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Business Combination and Asset Purchase - Summary of Carrying Amount of Loans (Parenthetical) (Detail) - USD ($) $ in Thousands | Jun. 30, 2019 | Jan. 01, 2019
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Business Combinations [Abstract] |  |
Carrying amount, allowance | $ 96 | $ 2,118

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- Definition Certain loans acquired in transfer accounted for as debt securities carrying amount allowance. + References No definition available. + Details
Name: | fmao_CertainLoansAcquiredInTransferAccountedForAsDebtSecuritiesCarryingAmountAllowance
Namespace Prefix: | fmao_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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