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Basis of Presentation and Other - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2020 USD ($)
Accounting Policies [Abstract]
Amount of reclassification effect	$ 0

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- Definition

Reclassification of gains losses to net income.

+ References

No definition available.

+ Details

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Namespace Prefix:	fmao_
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Balance Type:	credit
Period Type:	duration

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- References

No definition available.

+ Details

Name:	us-gaap_AccountingPoliciesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
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