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	Sep. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	$ 85,708	$ 199,927
Current	27,583,121	23,054,226
Loans Past Due 90 Days or More Still Accruing Interest	163	191
Total gross loans excluding impaired loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	53,196	156,278
Current	27,429,561	22,923,014
Loans Past Due 90 Days or More Still Accruing Interest	163	191
Impaired loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	32,512	43,649
Current	153,560	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	49,513	111,525
30 - 59 Days Past Due | Total gross loans excluding impaired loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	47,168	110,181
30 - 59 Days Past Due | Impaired loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	2,345	1,344
60 - 89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	6,835	57,808
60 - 89 Days Past Due | Total gross loans excluding impaired loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	5,865	45,906
60 - 89 Days Past Due | Impaired loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	970	11,902
Greater Than 90 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	29,360	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	163	191
Greater Than 90 Days Past Due | Impaired loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	29,197	30,403
Commercial loans | Software/internet
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	22,025	20,924
Current	6,175,651	6,101,147
Loans Past Due 90 Days or More Still Accruing Interest	116	141
Commercial loans | Hardware
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	590	1,272
Current	1,298,215	1,163,278
Loans Past Due 90 Days or More Still Accruing Interest	0	50
Commercial loans | Private equity/venture capital
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	7,558	125,146
Current	13,357,777	9,835,317
Loans Past Due 90 Days or More Still Accruing Interest	2	0
Commercial loans | Life science/healthcare
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	10,562	4,581
Current	2,330,483	1,841,692
Loans Past Due 90 Days or More Still Accruing Interest	45	0
Commercial loans | Premium wine
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	1,594	418
Current	910,356	871,074
Loans Past Due 90 Days or More Still Accruing Interest	0	0
Commercial loans | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	367	445
Current	400,547	490,292
Loans Past Due 90 Days or More Still Accruing Interest	0	0
Commercial loans | Commercial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	42,696	152,786
Current	24,473,029	20,302,800
Loans Past Due 90 Days or More Still Accruing Interest	163	191
Commercial loans | 30 - 59 Days Past Due | Software/internet
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	17,067	14,257
Commercial loans | 30 - 59 Days Past Due | Hardware
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	491	1,145
Commercial loans | 30 - 59 Days Past Due | Private equity/venture capital
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	7,556	86,566
Commercial loans | 30 - 59 Days Past Due | Life science/healthcare
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	9,620	4,390
Commercial loans | 30 - 59 Days Past Due | Premium wine
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	1,594	418
Commercial loans | 30 - 59 Days Past Due | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	340	445
Commercial loans | 30 - 59 Days Past Due | Commercial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	36,668	107,221
Commercial loans | 60 - 89 Days Past Due | Software/internet
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	4,842	6,526
Commercial loans | 60 - 89 Days Past Due | Hardware
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	99	77
Commercial loans | 60 - 89 Days Past Due | Private equity/venture capital
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	38,580
Commercial loans | 60 - 89 Days Past Due | Life science/healthcare
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	897	191
Commercial loans | 60 - 89 Days Past Due | Premium wine
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	0
Commercial loans | 60 - 89 Days Past Due | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	27	0
Commercial loans | 60 - 89 Days Past Due | Commercial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	5,865	45,374
Commercial loans | Greater Than 90 Days Past Due | Software/internet
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	116	141
Commercial loans | Greater Than 90 Days Past Due | Hardware
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	50
Commercial loans | Greater Than 90 Days Past Due | Private equity/venture capital
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	2	0
Commercial loans | Greater Than 90 Days Past Due | Life science/healthcare
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	45	0
Commercial loans | Greater Than 90 Days Past Due | Premium wine
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	0
Commercial loans | Greater Than 90 Days Past Due | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	0
Commercial loans | Greater Than 90 Days Past Due | Commercial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	163	191
Consumer loans | Real estate secured loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	8,389	2,696
Current	2,540,950	2,292,980
Loans Past Due 90 Days or More Still Accruing Interest	0	0
Consumer loans | Other consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	2,111	796
Current	415,582	327,234
Loans Past Due 90 Days or More Still Accruing Interest	0	0
Consumer loans | Consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	10,500	3,492
Current	2,956,532	2,620,214
Loans Past Due 90 Days or More Still Accruing Interest	0	0
Consumer loans | 30 - 59 Days Past Due | Real estate secured loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	8,389	2,164
Consumer loans | 30 - 59 Days Past Due | Other consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	2,111	796
Consumer loans | 30 - 59 Days Past Due | Consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	10,500	2,960
Consumer loans | 60 - 89 Days Past Due | Real estate secured loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	532
Consumer loans | 60 - 89 Days Past Due | Other consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	0
Consumer loans | 60 - 89 Days Past Due | Consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	532
Consumer loans | Greater Than 90 Days Past Due | Real estate secured loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	0
Consumer loans | Greater Than 90 Days Past Due | Other consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	0
Consumer loans | Greater Than 90 Days Past Due | Consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	$ 0	$ 0