Allowance for Credit Losses - Summary of Changes in Allowance for Credit Losses and Related Loan Balance by Product (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 Tdr [Member]	Dec. 31, 2011 Tdr [Member]
Loans balance above excludes TDR loans	$ 1,279	$ 1,052	$ 516	$ 303