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Loans - Additional Information (Detail) - USD ($) $ in Millions | Sep. 30, 2016 | Dec. 31, 2015
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Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Residential mortgage loan | $ 21.3 | $ 20.0
Consumer Real Estate [Member] | Agricultural Real Estate [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Loans held for sale | $ 1.6 | $ 1.2

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

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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
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount, after allowance for bad debt, of loan receivables held-for-sale that are not part of a disposal group. Excludes loans and leases covered under loss sharing agreements and loans classified as debt securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=65884687&loc=d3e4428-111522 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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

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