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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Financing Receivable, Impaired [Line Items]
Average impaired loans	$ 177,163	$ 209,473	$ 176,969	$ 200,355
Interest income on impaired loans	596	1,453	942	2,253
Private equity/venture capital
Financing Receivable, Impaired [Line Items]
Average impaired loans	72	327		342
Commercial loans | Software/internet
Financing Receivable, Impaired [Line Items]
Average impaired loans	110,101	136,374	109,444	123,145
Interest income on impaired loans	315	711	562	938
Commercial loans | Hardware
Financing Receivable, Impaired [Line Items]
Average impaired loans	37,058	29,771	37,742	31,940
Interest income on impaired loans	237	510	289	943
Commercial loans | Private equity/venture capital
Financing Receivable, Impaired [Line Items]
Average impaired loans			187
Interest income on impaired loans	0	3	0	5
Commercial loans | Life science/healthcare
Financing Receivable, Impaired [Line Items]
Average impaired loans	21,790	36,033	22,234	37,488
Interest income on impaired loans	5	191	11	291
Commercial loans | Premium wine
Financing Receivable, Impaired [Line Items]
Average impaired loans	2,604	3,221	2,686	3,217
Interest income on impaired loans	36	38	72	76
Commercial loans | Other
Financing Receivable, Impaired [Line Items]
Average impaired loans	379	708	195	885
Interest income on impaired loans	0	0	0	0
Commercial loans | Commercial loans
Financing Receivable, Impaired [Line Items]
Average impaired loans	172,004	206,434	172,488	197,017
Interest income on impaired loans	593	1,453	934	2,253
Consumer loans | Real estate secured loans
Financing Receivable, Impaired [Line Items]
Average impaired loans	4,466	1,360	3,765	1,424
Interest income on impaired loans	3	0	8	0
Consumer loans | Other consumer loans
Financing Receivable, Impaired [Line Items]
Average impaired loans	693	1,679	716	1,914
Interest income on impaired loans	0	0	0	0
Consumer loans | Consumer loans
Financing Receivable, Impaired [Line Items]
Average impaired loans	5,159	3,039	4,481	3,338
Interest income on impaired loans	$ 3	$ 0	$ 8	$ 0