Loans, Allowance for Loan Losses and Allowance for Unfunded Credit Commitments - Composition of Loans, Net of Unearned Income, Broken Out by Portfolio Segment and Class of Financing Receivable (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Unearned income on loans	$ 158,000	$ 148,000
Loans, net of unearned income	24,587,944	23,106,316
Commercial Portfolio Segment
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	21,097,918	19,697,163
Commercial Portfolio Segment | Software/internet
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	6,209,084	6,172,531
Commercial Portfolio Segment | Hardware
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	1,336,000	1,193,599
Commercial Portfolio Segment | Private equity/venture capital
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	11,211,816	9,952,377
Commercial Portfolio Segment | Life science/healthcare
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	1,932,608	1,808,827
Commercial Portfolio Segment | Premium wine
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	865,024	873,158
Commercial Portfolio Segment | Other
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	320,125	476,338
Commercial and Commercial Real Estate Portfolio Segment
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	21,874,657	20,476,830
Consumer Portfolio Segment
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	2,713,287	2,629,486
Consumer Portfolio Segment | Other
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	350,947	328,980
Consumer Portfolio Segment | Real estate secured loans
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	$ 2,362,340	$ 2,300,506