Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net income attributable to Brookline Bancorp, Inc.	$ 87,717	$ 83,062	$ 50,518
Adjustments to reconcile net income to net cash provided from operating activities:
Net income attributable to noncontrolling interest in subsidiary	43	3,489	3,101
Provision for credit losses	9,583	4,951	18,988
Origination of loans and leases held-for-sale	(15,361)	(25,370)	(27,425)
Proceeds from sales of loans and leases held-for-sale, net	19,930	26,519	32,073
Deferred income tax expense (benefit)	(2)	(5,453)	10,798
Depreciation of premises and equipment	6,861	7,382	7,232
Amortization of investment securities premiums and discounts, net	1,810	2,200	2,042
Amortization of deferred loan and lease origination costs, net	7,045	6,971	6,695
Amortization of identified intangible assets	1,663	2,080	2,089
Amortization of debt issuance costs	100	100	100
(Accretion) amortization of acquisition fair value adjustments, net	(809)	354	(1,583)
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)
Loss (gain) on sales of OREO and other repossessed assets, net	108	0	(79)
Write-down of OREO and other repossessed assets	903	1,234	458
Compensation under recognition and retention plans	2,683	2,546	2,308
ESOP shares committed to be released	458	543	501
Net change in:
Cash surrender value of bank-owned life insurance	(1,030)	(1,039)	(1,041)
Equity securities held-for-trading	1,215	(5,371)	0
Other assets	(63,235)	(10,855)	(2,413)
Accrued expenses and other liabilities	45,254	31,796	(5,378)
Net cash provided from operating activities	102,719	123,029	84,947
Cash flows from investing activities:
Proceeds from sales of investment securities available-for-sale	0	22,210	0
Proceeds from maturities, calls, and principal repayments of investment securities available-for-sale	68,165	82,896	71,611
Purchases of investment securities available-for-sale	(50,400)	(73,852)	(90,971)
Proceeds from maturities, calls, and principal repayments of investment securities held to maturity	28,858	3,290	3,817
Purchases of investment securities held-to-maturity	(1,430)	(8,915)	(26,873)
Proceeds from redemption/sales of restricted equity securities	20,857	12,110	24,462
Purchase of restricted equity securities	(12,924)	(13,262)	(7,927)
Proceeds from sales of loans and leases held-for-investment, net	13,860	7,294	28,608
Net increase in loans and leases	(467,493)	(593,968)	(378,906)
Acquisitions, net of cash and cash equivalents acquired	0	(24,659)	0
Purchase of premises and equipment, net	(4,997)	(3,352)	(11,557)
Proceeds from sales of OREO and other repossessed assets	6,482	2,186	3,762
Net cash used for investing activities	(399,022)	(588,022)	(383,974)
Cash flows from financing activities:
Increase in demand checking, NOW, savings and money market accounts	143,551	1,006	93,819
Increase (decrease) in certificates of deposit	232,911	580,878	166,448
Proceeds from FHLBB advances	4,029,662	6,607,745	4,685,706
Repayment of FHLBB advances	(4,055,568)	(6,713,279)	(4,705,543)
Increase (decrease) in other borrowed funds, net	7,955	5,095	(2,568)
(Decrease) increase in mortgagors' escrow accounts, net	(194)	(260)	41
Proceeds from exercise of stock options	0	490	1,469
Repurchases of common stock	(1,871)	(10,000)	0
Proceeds from issuance of common stock	0	0	81,943
Common stock issued for acquisition	0	55,182	0
Payment of dividends on common stock	(35,110)	(31,441)	(27,035)
Payment of income taxes for shares withheld in share based activity	(46)	(81)	(352)
Redemption of noncontrolling interest in subsidiary	(35,851)	0	0
Proceeds from issuance of noncontrolling units	0	130	118
Payment of dividends to owners of noncontrolling interest in subsidiary	(930)	(1,893)	(1,671)
Net cash provided from financing activities	284,509	493,572	292,375
Net (decrease) increase in cash and cash equivalents	(11,794)	28,579	(6,652)
Cash and cash equivalents at beginning of year	89,584	61,005	67,657
Cash and cash equivalents at end of year	77,790	89,584	61,005
Cash paid during the year for:
Interest on deposits, borrowed funds and subordinated debt	94,533	65,182	40,785
Income taxes	27,345	21,129	34,026
Non-cash investing activities:
Transfer from loans and leases to loan and leases held-for-sale	0	0	7,500
Transfer from loans to other real estate owned	6,105	3,020	7,161
First Commons Bank
Non-cash investing activities:
Fair value of assets acquired, net of cash and cash equivalents acquired	0	292,025	0
Fair value of liabilities assumed	$ 0	$ 278,988	$ 0