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, 2013	Dec. 31, 2012	Sep. 30, 2013 Tdr [Member]	Sep. 30, 2012 Tdr [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Residential mortgage loan carried at lower of amortized cost or fair value less cost to sell	$ 687	$ 608	$ 687	$ 516