Business Combination & Asset Purchase - Summary of Carrying Amount of Loans (Parenthetical) (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Jan. 01, 2019
Business Combinations [Abstract]
Fair value adjustment	$ 0	$ 62	$ 62	$ 2,118

Certain loans acquired in transfer accounted for as debt securities fair value adjustment.

No definition available.

Name:	fmao_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesFairValueAdjustment
Namespace Prefix:	fmao_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant