Investments - Schedule of Contractual Maturities of Investments (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Amortized cost
Due in one year or less	$ 598
Due after one year through five years	2,977
Due after five years through ten years	3,656
Due after ten years	3,503
Total	10,734	$ 10,643
Fair value
Due in one year or less	604
Due after one year through five years	3,039
Due after five years through ten years	3,742
Due after ten years	3,637
Total	$ 11,022	$ 10,689
% of fair value
Due in one year or less	5.50%
Due after one year through five years	27.60%
Due after five years through ten years	33.90%
Due after ten years	33.00%
Total	100.00%