Boston Private Financial Holdings, Inc. (Parent Company Only) Income Statement (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
Condensed Financial Statements, Captions [Line Items]
Gain/ (loss) on sale of affiliates or offices									$ 18,142	$ (1,264)	$ 2,862
Other									853	2,098	3,410
Total income	$ 105,842	$ 91,955	$ 89,640	$ 97,126	$ 96,447	$ 96,894	$ 95,163	$ 90,148	384,563	378,652	359,225
Salaries and employee benefits									161,468	178,501	164,683
Professional services									13,155	13,763	11,576
Junior subordinated debentures									3,925	2,919	2,427
Other									14,161	14,474	12,521
Income before income taxes	42,192	24,347	24,803	28,064	3,400	27,980	33,456	21,549	119,406	86,385	101,207
Income tax expense	8,651	5,461	17,399	6,026	21,391	8,289	9,963	6,553	37,537	46,196	30,963
Net income from discontinued operations	306	0	(2)	1,698	989	1,186	1,063	1,632	2,002	4,870	5,541
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
Parent Company
Condensed Financial Statements, Captions [Line Items]
Interest income									107	180	130
Gain/ (loss) on sale of affiliates or offices									18,142	(1,264)	0
Other									59	193	(2,552)
Total income									63,756	48,425	47,184
Salaries and employee benefits									2,121	4,061	4,620
Professional services									1,381	1,369	1,337
Junior subordinated debentures									3,925	2,919	2,427
Other									1,406	1,424	1,381
Total operating expense									8,833	9,773	9,765
Income before income taxes									54,923	38,652	37,419
Income tax expense									14,628	(6,880)	(3,751)
Net income from discontinued operations									2,002	4,870	5,541
Income before equity in undistributed earnings of subsidiaries									42,297	50,402	46,711
Equity/(loss) in undistributed earnings of subsidiaries									38,087	(9,811)	24,917
Net income attributable to the Company									80,384	40,591	71,628
Parent Company | Bank
Condensed Financial Statements, Captions [Line Items]
Dividends from subsidiaries									27,500	25,200	22,700
Equity/(loss) in undistributed earnings of subsidiaries									(70,930)	(50,476)	(58,184)
Parent Company | Non-banks
Condensed Financial Statements, Captions [Line Items]
Dividends from subsidiaries									17,948	24,116	24,044
Equity/(loss) in undistributed earnings of subsidiaries									$ (12,302)	$ 11,150	$ (13,477)