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	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	$ 183,639	$ 174,861
Total unpaid principal of impaired loans	211,911	205,295
Impaired loans for which there is a related allowance for loan losses
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	96,061	88,246
Impaired loans for which there is no related allowance for loan losses
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	87,578	86,615
Commercial Portfolio Segment | Software/internet
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	116,142	110,654
Total unpaid principal of impaired loans	137,467	129,006
Commercial Portfolio Segment | Hardware
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	36,821	36,350
Total unpaid principal of impaired loans	37,450	41,721
Commercial Portfolio Segment | Private equity/venture capital
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	108	658
Total unpaid principal of impaired loans	108	984
Commercial Portfolio Segment | Life science/healthcare
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	22,289	21,687
Total unpaid principal of impaired loans	26,913	26,360
Commercial Portfolio Segment | Premium wine
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	2,722	2,877
Total unpaid principal of impaired loans	2,762	2,911
Commercial Portfolio Segment | Other
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	14	32
Total unpaid principal of impaired loans	94	165
Commercial Portfolio Segment | Commercial loans
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	178,096	172,258
Total unpaid principal of impaired loans	204,794	201,147
Commercial Portfolio Segment | Impaired loans for which there is a related allowance for loan losses | Software/internet
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	56,600	49,645
Commercial Portfolio Segment | Impaired loans for which there is a related allowance for loan losses | Hardware
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	14,833	15,637
Commercial Portfolio Segment | Impaired loans for which there is a related allowance for loan losses | Private equity/venture capital
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	108	658
Commercial Portfolio Segment | Impaired loans for which there is a related allowance for loan losses | Life science/healthcare
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	19,672	20,521
Commercial Portfolio Segment | Impaired loans for which there is a related allowance for loan losses | Premium wine
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	372	0
Commercial Portfolio Segment | Impaired loans for which there is a related allowance for loan losses | Other
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	14	32
Commercial Portfolio Segment | Impaired loans for which there is a related allowance for loan losses | Commercial loans
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	91,599	86,493
Commercial Portfolio Segment | Impaired loans for which there is no related allowance for loan losses | Software/internet
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	59,542	61,009
Commercial Portfolio Segment | Impaired loans for which there is no related allowance for loan losses | Hardware
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	21,988	20,713
Commercial Portfolio Segment | Impaired loans for which there is no related allowance for loan losses | Private equity/venture capital
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	0	0
Commercial Portfolio Segment | Impaired loans for which there is no related allowance for loan losses | Life science/healthcare
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	2,617	1,166
Commercial Portfolio Segment | Impaired loans for which there is no related allowance for loan losses | Premium wine
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	2,350	2,877
Commercial Portfolio Segment | Impaired loans for which there is no related allowance for loan losses | Other
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	0	0
Commercial Portfolio Segment | Impaired loans for which there is no related allowance for loan losses | Commercial loans
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	86,497	85,765
Consumer Portfolio Segment | Real estate secured loans
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	4,490	2,181
Total unpaid principal of impaired loans	6,043	3,712
Consumer Portfolio Segment | Other consumer loans
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	1,053	422
Total unpaid principal of impaired loans	1,074	436
Consumer Portfolio Segment | Consumer loans
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	5,543	2,603
Total unpaid principal of impaired loans	7,117	4,148
Consumer Portfolio Segment | Impaired loans for which there is a related allowance for loan losses | Real estate secured loans
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	4,164	1,331
Consumer Portfolio Segment | Impaired loans for which there is a related allowance for loan losses | Other consumer loans
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	298	422
Consumer Portfolio Segment | Impaired loans for which there is a related allowance for loan losses | Consumer loans
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	4,462	1,753
Consumer Portfolio Segment | Impaired loans for which there is no related allowance for loan losses | Real estate secured loans
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	326	850
Consumer Portfolio Segment | Impaired loans for which there is no related allowance for loan losses | Other consumer loans
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	755	0
Consumer Portfolio Segment | Impaired loans for which there is no related allowance for loan losses | Consumer loans
Financing Receivable, Impaired [Line Items]
Carrying value of impaired loans	$ 1,081	$ 850