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, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Residential mortgage loans measured at fair value	$ 81	$ 76
Aggregate Fair Value Under Fair Value Option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Residential mortgage loans measured at fair value	2,641	2,932
Past due loans of 90 days or more	4	3
Aggregate Unpaid Principal Balance Under Fair Value Option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Residential mortgage loans measured at fair value	2,527	2,775
Past due loans of 90 days or more	4	4
Nonaccrual loans	1	1
Fair Value Over/(Under) Unpaid Principal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Residential mortgage loans measured at fair value	114	157
Past due loans of 90 days or more		(1)
Nonaccrual loans	$ (1)	$ (1)