Restrictions of Dividends & Inter-company Borrowings - Additional Information (Detail) $ in Millions	12 Months Ended
Dec. 31, 2019 USD ($)
Equity [Abstract]
Dividends may be paid without prior regulatory approval	$ 24.5

Dividends may be paid without prior regulatory approval.

No definition available.

Name:	fmao_DividendsMayBePaidWithoutPriorRegulatoryApproval
Namespace Prefix:	fmao_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration