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	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Residential mortgage loans measured at fair value	$ 67	$ 65	$ 54
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	1,496	2,816	1,071
Past due loans of 90 days or more	4	4	4
Nonaccrual loans			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	1,443	2,693	1,040
Past due loans of 90 days or more	4	5	4
Nonaccrual loans			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	53	123	31
Past due loans of 90 days or more		$ (1)