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Securities - Amortized Cost and Fair Value of Debt Securities, by Contractual Maturity (Detail) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Available-for-sale Securities, Debt Maturities [Abstract]    
Amortized Cost, One year or less $ 23,288us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost  
Amortized Cost, After one year through five years 157,000us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost  
Amortized Cost, After five years through ten years 31,333us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost  
Amortized Cost, After ten years 7,131us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost  
Amortized Cost, Total 218,752us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis  
Amortized Cost, Mortgage-backed securities 29,067fmb_MortgageBackedSecuritiesAvailableForSaleAmortizedCost  
Total, Amortized Cost 247,819fmb_AvailableForSaleSecuritiesDebtMaturitiesAmortizedCostIncludingMortgageBackedSecurities  
Fair Value, One year or less 23,505us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue  
Fair Value, After one year through five years 156,781us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue  
Fair Value, After five years through ten years 31,470us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue  
Fair Value, After ten years 7,174us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue  
Fair Value, Total 218,930fmb_AvailableForSaleSecuritiesDebtMaturitiesFairValueExcludingMortgageBackedSecurities  
Fair Value, Mortgage-backed securities 29,562us-gaap_MortgageBackedSecuritiesAvailableForSaleFairValueDisclosure  
Total, Fair Value $ 248,492us-gaap_AvailableForSaleSecuritiesDebtSecurities $ 324,509us-gaap_AvailableForSaleSecuritiesDebtSecurities