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Off Balance Sheet Activities - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013
Commitments and Contingencies Disclosure [Abstract] |  |
Allowance for credit losses relates to unfunded loan commitments | $ 207 fmb_AllowanceForCreditLossesRelatesToUnfundedLoanCommitments | $ 163 fmb_AllowanceForCreditLossesRelatesToUnfundedLoanCommitments

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- Definition Allowance For Credit Losses Relates To Unfunded Loan Commitments + References No definition available. + Details
Name: | fmb_AllowanceForCreditLossesRelatesToUnfundedLoanCommitments
Namespace Prefix: | fmb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
Name: | us-gaap_CommitmentsAndContingenciesDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration