Fair Value of Financial Instruments - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Debt Disclosure [Line Items]
Fair Value Measurements Impact Of Changes To Significant Unobservable Unputs Level 3 Measuements	$ 3,400,000
Fair Value Measurements, Recurring Basis, Transfers From Level 2 To Level 1	3,100,000		3,900,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	(9,011,000)		(717,000)		(11,478,000)
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	5.70%		5.70%
5.375% Senior Notes
Debt Disclosure [Line Items]
Stated interest rate	5.375%		5.375%
7.0% Junior Subordinated Debentures
Debt Disclosure [Line Items]
Stated interest rate	7.00%		7.00%
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	$ (7,590,000)	[1]	$ 0	[1]	$ (10,791,000)	[1]

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