Regulatory Capital Matters (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Common equity tier 1 capital ratio
Actual, Amount	$ 315,850	$ 279,864
Actual, Ratio	14.55%	13.22%
Requirement for Capital Adequacy Purposes, Amount	$ 97,715	$ 95,211
Requirement for Capital Adequacy Purposes, Ratio	4.50%	4.50%
Total risk based capital ratio
Actual, Amount	$ 351,987	$ 311,413
Actual, Ratio	0.1621	0.1472
Requirement for Capital Adequacy Purposes, Amount	$ 173,715	$ 169,264
Requirement for Capital Adequacy Purposes, Ratio	0.080	0.080
Tier 1 risk based capital ratio
Actual, Amount	$ 328,850	$ 289,269
Actual, Ratio	0.1514	0.1367
Requirement for Capital Adequacy Purposes, Amount	$ 130,286	$ 126,948
Requirement for Capital Adequacy Purposes, Ratio	0.060	0.060
Tier 1 leverage ratio
Actual, Amount	$ 328,850	$ 289,269
Actual, Ratio	0.1017	0.0977
Requirement for Capital Adequacy Purposes, Amount	$ 129,307	$ 118,464
Requirement for Capital Adequacy Purposes, Ratio	0.040	0.040
Farmers National Banc Corp
Common equity tier 1 capital ratio
Actual, Amount	$ 265,224	$ 268,041
Actual, Ratio	12.23%	12.71%
Requirement for Capital Adequacy Purposes, Amount	$ 97,606	$ 94,903
Requirement for Capital Adequacy Purposes, Ratio	4.50%	4.50%
To be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 140,986	$ 137,083
To be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	6.50%	6.50%
Total risk based capital ratio
Actual, Amount	$ 288,360	$ 290,185
Actual, Ratio	0.1329	0.1376
Requirement for Capital Adequacy Purposes, Amount	$ 173,521	$ 168,717
Requirement for Capital Adequacy Purposes, Ratio	0.080	0.080
To be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 216,901	$ 210,897
To be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	0.100	0.100
Tier 1 risk based capital ratio
Actual, Amount	$ 265,224	$ 268,041
Actual, Ratio	0.1223	0.1271
Requirement for Capital Adequacy Purposes, Amount	$ 130,141	$ 126,538
Requirement for Capital Adequacy Purposes, Ratio	0.060	0.060
To be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 173,521	$ 168,717
To be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	0.080	0.080
Tier 1 leverage ratio
Actual, Amount	$ 265,224	$ 268,041
Actual, Ratio	0.0825	0.0910
Requirement for Capital Adequacy Purposes, Amount	$ 128,647	$ 117,877
Requirement for Capital Adequacy Purposes, Ratio	0.040	0.040
To be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 160,809	$ 147,346
To be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	0.050	0.050