Regulatory Capital Matters (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Common equity tier 1 capital ratio
Actual, Amount	$ 199,491	$ 180,475
Actual, Ratio	12.00%	11.69%
Requirement for Capital Adequacy Purposes, Amount	$ 74,823	$ 69,474
Requirement for Capital Adequacy Purposes, Ratio	4.50%	4.50%
Total risk based capital ratio
Actual, Amount	$ 213,792	$ 193,487
Actual, Ratio	12.86%	12.53%
Requirement for Capital Adequacy Purposes, Amount	$ 133,018	$ 123,509
Requirement for Capital Adequacy Purposes, Ratio	8.00%	8.00%
Tier 1 risk based capital ratio
Actual, Amount	$ 201,688	$ 182,635
Actual, Ratio	12.13%	11.83%
Requirement for Capital Adequacy Purposes, Amount	$ 99,763	$ 92,632
Requirement for Capital Adequacy Purposes, Ratio	6.00%	6.00%
Tier 1 leverage ratio
Actual, Amount	$ 201,688	$ 182,635
Actual, Ratio	9.70%	9.41%
Requirement for Capital Adequacy Purposes, Amount	$ 83,153	$ 77,596
Requirement for Capital Adequacy Purposes, Ratio	4.00%	4.00%
Farmers National Banc Corp
Common equity tier 1 capital ratio
Actual, Amount	$ 189,227	$ 171,064
Actual, Ratio	11.41%	11.12%
Requirement for Capital Adequacy Purposes, Amount	$ 74,622	$ 69,244
Requirement for Capital Adequacy Purposes, Ratio	4.50%	4.50%
To be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 107,788	$ 100,020
To be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	6.50%	6.50%
Total risk based capital ratio
Actual, Amount	$ 201,331	$ 181,916
Actual, Ratio	12.14%	11.82%
Requirement for Capital Adequacy Purposes, Amount	$ 132,662	$ 123,101
Requirement for Capital Adequacy Purposes, Ratio	8.00%	8.00%
To be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 165,827	$ 153,877
To be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	10.00%	10.00%
Tier 1 risk based capital ratio
Actual, Amount	$ 189,227	$ 171,064
Actual, Ratio	11.41%	11.12%
Requirement for Capital Adequacy Purposes, Amount	$ 99,496	$ 92,326
Requirement for Capital Adequacy Purposes, Ratio	6.00%	6.00%
To be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 132,662	$ 123,101
To be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	8.00%	8.00%
Tier 1 leverage ratio
Actual, Amount	$ 189,227	$ 171,064
Actual, Ratio	9.16%	8.91%
Requirement for Capital Adequacy Purposes, Amount	$ 82,612	$ 76,792
Requirement for Capital Adequacy Purposes, Ratio	4.00%	4.00%
To be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 103,265	$ 95,990
To be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	5.00%	5.00%