Investments - Schedule of Contractual Maturities of Investments (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Amortized cost
Due in one year or less	$ 720
Due after one year through five years	2,745
Due after five years through ten years	3,630
Due after ten years	3,457
Total	10,552	$ 10,314
Fair value
Due in one year or less	733
Due after one year through five years	2,781
Due after five years through ten years	3,641
Due after ten years	3,481
Total	$ 10,636	$ 10,699
% of fair value
Due in one year or less	6.90%
Due after one year through five years	26.10%
Due after five years through ten years	34.30%
Due after ten years	32.70%
Total	100.00%