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Restrictions of Dividends & Inter-company Borrowings - Additional Information (Detail) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2019 USD ($)
Equity [Abstract] |
Dividends may be paid without prior regulatory approval | $ 19.0

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- Definition Dividends may be paid without prior regulatory approval. + 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
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration