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Commitments and Contingent Liabilities - Summary of Financial Instruments with Off-Balance-Sheet Risks (Detail) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
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Commitments And Contingencies Disclosure [Abstract] |  |
Loan commitments | $ 1,779,857 | $ 1,618,001
Stand-by letters of credit | $ 96,311 | $ 64,702

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- Definition Unfunded loan commitments. + References No definition available. + Details
Name: | banf_UnfundedLoanCommitments
Namespace Prefix: | banf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_CommitmentsAndContingenciesDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition The total amount of the contingent obligation under letters of credit outstanding as of the reporting date. + References No definition available. + Details
Name: | us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant