Loans, Allowance for Loan Losses and Allowance for Unfunded Credit Commitments - Recorded Investment in Loans Modified in TDRs (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans modified in TDRs during the period	$ 53,000,000	$ 26,000,000	$ 66,000,000	$ 32,000,000
Partial charge-offs on loans classified as TDRs	0	14,000,000	7,000,000	31,000,000
Boston Private
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans modified in TDRs during the period	45,000,000		45,000,000
Payment deferrals
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans modified in TDRs during the period	17,000,000	25,000,000	30,000,000	31,000,000
Principal forgiveness
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans modified in TDRs during the period		1,000,000		1,000,000
Extended Maturity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans modified in TDRs during the period	14,000,000		14,000,000
Investor dependent
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans modified in TDRs during the period	8,000,000	4,000,000	9,000,000	8,000,000
Investor dependent | Early stage
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans modified in TDRs during the period	0	4,000,000	1,000,000	4,000,000
Investor dependent | Growth stage
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans modified in TDRs during the period	8,000,000	0	8,000,000	4,000,000
Cash flow dependent and innovation C&I (Commercial and Industrial) | Cash flow dependent - SLBO
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans modified in TDRs during the period	0	21,000,000	12,000,000	21,000,000
Cash flow dependent and innovation C&I (Commercial and Industrial) | Innovation C&I
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans modified in TDRs during the period	0	1,000,000	0	2,000,000
Private bank
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans modified in TDRs during the period	2,000,000	0	2,000,000	0
CRE
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans modified in TDRs during the period	43,000,000	0	43,000,000	0
Premium wine and other | Premium wine
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans modified in TDRs during the period	$ 0	$ 0	$ 0	$ 1,000,000