Loans, Allowance for Loan Losses and Allowance for Unfunded Credit Commitments - Summary of Loans Modified in Troubled Debt Restructurings ("TDRs") by Portfolio Segment and Class of Financing Receivables (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) troubled_debt_restructuring	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Financing Receivable, Modifications [Line Items]
Financing Receivable, Modifications, Number of Contracts | troubled_debt_restructuring	21
Loans modified in TDRs	$ 140,986		$ 147,793
Modifications, Unfunded Commitments Available For Funding	400
Loans classified as TDRs, charge-offs	0	$ 6,200
Financing receivable modifications recorded investment modified during period	3,880	6,309
Software/internet | Commercial Portfolio Segment
Financing Receivable, Modifications [Line Items]
Loans modified in TDRs	67,205		73,455
Financing receivable modifications recorded investment modified during period	756	6,309
Hardware | Commercial Portfolio Segment
Financing Receivable, Modifications [Line Items]
Loans modified in TDRs	50,030		51,132
Financing receivable modifications recorded investment modified during period	1,559	0
Private equity/venture capital | Commercial Portfolio Segment
Financing Receivable, Modifications [Line Items]
Loans modified in TDRs	0		350
Life science/healthcare | Commercial Portfolio Segment
Financing Receivable, Modifications [Line Items]
Loans modified in TDRs	20,037		19,235
Financing receivable modifications recorded investment modified during period	1,239	0
Premium wine | Commercial Portfolio Segment
Financing Receivable, Modifications [Line Items]
Loans modified in TDRs	3,137		3,198
Other | Consumer Portfolio Segment
Financing Receivable, Modifications [Line Items]
Loans modified in TDRs	577		423
Financing receivable modifications recorded investment modified during period	326	0
Commercial loans
Financing Receivable, Modifications [Line Items]
Financing receivable modifications recorded investment modified during period	3,554	6,309
Commercial loans | Commercial Portfolio Segment
Financing Receivable, Modifications [Line Items]
Loans modified in TDRs	140,409		$ 147,370
Payment deferrals granted
Financing Receivable, Modifications [Line Items]
Financing receivable modifications recorded investment modified during period	$ 3,900	$ 6,300