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	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
Impaired loans for which there is a related allowance for loan losses	$ 104,992	$ 88,246
Impaired loans for which there is no related allowance for loan losses	73,187	86,615
Total carrying value of impaired loans	178,179	174,861
Total unpaid principal of impaired loans	210,296	205,295
Commercial loans | Software/internet
Financing Receivable, Impaired [Line Items]
Impaired loans for which there is a related allowance for loan losses	58,136	49,645
Impaired loans for which there is no related allowance for loan losses	44,384	61,009
Total carrying value of impaired loans	102,520	110,654
Total unpaid principal of impaired loans	126,752	129,006
Commercial loans | Hardware
Financing Receivable, Impaired [Line Items]
Impaired loans for which there is a related allowance for loan losses	17,093	15,637
Impaired loans for which there is no related allowance for loan losses	25,201	20,713
Total carrying value of impaired loans	42,294	36,350
Total unpaid principal of impaired loans	43,403	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	0	0
Total carrying value of impaired loans	0	658
Total unpaid principal of impaired loans	0	984
Commercial loans | Life science/healthcare
Financing Receivable, Impaired [Line Items]
Impaired loans for which there is a related allowance for loan losses	26,170	20,521
Impaired loans for which there is no related allowance for loan losses	250	1,166
Total carrying value of impaired loans	26,420	21,687
Total unpaid principal of impaired loans	31,517	26,360
Commercial loans | Premium wine
Financing Receivable, Impaired [Line Items]
Impaired loans for which there is a related allowance for loan losses	331	0
Impaired loans for which there is no related allowance for loan losses	2,193	2,877
Total carrying value of impaired loans	2,524	2,877
Total unpaid principal of impaired loans	2,576	2,911
Commercial loans | Other
Financing Receivable, Impaired [Line Items]
Impaired loans for which there is a related allowance for loan losses	8	32
Impaired loans for which there is no related allowance for loan losses	0	0
Total carrying value of impaired loans	8	32
Total unpaid principal of impaired loans	36	165
Commercial loans | Commercial loans
Financing Receivable, Impaired [Line Items]
Impaired loans for which there is a related allowance for loan losses	101,738	86,493
Impaired loans for which there is no related allowance for loan losses	72,028	85,765
Total carrying value of impaired loans	173,766	172,258
Total unpaid principal of impaired loans	204,284	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	3,254	1,331
Impaired loans for which there is no related allowance for loan losses	1,159	850
Total carrying value of impaired loans	4,413	2,181
Total unpaid principal of impaired loans	6,012	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	3,254	1,753
Impaired loans for which there is no related allowance for loan losses	1,159	850
Total carrying value of impaired loans	4,413	2,603
Total unpaid principal of impaired loans	$ 6,012	$ 4,148