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Restrictions of Dividends & Inter-company Borrowings - Additional Information (Detail) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014
Equity [Abstract] |
Dividends may be paid without prior regulatory approval | $ 14.7 fmb_DividendsMayBePaidWithoutPriorRegulatoryApproval

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- Definition Dividends May Be Paid Without Prior Regulatory Approval + References No definition available. + Details
Name: | fmb_DividendsMayBePaidWithoutPriorRegulatoryApproval
Namespace Prefix: | fmb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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