Investments - Schedule of Contractual Maturities of Investments (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Amortized cost
Due in one year or less	$ 449
Due after one year through five years	2,714
Due after five years through ten years	3,986
Due after ten years	3,215
Total	10,364	$ 10,314
Fair value
Due in one year or less	457
Due after one year through five years	2,768
Due after five years through ten years	4,046
Due after ten years	3,257
Total	$ 10,528	$ 10,699
% of fair value
Due in one year or less	4.40%
Due after one year through five years	26.30%
Due after five years through ten years	38.40%
Due after ten years	30.90%
Total	100.00%