Credit Quality Indicators (Details 2) - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021
Financing Receivable Recorded Investment [Line Items]
Total	$ 2,399,981	$ 2,331,082
Commercial real estate
Financing Receivable Recorded Investment [Line Items]
2022	140,403
2021	172,020
2020	134,025
2019	158,774
2018	106,429
Prior	297,256
Revolving Loans	18,373
Total	1,027,280
Commercial real estate \| Pass
Financing Receivable Recorded Investment [Line Items]
2022	140,403
2021	171,271
2020	131,878
2019	150,987
2018	103,825
Prior	264,766
Revolving Loans	17,198
Total	980,328
Commercial real estate \| Special Mention
Financing Receivable Recorded Investment [Line Items]
2022	0
2021	749
2020	1,877
2019	5,340
2018	2,604
Prior	11,413
Revolving Loans	508
Total	22,491
Commercial real estate \| Substandard
Financing Receivable Recorded Investment [Line Items]
2022	0
2021	0
2020	270
2019	2,447
2018	0
Prior	21,077
Revolving Loans	667
Total	24,461
Commercial
Financing Receivable Recorded Investment [Line Items]
2022	85,050
2021	52,135
2020	38,510
2019	19,392
2018	15,703
Prior	17,499
Revolving Loans	69,133
Total	297,422
Commercial \| Pass
Financing Receivable Recorded Investment [Line Items]
2022	85,050
2021	50,425
2020	37,834
2019	19,152
2018	15,502
Prior	16,006
Revolving Loans	68,019
Total	291,988
Commercial \| Special Mention
Financing Receivable Recorded Investment [Line Items]
2022	0
2021	333
2020	0
2019	116
2018	0
Prior	1
Revolving Loans	429
Total	879
Commercial \| Substandard
Financing Receivable Recorded Investment [Line Items]
2022	0
2021	1,377
2020	676
2019	124
2018	201
Prior	1,492
Revolving Loans	685
Total	4,555
Agricultural Portfolio Segment
Financing Receivable Recorded Investment [Line Items]
2022	39,434
2021	38,524
2020	45,765
2019	24,880
2018	24,672
Prior	48,792
Revolving Loans	17,120
Total	239,187
Agricultural Portfolio Segment \| Pass
Financing Receivable Recorded Investment [Line Items]
2022	39,434
2021	38,169
2020	45,529
2019	24,802
2018	24,672
Prior	46,716
Revolving Loans	16,740
Total	236,062
Agricultural Portfolio Segment \| Special Mention
Financing Receivable Recorded Investment [Line Items]
2022	0
2021	0
2020	0
2019	0
2018	0
Prior	0
Revolving Loans	250
Total	250
Agricultural Portfolio Segment \| Substandard
Financing Receivable Recorded Investment [Line Items]
2022	0
2021	355
2020	236
2019	78
2018	0
Prior	2,076
Revolving Loans	130
Total	2,875
Residential real estate
Financing Receivable Recorded Investment [Line Items]
2022	69,471
2021	111,431
2020	78,306
2019	33,442
2018	24,043
Prior	152,810
Revolving Loans	3,984
Total	473,487
Residential real estate \| Pass
Financing Receivable Recorded Investment [Line Items]
2022	69,471
2021	111,389
2020	77,984
2019	33,196
2018	23,947
Prior	144,706
Revolving Loans	3,984
Total	464,677
Residential real estate \| Special Mention
Financing Receivable Recorded Investment [Line Items]
2022	0
2021	0
2020	71
2019	121
2018	77
Prior	129
Revolving Loans	0
Total	398
Residential real estate \| Substandard
Financing Receivable Recorded Investment [Line Items]
2022	0
2021	42
2020	251
2019	125
2018	19
Prior	7,975
Revolving Loans	0
Total	8,412
Home Equity Lines Of Credit
Financing Receivable Recorded Investment [Line Items]
2022	0
2021	13
2020	137
2019	49
2018	81
Prior	3,522
Revolving Loans	128,494
Total	132,296
Home Equity Lines Of Credit \| Pass
Financing Receivable Recorded Investment [Line Items]
2022	0
2021	0
2020	0
2019	0
2018	17
Prior	1,532
Revolving Loans	128,345
Total	129,894
Home Equity Lines Of Credit \| Special Mention
Financing Receivable Recorded Investment [Line Items]
2022	0
2021	0
2020	0
2019	0
2018	0
Prior	0
Revolving Loans	48
Total	48
Home Equity Lines Of Credit \| Substandard
Financing Receivable Recorded Investment [Line Items]
2022	0
2021	13
2020	137
2019	49
2018	64
Prior	1,990
Revolving Loans	101
Total	2,354
Consumer
Financing Receivable Recorded Investment [Line Items]
2022	86,771
2021	50,718
2020	35,291
2019	23,707
2018	12,875
Prior	12,960
Revolving Loans	7,987
Total	230,309
Consumer \| Pass
Financing Receivable Recorded Investment [Line Items]
2022	86,710
2021	50,580
2020	35,022
2019	23,530
2018	12,791
Prior	12,744
Revolving Loans	7,987
Total	229,364
Consumer \| Substandard
Financing Receivable Recorded Investment [Line Items]
2022	61
2021	138
2020	269
2019	177
2018	84
Prior	216
Revolving Loans	0
Total	$ 945

