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Earnings Per Share - Additional Information (Detail) - USD ($) | Nov. 20, 2020 | Jun. 05, 2020
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Earnings Per Share [Abstract] |  |
Compensation annual basis committee award |  | $ 4,000
Number of directors received shares | 20 | 176

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- Definition Compensation annual basis committee award. + References No definition available. + Details
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- Definition Number of directors received shares. + References No definition available. + Details
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Namespace Prefix: | fmao_
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Balance Type: | na
Period Type: | instant

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- References No definition available. + Details
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Balance Type: | na
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