Off Balance Sheet Activities - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Commitments And Contingencies Disclosure [Abstract]
Allowance for credit losses relates to unfunded loan commitments	$ 641	$ 479

Allowance for credit losses relates to unfunded loan commitments.

No definition available.

Name:	fmao_AllowanceForCreditLossesRelatesToUnfundedLoanCommitments
Namespace Prefix:	fmao_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration