Represents the weighted average principal deferred for modified financing receivables.

No definition available.

Name:	fmnb_FinancingReceivableModifiedPaymentDeferralPrincipal
Namespace Prefix:	fmnb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Percentage of financing receivable modified during period for debtor experiencing financial difficulty to total financing receivables.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationToTotalFinancingReceivablesPercent
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amortized cost of financing receivable modified during period for debtor experiencing financial difficulty.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 12A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481933/310-10-55-12A

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 33
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-33

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

+ Details

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

- Definition

Percentage, after modification, of weighted-average interest rate of financing receivable for debtor experiencing financial difficulty.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModifiedWeightedAverageInterestRateAfterModification
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage, before modification, of weighted-average interest rate of financing receivable for debtor experiencing financial difficulty.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModifiedWeightedAverageInterestRateBeforeModification
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Period of increase in weighted-average maturity of financing receivable modified for debtor experiencing financial difficulty, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModifiedWeightedAverageTermIncreaseFromModification
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Note 5 - Loan Restructurings - Schedule of Loan Modifications (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Loans, modified	$ 201	$ 1,380	$ 7,378	$ 12,882
Loans, modified, percentage	0.00%	0.04%	0.15%	0.39%
Payment Deferral [Member]
Loans, modified	$ 190	$ 111	$ 4,511	$ 338
Extended Maturity [Member]
Loans, modified	0	0	0	11,142
Contractual Interest Rate Reduction [Member]
Loans, modified	0	759	0	759
Combination Payment Deferral and Interest Rate Reduction [Member]
Loans, modified	0	510	79	510
Combination Term Extension and Interest Rate Reduction [Member]
Loans, modified		0		133
Combination Term Extension and Payment Deferral [Member]
Loans, modified	11		2,788
Commercial Real Estate Portfolio Segment [Member] \| Nonowner Occupied [Member]
Loans, modified		$ 0	$ 7,098	$ 11,128
Loans, modified, percentage		0.00%	0.68%	1.52%
Commercial Real Estate Portfolio Segment [Member] \| Nonowner Occupied [Member] \| Payment Deferral [Member]
Loans, modified		$ 0	$ 4,321	$ 0
1-4 family residential			64
Commercial Real Estate Portfolio Segment [Member] \| Nonowner Occupied [Member] \| Extended Maturity [Member]
Loans, modified		0	$ 0	$ 11,128
Term extension, years added (Year)			1 year	1 year
Commercial Real Estate Portfolio Segment [Member] \| Nonowner Occupied [Member] \| Contractual Interest Rate Reduction [Member]
Loans, modified		0	$ 0	$ 0
Commercial Real Estate Portfolio Segment [Member] \| Nonowner Occupied [Member] \| Combination Payment Deferral and Interest Rate Reduction [Member]
Loans, modified		0	0	0
Commercial Real Estate Portfolio Segment [Member] \| Nonowner Occupied [Member] \| Combination Term Extension and Interest Rate Reduction [Member]
Loans, modified		$ 0		0
Commercial Real Estate Portfolio Segment [Member] \| Nonowner Occupied [Member] \| Combination Term Extension and Payment Deferral [Member]
Loans, modified			2,777
Commercial Real Estate Portfolio Segment [Member] \| Other Loans [Member] \| Payment Deferral [Member]
1-4 family residential				$ 16
Commercial Real Estate Portfolio Segment [Member] \| Other Loans [Member] \| Contractual Interest Rate Reduction [Member]
Interest rate before modification		6.81%		6.81%
Interest rate after modification		5.25%		5.25%
Residential Portfolio Segment [Member] \| 1-4 Family Residential [Member]
Loans, modified	$ 201	$ 0	$ 280	$ 103
Loans, modified, percentage	0.02%	0.00%	0.02%	0.01%
Residential Portfolio Segment [Member] \| 1-4 Family Residential [Member] \| Payment Deferral [Member]
Loans, modified	$ 190	$ 0	$ 190	$ 103
1-4 family residential	8		13	6
Residential Portfolio Segment [Member] \| 1-4 Family Residential [Member] \| Extended Maturity [Member]
Loans, modified	$ 0	0	$ 0	0
Term extension, years added (Year)	2 years		2 years
Residential Portfolio Segment [Member] \| 1-4 Family Residential [Member] \| Contractual Interest Rate Reduction [Member]
Loans, modified	$ 0	0	$ 0	0
Interest rate before modification	5.976%		5.976%
Interest rate after modification	4.032%		4.032%
Residential Portfolio Segment [Member] \| 1-4 Family Residential [Member] \| Combination Payment Deferral and Interest Rate Reduction [Member]
Loans, modified	$ 0	0	$ 79	0
Residential Portfolio Segment [Member] \| 1-4 Family Residential [Member] \| Combination Term Extension and Interest Rate Reduction [Member]
Loans, modified		0		0
Residential Portfolio Segment [Member] \| 1-4 Family Residential [Member] \| Combination Term Extension and Payment Deferral [Member]
Loans, modified	$ 11		$ 11
Residential Portfolio Segment [Member] \| Home Equity Loan [Member]
Loans, modified		$ 277		$ 366
Loans, modified, percentage		0.16%		0.21%
Residential Portfolio Segment [Member] \| Home Equity Loan [Member] \| Payment Deferral [Member]
Loans, modified		$ 0		$ 0
Residential Portfolio Segment [Member] \| Home Equity Loan [Member] \| Extended Maturity [Member]
Loans, modified		0		$ 14
Term extension, years added (Year)				9 years
Residential Portfolio Segment [Member] \| Home Equity Loan [Member] \| Contractual Interest Rate Reduction [Member]
Loans, modified		$ 277		$ 277
Interest rate before modification		7.75%		7.75%
Interest rate after modification		4.79%		4.79%
Residential Portfolio Segment [Member] \| Home Equity Loan [Member] \| Combination Payment Deferral and Interest Rate Reduction [Member]
Loans, modified		$ 0		$ 0
Residential Portfolio Segment [Member] \| Home Equity Loan [Member] \| Combination Term Extension and Interest Rate Reduction [Member]
Loans, modified		0		75
Other Loans [Member]
Loans, modified		$ 1,103		$ 1,103
Loans, modified, percentage		0.37%		0.37%
Other Loans [Member] \| Payment Deferral [Member]
Loans, modified		$ 111		$ 111
1-4 family residential		16
Other Loans [Member] \| Extended Maturity [Member]
Loans, modified		0		0
Other Loans [Member] \| Contractual Interest Rate Reduction [Member]
Loans, modified		$ 482		$ 482
Interest rate before modification		6.81%		6.81%
Interest rate after modification		5.25%		5.25%
Other Loans [Member] \| Combination Payment Deferral and Interest Rate Reduction [Member]
Loans, modified		$ 510		$ 510
Other Loans [Member] \| Combination Term Extension and Interest Rate Reduction [Member]
Loans, modified		0		0
Commercial Portfolio Segment [Member] \| Commercial and Industrial [Member]
Loans, modified		$ 0		$ 182
Loans, modified, percentage		0.00%		0.05%
Commercial Portfolio Segment [Member] \| Commercial and Industrial [Member] \| Payment Deferral [Member]
Loans, modified		$ 0		$ 124
1-4 family residential				112
Commercial Portfolio Segment [Member] \| Commercial and Industrial [Member] \| Extended Maturity [Member]
Loans, modified		0		$ 0
Term extension, years added (Year)				6 years
Commercial Portfolio Segment [Member] \| Commercial and Industrial [Member] \| Contractual Interest Rate Reduction [Member]
Loans, modified		$ 0		$ 0
Interest rate before modification		10.25%		10.25%
Interest rate after modification		8.00%		8.00%
Commercial Portfolio Segment [Member] \| Commercial and Industrial [Member] \| Combination Payment Deferral and Interest Rate Reduction [Member]
Loans, modified		$ 0		$ 0
Commercial Portfolio Segment [Member] \| Commercial and Industrial [Member] \| Combination Term Extension and Interest Rate Reduction [Member]
Loans, modified		$ 0		$ 58