Boston Private Financial Holdings, Inc. (Parent Company Only) Cash Flow (Details) - USD ($) $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017	[1]	Jun. 30, 2017	[1]	Mar. 31, 2017		Dec. 31, 2016		Sep. 30, 2016	[1]	Jun. 30, 2016	[1]	Mar. 31, 2016		Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Net Income Attributable to Parent	$ (18,280)	[1]	$ 19,803		$ 23,406		$ 15,662	[1]	$ 17,552	[1]	$ 19,636		$ 16,398		$ 18,041	[1]	$ 40,591		$ 71,628		$ 64,925
Net income from discontinued operations	989	[1]	$ 1,186		$ 1,063		1,632	[1]	1,184	[1]	$ 1,047		$ 1,245		2,065	[1]	4,870	[2]	5,541	[2]	6,411	[2]
Net income from continuing operations																	40,189		70,244		62,921
Deferred income tax expense/ (benefit)																	22,845		2,777		(4,575)
Depreciation and amortization																	21,023		23,441		22,187
Net decrease/(increase) in other operating activities																	(8,466)		(817)		2,852
Net Cash Provided by (Used in) Operating Activities, Continuing Operations																	97,115		100,606		87,477
Cash Provided by (Used in) Operating Activities, Discontinued Operations																	4,870		5,541		6,411
Net Cash Provided by (Used in) Operating Activities																	101,985		106,147		93,888
Net Cash Provided by (Used in) Investing Activities																	(354,315)		(692,780)		(694,790)
Repurchase of common stock																	0		(9,323)		0
Dividends paid to common shareholders																	(37,054)		(33,110)		(29,608)
Dividends paid to preferred shareholders																	(3,475)		(3,475)		(3,475)
Tax savings/ (deficiency) from certain stock compensation awards																	0		(620)		(1,262)
Proceeds from stock option exercises																	882		1,725		1,206
Proceeds from issuance of common stock, net																	171		842		(515)
Other equity adjustments																	26		(82)		(199)
Net Cash Provided by (Used in) Financing Activities																	266,314		454,496		666,987
Cash and Cash Equivalents, Period Increase (Decrease)																	13,984		(132,137)		66,085
Cash and cash equivalents at beginning of year							106,557								238,694		106,557		238,694		172,609
Cash and cash equivalents at end of period	120,541								106,557								120,541		106,557		238,694
Parent Company [Member]
Net Income Attributable to Parent																	50,402		46,711		46,187
Net income from discontinued operations																	4,870		5,541		6,411
Net income from continuing operations																	35,721		66,087		58,514
Deferred income tax expense/ (benefit)																	(3,075)		10,000		(4,575)
Depreciation and amortization																	3,553		(738)		5,097
Net decrease/(increase) in other operating activities																	1,899		(16,633)		2,167
Net Cash Provided by (Used in) Operating Activities, Continuing Operations																	48,088		33,799		42,465
Cash Provided by (Used in) Operating Activities, Discontinued Operations																	4,870		5,541		6,411
Net Cash Provided by (Used in) Operating Activities																	52,958		39,340		48,876
Payments to Acquire Businesses, Net of Cash Acquired																	1,410		1,890		1,821
Payments for (Proceeds from) Other Investing Activities																	0		1		1
Net Cash Provided by (Used in) Investing Activities																	1,356		671		99
Repurchase of common stock																	0		9,323		0
Dividends paid to common shareholders																	(37,054)		(33,110)		(29,608)
Dividends paid to preferred shareholders																	(3,475)		(3,475)		(3,475)
Tax savings/ (deficiency) from certain stock compensation awards																	0		(620)		(1,262)
Proceeds from stock option exercises																	882		1,725		1,206
Proceeds from issuance of common stock, net																	2,740		7,786		160
Other equity adjustments																	(5,740)		(3,685)		(5,204)
Net Cash Provided by (Used in) Financing Activities																	(42,647)		(40,702)		(38,183)
Cash and Cash Equivalents, Period Increase (Decrease)																	11,667		(691)		10,792
Cash and cash equivalents at beginning of year							$ 54,694								$ 55,385		54,694		55,385		44,593
Cash and cash equivalents at end of period	$ 66,361								$ 54,694								66,361		54,694		55,385
Private Banking Segment [Member] | Parent Company [Member]
Net Income Attributable to Parent																	(50,476)		(58,184)		(57,201)
Cash Dividends Paid to Parent Company by Consolidated Subsidiaries																	25,200		22,700		22,700
Non Banking Segments [Member] | Parent Company [Member]
Net Income Attributable to Parent																	11,150		(13,477)		(13,026)
Cash Dividends Paid to Parent Company by Consolidated Subsidiaries																	24,116		24,044		28,789
Private Banking Segment [Member]
Net Income Attributable to Parent																	53,441		70,348		57,989
Net income from continuing operations																	53,441		70,348		57,989
Private Banking Segment [Member] | Parent Company [Member]
Payments of Distributions to Affiliates																	0		0		0
Non Banking Segments [Member] | Parent Company [Member]
Payments of Distributions to Affiliates																	$ (54)		$ (1,220)		$ (1,723)

[1] Due to rounding, the sum of the four quarters may not add to the year to date total.

[2] The Holding Company and Eliminations calculation of Net Income attributable to the Company includes net income from Discontinued Operations for the years ended December 31, 2017, 2016 and 2015 of $4.9 million, $5.5 million, and $6.4 million, respectively