Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income before noncontrolling interest in subsidiary	$ 87,760	$ 86,551	$ 53,619
Investment securities available-for-sale:
Unrealized securities holding gains (losses)	15,290	(4,444)	(1,274)
Income tax (expense) benefit	(3,379)	980	457
Net unrealized securities holding gains (losses) before reclassification adjustments	11,911	(3,464)	(817)
Less reclassification adjustments for securities gains included in net income:
Gain on sales of securities, net	0	173	0
Income tax expense	0	(38)	0
Net reclassification adjustments for securities gains included in net income	0	135	0
Net unrealized securities holding losses	11,911	(3,599)	(817)
Postretirement benefits:
Adjustment of accumulated obligation for postretirement benefits	(227)	121	(422)
Income tax benefit (expense)	59	(32)	170
Net adjustment of accumulated obligation for postretirement benefits	(168)	89	(252)
Other comprehensive loss, net of taxes	11,743	(3,510)	(1,069)
Comprehensive income	99,503	83,041	52,550
Net income attributable to noncontrolling interest in subsidiary	43	3,489	3,101
Comprehensive income attributable to Brookline Bancorp, Inc.	$ 99,460	$ 79,552	$ 49,449