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Loans (Tier Risk Rating System) - Additional Information (Detail) | 12 Months Ended
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Dec. 31, 2018
Minimum [Member] |
Accounts, Notes, Loans and Financing Receivable [Line Items] |
Repayment period of loan with a reasonable reduction of principal balance | 2 years
Good [Member] | Maximum [Member] |
Accounts, Notes, Loans and Financing Receivable [Line Items] |
Leverage position | 1.50%
Satisfactory [Member] | Maximum [Member] |
Accounts, Notes, Loans and Financing Receivable [Line Items] |
Leverage position | 2.00%

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- Definition Leverage ratio. + References No definition available. + Details
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Namespace Prefix: | fmao_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Repayment period of loan with reasonable reduction of principal balance. + References No definition available. + Details
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Namespace Prefix: | fmao_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_AccountsNotesAndLoansReceivableLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_InternalCreditAssessmentAxis=fmao_GoodMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_InternalCreditAssessmentAxis=fmao_SatisfactoryMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |