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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 | Mar. 31, 2021 USD ($) | Dec. 31, 2020 USD ($)
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Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair Value | $ 352,974 | $ 307,812
State and Local Governments [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair Value | 62,487 | 70,515
Level 3 [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fair Value | 1,539 | 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 | $ 1,539 | $ 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 | $ 4,130 | $ 7,703
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 | 1.32 | 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 | 20.00 | 0.71
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 | 1.32 | 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 | 41.61 | 100
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 | 1.32 | 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 | 40.35 | 17.71

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