Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Total Brookline Bancorp, Inc. Stockholders' Equity	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Loss	Treasury Stock	Unallocated Common Stock Held by ESOP	Noncontrolling Interest in Subsidiary
Balance beginning of period at Dec. 31, 2016	$ 702,749	$ 695,544	$ 757	$ 616,734	$ 136,671	$ (3,818)	$ (53,837)	$ (963)	$ 7,205
Increase (Decrease) in Stockholders' Equity
Reclassification due to the adoption of ASU No. 2018-02					1,063	(1,063)
Net income attributable to Brookline Bancorp, Inc.	50,518	50,518			50,518
Net income attributable to noncontrolling interest in subsidiary	3,101								3,101
Issuance of common stock	81,943	81,943	60	81,883
Issuance of noncontrolling interest	118								118
Other comprehensive income (loss)	(1,069)	(1,069)				(1,069)
Common stock dividends	(27,035)	(27,035)			(27,035)
Dividend distribution to owners of noncontrolling interest in subsidiary	(1,671)								(1,671)
Compensation under recognition and retention plans	3,428	3,428		1,045			2,383
Common stock held by ESOP committed to be released	501	501		314				187
Balance end of period at Dec. 31, 2017	812,583	803,830	817	699,976	161,217	(5,950)	(51,454)	(776)	8,753
Increase (Decrease) in Stockholders' Equity
Reclassification due to the adoption of ASU No. 2018-02						0
Net income attributable to Brookline Bancorp, Inc.	83,062	83,062			83,062
Net income attributable to noncontrolling interest in subsidiary	3,489								3,489
Common stock issued for acquisition	55,183	55,183	35	55,148
Issuance of noncontrolling interest	130								130
Other comprehensive income (loss)	(3,510)	(3,510)				(3,510)
Common stock dividends	(31,441)	(31,441)			(31,441)
Dividend distribution to owners of noncontrolling interest in subsidiary	(1,893)								(1,893)
Restricted stock awards, net of awards surrendered	2,473	2,473		139			2,334
Treasury stock, repurchase shares	(10,000)	(10,000)					(10,000)
Common stock held by ESOP committed to be released	543	543		366				177
Balance end of period at Dec. 31, 2018	910,619	900,140	852	755,629	212,838	(9,460)	(59,120)	(599)	10,479
Increase (Decrease) in Stockholders' Equity
Net income attributable to Brookline Bancorp, Inc.	87,717	87,717			87,717
Net income attributable to noncontrolling interest in subsidiary	43								43
Other comprehensive income (loss)	11,743	11,743				11,743
Common stock dividends	(35,110)	(35,110)			(35,110)
Dividend distribution to owners of noncontrolling interest in subsidiary	(930)	(930)		(930)
Redemption of noncontrolling interest in subsidiary	(28,992)	(18,470)		(18,470)					(10,522)
Restricted stock awards, net of awards surrendered	1,929	1,929		80	(69)		1,918
Treasury stock, repurchase shares	(1,871)	(1,871)					(1,871)
Common stock held by ESOP committed to be released	458	458		292				166
Balance end of period at Dec. 31, 2019	$ 945,606	$ 945,606	$ 852	$ 736,601	$ 265,376	$ 2,283	$ (59,073)	$ (433)	$ 0