Fair Value of Financial Instruments - Quantitative Information about Unobservable Inputs Used in Recurring and Nonrecurring Level 3 Fair Value Measurements (Detail) $ in Thousands	Jun. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair Value	$ 407,500	$ 307,812
State and Local Governments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair Value	64,963	70,515
Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair Value	3,948	1,562
Level 3 [Member] | State and Local Governments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair Value	$ 3,948	$ 1,562
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	$ 7,516	$ 7,703
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	$ 137
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	0.34	1.19
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	34.54	0.71
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] | Maximum [Member] | State and Local Governments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Discount rate	1.34	1.19
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	81.41	100
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	7.32
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	0.86	1.19
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	47.96	17.71
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	7.32