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Loans - Analysis of Allowance for Credit Losses (Detail) - Unfunded Loan Commitment & Letters of Credit [Member] - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended | 12 Months Ended
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Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024 | Dec. 31, 2024
ALLOWANCE FOR CREDIT LOSSES: |  |  |  |  |
Balance at beginning of year | $ 1,308 | $ 1,928 | $ 1,541 | $ 2,212 | $ 2,212
Provision for (recovery of) credit losses - off balance sheet credit exposures | (272) | (267) | (505) | (551) | (671)
Balance at ending of year | $ 1,036 | $ 1,661 | $ 1,036 | $ 1,661 | $ 1,541

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- Definition Allowance For Credit Losses [Abstract] + References No definition available. + Details
Name: | fmao_AllowanceForCreditLossesAbstract
Namespace Prefix: | fmao_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Financing receivable allowance for expected credit losses. + References No definition available. + Details
Name: | fmao_FinancingReceivableAllowanceForExpectedCreditLosses
Namespace Prefix: | fmao_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Provision for credit losses-off balance sheet credit exposures + References No definition available. + Details
Name: | fmao_ProvisionForCreditLossesoffBalanceSheetCreditExposures
Namespace Prefix: | fmao_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fmao_UnfundedLoanCommitmentAndLettersOfCreditMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |