Difference Between the Aggregate Fair Value and the Aggregate Unpaid Principal Balance for Residential Mortgage Loans Measured at Fair Value (Detail) (Residential mortgage loans, USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Residential mortgage loans
Aggregate fair value
Residential mortgage loans measured at fair value	$ 688	$ 982
Past due loans of 90 days or more	2	1
Nonaccrual loans	2	2
Aggregate unpaid principal balance
Residential mortgage loans measured at fair value	665	962
Past due loans of 90 days or more	2	2
Nonaccrual loans	2	2
Difference
Residential mortgage loans measured at fair value	23	20
Past due loans of 90 days or more		$ (1)