Derivative Financial Instruments - Schedule of Amounts Related to Interest Rate Derivatives Included in Income Designated as Hedges of Cash Flows (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended	3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012 Designated as Hedging Instrument [Member] Junior Subordinated Deferred Interest Debentures [Member]	Mar. 31, 2013 Designated as Hedging Instrument [Member] Interest Income on Loans [Member] Interest Rate Swaps on Variable-Rate Loans [Member]	Mar. 31, 2012 Designated as Hedging Instrument [Member] Interest Income on Loans [Member] Interest Rate Swaps on Variable-Rate Loans [Member]	Mar. 31, 2013 Designated as Hedging Instrument [Member] Interest Expense [Member] Junior Subordinated Deferred Interest Debentures [Member]	Mar. 31, 2012 Designated as Hedging Instrument [Member] Interest Expense [Member] Junior Subordinated Deferred Interest Debentures [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount reclassified from accumulated other comprehensive income				$ 9,345	$ 9,345	$ 1,085	$ 1,033
Amount of gain (loss) recognized in other comprehensive income	$ 38,100	$ 44,200	$ (427)