Troubled Debt Restructurings (Details) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2022 USD ($) Loan	Jun. 30, 2021 USD ($) Loan	Jun. 30, 2022 USD ($) Loan	Jun. 30, 2021 USD ($) Loan
Financing Receivable Modifications [Line Items]
Number of Loans \| Loan	7	9	10	31
Pre-Modification Outstanding Recorded Investment	$ 302	$ 108	$ 380	$ 582
Post-Modification Outstanding Recorded Investment	$ 305	$ 108	$ 385	$ 585
Commercial
Financing Receivable Modifications [Line Items]
Number of Loans \| Loan				1
Pre-Modification Outstanding Recorded Investment				$ 21
Post-Modification Outstanding Recorded Investment				$ 21
Commercial real estate, Non-owner occupied
Financing Receivable Modifications [Line Items]
Number of Loans \| Loan	1		1
Pre-Modification Outstanding Recorded Investment	$ 140		$ 140
Post-Modification Outstanding Recorded Investment	$ 140		$ 140
Residential real estate, 1-4 family residential
Financing Receivable Modifications [Line Items]
Number of Loans \| Loan	3	2	4	8
Pre-Modification Outstanding Recorded Investment	$ 142	$ 48	$ 192	$ 325
Post-Modification Outstanding Recorded Investment	$ 145	$ 48	$ 197	$ 326
Residential real estate, Home equity lines of credit
Financing Receivable Modifications [Line Items]
Number of Loans \| Loan			1	4
Pre-Modification Outstanding Recorded Investment			$ 14	$ 100
Post-Modification Outstanding Recorded Investment			$ 14	$ 102
Consumer, Indirect
Financing Receivable Modifications [Line Items]
Number of Loans \| Loan	3	7	4	17
Pre-Modification Outstanding Recorded Investment	$ 20	$ 60	$ 34	$ 121
Post-Modification Outstanding Recorded Investment	$ 20	$ 60	$ 34	$ 121
Consumer, Direct
Financing Receivable Modifications [Line Items]
Number of Loans \| Loan			0
Pre-Modification Outstanding Recorded Investment			$ 0
Post-Modification Outstanding Recorded Investment			$ 0
Consumer, Other
Financing Receivable Modifications [Line Items]
Number of Loans \| Loan				1
Pre-Modification Outstanding Recorded Investment				$ 15
Post-Modification Outstanding Recorded Investment				$ 15

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

+ Details

Name:	us-gaap_FinancingReceivableModificationsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI https://asc.fasb.org/extlink&oid=123577603&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after troubled debt restructuring, of modified financing receivable.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount, before troubled debt restructuring, of financing receivable to be modified.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fmnb_CommercialRealEstateNonOwnerOccupiedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fmnb_ResidentialRealEstateOneToFourFamilyResidentialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fmnb_ResidentialRealEstateHomeEquityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fmnb_ConsumerIndirectMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fmnb_ConsumerDirectMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fmnb_ConsumerOtherMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: