| Cover Page | Oct. 19, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Oct. 19, 2020 |
| Entity Registrant Name | S&T BANCORP, INC |
| Entity Incorporation, State or Country Code | PA |
| Entity File Number | 0-12508 |
| Entity Tax Identification Number | 25-1434426 |
| Entity Address, Address Line One | 800 Philadelphia Street |
| Entity Address, City or Town | Indiana |
| Entity Address, State or Province | PA |
| Entity Address, Postal Zip Code | 15701 |
| City Area Code | 800 |
| Local Phone Number | 325-2265 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, $2.50 par value |
| Trading Symbol | STBA |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0000719220 |
| Amendment Flag | false |
