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