Condensed Parent Company Financial Statements (Condensed Income Statements) (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
Interest and dividend income:
Marketable and restricted equity securities									$ 3,477	$ 3,964	$ 3,062
Total interest and dividend income	$ 87,450	$ 87,906	$ 87,184	$ 85,086	$ 83,544	$ 80,636	$ 78,074	$ 71,639	347,626	313,893	263,050
Interest expense:
Net interest income	63,931	63,236	63,134	62,999	63,159	62,332	62,717	59,491	253,300	247,699	223,181
Gain on investment securities, net	133	(116)	357	134	(692)	(243)	0	1,162	508	227	11,393
Non-interest income:
Other									6,594	5,847	4,789
Total non-interest income									29,793	25,224	32,173
Non-interest expense:
Compensation and employee benefits									96,554	91,535	82,413
Occupancy									15,696	14,991	14,546
Equipment and data processing									18,652	18,213	16,854
Professional services									4,366	4,404	4,315
Advertising and marketing									4,044	4,016	3,369
Merger and acquisition expense									1,125	3,787	411
Other	38,395	39,770	39,184	38,469	39,745	36,773	37,163	39,471	13,936	13,484	11,788
Total non-interest expense									157,481	155,232	139,111
Credit for income taxes	7,087	7,507	6,780	6,895	7,055	6,140	7,342	5,652	28,269	26,189	43,636
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
Parent Company
Interest and dividend income:
Dividend income from subsidiaries									0	0	7,317
Marketable and restricted equity securities									147	37	0
ESOP loan to Brookline Bank									77	98	120
Intercompany loan to Brookline Bank									1,183	1,722	532
Total interest and dividend income									1,407	1,857	7,969
Interest expense:
Borrowed funds									5,274	5,223	5,123
Net interest income									(3,867)	(3,366)	2,846
Gain on investment securities, net									467	71	0
Non-interest income:
Other									0	16	0
Total non-interest income									467	87	0
Non-interest expense:
Compensation and employee benefits									(507)	345	391
Occupancy									1,589	1,586	1,584
Equipment and data processing									(770)	(798)	(1,011)
Directors' fees									461	417	453
Franchise taxes									306	321	180
Insurance									543	534	528
Professional services									548	364	470
Advertising and marketing									31	19	4
Merger and acquisition expense									0	452	411
Other									(1,139)	(1,140)	(1,224)
Total non-interest expense									1,062	2,100	1,786
Loss before income taxes									(4,462)	(5,379)	1,060
Credit for income taxes									(969)	(1,976)	(1,944)
Loss before equity in undistributed income of subsidiaries									(3,493)	(3,403)	3,004
Equity in undistributed income of subsidiaries									91,210	86,465	47,514
Net income attributable to Brookline Bancorp, Inc.									$ 87,717	$ 83,062	$ 50,518