Loans, Allowance for Loan Losses and Allowance for Unfunded Credit Commitments - Credit Quality Indicators, Broken out by Portfolio Segment and Class of Financing Receivables (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	$ 27,668,829	$ 23,254,153
Pass
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	26,601,154	22,357,492
Performing (Criticized)
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	881,603	721,800
Performing Impaired (Criticized)
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	70,910	55,602
Nonperforming Impaired (Nonaccrual)
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	115,162	119,259
Commercial loans | Software/internet
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	6,317,315	6,232,725
Commercial loans | Hardware
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	1,318,485	1,200,900
Commercial loans | Private equity/venture capital
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	13,369,035	9,961,121
Commercial loans | Life science/healthcare
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	2,377,470	1,867,960
Commercial loans | Premium wine
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	914,266	874,369
Commercial loans | Other
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	400,914	490,769
Commercial loans | Commercial loans
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	24,697,485	20,627,844
Commercial loans | Pass | Software/internet
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	5,613,023	5,655,739
Commercial loans | Pass | Hardware
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	1,245,532	1,112,574
Commercial loans | Pass | Private equity/venture capital
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	13,362,294	9,955,082
Commercial loans | Pass | Life science/healthcare
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	2,172,679	1,720,613
Commercial loans | Pass | Premium wine
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	862,311	834,537
Commercial loans | Pass | Other
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	398,263	469,721
Commercial loans | Pass | Commercial loans
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	23,654,102	19,748,266
Commercial loans | Performing (Criticized) | Software/internet
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	584,653	466,332
Commercial loans | Performing (Criticized) | Hardware
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	53,273	51,976
Commercial loans | Performing (Criticized) | Private equity/venture capital
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	3,041	5,381
Commercial loans | Performing (Criticized) | Life science/healthcare
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	168,366	125,660
Commercial loans | Performing (Criticized) | Premium wine
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	49,639	36,955
Commercial loans | Performing (Criticized) | Other
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	2,651	21,016
Commercial loans | Performing (Criticized) | Commercial loans
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	861,623	707,320
Commercial loans | Performing Impaired (Criticized) | Software/internet
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	37,204	31,794
Commercial loans | Performing Impaired (Criticized) | Hardware
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	17,249	20,165
Commercial loans | Performing Impaired (Criticized) | Private equity/venture capital
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	0	0
Commercial loans | Performing Impaired (Criticized) | Life science/healthcare
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	14,125	1,167
Commercial loans | Performing Impaired (Criticized) | Premium wine
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	2,010	2,476
Commercial loans | Performing Impaired (Criticized) | Other
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	0	0
Commercial loans | Performing Impaired (Criticized) | Commercial loans
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	70,588	55,602
Commercial loans | Nonperforming Impaired (Nonaccrual) | Software/internet
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	82,435	78,860
Commercial loans | Nonperforming Impaired (Nonaccrual) | Hardware
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	2,431	16,185
Commercial loans | Nonperforming Impaired (Nonaccrual) | Private equity/venture capital
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	3,700	658
Commercial loans | Nonperforming Impaired (Nonaccrual) | Life science/healthcare
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	22,300	20,520
Commercial loans | Nonperforming Impaired (Nonaccrual) | Premium wine
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	306	401
Commercial loans | Nonperforming Impaired (Nonaccrual) | Other
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	0	32
Commercial loans | Nonperforming Impaired (Nonaccrual) | Commercial loans
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	111,172	116,656
Consumer loans | Real estate secured loans
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	2,553,651	2,297,857
Consumer loans | Other consumer loans
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	417,693	328,452
Consumer loans | Consumer loans
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	2,971,344	2,626,309
Consumer loans | Pass | Real estate secured loans
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	2,529,708	2,282,375
Consumer loans | Pass | Other consumer loans
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	417,344	326,851
Consumer loans | Pass | Consumer loans
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	2,947,052	2,609,226
Consumer loans | Performing (Criticized) | Real estate secured loans
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	19,631	13,301
Consumer loans | Performing (Criticized) | Other consumer loans
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	349	1,179
Consumer loans | Performing (Criticized) | Consumer loans
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	19,980	14,480
Consumer loans | Performing Impaired (Criticized) | Real estate secured loans
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	322	0
Consumer loans | Performing Impaired (Criticized) | Other consumer loans
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	0	0
Consumer loans | Performing Impaired (Criticized) | Consumer loans
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	322	0
Consumer loans | Nonperforming Impaired (Nonaccrual) | Real estate secured loans
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	3,990	2,181
Consumer loans | Nonperforming Impaired (Nonaccrual) | Other consumer loans
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	0	422
Consumer loans | Nonperforming Impaired (Nonaccrual) | Consumer loans
Financing Receivable, Recorded Investment [Line Items]
Financing receivable	$ 3,990	$ 2,603