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Off Balance Sheet Activities - Additional Information (Detail) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract] |  |
Allowance for credit losses relates to unfunded loan commitments | $ 479 | $ 274

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- Definition Allowance for credit losses relates to unfunded loan commitments. + References No definition available. + Details
Name: | fmao_AllowanceForCreditLossesRelatesToUnfundedLoanCommitments
Namespace Prefix: | fmao_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_CommitmentsAndContingenciesDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration