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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
	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

X

- 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

X

- Details

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