Business Combination and Asset Purchase - Summary of Carrying Amount of Loans (Parenthetical) (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Jan. 01, 2019
Business Combinations [Abstract]
Carrying amount, allowance	$ 2,081	$ 2,118

Certain loans acquired in transfer accounted for as debt securities carrying amount allowance.

No definition available.

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