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	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Residential mortgage loans measured at fair value	$ 76	$ 65
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	2,932	2,816
Past due loans of 90 days or more	3	4
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	2,775	2,693
Past due loans of 90 days or more	4	5
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	157	123
Past due loans of 90 days or more	(1)	(1)
Nonaccrual loans	$ (1)