BOSTON PRIVATE FINANCIAL HOLDINGS, INC. (PARENT COMPANY ONLY) - Income Statement (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
Condensed Financial Statements, Captions [Line Items]
Gain on sale of affiliate									$ 0	$ 0	$ 18,142
Other									2,198	2,974	853
Total revenue	$ 86,063	$ 80,869	$ 81,600	$ 78,778	$ 82,918	$ 81,279	$ 81,840	$ 83,586	327,310	329,623	384,563
Salaries and employee benefits									139,726	134,302	161,468
Professional services									13,816	15,228	13,155
Junior subordinated debentures									2,670	4,189	3,925
Other									15,485	13,935	14,161
Income/(loss) before income taxes	31,095	24,501	(2,457)	908	28,129	25,575	24,818	24,459	54,047	102,981	119,406
Income tax expense/(benefit)	6,124	1,821	841	102	6,788	5,517	5,369	4,917	8,888	22,591	37,537
Net income from discontinued operations									0	0	2,002
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
Parent Company
Condensed Financial Statements, Captions [Line Items]
Interest income									31	62	107
Dividends from subsidiaries									57,095	54,606	45,448
Gain on sale of affiliate									0	0	18,142
Other									900	1,557	59
Total revenue									58,026	56,225	63,756
Salaries and employee benefits									1,447	2,106	2,121
Professional services									29	1,241	1,381
Junior subordinated debentures									2,670	4,189	3,925
Other									2,378	2,185	1,406
Total operating expense									6,524	9,721	8,833
Income/(loss) before income taxes									51,502	46,504	54,923
Income tax expense/(benefit)									(896)	(2,639)	14,628
Net income from discontinued operations									0	0	2,002
Income before equity in undistributed earnings of subsidiaries									52,398	49,143	42,297
Equity/(loss) in undistributed earnings of subsidiaries									(7,245)	30,885	38,087
Net income/(loss) attributable to the Company									$ 45,153	$ 80,028	$ 80,384