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Restrictions of Dividends & Inter-company Borrowings - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)
Equity [Abstract]
Dividends may be paid without prior regulatory approval	$ 18.3

X

- Definition

Dividends may be paid without prior regulatory approval.

+ References

No definition available.

+ Details

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

X

- References

No definition available.

+ Details

Name:	us-gaap_EquityAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration