Asset Quality - Allowance for Loan and Lease Losses and Corresponding Loan Balances (Parenthetical) (Detail) (USD $) In Billions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Disclosure Asset Quality Allowance For Loan And Lease Losses And Corresponding Loan Balances [Abstract]
Loans carried at fair value	$ 2.5	$ 2.5	$ 2.8