Boston Private Financial Holdings, Inc. (Parent Company Only) Balance Sheet (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Cash and cash equivalents		$ 120,541	$ 106,557	$ 238,694	$ 172,609
Deferred income taxes, net		44,073	73,010
Other assets		259,515	130,753
Assets		8,311,744	7,970,474	7,542,508
Junior Subordinated Debenture Owed to Unconsolidated Subsidiary Trust		106,363	106,363
Deferred Income Tax Liabilities, Net		5,322	8,638
Other liabilities		135,880	119,683
Total liabilities		7,508,339	7,185,021
Stockholders' Equity Attributable to Noncontrolling Interest		22,647	21,133
Stockholders' Equity Attributable to Parent		780,758	764,320
Total liabilities, redeemable noncontrolling interests and shareholders’ equity		8,311,744	7,970,474
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Cash and cash equivalents		66,361	54,694	$ 55,385	$ 44,593
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures		6,340	6,340
Other assets		11,046	17,131
Assets		926,418	909,944
Junior Subordinated Debenture Owed to Unconsolidated Subsidiary Trust		106,363	106,363
Other liabilities		11,328	9,490
Total liabilities		123,013	124,491
Stockholders' Equity Attributable to Noncontrolling Interest	[1]	22,647	21,133
Stockholders' Equity Attributable to Parent		780,758	764,320
Total liabilities, redeemable noncontrolling interests and shareholders’ equity		926,418	909,944
Parent Company [Member] | Private Banking Segment [Member]
Condensed Financial Statements, Captions [Line Items]
Investments in and Advances to Affiliates, Balance, Principal Amount		731,907	694,517
Parent Company [Member] | Non Banking Segments [Member]
Condensed Financial Statements, Captions [Line Items]
Investments in and Advances to Affiliates, Balance, Principal Amount		$ 110,764	$ 137,262

[1] Includes noncontrolling interests and the maximum redemption value of redeemable noncontrolling interests.