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	Sep. 30, 2015	Dec. 31, 2014
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	$ 15,314,580	$ 14,384,276
Commercial and Commercial Real Estate Portfolio Segment
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	13,657,688	13,105,641
Commercial and Commercial Real Estate Portfolio Segment | Software and internet
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	5,272,825	4,954,676
Commercial and Commercial Real Estate Portfolio Segment | Hardware
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	962,295	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	4,571,102	4,582,906
Commercial and Commercial Real Estate Portfolio Segment | Life science & healthcare
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	1,598,362	1,289,904
Commercial and Commercial Real Estate Portfolio Segment | Premium wine
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	826,960	794,321
Commercial and Commercial Real Estate Portfolio Segment | Other
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	426,144	352,828
Consumer Portfolio Segment
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	1,656,892	1,278,635
Consumer Portfolio Segment | Other
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	212,825	160,520
Consumer Portfolio Segment | Real estate secured loans
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	$ 1,444,067	$ 1,118,115