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Fair Value Measurements - Additional Information (Detail) - USD ($) | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jun. 30, 2019 | Jun. 30, 2018 | Jun. 30, 2019 | Dec. 31, 2018
Debt Instrument Fair Value Disclosure [Abstract] |  |  |  |
Transfer of securities between levels of fair value hierarchy | $ 0 | $ 0 |  |
Days from origination after which mortgage loans are sold |  |  | 30 days |
Amount of non-financial assets (liabilities) measured at fair value on a recurring basis |  |  | $ 0 | $ 0
Non-financial assets or liabilities for which no impairment was provided |  |  | $ 0 | $ 0

X
- Definition Non financial assets liabilities for which no impairment provided. + References No definition available. + Details
Name: | banf_NonFinancialAssetsLiabilitiesForWhichNoImpairmentProvided
Namespace Prefix: | banf_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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