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Premises and Equipment (Details Textual) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Premises and Equipment (Textual) [Abstract] |  |  |
Depreciation expense | $ 1.5 | $ 1.5 | $ 1.5

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- Definition Premises and Equipment (Textual) [Abstract]. + 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 The expense recognized in the current period that allocates the cost of nonproduction tangible assets over their useful lives. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229 + Details
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Balance Type: | debit
Period Type: | duration