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Revenue from Contracts with Customers (Details Textual) - ASC 606 | 12 Months Ended
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Dec. 31, 2019 USD ($)
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 | 2.20%
Percentage of retirement plan consulting fees representing total revenue | 1.10%
Percentage of investment commissions representing total revenue | 1.10%
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 Percentage of insurance commissions on revenues. + References No definition available. + Details
Name: | fmnb_PercentageOfInsuranceCommissionsOnRevenues
Namespace Prefix: | fmnb_
Data Type: | num: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: | num: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: | num: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: |