Allowance for Credit Losses - Summary of Activity in Allowance for Loan Losses (Parenthetical) (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Financing Receivable, Allowance for Credit Losses [Line Items]
Covered loans charge-offs	$ 4,000,000	$ 14,400,000
Provision for covered loan losses		$ 21,510,000	$ 6,378,000