Loans, Allowance for Loan Losses and Allowance for Unfunded Credit Commitments - Recorded Investment in Loans Modified in TDRs within Previous 12 months Subsequently Defaulted (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Financing Receivable, Modifications [Line Items]
TDRs modified within the previous 12 months that defaulted in the period	$ 26,920	$ 1,420	$ 54,256	$ 1,420
Commercial loans | Software/internet
Financing Receivable, Modifications [Line Items]
TDRs modified within the previous 12 months that defaulted in the period	18,911	1,234	41,568	1,234
Commercial loans | Hardware
Financing Receivable, Modifications [Line Items]
TDRs modified within the previous 12 months that defaulted in the period	2,100	0	5,549	0
Commercial loans | Life science/healthcare
Financing Receivable, Modifications [Line Items]
TDRs modified within the previous 12 months that defaulted in the period	5,909	0	7,139	0
Commercial loans | Premium wine
Financing Receivable, Modifications [Line Items]
TDRs modified within the previous 12 months that defaulted in the period	$ 0	$ 186	$ 0	$ 186