Loans, Allowance for Loan Losses and Allowance for Unfunded Credit Commitments - Summary of Loans Modified in Troubled Debt Restructurings (TDRs) (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) troubled_debt_restructuring	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of TDRs | troubled_debt_restructuring			57
Loans modified in TDRs	$ 122,000		$ 122,000		$ 61,000
Unfunded commitments available for funding associated with TDRs (less than)	1,000		1,000
Loans modified in TDRs during the period	53,000	$ 26,000	66,000	$ 32,000
Payment deferrals
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans modified in TDRs during the period	17,000	25,000	30,000	31,000
Extended Maturity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans modified in TDRs during the period	14,000		14,000
Payment Deferral and Extended Maturity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans modified in TDRs during the period	22,000		$ 22,000
Boston Private
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of TDRs | troubled_debt_restructuring			48
Loans modified in TDRs	60,000		$ 60,000
Loans modified in TDRs during the period	45,000		45,000
Investor dependent
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans modified in TDRs	26,000		26,000		36,000
Loans modified in TDRs during the period	8,000	4,000	9,000	8,000
Investor dependent | Early stage
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans modified in TDRs	0		0		7,000
Loans modified in TDRs during the period	0	4,000	1,000	4,000
Investor dependent | Growth stage
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans modified in TDRs	26,000		26,000		29,000
Loans modified in TDRs during the period	8,000	0	8,000	4,000
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	34,000		34,000		22,000
Loans modified in TDRs during the period	0	21,000	12,000	21,000
Cash flow dependent and innovation C&I (Commercial and Industrial) | Innovation C&I
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans modified in TDRs	0		0		1,000
Loans modified in TDRs during the period	0	1,000	0	2,000
Private bank
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans modified in TDRs	10,000		10,000		0
Loans modified in TDRs during the period	2,000	0	2,000	0
CRE
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans modified in TDRs	48,000		48,000		0
Loans modified in TDRs during the period	43,000	0	43,000	0
Premium wine and other | Premium wine
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans modified in TDRs	2,000		2,000		2,000
Loans modified in TDRs during the period	0	$ 0	0	$ 1,000
Other C&I
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans modified in TDRs	$ 2,000		$ 2,000		$ 0