Investments - Schedule of Contractual Maturities of Investments (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Amortized cost
Due in one year or less	$ 470
Due after one year through five years	3,058
Due after five years through ten years	3,868
Due after ten years	3,616
Total	11,012	$ 10,643
Fair value
Due in one year or less	475
Due after one year through five years	3,165
Due after five years through ten years	4,081
Due after ten years	3,879
Total	$ 11,600	$ 10,689
% of fair value
Due in one year or less	4.10%
Due after one year through five years	27.30%
Due after five years through ten years	35.20%
Due after ten years	33.40%
Total	100.00%