Loans and Allowance for Loan Losses - Composition of Loans, Net of Unearned Income (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	$ 23,106,316	$ 19,899,944
Commercial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	20,476,830	17,731,612
Commercial loans | Software and internet
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	6,172,531	5,627,031
Commercial loans | Hardware
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	1,193,599	1,180,398
Commercial loans | Private equity/venture capital
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	9,952,377	7,691,148
Commercial loans | Life science/healthcare
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	1,808,827	1,853,004
Commercial loans | Premium wine
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	873,158	878,322
Commercial loans | Commercial Other Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	365,724	393,551
Commercial loans | Commercial Loans Receivable excluding Construction and Real Estate Secured
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	19,697,163	16,945,288
Commercial loans | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	476,338	501,709
Real estate secured loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	3,011,627	2,648,621
Real estate secured loans | Premium wine
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	669,053	678,166
Real estate secured loans | Real estate secured consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	2,300,506	1,926,968
Real estate secured loans | Other Real Estate Commercial Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	42,068	43,487
Construction Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	68,546	64,671
Consumer loans | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	328,980	241,364
Consumer loans | Real estate secured consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	2,300,506	1,926,968
Non-real Estate Secured Commercial Loans | Commercial loans | Premium wine
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	204,105	200,156
Loans for personal residence | Consumer loans | Real estate secured consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	1,995,840	1,655,349
Loans to eligible employees | Consumer loans | Real estate secured consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	243,118	199,291
Home equity lines of credit | Consumer loans | Real estate secured consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	$ 61,548	$ 72,328