BOSTON PRIVATE FINANCIAL HOLDINGS, INC. (PARENT COMPANY ONLY) - Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Condensed Financial Statements, Captions [Line Items]
Cash and cash equivalents	$ 1,055,588	$ 292,479
Other assets	359,248	287,316
Total assets	10,048,733	8,830,501	$ 8,494,625
Junior subordinated debentures	106,363	106,363
Other liabilities	198,526	140,820
Total liabilities	9,180,725	8,010,100
Redeemable Noncontrolling Interests	0	1,383
Total liabilities, redeemable noncontrolling interests and shareholders’ equity	10,048,733	8,830,501
Parent Company
Condensed Financial Statements, Captions [Line Items]
Cash and cash equivalents	70,269	51,324
Investment in wholly-owned and majority-owned subsidiaries	887,847	857,062
Investment in partnerships and trusts	6,340	6,340
Other assets	23,036	27,453
Total assets	987,492	942,179
Junior subordinated debentures	106,363	106,363
Deferred income taxes	3,665	2,895
Other liabilities	9,456	12,520
Total liabilities	119,484	121,778
Redeemable Noncontrolling Interests	0	1,383
Total Shareholders’ Equity	868,008	819,018
Total liabilities, redeemable noncontrolling interests and shareholders’ equity	$ 987,492	$ 942,179