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	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Unearned income on loans	$ 165,000	$ 148,000
Loans, net of unearned income	25,996,192	23,106,316
Commercial loans
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	23,117,851	20,476,830
Commercial loans | Software/internet
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	6,230,358	6,172,531
Commercial loans | Hardware
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	1,269,241	1,193,599
Commercial loans | Private equity/venture capital
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	12,224,155	9,952,377
Commercial loans | Life science/healthcare
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	2,087,872	1,808,827
Commercial loans | Premium wine
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	884,170	873,158
Commercial loans | Other
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	422,055	476,338
Consumer loans
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	2,878,341	2,629,486
Consumer loans | Other
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	397,279	328,980
Consumer loans | Real estate secured loans
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	$ 2,481,062	$ 2,300,506