| Cover Page Document | Jul. 28, 2020 | Jul. 20, 2020 |
| Cover Page [Abstract] | ||
| Document Type | 8-K | |
| Document Period End Date | Jul. 28, 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 |
