Fair Value of Financial Instruments - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended								3 Months Ended				9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 6.05% Subordinated Notes	Dec. 31, 2011 6.05% Subordinated Notes	Sep. 30, 2012 5.375% Senior Notes	Dec. 31, 2011 5.375% Senior Notes	Sep. 30, 2012 7.0% Junior Subordinated Debentures	Dec. 31, 2011 7.0% Junior Subordinated Debentures	Sep. 30, 2012 Fair value accounting Non-marketable securities		Sep. 30, 2011 Fair value accounting Non-marketable securities		Sep. 30, 2012 Fair value accounting Non-marketable securities		Sep. 30, 2011 Fair value accounting Non-marketable securities
Debt Disclosure [Line Items]
Stated interest rate					6.05%	6.05%	5.375%	5.375%	7.00%	7.00%
Fair Value Measurements Impact Of Changes To Significant Unobservable Unputs Level 3 Measuements	$ 3,400,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	(1,146,000)	0	(2,256,000)	(628,000)							(978,000)	[1]	0	[1]	(1,573,000)	[1]	0	[1]
Fair value transfer from Level 2 to level 1	$ 2,100,000	$ 0	$ 2,100,000	$ 0

[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.