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Loans - Additional Information (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Receivables [Abstract]
Residential mortgage loan	$ 17.5	$ 17.3

X

- Definition

Residential mortgage loan.

+ References

No definition available.

+ Details

Name:	fmao_ResidentialMortgageLoan
Namespace Prefix:	fmao_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- References

No definition available.

+ Details

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