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Revenue from Contracts with Customers (Details Textual) - USD ($) | 12 Months Ended
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Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Revenue From Contract With Customer [Line Items] |  |  |
Legal settlement | $ 0 | $ 0 | $ 8,375,000
ASC 606 |  |  |
Revenue From Contract With Customer [Line Items] |  |  |
Contingent debit card interchange fees | 0 |  |
Contingent incentive fees | 0 |  |
Contingent commission | 0 |  |
ASC 606 | Cetera |  |  |
Revenue From Contract With Customer [Line Items] |  |  |
Contingent investment commissions to be refunded | $ 0 |  |

X
- Definition Contingent brokerage commissions revenue payable. + References No definition available. + Details
Name: | fmnb_ContingentBrokerageCommissionsRevenuePayable
Namespace Prefix: | fmnb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Contingent fees and commissions debit cards receivable. + References No definition available. + Details
Name: | fmnb_ContingentFeesAndCommissionsDebitCardsReceivable
Namespace Prefix: | fmnb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Contingent fees and commissions fiduciary and trust activities receivable. + References No definition available. + Details
Name: | fmnb_ContingentFeesAndCommissionsFiduciaryAndTrustActivitiesReceivable
Namespace Prefix: | fmnb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Contingent insurance commissions and fees receivable. + References No definition available. + Details
Name: | fmnb_ContingentInsuranceCommissionsAndFeesReceivable
Namespace Prefix: | fmnb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Legal settlement income. + References No definition available. + Details
Name: | fmnb_LegalSettlementIncome
Namespace Prefix: | fmnb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Revenue from contract with customer. + References No definition available. + Details
Name: | fmnb_RevenueFromContractWithCustomerLineItems
Namespace Prefix: | fmnb_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=fmnb_AccountingStandardsUpdates201409Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_CounterpartyNameAxis=fmnb_CeteraMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |