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Income Taxes - Additional Information (Detail) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Income Taxes [Line Items] |  |  |
Securities transactions income tax expense | $ 574,000 banf_IncomeTaxExpenseOnSecuritiesTransaction | $ 147,000 banf_IncomeTaxExpenseOnSecuritiesTransaction | $ 1,720,000 banf_IncomeTaxExpenseOnSecuritiesTransaction
Recognition of interest and penalties related to unrecognized tax benefits | $ 0 us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense | $ 0 us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense | $ 0 us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense
Earliest Tax Year [Member] |  |  |
Income Taxes [Line Items] |  |  |
Year under examination | 2012 |  |
Latest Tax Year [Member] |  |  |
Income Taxes [Line Items] |  |  |
Year under examination | 2014 |  |

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- Definition Income tax expense on securities transaction. + References No definition available. + Details
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- Definition Tax year that remains open to examination under enacted tax laws, in CCYY format. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32718-109319 + Details
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- Definition Amount of expense for interest on an underpayment of income taxes and penalties related to a tax position claimed or expected to be claimed in the tax return. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32718-109319 + Details
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