BOSTON PRIVATE FINANCIAL HOLDINGS, INC. (PARENT COMPANY ONLY) - Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net income/(loss) attributable to the Company	$ 24,971	$ 22,680	$ (3,298)	$ 800	$ 21,244	$ 19,962	$ 19,380	$ 19,442	$ 45,153	$ 80,028	$ 80,384
Net income from continuing operations									45,153	80,028	78,382
Depreciation and amortization									21,748	24,125	23,129
(Gain) on sale of affiliate									0	0	(18,142)
Net decrease/(increase) in other operating activities									(23,836)	(32,737)	(23,240)
Net cash provided by operating activities of continuing operations									60,748	73,282	91,497
Net cash provided by operating activities of discontinued operations									0	0	2,002
Net cash provided by operating activities									60,748	73,282	93,499
Cash flows from investing activities:
Net cash provided by investing activities									(375,001)	(18,996)	(198,405)
Cash flows from financing activities:
Repurchase of Series D preferred stock, including deemed dividend									0	0	(50,000)
Repurchase of common stock									(12,807)	(7,193)	(20,000)
Dividends paid to common shareholders									(29,683)	(40,380)	(40,685)
Dividends paid to preferred shareholders									0	0	(1,738)
Proceeds from stock option exercises									112	562	1,661
Proceeds from issuance of common stock									2,243	2,413	1,866
Other equity adjustments									44	(248)	3,786
Net cash (used in) financing activities									1,077,362	110,934	111,624
Net increase in Cash and cash equivalents									763,109	165,220	6,718
Cash and cash equivalents at beginning of year				292,479				127,259	292,479	127,259	120,541
Cash and cash equivalents at end of year	1,055,588				292,479				1,055,588	292,479	127,259
Parent Company
Cash flows from operating activities:
Net income/(loss) attributable to the Company									45,153	80,028	80,384
Net income from discontinued operations									0	0	2,002
Net income from continuing operations									45,153	80,028	78,382
Equity in earnings of subsidiaries:									(49,850)	(85,480)	(83,232)
Dividends from subsidiaries									57,095	54,606	45,448
Depreciation and amortization									34	201	(186)
(Gain) on sale of affiliate									0	0	(18,142)
Net decrease/(increase) in other operating activities									920	(8,108)	13,181
Net cash provided by operating activities of continuing operations									53,352	41,247	35,451
Net cash provided by operating activities of discontinued operations									0	0	2,002
Net cash provided by operating activities									53,352	41,247	37,453
Cash flows from investing activities:
Contingent consideration from divestitures									4,835	4,507	1,233
Cash received from divestitures									0	0	52,981
Net cash provided by investing activities									4,762	4,429	54,118
Cash flows from financing activities:
Repurchase of Series D preferred stock, including deemed dividend									0	0	(50,000)
Equity sales in minority-owned subsidiaries									0	0	1,021
Repurchase of common stock									(12,807)	(7,193)	(20,000)
Dividends paid to common shareholders									(29,683)	(40,380)	(40,685)
Dividends paid to preferred shareholders									0	0	(1,738)
Proceeds from stock option exercises									56	562	1,661
Proceeds from issuance of common stock									2,243	2,413	434
Other equity adjustments									1,022	(842)	2,463
Net cash (used in) financing activities									(39,169)	(45,440)	(106,844)
Net increase in Cash and cash equivalents									18,945	236	(15,273)
Cash and cash equivalents at beginning of year				$ 51,324				$ 51,088	51,324	51,088	66,361
Cash and cash equivalents at end of year	$ 70,269				$ 51,324				70,269	51,324	51,088
Parent Company | Non Banking Segments
Cash flows from investing activities:
Capital investments in subsidiaries:									$ (73)	$ (78)	$ (96)