Consolidated Statements of Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Interest and dividend income:
Loans and leases	$ 330,345	$ 295,269	$ 247,022
Debt securities	12,281	13,960	12,524
Marketable and restricted equity securities	3,477	3,964	3,062
Short-term investments	1,523	700	442
Total interest and dividend income	347,626	313,893	263,050
Interest expense:
Deposits	69,615	41,978	23,288
Borrowed funds	24,711	24,216	16,581
Total interest expense	94,326	66,194	39,869
Net interest income	253,300	247,699	223,181
Provision for credit losses	9,583	4,951	18,988
Net interest income after provision for credit losses	243,717	242,748	204,193
Non-interest income:
Loan level derivative income, net	8,262	5,440	2,187
Gain on investment securities, net	508	227	11,393
Gain on sales of loans and leases held-for-sale	1,709	1,883	2,644
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
FDIC insurance	1,445	2,722	3,326
Advertising and marketing	4,044	4,016	3,369
Amortization of identified intangible assets	1,663	2,080	2,089
Merger and acquisition expense	1,125	3,787	411
Other	13,936	13,484	11,788
Total non-interest expense	157,481	155,232	139,111
Income before provision for income taxes	116,029	112,740	97,255
Provision for income taxes	28,269	26,189	43,636
Net income before noncontrolling interest in subsidiary	87,760	86,551	53,619
Less net income attributable to noncontrolling interest in subsidiary	43	3,489	3,101
Net income attributable to Brookline Bancorp, Inc.	$ 87,717	$ 83,062	$ 50,518
Earnings per common share:
Basic (in dollars per share)	$ 1.10	$ 1.04	$ 0.68
Diluted (in dollars per share)	$ 1.10	$ 1.04	$ 0.68
Weighted average common shares outstanding during the year:
Basic (in shares)	79,679,781	79,669,668	74,459,508
Diluted (in shares)	79,856,921	79,909,251	74,811,408
Dividends declared per common share (in dollars per share)	$ 0.450	$ 0.395	$ 0.360
Deposit fees
Non-interest income:
Non-interest income, fees	$ 10,623	$ 10,400	$ 10,050
Loan fees
Non-interest income:
Non-interest income, fees	$ 2,097	$ 1,427	$ 1,110