Difference Between the Aggregate Fair Value and the Aggregate Unpaid Principal Balance for Residential Mortgage Loans Measured at Fair Value (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011
Aggregate Fair Value Under Fair Value Option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Residential mortgage loans measured at fair value	$ 1,864	$ 2,816	$ 1,037
Past due loans of 90 days or more	3	4	3
Aggregate Unpaid Principal Balance Under Fair Value Option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Residential mortgage loans measured at fair value	1,767	2,693	1,002
Past due loans of 90 days or more	4	5	4
Fair Value Over Under Unpaid Principal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Residential mortgage loans measured at fair value	97	123	35
Past due loans of 90 days or more	$ (1)	$ (1)	$ (1)