Loans and Allowance for Loan Losses - Composition of Loans, Net of Unearned Income (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unearned income on loans	$ 111,000	$ 115,000
Loans, net of unearned income	17,735,147	16,742,070
Commercial Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	15,080,602	14,201,115
Commercial Portfolio Segment | Software and internet
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	5,454,552	5,437,915
Commercial Portfolio Segment | Hardware
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	1,056,933	1,071,528
Commercial Portfolio Segment | Private equity/venture capital
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	6,299,608	5,467,577
Commercial Portfolio Segment | Life science/healthcare
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	1,727,992	1,710,642
Commercial Portfolio Segment | Premium wine
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	837,910	847,295
Commercial Portfolio Segment | Premium wine | Non-real Estate Secured Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	183,917	201,175
Commercial Portfolio Segment | Commercial Other Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	357,600	312,278
Commercial Portfolio Segment | Other consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	476,206	435,790
Commercial Real Estate Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	2,352,127	2,235,390
Commercial Real Estate Portfolio Segment | Premium wine
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	653,993	646,120
Commercial Real Estate Portfolio Segment | Real estate secured loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	1,653,531	1,544,440
Commercial Real Estate Portfolio Segment | Other real estate loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	44,603	44,830
Construction Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	74,003	78,682
Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	1,881,946	1,771,323
Consumer Portfolio Segment | Real estate secured loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	1,653,531	1,544,440
Consumer Portfolio Segment | Other consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	$ 228,415	$ 226,883