Loans and Allowances for Loan Losses - Allowance for Loan Losses by Portfolio Segment and Related Recorded Investment in Loans (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012 Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision expense for impairment covered loans	$ 6.6
Increase in allowance for losses related to impairment loans		$ 1.7