Investments - Contractual Maturities of Investments (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Amortized Cost
Due in one year or less	$ 1,151
Due after one year through five years	872
Due after five years through ten years	36
Amortized Cost	2,059	$ 1,761
Estimated Fair Value
Due in one year or less	1,151
Due after one year through five years	870
Due after five years through ten years	35
Total, Estimated Fair Value	$ 2,056