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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 | $ 29,500,000 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities | $ 29,500,000 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities |  |  |
Loans included in purchase |  | 11,400,000 fmb_LoansIncludedInPurchase |  |  |
Aggregate acquisition cost |  | (15,215,000)us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired |  |  |
Recognition of core deposit intangible asset with acquired purchase | 2,260,000 fmb_RecognitionOfCoreDepositIntangibleAssetWithAcquiredPurchase |  |  |  |
Number of years amortized over an estimated remaining economic useful life of deposits | 7 years |  |  |  |
Amortization expense | 480,000 us-gaap_AmortizationOfIntangibleAssets | 319,000 us-gaap_AmortizationOfIntangibleAssets | 312,000 us-gaap_AmortizationOfIntangibleAssets |  |
Custar [Member] |  |  |  |  |
Loans At Acquisition Date [Line Items] |  |  |  |  |
Aggregate acquisition cost |  | 1,200,000 us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired / us-gaap_BusinessAcquisitionAxis = fmb_CustarMember |  |  |
Recognition of core deposit intangible asset with acquired purchase |  |  |  | 1,170,000 fmb_RecognitionOfCoreDepositIntangibleAssetWithAcquiredPurchase / us-gaap_BusinessAcquisitionAxis = fmb_CustarMember |
Hicksville [Member] |  |  |  |  |
Loans At Acquisition Date [Line Items] |  |  |  |  |
Recognition of core deposit intangible asset with acquired purchase |  |  |  |  | 1,090,000 fmb_RecognitionOfCoreDepositIntangibleAssetWithAcquiredPurchase / us-gaap_BusinessAcquisitionAxis = fmb_HicksvilleMember
Knisely [Member] |  |  |  |  |
Loans At Acquisition Date [Line Items] |  |  |  |  |
Recognition of core deposit intangible asset with acquired purchase | $ 1,100,000 fmb_RecognitionOfCoreDepositIntangibleAssetWithAcquiredPurchase / us-gaap_BusinessAcquisitionAxis = fmb_KniselyMember |  |  |  |

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- Definition The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6388964&loc=d3e16225-109274 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275 + Details
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- Definition The cash outflow associated with the acquisition of a business, net of the cash acquired from the purchase. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Investing Activities -URI http://asc.fasb.org/extlink&oid=6516133 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3213-108585 + Details
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