Boston Private Financial Holdings, Inc. (Parent Company Only) Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net income attributable to the Company	$ 33,302	$ 17,962	$ 6,434	$ 22,686	$ (18,280)	$ 19,803	$ 23,406	$ 15,662	$ 80,384	$ 40,591	$ 71,628
Net income from continuing operations									78,382	35,721	66,087
Depreciation and amortization									23,129	21,023	23,441
(Gain)/loss on sale of affiliates or offices									(18,142)	1,264	(2,862)
Net decrease/ (increase) in other operating activities									(23,240)	(8,466)	(817)
Net cash provided by/ (used in) operating activities of continuing operations									91,497	97,115	100,606
Net cash provided by/ (used in) operating activities of discontinued operations									2,002	4,870	5,541
Net cash provided by operating activities									93,499	101,985	106,147
Cash flows from investing activities:
Cash received from/(provided for) divestitures									52,981	0
Net cash provided by/ (used in) investing activities									(198,405)	(354,315)	(692,780)
Cash flows from financing activities:
Repurchase of Series D preferred stock, including deemed dividend									(50,000)	0	0
Repurchase of common stock									(20,000)	0	(9,323)
Dividends paid to common shareholders									(40,685)	(37,054)	(33,110)
Dividends paid to preferred shareholders									(1,738)	(3,475)	(3,475)
Tax benefit/ (deficiency) from certain stock compensation awards									0	0	(620)
Proceeds from stock option exercises									1,661	882	1,725
Proceeds from issuance of common stock									1,866	1,601	1,579
Other equity adjustments									3,786	26	(82)
Net cash provided by financing activities									111,624	266,314	454,496
Net increase/ (decrease) in cash and cash equivalents									6,718	13,984	(132,137)
Cash and cash equivalents at beginning of year				120,541				106,557	120,541	106,557	238,694
Cash and cash equivalents at end of year	127,259				120,541				127,259	120,541	106,557
Parent Company
Cash flows from operating activities:
Net income attributable to the Company									80,384	40,591	71,628
Net income from discontinued operations									2,002	4,870	5,541
Net income from continuing operations									78,382	35,721	66,087
Equity in earnings/ (loss) of subsidiaries									38,087	(9,811)	24,917
Depreciation and amortization									(186)	3,553	(738)
(Gain)/loss on sale of affiliates or offices									(18,142)	1,264	0
Net decrease/ (increase) in other operating activities									13,181	(2,440)	(6,633)
Net cash provided by/ (used in) operating activities of continuing operations									35,451	48,088	33,799
Net cash provided by/ (used in) operating activities of discontinued operations									2,002	4,870	5,541
Net cash provided by operating activities									37,453	52,958	39,340
Cash flows from investing activities:
Contingent consideration from divestitures									(1,233)	0	0
Cash received from/(provided for) divestitures									52,981	0	0
Net cash (used in)/ provided by in other investing activities									0	0	1
Net cash provided by/ (used in) investing activities									54,118	(54)	(1,219)
Cash flows from financing activities:
Repurchase of Series D preferred stock, including deemed dividend									(50,000)	0	0
Repurchase of common stock									(20,000)	0	(9,323)
Dividends paid to common shareholders									(40,685)	(37,054)	(33,110)
Dividends paid to preferred shareholders									(1,738)	(3,475)	(3,475)
Tax benefit/ (deficiency) from certain stock compensation awards									0	0	(620)
Proceeds from stock option exercises									1,661	882	1,725
Proceeds from issuance of common stock									434	2,740	7,786
Other equity adjustments									2,463	(5,740)	(3,685)
Net cash provided by financing activities									(106,844)	(41,237)	(38,812)
Net increase/ (decrease) in cash and cash equivalents									(15,273)	11,667	(691)
Cash and cash equivalents at beginning of year				$ 66,361				$ 54,694	66,361	54,694	55,385
Cash and cash equivalents at end of year	$ 51,088				$ 66,361				51,088	66,361	54,694
Parent Company | Private Banking
Cash flows from operating activities:
Equity in earnings/ (loss) of subsidiaries									(70,930)	(50,476)	(58,184)
Dividends from subsidiaries									27,500	25,200	22,700
Parent Company | Non Banking Segments
Cash flows from operating activities:
Equity in earnings/ (loss) of subsidiaries									(12,302)	11,150	(13,477)
Dividends from subsidiaries									17,948	24,116	24,044
Cash flows from investing activities:
Capital investments in subsidiaries:									$ (96)	$ (54)	$ (1,220)