| Cover Page Cover Page | May 27, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | May 27, 2021 |
| Entity Registrant Name | Boston Private Financial Holdings, Inc. |
| Entity Incorporation, State or Country Code | MA |
| Entity File Number | 001-35070 |
| Entity Tax Identification Number | 04-2976299 |
| Entity Address, Address Line One | Ten Post Office Square |
| Entity Address, City or Town | Boston |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02109 |
| City Area Code | 617 |
| Local Phone Number | 912-1900 |
| 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 | 0000821127 |
| Amendment Flag | false |
