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Loans and Allowance for Loan Losses - ALLL for Impaired Loans with Specific Reserves (Detail) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Receivables [Abstract]      
Allowance for loss on impaired loans $ 1,559banf_ImpairedFinancingLoanWithSpecificReserveRelatedAllowance $ 1,578banf_ImpairedFinancingLoanWithSpecificReserveRelatedAllowance $ 2,170banf_ImpairedFinancingLoanWithSpecificReserveRelatedAllowance
Recorded balance of impaired loans 3,673us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment 5,283us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment 6,185us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment
Average recorded investment $ 4,478banf_ImpairedFinancingReceivableAverageRecordedValueOfInvestment $ 5,734banf_ImpairedFinancingReceivableAverageRecordedValueOfInvestment $ 8,416banf_ImpairedFinancingReceivableAverageRecordedValueOfInvestment