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	Sep. 30, 2015	Dec. 31, 2014
Aggregate fair value
Residential mortgage loans measured at fair value	$ 790	$ 669
Past due loans of 90 days or more	2	2
Nonaccrual loans	2	3
Aggregate unpaid principal balance
Residential mortgage loans measured at fair value	758	643
Past due loans of 90 days or more	2	2
Nonaccrual loans	2	3
Difference
Residential mortgage loans measured at fair value	$ 32	$ 26