financial instrument classified as a liability into (out of) level 3 of the fair value hierarchy.

+ References

No definition available.

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityTransfersNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration