Loans, Allowance for Loan Losses and Allowance for Unfunded Credit Commitments - Impaired Loans and Allowance for Loan Losses, Broken out by Portfolio Segment and Class of Financing Receivable (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
Impaired loans for which there is a related allowance for loan losses	$ 93,224	$ 88,246
Impaired loans for which there is no related allowance for loan losses	92,848	86,615
Total carrying value of impaired loans	186,072	174,861
Total unpaid principal of impaired loans	233,062	205,295
Commercial loans | Software/internet
Financing Receivable, Impaired [Line Items]
Impaired loans for which there is a related allowance for loan losses	64,197	49,645
Impaired loans for which there is no related allowance for loan losses	55,442	61,009
Total carrying value of impaired loans	119,639	110,654
Total unpaid principal of impaired loans	145,122	129,006
Commercial loans | Hardware
Financing Receivable, Impaired [Line Items]
Impaired loans for which there is a related allowance for loan losses	2,431	15,637
Impaired loans for which there is no related allowance for loan losses	17,249	20,713
Total carrying value of impaired loans	19,680	36,350
Total unpaid principal of impaired loans	31,804	41,721
Commercial loans | Private equity/venture capital
Financing Receivable, Impaired [Line Items]
Impaired loans for which there is a related allowance for loan losses	0	658
Impaired loans for which there is no related allowance for loan losses	3,700	0
Total carrying value of impaired loans	3,700	658
Total unpaid principal of impaired loans	3,700	984
Commercial loans | Life science/healthcare
Financing Receivable, Impaired [Line Items]
Impaired loans for which there is a related allowance for loan losses	22,300	20,521
Impaired loans for which there is no related allowance for loan losses	14,125	1,166
Total carrying value of impaired loans	36,425	21,687
Total unpaid principal of impaired loans	44,065	26,360
Commercial loans | Premium wine
Financing Receivable, Impaired [Line Items]
Impaired loans for which there is a related allowance for loan losses	306	0
Impaired loans for which there is no related allowance for loan losses	2,010	2,877
Total carrying value of impaired loans	2,316	2,877
Total unpaid principal of impaired loans	2,375	2,911
Commercial loans | Other
Financing Receivable, Impaired [Line Items]
Impaired loans for which there is a related allowance for loan losses	0	32
Impaired loans for which there is no related allowance for loan losses	0	0
Total carrying value of impaired loans	0	32
Total unpaid principal of impaired loans	0	165
Commercial loans | Commercial loans
Financing Receivable, Impaired [Line Items]
Impaired loans for which there is a related allowance for loan losses	89,234	86,493
Impaired loans for which there is no related allowance for loan losses	92,526	85,765
Total carrying value of impaired loans	181,760	172,258
Total unpaid principal of impaired loans	227,066	201,147
Consumer loans | Real estate secured loans
Financing Receivable, Impaired [Line Items]
Impaired loans for which there is a related allowance for loan losses	3,990	1,331
Impaired loans for which there is no related allowance for loan losses	322	850
Total carrying value of impaired loans	4,312	2,181
Total unpaid principal of impaired loans	5,996	3,712
Consumer loans | Other consumer loans
Financing Receivable, Impaired [Line Items]
Impaired loans for which there is a related allowance for loan losses	0	422
Impaired loans for which there is no related allowance for loan losses	0	0
Total carrying value of impaired loans	0	422
Total unpaid principal of impaired loans	0	436
Consumer loans | Consumer loans
Financing Receivable, Impaired [Line Items]
Impaired loans for which there is a related allowance for loan losses	3,990	1,753
Impaired loans for which there is no related allowance for loan losses	322	850
Total carrying value of impaired loans	4,312	2,603
Total unpaid principal of impaired loans	$ 5,996	$ 4,148