Regulatory Matters (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Common equity tier 1 capital ratio
Actual, Amount	$ 279,864	$ 247,395
Actual, Ratio	13.22%	12.94%
Requirement for Capital Adequacy Purposes, Amount	$ 95,211	$ 86,039
Requirement for Capital Adequacy Purposes, Ratio	4.50%	4.50%
Total risk based capital ratio
Actual, Amount	$ 311,413	$ 264,188
Actual, Ratio	0.1472	0.1382
Requirement for Capital Adequacy Purposes, Amount	$ 169,264	$ 152,958
Requirement for Capital Adequacy Purposes, Ratio	0.080	0.080
Tier 1 risk based capital ratio
Actual, Amount	$ 289,269	$ 249,701
Actual, Ratio	0.1367	0.1306
Requirement for Capital Adequacy Purposes, Amount	$ 126,948	$ 114,719
Requirement for Capital Adequacy Purposes, Ratio	0.060	0.060
Tier 1 leverage ratio
Actual, Amount	$ 289,269	$ 249,701
Actual, Ratio	0.0977	0.1069
Requirement for Capital Adequacy Purposes, Amount	$ 118,464	$ 93,406
Requirement for Capital Adequacy Purposes, Ratio	0.040	0.040
Farmers National Banc Corp
Common equity tier 1 capital ratio
Actual, Amount	$ 268,041	$ 213,507
Actual, Ratio	12.71%	11.19%
Requirement for Capital Adequacy Purposes, Amount	$ 94,903	$ 85,854
Requirement for Capital Adequacy Purposes, Ratio	4.50%	4.50%
To be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 137,083	$ 124,011
To be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	6.50%	6.50%
Total risk based capital ratio
Actual, Amount	$ 290,185	$ 227,994
Actual, Ratio	0.1376	0.1195
Requirement for Capital Adequacy Purposes, Amount	$ 168,717	$ 152,629
Requirement for Capital Adequacy Purposes, Ratio	0.080	0.080
To be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 210,897	$ 190,787
To be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	0.100	0.100
Tier 1 risk based capital ratio
Actual, Amount	$ 268,041	$ 213,507
Actual, Ratio	0.1271	0.1119
Requirement for Capital Adequacy Purposes, Amount	$ 126,538	$ 114,472
Requirement for Capital Adequacy Purposes, Ratio	0.060	0.060
To be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 168,717	$ 152,629
To be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	0.080	0.080
Tier 1 leverage ratio
Actual, Amount	$ 268,041	$ 213,507
Actual, Ratio	0.0910	0.0906
Requirement for Capital Adequacy Purposes, Amount	$ 117,877	$ 94,304
Requirement for Capital Adequacy Purposes, Ratio	0.040	0.040
To be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 147,346	$ 117,881
To be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	0.050	0.050