Difference Between the Aggregate Fair Value and the Aggregate Unpaid Principal Balance for Residential Mortgage Loans Measured at Fair Value (Detail) (USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010	Sep. 30, 2010
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	$ 1,655	$ 1,938	$ 1,921
Past due loans of 90 days or more	4	5	5
Nonaccrual loans		1	1
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	1,578	1,913	1,832
Past due loans of 90 days or more	4	6	6
Nonaccrual loans		1	1
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	77	25	89
Past due loans of 90 days or more		$ (1)	$ (1)