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Fair Value of Financial Instruments - Quantitative Information about Unobservable Inputs Used in Recurring and Nonrecurring Level 3 Fair Value Measurements (Detail)
$ in Thousands
Sep. 30, 2018
USD ($)
Dec. 31, 2017
USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair Value $ 183,075 $ 196,398
State and Local Governments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair Value 51,937 55,444
Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair Value 1,393 1,428
Level 3 [Member] | State and Local Governments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair Value $ 1,393 $ 1,428
Valuation Technique us-gaap:ValuationTechniqueDiscountedCashFlowMember us-gaap:ValuationTechniqueDiscountedCashFlowMember
Unobservable Inputs us-gaap:MeasurementInputDiscountRateMember us-gaap:MeasurementInputDiscountRateMember
Level 3 [Member] | Collateral Dependent Impaired Loans [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair Value $ 822 $ 508
Valuation Technique us-gaap:MarketApproachValuationTechniqueMember us-gaap:MarketApproachValuationTechniqueMember
Unobservable Inputs fmao:MeasurementInputCollateralBasedPriceMember fmao:MeasurementInputCollateralBasedPriceMember
Level 3 [Member] | Other Real Estate Owned-Residential [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair Value   $ 22
Valuation Technique us-gaap:MarketApproachValuationTechniqueMember us-gaap:MarketApproachValuationTechniqueMember
Unobservable Inputs us-gaap:MeasurementInputAppraisedValueMember us-gaap:MeasurementInputAppraisedValueMember
Level 3 [Member] | Other Real Estate Owned-Commercial [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair Value   $ 266
Valuation Technique us-gaap:MarketApproachValuationTechniqueMember us-gaap:MarketApproachValuationTechniqueMember
Unobservable Inputs us-gaap:MeasurementInputAppraisedValueMember us-gaap:MeasurementInputAppraisedValueMember
Level 3 [Member] | Minimum [Member] | State and Local Governments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Discount rate 0 0
Level 3 [Member] | Minimum [Member] | Collateral Dependent Impaired Loans [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Discount rate 0 0
Level 3 [Member] | Minimum [Member] | Other Real Estate Owned-Residential [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Discount rate   0
Level 3 [Member] | Minimum [Member] | Other Real Estate Owned-Commercial [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Discount rate   0
Level 3 [Member] | Maximum [Member] | State and Local Governments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Discount rate 5 5
Level 3 [Member] | Maximum [Member] | Collateral Dependent Impaired Loans [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Discount rate 50 50
Level 3 [Member] | Maximum [Member] | Other Real Estate Owned-Residential [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Discount rate   20
Level 3 [Member] | Maximum [Member] | Other Real Estate Owned-Commercial [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Discount rate   20
Level 3 [Member] | Weighted Average [Member] | State and Local Governments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Discount rate 3.92 3.68
Level 3 [Member] | Weighted Average [Member] | Collateral Dependent Impaired Loans [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Discount rate 15.28 17.28
Level 3 [Member] | Weighted Average [Member] | Other Real Estate Owned-Residential [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Discount rate   2.22
Level 3 [Member] | Weighted Average [Member] | Other Real Estate Owned-Commercial [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Discount rate   5.15