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Off Balance Sheet Activities - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Allowance for credit losses relates to unfunded loan commitments	$ 274	$ 227

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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