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	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
Individually evaluated for impairment, allowance for loan losses	$ 44,261	$ 41,793
Individually evaluated for impairment, recorded investment in loans	183,639	174,861
Collectively evaluated for impairment, allowance for loan losses	230,033	213,231
Collectively evaluated for impairment, allowance for loan losses	24,404,305	22,931,455
Commercial Portfolio Segment \| Software/internet
Financing Receivable, Impaired [Line Items]
Individually evaluated for impairment, allowance for loan losses	27,379	23,088
Individually evaluated for impairment, recorded investment in loans	116,142	110,654
Collectively evaluated for impairment, allowance for loan losses	75,916	73,016
Collectively evaluated for impairment, allowance for loan losses	6,092,942	6,061,877
Commercial Portfolio Segment \| Hardware
Financing Receivable, Impaired [Line Items]
Individually evaluated for impairment, allowance for loan losses	7,087	8,450
Individually evaluated for impairment, recorded investment in loans	36,821	36,350
Collectively evaluated for impairment, allowance for loan losses	21,385	19,164
Collectively evaluated for impairment, allowance for loan losses	1,299,179	1,157,249
Commercial Portfolio Segment \| Private equity/venture capital
Financing Receivable, Impaired [Line Items]
Individually evaluated for impairment, allowance for loan losses	108	330
Individually evaluated for impairment, recorded investment in loans	108	658
Collectively evaluated for impairment, allowance for loan losses	91,510	82,138
Collectively evaluated for impairment, allowance for loan losses	11,211,708	9,951,719
Commercial Portfolio Segment \| Life science/healthcare
Financing Receivable, Impaired [Line Items]
Individually evaluated for impairment, allowance for loan losses	8,149	9,315
Individually evaluated for impairment, recorded investment in loans	22,289	21,687
Collectively evaluated for impairment, allowance for loan losses	17,657	15,609
Collectively evaluated for impairment, allowance for loan losses	1,910,319	1,787,140
Commercial Portfolio Segment \| Premium wine
Financing Receivable, Impaired [Line Items]
Individually evaluated for impairment, allowance for loan losses	0	0
Individually evaluated for impairment, recorded investment in loans	2,722	2,877
Collectively evaluated for impairment, allowance for loan losses	3,365	3,532
Collectively evaluated for impairment, allowance for loan losses	862,302	870,281
Commercial Portfolio Segment \| Other
Financing Receivable, Impaired [Line Items]
Individually evaluated for impairment, allowance for loan losses	14	32
Individually evaluated for impairment, recorded investment in loans	14	32
Collectively evaluated for impairment, allowance for loan losses	3,468	3,909
Collectively evaluated for impairment, allowance for loan losses	320,111	476,306
Commercial Portfolio Segment \| Commercial loans
Financing Receivable, Impaired [Line Items]
Individually evaluated for impairment, allowance for loan losses	42,737	41,215
Individually evaluated for impairment, recorded investment in loans	178,096	172,258
Collectively evaluated for impairment, allowance for loan losses	213,301	197,368
Collectively evaluated for impairment, allowance for loan losses	21,696,561	20,304,572
Consumer Portfolio Segment \| Consumer loans
Financing Receivable, Impaired [Line Items]
Individually evaluated for impairment, allowance for loan losses	1,524	578
Individually evaluated for impairment, recorded investment in loans	5,543	2,603
Collectively evaluated for impairment, allowance for loan losses	16,732	15,863
Collectively evaluated for impairment, allowance for loan losses	$ 2,707,744	$ 2,626,883

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

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