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

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- Definition Amount of borrowings as of the balance sheet date from the Federal Home Loan Bank, which are primarily used to cover shortages in the required reserve balance and liquidity shortages. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 470 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6479118&loc=d3e64650-112822 + Details
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Balance Type: | credit
Period Type: | instant

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- Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
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- Definition The carrying amount, as of the date of the latest financial statement presented, of loans receivable which are owned but transferred to serve as collateral for the payment of FHLB debt obligations, and for which the transferee is not permitted to sell or re-pledge them to an unrelated party. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 30 -Section 50 -Paragraph 1A -Subparagraph (b)(1)(i) -URI http://asc.fasb.org/extlink&oid=34720971&loc=SL6224234-111729 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
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Balance Type: |
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