Loans, Allowance for Loan Losses and Allowance for Unfunded Credit Commitments - Composition of Loans, Net of Unearned Income (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unearned income on loans	$ 174,000	$ 148,000
Loans, net of unearned income	27,494,915	23,106,316
Commercial loans, excluding real estate secured and construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	23,712,308	19,697,163
Commercial loans, excluding real estate secured and construction | Software/internet
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	6,258,724	6,172,531
Commercial loans, excluding real estate secured and construction | Hardware
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	1,306,709	1,193,599
Commercial loans, excluding real estate secured and construction | Private equity/venture capital
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	13,360,511	9,952,377
Commercial loans, excluding real estate secured and construction | Life science/healthcare
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	2,302,453	1,808,827
Commercial loans, excluding real estate secured and construction | Premium wine | Non-real estate secured commercial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	227,498	204,105
Commercial loans, excluding real estate secured and construction | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	256,413	365,724
Real estate secured loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	3,283,726	3,011,627
Real estate secured loans | Premium wine
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	685,941	669,053
Real estate secured loans | Real estate secured loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	2,556,906	2,300,506
Real estate secured loans | Other real estate loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	40,879	42,068
Construction loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	81,163	68,546
Consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	2,974,624	2,629,486
Consumer loans | Real estate secured loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	2,556,906	2,300,506
Consumer loans | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income	$ 417,718	$ 328,980