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Business Combination & Asset Purchase - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 13, 2014	Jul. 09, 2010
Loans At Acquisition Date [Line Items]
Deposits at close included in purchase	$ 29,500,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities	$ 29,500,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities
Loans included in purchase		11,400,000fmb_LoansIncludedInPurchase
Aggregate acquisition cost		(15,215,000)us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired
Recognition of core deposit intangible asset with acquired purchase	2,260,000fmb_RecognitionOfCoreDepositIntangibleAssetWithAcquiredPurchase
Number of years amortized over an estimated remaining economic useful life of deposits	7 years
Amortization expense	480,000us-gaap_AmortizationOfIntangibleAssets	319,000us-gaap_AmortizationOfIntangibleAssets	312,000us-gaap_AmortizationOfIntangibleAssets
Custar [Member]
Loans At Acquisition Date [Line Items]
Aggregate acquisition cost		1,200,000us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired / us-gaap_BusinessAcquisitionAxis = fmb_CustarMember
Recognition of core deposit intangible asset with acquired purchase				1,170,000fmb_RecognitionOfCoreDepositIntangibleAssetWithAcquiredPurchase / us-gaap_BusinessAcquisitionAxis = fmb_CustarMember
Hicksville [Member]
Loans At Acquisition Date [Line Items]
Recognition of core deposit intangible asset with acquired purchase					1,090,000fmb_RecognitionOfCoreDepositIntangibleAssetWithAcquiredPurchase / us-gaap_BusinessAcquisitionAxis = fmb_HicksvilleMember
Knisely [Member]
Loans At Acquisition Date [Line Items]
Recognition of core deposit intangible asset with acquired purchase	$ 1,100,000fmb_RecognitionOfCoreDepositIntangibleAssetWithAcquiredPurchase / us-gaap_BusinessAcquisitionAxis = fmb_KniselyMember

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Amortized over estimated remaining economic useful life of deposits term.

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

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Amount of liabilities assumed at the acquisition date.

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The cash outflow associated with the acquisition of a business, net of the cash acquired from the purchase.

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