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Servicing - Additional Information (Detail) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013
Transfers and Servicing [Abstract] |  |
Loans serviced for others unpaid principal amount | $ 275 fmb_LoansServicedForOthersUnpaidPrincipalAmount | $ 282 fmb_LoansServicedForOthersUnpaidPrincipalAmount
Capitalized service right assets included in other assets | 2.0 fmb_CapitalizedServiceRightAssetsIncludedInOtherAssets | 2.1 fmb_CapitalizedServiceRightAssetsIncludedInOtherAssets
Fair market value of the capitalized servicing rights | $ 2.9 fmb_CapitalizedServiceRightAssetsFairMarketValue | $ 2.8 fmb_CapitalizedServiceRightAssetsFairMarketValue
Prepayment rate | 9.60%us-gaap_ServicingAssetsAndServicingLiabilitiesAtFairValueAssumptionsUsedToEstimateFairValuePrepaymentSpeed | 11.20%us-gaap_ServicingAssetsAndServicingLiabilitiesAtFairValueAssumptionsUsedToEstimateFairValuePrepaymentSpeed
Discount yield | 6.48%us-gaap_ServicingAssetsAndServicingLiabilitiesAtFairValueAssumptionsUsedToEstimateFairValueDiscountRate | 8.25%us-gaap_ServicingAssetsAndServicingLiabilitiesAtFairValueAssumptionsUsedToEstimateFairValueDiscountRate

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- Definition Capitalized service right assets fair market value. + References No definition available. + Details
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Balance Type: | debit
Period Type: | instant

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- Definition Capitalized service right assets included in other assets. + References No definition available. + Details
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Namespace Prefix: | fmb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Loans serviced for others unpaid principal amount. + References No definition available. + Details
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Namespace Prefix: | fmb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Discount rate used to estimate the fair value of servicing assets and servicing liabilities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 50 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=7882072&loc=d3e122596-111746 + Details
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- Definition Estimated rate of prepayments of principal on servicing assets and servicing liabilities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 50 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=7882072&loc=d3e122596-111746 + Details
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Balance Type: | na
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- Details
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