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Fair Value of Instruments - Quantitative Information about Unobservable Inputs Used in Recurring and Nonrecurring Level 3 Fair Value Measurements (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
U.S. Government Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 7,931us-gaap_AssetsFairValueDisclosure / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember	$ 10,346us-gaap_AssetsFairValueDisclosure / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Valuation Technique	Discounted Cash Flow	Discounted Cash Flow
Unobservable Inputs	Credit strength of underlying project or entity / Discount rate	Credit strength of underlying project or entity / Discount rate
Impaired Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,456us-gaap_AssetsFairValueDisclosure / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fmb_ImpairedLoansMember	1,924us-gaap_AssetsFairValueDisclosure / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fmb_ImpairedLoansMember
Valuation Technique	Collateral based measurements	Collateral based measurements
Unobservable Inputs	Discount to reflect current market conditions and ultimate collectability	Discount to reflect current market conditions and ultimate collectability
Other real estate owned - residential [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	21us-gaap_AssetsFairValueDisclosure / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fmb_OtherRealEstateResidentialMember	964us-gaap_AssetsFairValueDisclosure / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fmb_OtherRealEstateResidentialMember
Valuation Technique	Appraisals	Appraisals
Unobservable Inputs	Discount to reflect current market	Discount to reflect current market
Other real estate owned - commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 23us-gaap_AssetsFairValueDisclosure / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fmb_OtherRealEstateCommercialMember	$ 1,127us-gaap_AssetsFairValueDisclosure / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fmb_OtherRealEstateCommercialMember
Valuation Technique	Appraisals	Appraisals
Unobservable Inputs	Discount to reflect current market	Discount to reflect current market
Minimum [Member] | U.S. Government Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weighted Average	0.00%us-gaap_FairValueInputsDiscountRate / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_RangeAxis = us-gaap_MinimumMember	0.00%us-gaap_FairValueInputsDiscountRate / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_RangeAxis = us-gaap_MinimumMember
Minimum [Member] | Impaired Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weighted Average	0.00%us-gaap_FairValueInputsDiscountRate / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fmb_ImpairedLoansMember / us-gaap_RangeAxis = us-gaap_MinimumMember	0.00%us-gaap_FairValueInputsDiscountRate / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fmb_ImpairedLoansMember / us-gaap_RangeAxis = us-gaap_MinimumMember
Minimum [Member] | Other real estate owned - residential [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weighted Average	0.00%us-gaap_FairValueInputsDiscountRate / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fmb_OtherRealEstateResidentialMember / us-gaap_RangeAxis = us-gaap_MinimumMember	0.00%us-gaap_FairValueInputsDiscountRate / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fmb_OtherRealEstateResidentialMember / us-gaap_RangeAxis = us-gaap_MinimumMember
Minimum [Member] | Other real estate owned - commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weighted Average	0.00%us-gaap_FairValueInputsDiscountRate / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fmb_OtherRealEstateCommercialMember / us-gaap_RangeAxis = us-gaap_MinimumMember	0.00%us-gaap_FairValueInputsDiscountRate / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fmb_OtherRealEstateCommercialMember / us-gaap_RangeAxis = us-gaap_MinimumMember
Maximum [Member] | U.S. Government Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weighted Average	5.00%us-gaap_FairValueInputsDiscountRate / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_RangeAxis = us-gaap_MaximumMember	5.00%us-gaap_FairValueInputsDiscountRate / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_RangeAxis = us-gaap_MaximumMember
Maximum [Member] | Impaired Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weighted Average	50.00%us-gaap_FairValueInputsDiscountRate / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fmb_ImpairedLoansMember / us-gaap_RangeAxis = us-gaap_MaximumMember	50.00%us-gaap_FairValueInputsDiscountRate / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fmb_ImpairedLoansMember / us-gaap_RangeAxis = us-gaap_MaximumMember
Maximum [Member] | Other real estate owned - residential [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weighted Average	20.00%us-gaap_FairValueInputsDiscountRate / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fmb_OtherRealEstateResidentialMember / us-gaap_RangeAxis = us-gaap_MaximumMember	20.00%us-gaap_FairValueInputsDiscountRate / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fmb_OtherRealEstateResidentialMember / us-gaap_RangeAxis = us-gaap_MaximumMember
Maximum [Member] | Other real estate owned - commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weighted Average	20.00%us-gaap_FairValueInputsDiscountRate / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fmb_OtherRealEstateCommercialMember / us-gaap_RangeAxis = us-gaap_MaximumMember	20.00%us-gaap_FairValueInputsDiscountRate / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fmb_OtherRealEstateCommercialMember / us-gaap_RangeAxis = us-gaap_MaximumMember

X

- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)

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- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

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Description of significant assumptions used by management to measure fair value.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

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Description of the inputs and valuation technique(s) used to measure fair value.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

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