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Revenue from Contracts with Customers (Details Textual) - ASC 606	3 Months Ended
	Mar. 31, 2021 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	2.60%
Percentage of retirement plan consulting fees representing total revenue	0.80%
Percentage of investment commissions representing total revenue	1.30%
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: