Loans, Allowance for Loan Losses and Allowance for Unfunded Credit Commitments - Aging of Gross Loans, Broken out by Portfolio Segment and Class of Financing Receivable (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	$ 502,598	$ 199,927
Current	24,243,154	23,054,226
Loans Past Due 90 Days or More Still Accruing Interest	7	191
Total gross loans excluding impaired loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	460,030	156,278
Current	24,102,083	22,923,014
Loans Past Due 90 Days or More Still Accruing Interest	7	191
Impaired loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	42,568	43,649
Current	141,071	131,212
Loans Past Due 90 Days or More Still Accruing Interest	0	0
30 - 59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	408,165	111,525
30 - 59 Days Past Due | Total gross loans excluding impaired loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	400,052	110,181
30 - 59 Days Past Due | Impaired loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	8,113	1,344
60 - 89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	61,217	57,808
60 - 89 Days Past Due | Total gross loans excluding impaired loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	59,971	45,906
60 - 89 Days Past Due | Impaired loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	1,246	11,902
Greater Than 90 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	33,216	30,594
Greater Than 90 Days Past Due | Total gross loans excluding impaired loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	7	191
Greater Than 90 Days Past Due | Impaired loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	33,209	30,403
Commercial Portfolio Segment | Software/internet
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	41,230	20,924
Current	6,113,585	6,101,147
Loans Past Due 90 Days or More Still Accruing Interest	3	141
Commercial Portfolio Segment | Hardware
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	5,547	1,272
Current	1,306,873	1,163,278
Loans Past Due 90 Days or More Still Accruing Interest	0	50
Commercial Portfolio Segment | Private equity/venture capital
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	407,143	125,146
Current	10,814,598	9,835,317
Loans Past Due 90 Days or More Still Accruing Interest	0	0
Commercial Portfolio Segment | Life science/healthcare
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	4,246	4,581
Current	1,972,241	1,841,692
Loans Past Due 90 Days or More Still Accruing Interest	4	0
Commercial Portfolio Segment | Premium wine
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	708	418
Current	862,530	871,074
Loans Past Due 90 Days or More Still Accruing Interest	0	0
Commercial Portfolio Segment | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	445
Current	328,847	490,292
Loans Past Due 90 Days or More Still Accruing Interest	0	0
Commercial Portfolio Segment | Commercial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	458,874	152,786
Current	21,398,674	20,302,800
Loans Past Due 90 Days or More Still Accruing Interest	7	191
Commercial Portfolio Segment | 30 - 59 Days Past Due | Software/internet
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	30,557	14,257
Commercial Portfolio Segment | 30 - 59 Days Past Due | Hardware
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	5,502	1,145
Commercial Portfolio Segment | 30 - 59 Days Past Due | Private equity/venture capital
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	357,936	86,566
Commercial Portfolio Segment | 30 - 59 Days Past Due | Life science/healthcare
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	4,193	4,390
Commercial Portfolio Segment | 30 - 59 Days Past Due | Premium wine
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	708	418
Commercial Portfolio Segment | 30 - 59 Days Past Due | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	445
Commercial Portfolio Segment | 30 - 59 Days Past Due | Commercial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	398,896	107,221
Commercial Portfolio Segment | 60 - 89 Days Past Due | Software/internet
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	10,670	6,526
Commercial Portfolio Segment | 60 - 89 Days Past Due | Hardware
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	45	77
Commercial Portfolio Segment | 60 - 89 Days Past Due | Private equity/venture capital
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	49,207	38,580
Commercial Portfolio Segment | 60 - 89 Days Past Due | Life science/healthcare
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	49	191
Commercial Portfolio Segment | 60 - 89 Days Past Due | Premium wine
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	0
Commercial Portfolio Segment | 60 - 89 Days Past Due | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	0
Commercial Portfolio Segment | 60 - 89 Days Past Due | Commercial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	59,971	45,374
Commercial Portfolio Segment | Greater Than 90 Days Past Due | Software/internet
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	3	141
Commercial Portfolio Segment | Greater Than 90 Days Past Due | Hardware
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	50
Commercial Portfolio Segment | Greater Than 90 Days Past Due | Private equity/venture capital
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	0
Commercial Portfolio Segment | Greater Than 90 Days Past Due | Life science/healthcare
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	4	0
Commercial Portfolio Segment | Greater Than 90 Days Past Due | Premium wine
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	0
Commercial Portfolio Segment | Greater Than 90 Days Past Due | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	0
Commercial Portfolio Segment | Greater Than 90 Days Past Due | Commercial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	7	191
Consumer Portfolio Segment | Real estate secured loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	599	2,696
Current	2,354,108	2,292,980
Loans Past Due 90 Days or More Still Accruing Interest	0	0
Consumer Portfolio Segment | Other consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	557	796
Current	349,301	327,234
Loans Past Due 90 Days or More Still Accruing Interest	0	0
Consumer Portfolio Segment | Consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	1,156	3,492
Current	2,703,409	2,620,214
Loans Past Due 90 Days or More Still Accruing Interest	0	0
Consumer Portfolio Segment | 30 - 59 Days Past Due | Real estate secured loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	599	2,164
Consumer Portfolio Segment | 30 - 59 Days Past Due | Other consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	557	796
Consumer Portfolio Segment | 30 - 59 Days Past Due | Consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	1,156	2,960
Consumer Portfolio Segment | 60 - 89 Days Past Due | Real estate secured loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	532
Consumer Portfolio Segment | 60 - 89 Days Past Due | Other consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	0
Consumer Portfolio Segment | 60 - 89 Days Past Due | Consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	532
Consumer Portfolio Segment | Greater Than 90 Days Past Due | Real estate secured loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	0
Consumer Portfolio Segment | Greater Than 90 Days Past Due | Other consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	0
Consumer Portfolio Segment | Greater Than 90 Days Past Due | Consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	$ 0	$ 0