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	Jun. 30, 2015	Dec. 31, 2014
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	$ 14,261,430	$ 14,384,276
Commercial and Commercial Real Estate Portfolio Segment
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	12,694,394	13,105,641
Commercial and Commercial Real Estate Portfolio Segment | Software and internet
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	4,980,553	4,954,676
Commercial and Commercial Real Estate Portfolio Segment | Hardware
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	1,048,848	1,131,006
Commercial and Commercial Real Estate Portfolio Segment | Private equity/venture capital
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	3,987,448	4,582,906
Commercial and Commercial Real Estate Portfolio Segment | Life science & healthcare
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	1,475,568	1,289,904
Commercial and Commercial Real Estate Portfolio Segment | Premium wine
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	823,407	794,321
Commercial and Commercial Real Estate Portfolio Segment | Other
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	378,570	352,828
Consumer Portfolio Segment
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	1,567,036	1,278,635
Consumer Portfolio Segment | Other
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	226,720	160,520
Consumer Portfolio Segment | Real estate secured loans
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	$ 1,340,316	$ 1,118,115