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Loans - Additional Information (Detail) - USD ($) $ in Millions | Jun. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Receivables [Abstract] |  |
Residential mortgage loan | $ 15.8 | $ 17.3

X
- Definition Residential mortgage loan. + References No definition available. + Details
Name: | fmao_ResidentialMortgageLoan
Namespace Prefix: | fmao_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_ReceivablesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration