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Fair Value Measurements - Additional Information (Detail) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Debt Instrument Fair Value Disclosure [Abstract]    
Days from origination after which mortgage loan are sold 30 days  
Transfer of securities between levels of fair value hierarchy $ 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityTransfersNet $ 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityTransfersNet
Amount of non-financial assets (liabilities) 0banf_NonFinancialAssetsLiabilitiesValue 0banf_NonFinancialAssetsLiabilitiesValue
Non-financial assets or liabilities for which no impairment was provided $ 0banf_NonFinancialAssetsLiabilitiesForWhichNoImpairmentProvided $ 0banf_NonFinancialAssetsLiabilitiesForWhichNoImpairmentProvided