Loans and Allowance for Loan Losses - Composition of Loans, Net of Unearned Income (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unearned income on loans	$ 115,000	$ 104,000
Loans, net of unearned income	15,314,580	14,384,276
Commercial Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	12,893,846	12,380,611
Commercial Portfolio Segment | Software and internet
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	5,272,825	4,954,676
Commercial Portfolio Segment | Hardware
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	962,295	1,131,006
Commercial Portfolio Segment | Private equity/venture capital
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	4,571,102	4,582,906
Commercial Portfolio Segment | Life science & healthcare
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	1,598,362	1,289,904
Commercial Portfolio Segment | Premium wine
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	193,712	187,568
Commercial Portfolio Segment | Other consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	295,550	234,551
Commercial Real Estate Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	2,115,358	1,764,519
Commercial Real Estate Portfolio Segment | Premium wine
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	633,248	606,753
Commercial Real Estate Portfolio Segment | Real estate secured loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	1,444,067	1,118,115
Commercial Real Estate Portfolio Segment | Other real estate loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	38,043	39,651
Construction Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	92,551	78,626
Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	1,656,892	1,278,635
Consumer Portfolio Segment | Real estate secured loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	1,444,067	1,118,115
Consumer Portfolio Segment | Other consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	$ 212,825	$ 160,520