Regulatory Capital Matters (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Common equity tier 1 capital ratio
Actual, Amount	$ 117,890fmnb_TierOneCommonCapital
Actual, Ratio	15.03%fmnb_TierOneCommonCapitalRatio
Requirement for Capital Adequacy Purposes, Amount	35,291
Requirement for Capital Adequacy Purposes, Ratio	4.50%
Total risk based capital ratio
Actual, Amount	125,667us-gaap_Capital	121,340us-gaap_Capital
Actual, Ratio	16.02%us-gaap_CapitalToRiskWeightedAssets	16.48%us-gaap_CapitalToRiskWeightedAssets
Requirement for Capital Adequacy Purposes, Amount	62,740	58,523
Requirement for Capital Adequacy Purposes, Ratio	8.00%	8.00%
Tier 1 risk based capital ratio
Actual, Amount	117,890us-gaap_TierOneRiskBasedCapital	113,654us-gaap_TierOneRiskBasedCapital
Actual, Ratio	15.03%	15.43%
Requirement for Capital Adequacy Purposes, Amount	47,055	29,262
Requirement for Capital Adequacy Purposes, Ratio	6.00%	4.00%
Tier 1 leverage ratio
Actual, Amount	117,890us-gaap_TierOneLeverageCapital	113,654us-gaap_TierOneLeverageCapital
Actual, Ratio	10.44%	10.03%
Requirement for Capital Adequacy Purposes, Amount	45,152	45,313
Requirement for Capital Adequacy Purposes, Ratio	4.00%	4.00%
Farmers National Banc Corp
Common equity tier 1 capital ratio
Actual, Amount	107,914
Actual, Ratio	13.85%
Requirement for Capital Adequacy Purposes, Amount	35,071
Requirement for Capital Adequacy Purposes, Ratio	4.50%
To be Well Capitalized Under Prompt Corrective Action Provisions, Amount	50,659
To be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	6.50%
Total risk based capital ratio
Actual, Amount	115,637	114,321
Actual, Ratio	14.84%	15.56%
Requirement for Capital Adequacy Purposes, Amount	62,349	58,773
Requirement for Capital Adequacy Purposes, Ratio	8.00%	8.00%
To be Well Capitalized Under Prompt Corrective Action Provisions, Amount	77,936	73,466
To be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	10.00%	10.00%
Tier 1 risk based capital ratio
Actual, Amount	107,914	106,689
Actual, Ratio	13.85%	14.52%
Requirement for Capital Adequacy Purposes, Amount	46,762	29,386
Requirement for Capital Adequacy Purposes, Ratio	6.00%	4.00%
To be Well Capitalized Under Prompt Corrective Action Provisions, Amount	62,349	44,079
To be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	8.00%	6.00%
Tier 1 leverage ratio
Actual, Amount	107,914	106,689
Actual, Ratio	9.38%	9.37%
Requirement for Capital Adequacy Purposes, Amount	46,032	45,565
Requirement for Capital Adequacy Purposes, Ratio	4.00%	4.00%
To be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 57,540	$ 56,956
To be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	5.00%	5.00%

Tier one common capital.

No definition available.

Name:	fmnb_TierOneCommonCapital
Namespace Prefix:	fmnb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Tier one common capital ratio.

+ References

No definition available.

+ Details

Name:	fmnb_TierOneCommonCapitalRatio
Namespace Prefix:	fmnb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	fmnb_TierOneCommonCapitalRatioAbstract
Namespace Prefix:	fmnb_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Tier one common capital required for capital adequacy.

+ References

No definition available.

+ Details

Name:	fmnb_TierOneRiskBasedCommonCapitalRequiredForCapitalAdequacy
Namespace Prefix:	fmnb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Tier one risk based common capital required for capital adequacy to risk weighted assets.

+ References

No definition available.

+ Details

Name:	fmnb_TierOneRiskBasedCommonCapitalRequiredForCapitalAdequacyToRiskWeightedAssets
Namespace Prefix:	fmnb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Tier one risk based common capital required to be well capitalized.

+ References

No definition available.

+ Details

Name:	fmnb_TierOneRiskBasedCommonCapitalRequiredToBeWellCapitalized
Namespace Prefix:	fmnb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Tier one common capital required to be well capitalized to risk weighted assets.

+ References

No definition available.

+ Details

Name:	fmnb_TierOneRiskBasedCommonCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
Namespace Prefix:	fmnb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The amount of total capital as defined in the regulations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 948

-SubTopic 10

-Section 50

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=6490092&loc=d3e47304-110998

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 505

-Section 50

-Paragraph 1H

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 505

-Section 50

-Paragraph 1H

-Subparagraph (c)(2)

-URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 948

-SubTopic 10

-Section 50

-Paragraph 3

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=6490092&loc=d3e47080-110998

+ Details

Name:	us-gaap_Capital
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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

- Definition

The minimum amount of Total Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_CapitalRequiredForCapitalAdequacy
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The minimum Total risk based capital ratio (total capital divided by risk weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 505

-Section 50

-Paragraph 1H

-Subparagraph (c)(2)

-URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 505

-Section 50

-Paragraph 1

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 505

-Section 50

-Paragraph 1B

-URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958570-112826

+ Details

Name:	us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The amount of Total Capital required to be categorized as well as capitalized under the regulatory framework for prompt corrective action.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 505

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826

+ Details

Name:	us-gaap_CapitalRequiredToBeWellCapitalized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The Total risk based capital ratio (total capital divided by risk weighted assets) required to be categorized as "well capitalized", in the regulatory framework for prompt corrective action.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 505

-Section 50

-Paragraph 1

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826

+ Details

Name:	us-gaap_CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Total Capital divided by risk weighted assets as defined in the regulations.

+ References

+ Details

Name:	us-gaap_CapitalToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Tier 1 Leverage Capital as defined in the regulations. Also called Core Capital.

+ References

+ Details

Name:	us-gaap_TierOneLeverageCapital
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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

- Definition

The minimum amount of Tier 1 Leverage Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacy
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The minimum amount of Tier 1 Leverage Capital for capital adequacy purposes divided by average assets as defined in the regulations.

+ References

+ Details

Name:	us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The amount of Tier 1 Leverage Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The amount of Tier 1 Leverage Capital required to be categorized as well capitalized divided by average assets as defined in the regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalizedToAverageAssets
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Tier 1 Leverage Capital divided by average assets as defined in the regulations.

+ References

+ Details

Name:	us-gaap_TierOneLeverageCapitalToAverageAssets
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Tier 1 Risk Based Capital as defined in the regulations.

+ References

+ Details

Name:	us-gaap_TierOneRiskBasedCapital
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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

- Definition

The minimum amount of Tier 1 Risk Based Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacy
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The minimum Tier One Capital Ratio (Tier one capital divided by risk-weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The amount of Tier 1 Risk Based Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The Tier 1 capital ratio (Tier 1 capital divided by risk weighted assets) required to be categorized as "well capitalized" under the regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Tier 1 capital divided by risk weighted assets as defined by regulations.

+ References

+ Details

Name:	us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	dei_LegalEntityAxis=us-gaap_ParentCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: