Schedule H, Line 4i - Schedule of Assets (Held at End of Year) 2 (Details) - EBP 34-0214400 002 [Member]	12 Months Ended
Dec. 31, 2025 USD ($)
EBP, Asset Held for Investment	$ 67,643,938
Minimum [Member]
EBP, Asset Held for Investment, Participant Loan, Interest Rate	4.25%
Maximum [Member]
EBP, Asset Held for Investment, Participant Loan, Interest Rate	9.50%
EBP, Asset Held for Investment, Participant Loan, Maturity Date	Dec. 31, 2038
Mutual Fund [Member]
Current value	$ 33,113,532
EBP, Pooled Separate Account [Member]
Current value	$ 27,513,354

Amount of total assets in schedule of asset held for investment of employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Includes, but is not limited to, current value of investment, participant loan, and plan interest in master trust.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 962
-SubTopic 205
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6A-05(a)(Schedule I))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477804/962-205-S99-1

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Name Employee Retirement Income Security Act of 1974
-Number 2520
-Section 103
-Subsection 10
-Paragraph b
-Subparagraph (1)
-Publisher DOL-EBSA

+ Details

Name:	us-gaap-ebp_EmployeeBenefitPlanAssetHeldForInvestment
Namespace Prefix:	us-gaap-ebp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Current value of investment, excluding plan interest in master trust, in schedule of asset held for investment of employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Includes, but is not limited to, derivative, investment measured at net asset value per share as practical expedient, and insurance contract.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name Employee Retirement Income Security Act of 1974
-Number 2520
-Section 103
-Subsection 10
-Paragraph b
-Subparagraph (1)(i)(D)
-Publisher DOL-EBSA

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 962
-SubTopic 205
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6A-05(a)(Schedule I))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477804/962-205-S99-1

+ Details

Name:	us-gaap-ebp_EmployeeBenefitPlanAssetHeldForInvestmentInvestmentExcludingPlanInterestInMasterTrustCurrentValue
Namespace Prefix:	us-gaap-ebp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Interest rate on participant loan in schedule of asset held for investment of employee stock purchase, saving, or similar plan (employee benefit plan (EBP)).

+ References

+ Details

Name:	us-gaap-ebp_EmployeeBenefitPlanParticipantLoanInterestRate
Namespace Prefix:	us-gaap-ebp_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Maturity date of participant loan in schedule of asset held for investment of employee stock purchase, saving, or similar plan (employee benefit plan (EBP)), in YYYY-MM-DD format.

+ References

+ Details

Name:	us-gaap-ebp_EmployeeBenefitPlanParticipantLoanMaturityDate
Namespace Prefix:	us-gaap-ebp_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	dei_LegalEntityAxis=fmnb_EBP340214400002Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_MutualFundMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap-ebp_EmployeeBenefitPlanPooledSeparateAccountMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: