Investments - Schedule of Contractual Maturities of Investments (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Amortized cost
Due in one year or less	$ 491
Due after one year through five years	3,004
Due after five years through ten years	3,750
Due after ten years	3,587
Total	10,832	$ 10,643
Fair value
Due in one year or less	496
Due after one year through five years	3,097
Due after five years through ten years	3,929
Due after ten years	3,798
Total	$ 11,320	$ 10,689
% of fair value
Due in one year or less	4.40%
Due after one year through five years	27.40%
Due after five years through ten years	34.70%
Due after ten years	33.50%
Total	100.00%