Loans and Allowance for Loan Losses - Impaired Loans and Allowance for Loan Losses, Broken out by Portfolio Segment and Class of Financing Receivable (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	$ 174,861	$ 234,460
Impaired Financing Receivable, Unpaid Principal Balance	205,295	250,692
Impaired Financing Receivables With Related Allowance
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	88,246	233,370
Impaired Financing Receivables With No Related Allowance
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	86,615	1,090
Commercial loans
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	172,258	230,558
Impaired Financing Receivable, Unpaid Principal Balance	201,147	245,515
Commercial loans | Software and internet
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	110,654	122,748
Impaired Financing Receivable, Unpaid Principal Balance	129,006	129,648
Commercial loans | Hardware
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	36,350	65,395
Impaired Financing Receivable, Unpaid Principal Balance	41,721	70,683
Commercial loans | Private equity/venture capital
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	658	0
Impaired Financing Receivable, Unpaid Principal Balance	984	0
Commercial loans | Life science/healthcare
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	21,687	38,361
Impaired Financing Receivable, Unpaid Principal Balance	26,360	41,130
Commercial loans | Premium wine
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	2,877	3,187
Impaired Financing Receivable, Unpaid Principal Balance	2,911	3,187
Commercial loans | Other
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	32	867
Impaired Financing Receivable, Unpaid Principal Balance	165	867
Commercial loans | Impaired Financing Receivables With Related Allowance
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	86,493	229,468
Commercial loans | Impaired Financing Receivables With Related Allowance | Software and internet
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	49,645	121,658
Commercial loans | Impaired Financing Receivables With Related Allowance | Hardware
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	15,637	65,395
Commercial loans | Impaired Financing Receivables With Related Allowance | Private equity/venture capital
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	658	0
Commercial loans | Impaired Financing Receivables With Related Allowance | Life science/healthcare
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	20,521	38,361
Commercial loans | Impaired Financing Receivables With Related Allowance | Premium wine
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	0	3,187
Commercial loans | Impaired Financing Receivables With Related Allowance | Other
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	32	867
Commercial loans | Impaired Financing Receivables With No Related Allowance
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	85,765	1,090
Commercial loans | Impaired Financing Receivables With No Related Allowance | Software and internet
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	61,009	1,090
Commercial loans | Impaired Financing Receivables With No Related Allowance | Hardware
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	20,713	0
Commercial loans | Impaired Financing Receivables With No Related Allowance | Private equity/venture capital
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	0	0
Commercial loans | Impaired Financing Receivables With No Related Allowance | Life science/healthcare
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	1,166	0
Commercial loans | Impaired Financing Receivables With No Related Allowance | Premium wine
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	2,877	0
Commercial loans | Impaired Financing Receivables With No Related Allowance | Other
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	0	0
Consumer loans
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	2,603	3,902
Impaired Financing Receivable, Unpaid Principal Balance	4,148	5,177
Consumer loans | Real estate secured consumer loans
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	2,181	1,504
Impaired Financing Receivable, Unpaid Principal Balance	3,712	2,779
Consumer loans | Other Loans
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	422	2,398
Impaired Financing Receivable, Unpaid Principal Balance	436	2,398
Consumer loans | Impaired Financing Receivables With Related Allowance
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	1,753	3,902
Consumer loans | Impaired Financing Receivables With Related Allowance | Real estate secured consumer loans
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	1,331	1,504
Consumer loans | Impaired Financing Receivables With Related Allowance | Other Loans
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	422	2,398
Consumer loans | Impaired Financing Receivables With No Related Allowance
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	850	0
Consumer loans | Impaired Financing Receivables With No Related Allowance | Real estate secured consumer loans
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	850	0
Consumer loans | Impaired Financing Receivables With No Related Allowance | Other Loans
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Recorded Investment	$ 0	$ 0