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v2.4.1.9

Summary of Significant Accounting Policies - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2014 Multiplier | Dec. 31, 2013 | Jan. 16, 2015
Summary Of Significant Accounting Policies [Line Items] |  |  |
Period for federal funds purchased and sold | 1 day |  |
Average aggregate reserve | $ 7.1 fmb_ReserveFederalReserveBank | $ 6.5 fmb_ReserveFederalReserveBank |
Loan non-accrual classification period | 90 days |  |
Days to charged consumer loans to the value of the collateral | 120 days |  |
Maximum time for re-evaluation (in months) | 12 months |  |
Re-evaluation period for real estate | 2 years |  |
Excess fair value of goodwill amount | 48 us-gaap_ReportingUnitAmountOfFairValueInExcessOfCarryingAmount |  |
Percentage of carrying value for excess fair value | 50.16%us-gaap_ReportingUnitPercentageOfFairValueInExcessOfCarryingAmount |  |
Factor multiplication of value of goodwill | 12 fmb_FactorMultiplicationOfValueOfGoodwill |  |
Other real estate owned | $ 1.1 us-gaap_OtherRealEstate | $ 2.1 us-gaap_OtherRealEstate |
Percentage of likelihood of realization of recognized tax benefit, minimum | 50.00%fmb_PercentageOfLikelihoodOfRealizationOfRecognizedTaxBenefitMinimum |  |
Subsequent Event [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Stock Repurchase Program, Number of Shares Authorized to be Repurchased |  |  | 200,000 us-gaap_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
Maximum [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Maturity period of cash equivalents | 3 months |  |

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- Definition Original Maturity Period To Consider All Highly Liquid Investments Purchased As Cash Equivalents + References No definition available. + Details
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- Definition Period For Federal Funds Purchased And Sold + References No definition available. + Details
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- Definition Carrying amount of other real estate not separately disclosed, including, but not limited to, real estate investments, real estate loans that qualify as investments in real estate, and premises that are no longer used in operations may also be included in real estate owned. This does not include real estate assets taken in settlement of troubled loans through surrender or foreclosure. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.10(4)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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