| Cover Page Document | Nov. 27, 2020 |
| Cover Page [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Nov. 27, 2020 |
| Entity Registrant Name | WASHINGTON TRUST BANCORP, INC. |
| Entity Incorporation, State or Country Code | RI |
| Entity File Number | 001-32991 |
| Entity Tax Identification Number | 05-0404671 |
| Entity Address, Address Line One | 23 Broad Street |
| Entity Address, City or Town | Westerly, |
| Entity Address, State or Province | RI |
| Entity Address, Postal Zip Code | 02891 |
| City Area Code | (401) |
| Local Phone Number | 348-1200 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0000737468 |
| Amendment Flag | false |
