Investments - Schedule of Contractual Maturities of Investments (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Amortized cost
Due in one year or less	$ 674
Due after one year through five years	2,668
Due after five years through ten years	3,853
Due after ten years	2,940
Total	10,135	$ 9,799
Fair value
Due in one year or less	686
Due after one year through five years	2,803
Due after five years through ten years	4,009
Due after ten years	3,042
Total	$ 10,540	$ 10,085
% of fair value
Due in one year or less	6.50%
Due after one year through five years	26.60%
Due after five years through ten years	38.00%
Due after ten years	28.90%
Total	100.00%