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Quarterly Financial Data (Unaudited) (Parenthetical) (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract] |  |  |  |
Impact of enactment of federal tax reform | $ 1,800 | $ 1,793 | $ 0 | $ 0

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- Definition Tax cuts and jobs act of 2017 change in tax rate income tax expense benefit. + References No definition available. + Details
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Namespace Prefix: | fmnb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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