Condensed Parent Company Financial Statements (Condensed Cash Flows) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net income attributable to Brookline Bancorp, Inc.	$ 22,183	$ 22,596	$ 20,471	$ 22,467	$ 21,138	$ 22,460	$ 20,831	$ 18,633	$ 87,717	$ 83,062	$ 50,518
Adjustments to reconcile net income to net cash provided from operating activities:
Depreciation of premises and equipment									6,861	7,382	7,232
Amortization of debt issuance costs									100	100	100
Equity securities held-for-trading									1,215	(5,371)	0
Net cash provided from operating activities									102,719	123,029	84,947
Cash flows from investing activities:
Purchase of restricted equity securities									(12,924)	(13,262)	(7,927)
Purchase of premises and equipment									(4,997)	(3,352)	(11,557)
Net cash used for investing activities									(399,022)	(588,022)	(383,974)
Cash flows from financing activities:
Proceeds from issuance of common stock									0	0	81,943
Common stock issued for acquisition									0	55,182	0
Redemption of noncontrolling interest in subsidiary									(35,851)	0	0
Payment of dividends to owners of noncontrolling interest in subsidiary									(930)	(1,893)	(1,671)
Payment of dividends on common stock									(35,110)	(31,441)	(27,035)
Net cash provided from financing activities									284,509	493,572	292,375
Cash and cash equivalents at beginning of year				89,584					89,584
Cash and cash equivalents at end of year	77,790				89,584				77,790	89,584
Parent Company
Cash flows from operating activities:
Net income attributable to Brookline Bancorp, Inc.									87,717	83,062	50,518
Adjustments to reconcile net income to net cash provided from operating activities:
Equity in undistributed income of subsidiaries									(91,210)	(86,465)	(47,514)
Depreciation of premises and equipment									2,824	3,073	2,856
Amortization of debt issuance costs									100	100	100
Equity securities held-for-trading									666	(3,235)	0
Other operating activities, net									47,296	(31,014)	(5,885)
Net cash provided from operating activities									47,393	(34,479)	75
Cash flows from investing activities:
Repayment of ESOP loan by Brookline Bank									250	250	250
Pay down (issuance) of intercompany loan to Brookline Bank									10,000	40,000	(80,000)
Purchase of restricted equity securities									(151)	(1)	0
Purchase of premises and equipment									(909)	(1,359)	(1,942)
Net cash used for investing activities									9,190	38,890	(81,692)
Cash flows from financing activities:
Proceeds from issuance of common stock									0	0	81,943
Common stock issued for acquisition									0	55,183	0
Redemption of noncontrolling interest in subsidiary									(18,470)	0	0
Payment of dividends to owners of noncontrolling interest in subsidiary									(930)	0	0
Payment of dividends on common stock									(35,110)	(31,441)	(27,035)
Net cash provided from financing activities									(54,510)	23,742	54,908
Net increase (decrease) in cash and cash equivalents									2,073	28,153	(26,709)
Cash and cash equivalents at beginning of year				$ 33,696				$ 5,543	33,696	5,543	32,252
Cash and cash equivalents at end of year	$ 35,769				$ 33,696				35,769	33,696	5,543
Supplemental disclosures of cash flow information:
Fair value of assets acquired, net of cash and cash equivalents acquired									0	292,025	0
Fair value of liabilities assumed									$ 0	$ 278,988	$ 0