Loans, Allowance for Loan Losses and Allowance for Unfunded Credit Commitments - Average Impaired Loans, Broken out by Portfolio Segment and Class of Financing Receivable (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Financing Receivable, Impaired [Line Items]
Average impaired loans	$ 176,777	$ 191,236
Interest income on impaired loans	973	1,196
Private equity/venture capital
Financing Receivable, Impaired [Line Items]
Average impaired loans	302	358
Commercial Portfolio Segment | Software/internet
Financing Receivable, Impaired [Line Items]
Average impaired loans	108,788	109,916
Interest income on impaired loans	532	422
Commercial Portfolio Segment | Hardware
Financing Receivable, Impaired [Line Items]
Average impaired loans	38,426	34,110
Interest income on impaired loans	363	572
Commercial Portfolio Segment | Private equity/venture capital
Financing Receivable, Impaired [Line Items]
Interest income on impaired loans	0	2
Commercial Portfolio Segment | Life science/healthcare
Financing Receivable, Impaired [Line Items]
Average impaired loans	22,679	38,942
Interest income on impaired loans	37	150
Commercial Portfolio Segment | Premium wine
Financing Receivable, Impaired [Line Items]
Average impaired loans	2,769	3,213
Interest income on impaired loans	36	38
Commercial Portfolio Segment | Other
Financing Receivable, Impaired [Line Items]
Average impaired loans	11	1,061
Interest income on impaired loans	0	4
Commercial Portfolio Segment | Commercial loans
Financing Receivable, Impaired [Line Items]
Average impaired loans	172,975	187,600
Interest income on impaired loans	968	1,188
Consumer Portfolio Segment | Real estate secured loans
Financing Receivable, Impaired [Line Items]
Average impaired loans	3,063	1,488
Interest income on impaired loans	0	0
Consumer Portfolio Segment | Other consumer loans
Financing Receivable, Impaired [Line Items]
Average impaired loans	739	2,148
Interest income on impaired loans	5	8
Consumer Portfolio Segment | Consumer loans
Financing Receivable, Impaired [Line Items]
Average impaired loans	3,802	3,636
Interest income on impaired loans	$ 5	$ 8