Loans, Allowance for Loan Losses and Allowance for Unfunded Credit Commitments - Allowance for Loan Losses Individually and Collectively Evaluated for Impairment (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
Individually evaluated for impairment, allowance for loan losses	$ 49,992	$ 41,793
Individually evaluated for impairment, recorded investment in loans	186,072	174,861
Collectively evaluated for impairment, allowance for loan losses	235,721	213,231
Collectively evaluated for impairment, allowance for loan losses	27,308,843	22,931,455
Commercial loans \| Software/internet
Financing Receivable, Impaired [Line Items]
Individually evaluated for impairment, allowance for loan losses	36,856	23,088
Individually evaluated for impairment, recorded investment in loans	119,639	110,654
Collectively evaluated for impairment, allowance for loan losses	76,634	73,016
Collectively evaluated for impairment, allowance for loan losses	6,139,085	6,061,877
Commercial loans \| Hardware
Financing Receivable, Impaired [Line Items]
Individually evaluated for impairment, allowance for loan losses	764	8,450
Individually evaluated for impairment, recorded investment in loans	19,680	36,350
Collectively evaluated for impairment, allowance for loan losses	19,734	19,164
Collectively evaluated for impairment, allowance for loan losses	1,287,029	1,157,249
Commercial loans \| Private equity/venture capital
Financing Receivable, Impaired [Line Items]
Individually evaluated for impairment, allowance for loan losses	0	330
Individually evaluated for impairment, recorded investment in loans	3,700	658
Collectively evaluated for impairment, allowance for loan losses	91,011	82,138
Collectively evaluated for impairment, allowance for loan losses	13,356,811	9,951,719
Commercial loans \| Life science/healthcare
Financing Receivable, Impaired [Line Items]
Individually evaluated for impairment, allowance for loan losses	11,951	9,315
Individually evaluated for impairment, recorded investment in loans	36,425	21,687
Collectively evaluated for impairment, allowance for loan losses	23,501	15,609
Collectively evaluated for impairment, allowance for loan losses	2,266,028	1,787,140
Commercial loans \| Premium wine
Financing Receivable, Impaired [Line Items]
Individually evaluated for impairment, allowance for loan losses	3	0
Individually evaluated for impairment, recorded investment in loans	2,316	2,877
Collectively evaluated for impairment, allowance for loan losses	3,557	3,532
Collectively evaluated for impairment, allowance for loan losses	911,123	870,281
Commercial loans \| Other
Financing Receivable, Impaired [Line Items]
Individually evaluated for impairment, allowance for loan losses	0	32
Individually evaluated for impairment, recorded investment in loans	0	32
Collectively evaluated for impairment, allowance for loan losses	2,655	3,909
Collectively evaluated for impairment, allowance for loan losses	378,455	476,306
Commercial loans \| Commercial loans
Financing Receivable, Impaired [Line Items]
Individually evaluated for impairment, allowance for loan losses	49,574	41,215
Individually evaluated for impairment, recorded investment in loans	181,760	172,258
Collectively evaluated for impairment, allowance for loan losses	217,092	197,368
Collectively evaluated for impairment, allowance for loan losses	24,338,531	20,304,572
Consumer loans \| Consumer loans
Financing Receivable, Impaired [Line Items]
Individually evaluated for impairment, allowance for loan losses	418	578
Individually evaluated for impairment, recorded investment in loans	4,312	2,603
Collectively evaluated for impairment, allowance for loan losses	18,629	15,863
Collectively evaluated for impairment, allowance for loan losses	$ 2,970,312	$ 2,626,883

The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

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Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

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The balance of financing receivables that were collectively evaluated for impairment.

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-Publisher FASB
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-Topic 310
-SubTopic 10
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-Paragraph 11B
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