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	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Financing Receivable, Allowance for Credit Losses [Line Items]
Residential mortgage loan carried at lower of amortized cost or fair value less cost to sell		$ 445
Loans balance above excludes TDR loans	1,552		758
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	632
Loans balance above excludes TDR loans			$ 445