Loans and Allowance for Loan Losses - Composition of Loans, Net of Unearned Income, Broken Out by Portfolio Segment and Class of Financing Receivable (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	$ 23,106,316	$ 19,899,944
Consumer loans
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	2,629,486	2,168,332
Commercial loans
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	20,476,830	17,731,612
Commercial loans | Software and internet
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	6,172,531	5,627,031
Commercial loans | Hardware
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	1,193,599	1,180,398
Commercial loans | Private equity/venture capital
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	9,952,377	7,691,148
Commercial loans | Life science/healthcare
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	1,808,827	1,853,004
Commercial loans | Premium wine
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	873,158	878,322
Commercial loans | Other
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	476,338	501,709
Consumer loans | Other
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	328,980	241,364
Consumer loans | Real estate secured consumer loans
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	$ 2,300,506	$ 1,926,968