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, 2020 USD ($)	Dec. 31, 2019 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 307,812	$ 222,293
Fair Value	71	214
State and Local Governments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	70,515	54,630
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,562	1,490
Level 3 [Member] | State and Local Governments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 1,562	$ 1,490
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	$ 7,703	$ 444
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	1.19	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	0.71	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	1.19	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	100	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	1.19	2.52
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	17.71	30.73
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