| Cover Page | Jan. 30, 2020 | Mar. 31, 2020 |
| Cover page. | ||
| Document Type | 8-K | |
| Document Period End Date | Apr. 29, 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 |
