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Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)
In Millions, except Share data, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Multiplier
Dec. 31, 2013
Jan. 16, 2015
Summary Of Significant Accounting Policies [Line Items]      
Period for federal funds purchased and sold 1 day    
Average aggregate reserve $ 7.1fmb_ReserveFederalReserveBank $ 6.5fmb_ReserveFederalReserveBank  
Loan non-accrual classification period 90 days    
Days to charged consumer loans to the value of the collateral 120 days    
Maximum time for re-evaluation (in months) 12 months    
Re-evaluation period for real estate 2 years    
Excess fair value of goodwill amount 48us-gaap_ReportingUnitAmountOfFairValueInExcessOfCarryingAmount    
Percentage of carrying value for excess fair value 50.16%us-gaap_ReportingUnitPercentageOfFairValueInExcessOfCarryingAmount    
Factor multiplication of value of goodwill 12fmb_FactorMultiplicationOfValueOfGoodwill    
Other real estate owned $ 1.1us-gaap_OtherRealEstate $ 2.1us-gaap_OtherRealEstate  
Percentage of likelihood of realization of recognized tax benefit, minimum 50.00%fmb_PercentageOfLikelihoodOfRealizationOfRecognizedTaxBenefitMinimum    
Subsequent Event [Member]      
Summary Of Significant Accounting Policies [Line Items]      
Stock Repurchase Program, Number of Shares Authorized to be Repurchased     200,000us-gaap_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
Maximum [Member]      
Summary Of Significant Accounting Policies [Line Items]      
Maturity period of cash equivalents 3 months