Loans, Allowance for Loan Losses and Allowance for Unfunded Credit Commitments - Activity Relating to our Allowance for Credit Losses for Loans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2021	Dec. 31, 2020
Financing Receivable, Allowance for Credit Loss [Line Items]
Increase in allowance for credit loss	$ 2
Allowance for credit loss, beginning balance	396	$ 590	$ 448	$ 305
Initial Allowance on PCD Loans					$ 22	$ 22
Charge-offs	(19)	(28)	(129)	(80)
Recoveries	8	4	16	16
(Reduction) Provision for Credit Losses	(9)	(54)	41	247
Foreign Currency Translation Adjustments		1		0
Allowance for credit loss, ending balance	398	513	398	513
Boston Private | Non-PCD Loans
Financing Receivable, Allowance for Credit Loss [Line Items]
(Reduction) Provision for Credit Losses	44
Impact of adopting ASC 326
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss, beginning balance				25
Accrued interest receivable and other assets | Loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Accrued interest receivable	160		160			126
Global fund banking
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss, beginning balance	66	54	46	107
Initial Allowance on PCD Loans					0	0
Charge-offs	0	0	(80)	0
Recoveries	0	0	0	0
(Reduction) Provision for Credit Losses	(7)	(15)	93	2
Foreign Currency Translation Adjustments		0		0
Allowance for credit loss, ending balance	59	39	59	39
Global fund banking | Impact of adopting ASC 326
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss, beginning balance				(70)
Investor dependent
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss, beginning balance	158	292	213	82
Initial Allowance on PCD Loans					0	0
Charge-offs	(17)	(28)	(37)	(67)
Recoveries	5	4	13	12
(Reduction) Provision for Credit Losses	(8)	(5)	(51)	165
Foreign Currency Translation Adjustments		0		(1)
Allowance for credit loss, ending balance	138	263	138	263
Investor dependent | Impact of adopting ASC 326
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss, beginning balance				72
Cash flow dependent and innovation C&I (Commercial and Industrial)
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss, beginning balance	119	123	125	81
Initial Allowance on PCD Loans					0	0
Charge-offs	(1)	0	(8)	(11)
Recoveries	3	0	3	3
(Reduction) Provision for Credit Losses	(12)	(2)	(11)	50
Foreign Currency Translation Adjustments		0		(1)
Allowance for credit loss, ending balance	109	121	109	121
Cash flow dependent and innovation C&I (Commercial and Industrial) | Impact of adopting ASC 326
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss, beginning balance				(1)
Private bank
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss, beginning balance	47	91	53	22
Initial Allowance on PCD Loans					1	1
Charge-offs	(1)	0	(3)	(2)
Recoveries	0	0	0	0
(Reduction) Provision for Credit Losses	(16)	(15)	(20)	44
Foreign Currency Translation Adjustments		0		0
Allowance for credit loss, ending balance	31	76	31	76
Private bank | Impact of adopting ASC 326
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss, beginning balance				12
CRE
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss, beginning balance	0		0
Initial Allowance on PCD Loans					17	17
Charge-offs	0		0
Recoveries	0		0
(Reduction) Provision for Credit Losses	23		23
Allowance for credit loss, ending balance	40		40
Other C&I
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss, beginning balance	0		0
Initial Allowance on PCD Loans					4	4
Charge-offs	0		0
Recoveries	0		0
(Reduction) Provision for Credit Losses	8		8
Allowance for credit loss, ending balance	12		12
Premium wine and other
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss, beginning balance	6	26	9	13
Initial Allowance on PCD Loans					$ 0	0
Charge-offs	0	0	(1)	0
Recoveries	0	0	0	1
(Reduction) Provision for Credit Losses	3	(16)	1	(17)
Foreign Currency Translation Adjustments		1		2
Allowance for credit loss, ending balance	9	11	9	11
Premium wine and other | Impact of adopting ASC 326
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss, beginning balance				12
PPP Loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss, beginning balance		4	2	0
Initial Allowance on PCD Loans						$ 0
Charge-offs		0	0	0
Recoveries		0	0	0
(Reduction) Provision for Credit Losses		(1)	(2)	3
Foreign Currency Translation Adjustments		0		0
Allowance for credit loss, ending balance	$ 0	$ 3	$ 0	3
PPP Loans | Impact of adopting ASC 326
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for credit loss, beginning balance				$ 0