Investments - Schedule of Contractual Maturities of Investments (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Amortized cost
Due in one year or less	$ 655
Due after one year through five years	2,764
Due after five years through ten years	3,862
Due after ten years	2,700
Total	9,981	$ 9,799
Fair value
Due in one year or less	669
Due after one year through five years	2,911
Due after five years through ten years	3,981
Due after ten years	2,740
Total	$ 10,301	$ 10,085
% of fair value
Due in one year or less	6.50%
Due after one year through five years	28.30%
Due after five years through ten years	38.60%
Due after ten years	26.60%
Total	100.00%