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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Financing Receivable, Impaired [Line Items]
Average impaired loans	$ 190,907	$ 169,359	$ 197,206	$ 165,062
Interest income on impaired loans	1,273	990	2,371	1,820
Private equity/venture capital
Financing Receivable, Impaired [Line Items]
Average impaired loans	644	0	443	0
Commercial Portfolio Segment | Software/internet
Financing Receivable, Impaired [Line Items]
Average impaired loans	121,290	61,481	122,527	84,005
Interest income on impaired loans	767	70	1,646	133
Commercial Portfolio Segment | Hardware
Financing Receivable, Impaired [Line Items]
Average impaired loans	35,932	45,353	33,271	31,000
Interest income on impaired loans	419	761	518	1,467
Commercial Portfolio Segment | Private equity/venture capital
Financing Receivable, Impaired [Line Items]
Interest income on impaired loans	3	0	8	0
Commercial Portfolio Segment | Life science/healthcare
Financing Receivable, Impaired [Line Items]
Average impaired loans	25,796	55,558	33,590	42,857
Interest income on impaired loans	21	128	60	128
Commercial Portfolio Segment | Premium wine
Financing Receivable, Impaired [Line Items]
Average impaired loans	3,625	1,291	3,353	1,834
Interest income on impaired loans	39	19	115	54
Commercial Portfolio Segment | Other
Financing Receivable, Impaired [Line Items]
Average impaired loans	348	3,768	706	4,369
Interest income on impaired loans	0	6	0	21
Commercial Portfolio Segment | Commercial loans
Financing Receivable, Impaired [Line Items]
Average impaired loans	187,635	167,451	193,890	164,065
Interest income on impaired loans	1,249	984	2,347	1,803
Consumer Portfolio Segment | Real estate secured loans
Financing Receivable, Impaired [Line Items]
Average impaired loans	1,306	584	1,385	282
Interest income on impaired loans	24	0	24	0
Consumer Portfolio Segment | Other consumer loans
Financing Receivable, Impaired [Line Items]
Average impaired loans	1,966	1,324	1,931	715
Interest income on impaired loans	0	6	0	17
Consumer Portfolio Segment | Consumer loans
Financing Receivable, Impaired [Line Items]
Average impaired loans	3,272	1,908	3,316	997
Interest income on impaired loans	$ 24	$ 6	$ 24	$ 17