Investments - Contractual Maturities of Investments (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Amortized Cost
Due in one year or less	$ 887
Due after one year through five years	817
Due after five years through ten years	26
Amortized Cost	1,730	$ 1,808
Estimated Fair Value
Due in one year or less	887
Due after one year through five years	821
Due after five years through ten years	27
Estimated Fair Value	$ 1,735