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Earnings Per Share - Additional Information (Detail) - USD ($) | Jun. 04, 2021 | Jun. 05, 2020
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Number of directors received shares |  | 176
Compensation annual basis committee award |  | $ 4,000
Forecast [Member] |  |
Compensation annual basis committee award | $ 6,000 |

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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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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |