Investment Securities (Narrative) (Details)			1 Months Ended	3 Months Ended	12 Months Ended
	Dec. 17, 2019 USD ($) shares	Mar. 06, 2018 USD ($) shares	Mar. 31, 2018 USD ($)	Mar. 31, 2018 USD ($) shares	Dec. 31, 2019 USD ($) security	Dec. 31, 2018 USD ($) security	Dec. 31, 2017 USD ($)
Investment [Line Items]
AFS, debt securities, estimated fair value					$ 498,995,000	$ 502,793,000
Debt securities, unrealized gain (loss)					2,800,000	(12,500,000)
AFS, debt securities, unrealized loss position					$ 205,599,000	$ 466,671,000
Percentage of securities in unrealized loss positions, available-for-sale securities					41.20%	92.80%
AFS, debt securities, unrealized loss position, accumulated loss					$ 1,768,000	$ 12,762,000
Investment securities held to maturity, fair value					87,561,000	112,830,000
Net unrealized gain (loss), held-to-maturity securities					800,000	(1,900,000)
Gross unrealized losses, fair value, held-to-maturity securities					$ 22,341,000	$ 102,104,000
Percentage of securities in unrealized loss positions, held-to-maturity securities					25.50%	90.50%
HTM, gross unrealized losses					$ 154,000	$ 1,978,000
Equity securities held-for-trading					3,581,000	4,207,000
Purchases of investment securities held-to-maturity					1,430,000	8,915,000	$ 26,873,000
Investment securities held-to-maturity					86,780,000	114,776,000
Estimated fair value of debt securities have right to call or prepay the obligations					37,600,000	19,100,000
Estimated fair value of debt securities have right to call or prepay the obligations, scheduled maturities within one year					3,000,000.0	0
Estimated fair value of debt securities have right to call or prepay the obligations, scheduled maturities of after one year through five years					34,600,000	8,400,000
Estimated fair value of debt securities have right to call or prepay the obligations, scheduled maturities of after five years					0
Estimated fair value of debt securities have right to call or prepay the obligations, scheduled maturities after five years through ten years						10,700,000
Estimated fair value of debt securities have right to call or prepay the obligations, scheduled maturities of which is after 10 years						0
Investment owned shares sold | shares	100,000
Loss on marketable equity securities	$ 232,000
Community Bank Systems, Inc. | Brookline Bank
Investment [Line Items]
Investment owned cash proceeds from settlement		$ 600,000
Investment owned shares received form settlement | shares		11,303
Investment owned shares sold | shares		11,303
Gain (loss) on sale of investments		$ 600,000
Longwood Securities | Brookline Bank
Investment [Line Items]
Investment owned shares sold | shares				3
Gain (loss) on sale of investments			$ (100,000)
Investment owned			$ 1,500,000	$ 1,500,000
GSE debt securities excluding specified securities
Investment [Line Items]
AFS, debt securities, estimated fair value					17,400,000	20,600,000
GSE debentures
Investment [Line Items]
AFS, debt securities, estimated fair value					185,803,000	181,079,000
Debt securities, unrealized gain (loss)					2,900,000	(3,000,000.0)
Investment securities held to maturity, fair value					31,290,000	49,601,000
Net unrealized gain (loss), held-to-maturity securities					100,000	(900,000)
Gross unrealized losses, fair value, held-to-maturity securities					31,300,000	49,600,000
HTM, gross unrealized losses					$ 51,000	$ 967,000
Available for sale number of securities | security					60	60
Number of securities in unrealized loss positions | security					5	51
Payments to acquired AFS debt securities					$ 0	$ 33,900,000
Number of securities | security					10	17
Number of securities in an unrealized loss position | security					4	14
Purchases of investment securities held-to-maturity					$ 0	$ 8,900,000
Investment securities held-to-maturity					31,228,000	50,546,000
GSE CMOs
Investment [Line Items]
AFS, debt securities, estimated fair value					85,900,000	103,100,000
Debt securities, unrealized gain (loss)					$ (1,100,000)	$ (4,200,000)
Available for sale number of securities | security					61	61
Number of securities in unrealized loss positions | security					45	46
Payments to acquired AFS debt securities					$ 0	$ 0
GSE mortgage-related securities
Investment [Line Items]
AFS, debt securities, estimated fair value					153,300,000	165,100,000
Debt securities, unrealized gain (loss)					$ 300,000	$ (4,200,000)
Available for sale number of securities | security					150	162
Number of securities in unrealized loss positions | security					48	93
Payments to acquired AFS debt securities					$ 19,600,000	$ 15,200,000
SBA commercial loan asset-backed securities
Investment [Line Items]
AFS, debt securities, estimated fair value					$ 34,000.0	$ 51,000.0
Available for sale number of securities | security					4	4
Number of securities in unrealized loss positions | security					3
Payments to acquired AFS debt securities					$ 0	$ 0
Corporate debt obligations
Investment [Line Items]
AFS, debt securities, estimated fair value					28,986,000	39,708,000
Debt securities, unrealized gain (loss)					$ (500,000)	$ (900,000)
Available for sale number of securities | security					8	11
Number of securities in unrealized loss positions | security					0	11
Payments to acquired AFS debt securities					$ 0	$ 0
U.S. Treasury bonds
Investment [Line Items]
AFS, debt securities, estimated fair value					44,900,000	13,700,000
Debt securities, unrealized gain (loss)					$ 200,000	$ (100,000)
Available for sale number of securities | security					9	7
Number of securities in unrealized loss positions | security					5	2
Payments to acquired AFS debt securities					$ 30,800,000	$ 24,700,000
GSE MBSs
Investment [Line Items]
AFS, debt securities, estimated fair value					153,343,000	165,089,000
Investment securities held to maturity, fair value					9,279,000	11,131,000
Net unrealized gain (loss), held-to-maturity securities					(100,000)	(300,000)
Gross unrealized losses, fair value, held-to-maturity securities					9,300,000	11,100,000
HTM, gross unrealized losses					$ 81,000	$ 295,000
Number of securities | security					8	8
Purchases of investment securities held-to-maturity					$ 0	$ 0
Investment securities held-to-maturity					9,360,000	11,426,000
Municipal obligations
Investment [Line Items]
Investment securities held to maturity, fair value					46,514,000	51,598,000
Gross unrealized losses, fair value, held-to-maturity securities					46,500,000	51,600,000
HTM, gross unrealized losses					$ 0	$ 716,000
Number of securities | security					93	98
Number of securities in an unrealized loss position | security					6	94
Purchases of investment securities held-to-maturity					$ 900,000	$ 0
Investment securities held-to-maturity					45,692,000	52,304,000
Foreign government obligations
Investment [Line Items]
Investment securities held to maturity, fair value					478,000	500,000
Gross unrealized losses, fair value, held-to-maturity securities					500,000	500,000
HTM, gross unrealized losses					$ 22,000	$ 0
Number of securities | security					1	1
Purchases of investment securities held-to-maturity					$ 0	$ 0
Investment securities held-to-maturity					500,000	500,000
Collateral Pledged
Investment [Line Items]
Available for sale securities pledged as collateral					$ 433,600,000	$ 442,500,000