Condensed Parent Company Financial Statements (Condensed Balance Sheets) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
ASSETS
Cash and due from banks	$ 33,589	$ 47,542
Short-term investments	44,201	42,042
Total cash and cash equivalents	77,790	89,584
Equity securities held-for-trading	3,581	4,207
Restricted equity securities	53,818	61,751
Premises and equipment, net	74,350	76,382
Deferred tax asset	25,017	21,495
Goodwill	160,427	160,427	$ 137,890	$ 137,890
Other assets	167,431	103,214
Total assets	7,856,853	7,392,805
LIABILITIES AND STOCKHOLDERS' EQUITY
Borrowed funds	902,749	920,542
Accrued expenses and other liabilities	146,318	100,174
Total liabilities	6,911,247	6,482,186
Stockholders' Equity:
Common stock, $0.01 par value; 200,000,000 shares authorized; 85,177,172 shares issued and 85,177,172 shares issued, respectively	852	852
Additional paid-in capital	736,601	755,629
Retained earnings, partially restricted	265,376	212,838
Accumulated other comprehensive loss	2,283	(9,460)
Treasury stock, at cost; 5,003,127 shares and 5,020,025 shares, respectively	(59,073)	(59,120)
Unallocated common stock held by ESOP; 79,548 shares and 109,950 shares, respectively	(433)	(599)
Total Brookline Bancorp, Inc. stockholders' equity	945,606	900,140
Total liabilities and stockholders' equity	7,856,853	7,392,805
Parent Company
ASSETS
Cash and due from banks	35,736	33,663
Short-term investments	33	33
Total cash and cash equivalents	35,769	33,696	$ 5,543	$ 32,252
Equity securities held-for-trading	2,569	3,235
ESOP loan to Brookline Bank	752	1,002
Intercompany loan to Brookline Bank	30,000	40,000
Restricted equity securities	252	101
Premises and equipment, net	2,403	4,318
Deferred tax asset	1,742	1,499
Investment in subsidiaries, at equity	924,352	864,798
Goodwill	35,267	35,267
Other assets	12,116	5,400
Total assets	1,045,222	989,316
LIABILITIES AND STOCKHOLDERS' EQUITY
Borrowed funds	83,591	83,433
Deferred tax liability	0	0
Accrued expenses and other liabilities	16,025	5,743
Total liabilities	99,616	89,176
Stockholders' Equity:
Common stock, $0.01 par value; 200,000,000 shares authorized; 85,177,172 shares issued and 85,177,172 shares issued, respectively	852	852
Additional paid-in capital	736,601	755,629
Retained earnings, partially restricted	265,376	212,838
Accumulated other comprehensive loss	2,283	(9,460)
Treasury stock, at cost; 5,003,127 shares and 5,020,025 shares, respectively	(59,073)	(59,120)
Unallocated common stock held by ESOP; 79,548 shares and 109,950 shares, respectively	(433)	(599)
Total Brookline Bancorp, Inc. stockholders' equity	945,606	900,140
Total liabilities and stockholders' equity	$ 1,045,222	$ 989,316