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	Mar. 31, 2016	Dec. 31, 2015
Aggregate fair value
Residential mortgage loans measured at fair value	$ 760	$ 686
Past due loans of 90 days or more	1	2
Nonaccrual loans	1	2
Aggregate unpaid principal balance
Residential mortgage loans measured at fair value	730	669
Past due loans of 90 days or more	1	2
Nonaccrual loans	1	2
Difference
Residential mortgage loans measured at fair value	$ 30	$ 17