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Revenue from Contracts with Customers (Details Textual) - USD ($) | 3 Months Ended
--- | ---
Mar. 31, 2022 | Mar. 31, 2021
Revenue From Contract With Customer [Line Items] |  |
Legal settlement | $ 8,400,000 | $ 0
ASC 606 |  |
Revenue From Contract With Customer [Line Items] |  |
Contingent debit card interchange fees | 0 |
Contingent incentive fees | 0 |
Contingent commission | $ 0 |
Percentage of insurance agency commissions representing total revenue | 2.10% |
Percentage of retirement plan consulting fees representing total revenue | 0.80% |
Percentage of investment commissions representing total revenue | 1.40% |
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 Percentage of insurance commissions on revenues. + References No definition available. + Details
Name: | fmnb_PercentageOfInsuranceCommissionsOnRevenues
Namespace Prefix: | fmnb_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage of investment commissions on revenues. + References No definition available. + Details
Name: | fmnb_PercentageOfInvestmentCommissionsOnRevenues
Namespace Prefix: | fmnb_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage of retirement plan consulting fees on revenues. + References No definition available. + Details
Name: | fmnb_PercentageOfRetirementPlanConsultingFeesOnRevenues
Namespace Prefix: | fmnb_
Data Type: | dtr-types:percentItemType
Balance Type: | na
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=us-gaap_AccountingStandardsUpdate201409Member
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: |