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Earnings Per Share - Additional Information (Detail) | Jun. 05, 2020 USD ($) shares
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Earnings Per Share [Abstract] |
Compensation annual basis committee award | $ | $ 4,000
Number of directors received shares | shares | 176

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- Definition Compensation annual basis committee award. + References No definition available. + Details
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Namespace Prefix: | fmao_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Number of directors received shares. + References No definition available. + Details
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Namespace Prefix: | fmao_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

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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