Regulatory Capital Matters (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Common equity tier 1 capital ratio
Actual, Amount	$ 269,283	$ 247,395
Actual, Ratio	12.98%	12.94%
Requirement for Capital Adequacy Purposes, Amount	$ 93,379	$ 86,039
Requirement for Capital Adequacy Purposes, Ratio	4.50%	4.50%
Total risk based capital ratio
Actual, Amount	$ 297,961	$ 264,188
Actual, Ratio	0.1436	0.1382
Requirement for Capital Adequacy Purposes, Amount	$ 166,007	$ 152,958
Requirement for Capital Adequacy Purposes, Ratio	0.080	0.080
Tier 1 risk based capital ratio
Actual, Amount	$ 278,620	$ 249,701
Actual, Ratio	0.1343	0.1306
Requirement for Capital Adequacy Purposes, Amount	$ 124,505	$ 114,719
Requirement for Capital Adequacy Purposes, Ratio	0.060	0.060
Tier 1 leverage ratio
Actual, Amount	$ 278,620	$ 249,701
Actual, Ratio	0.0967	0.1069
Requirement for Capital Adequacy Purposes, Amount	$ 115,266	$ 93,406
Requirement for Capital Adequacy Purposes, Ratio	0.040	0.040
Farmers National Banc Corp
Common equity tier 1 capital ratio
Actual, Amount	$ 258,385	$ 213,507
Actual, Ratio	12.49%	11.19%
Requirement for Capital Adequacy Purposes, Amount	$ 93,094	$ 85,854
Requirement for Capital Adequacy Purposes, Ratio	4.50%	4.50%
To be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 134,469	$ 124,011
To be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	6.50%	6.50%
Total risk based capital ratio
Actual, Amount	$ 277,726	$ 227,994
Actual, Ratio	0.1342	0.1195
Requirement for Capital Adequacy Purposes, Amount	$ 165,500	$ 152,629
Requirement for Capital Adequacy Purposes, Ratio	0.080	0.080
To be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 206,875	$ 190,787
To be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	0.100	0.100
Tier 1 risk based capital ratio
Actual, Amount	$ 258,385	$ 213,507
Actual, Ratio	0.1249	0.1119
Requirement for Capital Adequacy Purposes, Amount	$ 124,125	$ 114,472
Requirement for Capital Adequacy Purposes, Ratio	0.060	0.060
To be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 165,500	$ 152,629
To be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	0.080	0.080
Tier 1 leverage ratio
Actual, Amount	$ 258,385	$ 213,507
Actual, Ratio	0.0901	0.0906
Requirement for Capital Adequacy Purposes, Amount	$ 114,678	$ 94,304
Requirement for Capital Adequacy Purposes, Ratio	0.040	0.040
To be Well Capitalized Under Prompt Corrective Action Provisions, Amount	$ 143,348	$ 117,881
To be Well Capitalized Under Prompt Corrective Action Provisions, Ratio	0.050	0.050