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 http://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_FinancialInstrumentAxis=fmnb_OriginatedLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=fmnb_AcquiredLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
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_ConsumerIndirectMember
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_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:

Troubled Debt Restructurings (Details) $ in Thousands	3 Months Ended		9 Months Ended
Troubled Debt Restructurings (Details) $ in Thousands	Sep. 30, 2021 USD ($) Loan	Sep. 30, 2020 USD ($) Loan	Sep. 30, 2021 USD ($) Loan	Sep. 30, 2020 USD ($) Loan
Financing Receivable Modifications [Line Items]
Number of Loans \| Loan	9	3	30	34
Pre-Modification Outstanding Recorded Investment	$ 358	$ 63	$ 786	$ 645
Post-Modification Outstanding Recorded Investment	$ 376	$ 67	$ 774	$ 652
Originated Loans
Financing Receivable Modifications [Line Items]
Number of Loans \| Loan		2		31
Pre-Modification Outstanding Recorded Investment		$ 3		$ 505
Post-Modification Outstanding Recorded Investment		$ 3		$ 508
Acquired Loans
Financing Receivable Modifications [Line Items]
Number of Loans \| Loan		1		3
Pre-Modification Outstanding Recorded Investment		$ 60		$ 140
Post-Modification Outstanding Recorded Investment		$ 64		$ 144
Commercial
Financing Receivable Modifications [Line Items]
Number of Loans \| Loan			4
Pre-Modification Outstanding Recorded Investment			$ 22
Post-Modification Outstanding Recorded Investment			$ 22
Commercial \| Originated Loans
Financing Receivable Modifications [Line Items]
Number of Loans \| Loan				1
Pre-Modification Outstanding Recorded Investment				$ 21
Post-Modification Outstanding Recorded Investment				$ 21
Residential real estate, 1-4 family residential
Financing Receivable Modifications [Line Items]
Number of Loans \| Loan	2		6
Pre-Modification Outstanding Recorded Investment	$ 215		$ 426
Post-Modification Outstanding Recorded Investment	$ 233		$ 414
Residential real estate, 1-4 family residential \| Originated Loans
Financing Receivable Modifications [Line Items]
Number of Loans \| Loan				6
Pre-Modification Outstanding Recorded Investment				$ 245
Post-Modification Outstanding Recorded Investment				$ 246
Residential real estate, 1-4 family residential \| Acquired Loans
Financing Receivable Modifications [Line Items]
Number of Loans \| Loan		1		3
Pre-Modification Outstanding Recorded Investment		$ 60		$ 140
Post-Modification Outstanding Recorded Investment		$ 64		$ 144
Consumer, Indirect
Financing Receivable Modifications [Line Items]
Number of Loans \| Loan	5		12
Pre-Modification Outstanding Recorded Investment	$ 30		$ 121
Post-Modification Outstanding Recorded Investment	$ 30		$ 121
Consumer, Indirect \| Originated Loans
Financing Receivable Modifications [Line Items]
Number of Loans \| Loan		2		19
Pre-Modification Outstanding Recorded Investment		$ 3		$ 124
Post-Modification Outstanding Recorded Investment		$ 3		$ 124
Residential real estate, Home equity lines of credit
Financing Receivable Modifications [Line Items]
Number of Loans \| Loan	1		5
Pre-Modification Outstanding Recorded Investment	$ 103		$ 201
Post-Modification Outstanding Recorded Investment	$ 103		$ 201
Residential real estate, Home equity lines of credit \| Originated Loans
Financing Receivable Modifications [Line Items]
Number of Loans \| Loan				4
Pre-Modification Outstanding Recorded Investment				$ 100
Post-Modification Outstanding Recorded Investment				$ 102
Consumer, Direct
Financing Receivable Modifications [Line Items]
Number of Loans \| Loan	1		3
Pre-Modification Outstanding Recorded Investment	$ 10		$ 16
Post-Modification Outstanding Recorded Investment	$ 10		$ 16
Consumer, Other \| Originated Loans
Financing Receivable Modifications [Line Items]
Number of Loans \| Loan				1
Pre-Modification Outstanding Recorded Investment				$ 15
Post-Modification Outstanding Recorded Investment				$ 15