| Cover | Apr. 28, 2021 |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Apr. 28, 2021 |
| Entity File Number | 0-23695 |
| Entity Registrant Name | BROOKLINE BANCORP, INC. |
| Entity Central Index Key | 0001049782 |
| Entity Tax Identification Number | 04-3402944 |
| Entity Incorporation, State or Country Code | DE |
| Entity Address, Address Line One | 131 Clarendon Street |
| Entity Address, City or Town | Boston |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02116 |
| City Area Code | 617 |
| Local Phone Number | 425-4600 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, par value of $0.01 per share |
| Trading Symbol | BRKL |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
