Quarterly Results of Operations (Unaudited) (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
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	23,519	24,670	24,050	22,087	20,385	18,304	15,357	12,148	94,326	66,194	39,869
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
Provision for credit losses	3,602	871	3,757	1,353	123	2,717	1,470	641	9,583	4,951	18,988
Net interest income after provision for credit losses	60,329	62,365	59,377	61,646	63,036	59,615	61,247	58,850	243,717	242,748	204,193
Loan level derivative income, net	2,494	2,251	1,772	1,745	1,811	2,192	571	866	8,262	5,440	2,187
(Loss) gain on investment securities, net	133	(116)	357	134	(692)	(243)	0	1,162	508	227	11,393
Gain on sales of loans and leases held-for-sale	309	550	561	289	327	535	722	299	1,709	1,883	2,644
Other non-interest income	4,820	5,244	4,788	4,462	5,015	4,585	4,233	3,841
Amortization of identified intangible assets	(420)	(421)	(420)	(402)	(537)	(537)	(539)	(467)	(1,663)	(2,080)	(2,089)
Other non-interest expense	(38,395)	(39,770)	(39,184)	(38,469)	(39,745)	(36,773)	(37,163)	(39,471)	(13,936)	(13,484)	(11,788)
Income before provision for income taxes	29,270	30,103	27,251	29,405	29,215	29,374	29,071	25,080	116,029	112,740	97,255
Provision 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 before noncontrolling interest in subsidiary	22,183	22,596	20,471	22,510	22,160	23,234	21,729	19,428	87,760	86,551	53,619
Less net income attributable to noncontrolling interest in subsidiary	0	0	0	43	1,022	774	898	795	43	3,489	3,101
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
Earnings per share:
Basic (in dollars per share)	$ 0.28	$ 0.28	$ 0.26	$ 0.28	$ 0.26	$ 0.28	$ 0.26	$ 0.24	$ 1.10	$ 1.04	$ 0.68
Diluted (in dollars per share)	$ 0.28	$ 0.28	$ 0.26	$ 0.28	$ 0.26	$ 0.28	$ 0.26	$ 0.24	$ 1.10	$ 1.04	$ 0.68
Average common shares outstanding:
Basic (in shares)	79,682,724	79,700,403	79,669,922	79,658,583	80,258,489	80,315,050	80,184,977	77,879,593	79,679,781	79,669,668	74,459,508
Diluted (in shares)	79,845,447	79,883,510	79,886,292	79,843,578	80,407,558	80,515,467	80,505,614	78,167,800	79,856,921	79,909,251	74,811,408
Common stock price:
High (in dollars per share)	$ 16.83	$ 15.39	$ 15.92	$ 16.23	$ 16.97	$ 19.05	$ 19.25	$ 17.05
Low (in dollars per share)	14.36	13.73	14.35	13.90	12.86	16.70	15.90	15.60
Dividends paid:
Dividends per share (in dollars per share)	$ 0.115	$ 0.110	$ 0.110	$ 0.105	$ 0.105	$ 0.100	$ 0.100	$ 0.090	$ 0.44	$ 0.395	$ 0.360