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Loans - Additional Information (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Receivables [Abstract]
Residential mortgage loan	$ 17.7	$ 17.9

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

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- References

No definition available.

+ Details

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