Amortized cost of financing receivable originated more than five years prior to current fiscal year. Excludes net investment in lease.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 79
-URI https://asc.fasb.org/extlink&oid=124267575&loc=SL82922352-210448

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 6
-URI https://asc.fasb.org/extlink&oid=124255953&loc=SL82919232-210447

+ Details

Name:	us-gaap_FinancingReceivableOriginatedFiveOrMoreYearsBeforeLatestFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amortized cost of financing receivable originated four years prior to current fiscal year. Excludes net investment in lease.

+ References

+ Details

Name:	us-gaap_FinancingReceivableOriginatedFourYearsBeforeLatestFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amortized cost of financing receivable originated in current fiscal year. Excludes net investment in lease.

+ References

+ Details

Name:	us-gaap_FinancingReceivableOriginatedInCurrentFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amortized cost of financing receivable originated in fiscal year prior to current fiscal year. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 6
-URI https://asc.fasb.org/extlink&oid=124255953&loc=SL82919232-210447

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 79
-URI https://asc.fasb.org/extlink&oid=124267575&loc=SL82922352-210448

+ Details

Name:	us-gaap_FinancingReceivableOriginatedInFiscalYearBeforeLatestFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amortized cost of financing receivable originated three years prior to current fiscal year. Excludes net investment in lease.

+ References

+ Details

Name:	us-gaap_FinancingReceivableOriginatedThreeYearsBeforeLatestFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amortized cost of financing receivable originated two years prior to current fiscal year. Excludes net investment in lease.

+ References

+ Details

Name:	us-gaap_FinancingReceivableOriginatedTwoYearsBeforeLatestFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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.

+ References

No definition available.

+ Details

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

- Definition

Amortized cost of financing receivable that can be withdrawn, repaid, and redrawn.

+ References

+ Details

Name:	us-gaap_FinancingReceivableRevolving
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amortized cost, before allowance for credit loss, of financing receivable. Excludes financing receivable covered under loss sharing agreement and net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7A
-URI https://asc.fasb.org/extlink&oid=123577603&loc=SL6953401-111524

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 79
-URI https://asc.fasb.org/extlink&oid=124267575&loc=SL82922352-210448

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(3)(b))
-URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 14
-URI https://asc.fasb.org/extlink&oid=124255953&loc=SL82919253-210447

Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=124255206&loc=SL82895884-210446

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=124255953&loc=SL82919230-210447

Reference 7: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 80
-URI https://asc.fasb.org/extlink&oid=124267575&loc=SL82922355-210448

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph (a)
-Subparagraph (1)

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph (a)
-Subparagraph (2)

Reference 10: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher SEC
-Name Regulation S-K (SK)
-Number 229
-Section 1404
-Paragraph (a)

+ Details

Name:	us-gaap_NotesReceivableGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_PassMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_SpecialMentionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_SubstandardMember
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_FinancingReceivablePortfolioSegmentAxis=fmnb_AgriculturalPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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