Fair Value of Financial Instruments - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Debt Disclosure [Line Items]
Fair Value Measurements, Recurring Basis, Transfers From Level 2 To Level 1	$ 0		$ 0		$ 3,100
6.05% Subordinated Notes
Debt Disclosure [Line Items]
Stated interest rate	6.05%		6.05%
5.70% Senior Notes
Debt Disclosure [Line Items]
Stated interest rate
5.375% Senior Notes
Debt Disclosure [Line Items]
Stated interest rate			5.375%
7.0% Junior Subordinated Debentures
Debt Disclosure [Line Items]
Stated interest rate			7.00%
Other venture capital investments | Fair value accounting | Non-marketable securities
Debt Disclosure [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3			(3,600)
Fair Value, Measurements, Recurring
Debt Disclosure [Line Items]
Transfers Into Level 3	0		24,217		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	(381,982)		(10,903)		(9,011)
Fair Value, Measurements, Recurring | Fair value accounting | Non-marketable securities
Debt Disclosure [Line Items]
Transfers Into Level 3	0	[1]	0	[1]	0	[1]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	(380,262)	[1]	(3,561)	[1]	(7,590)	[1]
Fair Value, Measurements, Recurring | Other venture capital investments | Fair value accounting | Non-marketable securities
Debt Disclosure [Line Items]
Transfers Into Level 3	0		0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	$ (126)		$ (3,561)		$ (7,590)

[1] Realized and unrealized gains, net are recorded on the line items “gains on investment securities, net”, and “other noninterest income”, components of noninterest income.