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		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Financing Receivable, Impaired [Line Items]
Average impaired loans	$ 193,254	$ 190,907	$ 182,398	$ 197,206
Interest income on impaired loans	1,421	1,273	1,981	2,371
Private equity/venture capital
Financing Receivable, Impaired [Line Items]
Average impaired loans	1,233	644		443
Commercial loans | Software/internet
Financing Receivable, Impaired [Line Items]
Average impaired loans	118,840	121,290	112,576	122,527
Interest income on impaired loans	607	767	991	1,646
Commercial loans | Hardware
Financing Receivable, Impaired [Line Items]
Average impaired loans	27,922	35,932	34,469	33,271
Interest income on impaired loans	410	419	499	518
Commercial loans | Private equity/venture capital
Financing Receivable, Impaired [Line Items]
Average impaired loans			536
Interest income on impaired loans	0	3	0	8
Commercial loans | Life science/healthcare
Financing Receivable, Impaired [Line Items]
Average impaired loans	38,545	25,796	27,671	33,590
Interest income on impaired loans	365	21	376	60
Commercial loans | Premium wine
Financing Receivable, Impaired [Line Items]
Average impaired loans	2,384	3,625	2,586	3,353
Interest income on impaired loans	35	39	103	115
Commercial loans | Other
Financing Receivable, Impaired [Line Items]
Average impaired loans	0	348	130	706
Interest income on impaired loans	0	0	0	0
Commercial loans | Commercial loans
Financing Receivable, Impaired [Line Items]
Average impaired loans	188,924	187,635	177,968	193,890
Interest income on impaired loans	1,417	1,249	1,969	2,347
Consumer loans | Real estate secured loans
Financing Receivable, Impaired [Line Items]
Average impaired loans	4,330	1,306	3,953	1,385
Interest income on impaired loans	4	24	12	24
Consumer loans | Other consumer loans
Financing Receivable, Impaired [Line Items]
Average impaired loans	0	1,966	477	1,931
Interest income on impaired loans	0	0	0	0
Consumer loans | Consumer loans
Financing Receivable, Impaired [Line Items]
Average impaired loans	4,330	3,272	4,430	3,316
Interest income on impaired loans	$ 4	$ 24	$ 12	$ 24