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Cash Due from Banks, Interest-Bearing Deposits and Federal Funds Sold - Summary of Average Amount of Required Reserves (Detail) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
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Average Amount Of Required Reserves [Abstract] |  |
Vault cash and funds on deposit with the Federal Reserve Bank | $ 1,429,410 | $ 1,738,523
Average required reserves |  | $ 63,842

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- Definition Average amount of required reserves. + References No definition available. + Details
Name: | banf_AverageAmountOfRequiredReservesAbstract
Namespace Prefix: | banf_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Vault cash equivalents reserves. + References No definition available. + Details
Name: | banf_VaultCashEquivalentsReserves
Namespace Prefix: | banf_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Amount of pass-through reserves deposited with Federal Reserve Banks from member institutions of Federal Home Loan Banks (FHLBanks). + References No definition available. + Details
Name: | us-gaap_CashPassThroughReserveFederalHomeLoanBank
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant