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v3.20.1

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

X
- Definition Reclassification of gains losses to net income. + References No definition available. + Details
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Namespace Prefix: | fmao_
Data Type: | xbrli:monetaryItemType
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
Period Type: | duration