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Loan Servicing - Summary of Principal Balance for Mortgage Loans (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
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FHLMC |  |
Mortgage loan portfolio serviced for: |  |
Loan Portfolio Expense | $ 430,233 | $ 276,863

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- Definition Mortgage loan portfolios serviced for. + References No definition available. + Details
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Namespace Prefix: | fmnb_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Principal amount of transferred financial assets in which the transferor has continuing involvement with the assets underlying the transaction. Continuing involvement includes, but is not limited to, servicing, recourse, and restrictions on transferor's interests in transferred financial assets. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=121570589&loc=d3e107314-111719 + 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: |