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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
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair Value $ 222,293 $ 168,447
Fair Value 214 600
State and Local Governments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair Value 54,630 40,028
Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair Value 1,490 1,427
Level 3 [Member] | State and Local Governments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair Value $ 1,490 $ 1,427
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 $ 444 $ 215
Valuation Technique us-gaap:MarketApproachValuationTechniqueMember us-gaap:MarketApproachValuationTechniqueMember
Unobservable Inputs fmao:MeasurementInputCollateralBasedPriceMember fmao:MeasurementInputCollateralBasedPriceMember
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] | 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] | Weighted Average [Member] | State and Local Governments [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Discount rate 2.52 3.51
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 30.73 12.38
Level 3 [Member] | Other Real Estate Owned-Commercial [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair Value $ 164  
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] | Minimum [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Discount rate 0  
Level 3 [Member] | Other Real Estate Owned-Commercial [Member] | Maximum [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Discount rate 20  
Level 3 [Member] | Other Real Estate Owned-Commercial [Member] | Weighted Average [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Discount rate 23.31