Investments - Schedule of Contractual Maturities of Investments (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Amortized cost
Due in one year or less	$ 668
Due after one year through five years	2,797
Due after five years through ten years	3,660
Due after ten years	3,528
Total	10,653	$ 10,314
Fair value
Due in one year or less	678
Due after one year through five years	2,823
Due after five years through ten years	3,655
Due after ten years	3,504
Total	$ 10,660	$ 10,699
% of fair value
Due in one year or less	6.40%
Due after one year through five years	26.50%
Due after five years through ten years	34.20%
Due after ten years	32.90%
Total	100.00%