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Federal Home Loan Bank Advances - Additional Information (Detail) (USD $)
Dec. 31, 2014
Dec. 31, 2013
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]    
Total Borrowings $ 0us-gaap_AdvancesFromFederalHomeLoanBanks $ 4,500,000us-gaap_AdvancesFromFederalHomeLoanBanks
Mortgage Loans at Cost [Member]    
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]    
Advances, secured by mortgage loans $ 20,800,000us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedLoansReceivablePledgedForFederalHomeLoanBankDebt
/ us-gaap_FinancialInstrumentAxis
= fmb_MortgageLoansAtCostMember
$ 24,300,000us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedLoansReceivablePledgedForFederalHomeLoanBankDebt
/ us-gaap_FinancialInstrumentAxis
= fmb_MortgageLoansAtCostMember