| Document and Entity Information | Jun. 27, 2022 |
| Cover [Abstract] | |
| Entity Registrant Name | FARMERS NATIONAL BANC CORP /OH/ |
| Amendment Flag | false |
| Entity Central Index Key | 0000709337 |
| Document Type | 8-K |
| Document Period End Date | Jun. 27, 2022 |
| Entity Incorporation State Country Code | OH |
| Entity File Number | 001-35296 |
| Entity Tax Identification Number | 34-1371693 |
| Entity Address, Address Line One | 20 South Broad Street |
| Entity Address, Address Line Two | P.O. Box 555 |
| Entity Address, City or Town | Canfield |
| Entity Address, State or Province | OH |
| Entity Address, Postal Zip Code | 44406-0555 |
| City Area Code | (330) |
| Local Phone Number | 533-3341 |
| Written Communications | true |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, No Par Value |
| Trading Symbol | FMNB |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
