Investments - Schedule of Contractual Maturities of Investments (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Amortized cost
Due in one year or less	$ 446
Due after one year through five years	3,049
Due after five years through ten years	3,856
Due after ten years	2,306
Total	9,657	$ 9,324
Fair value
Due in one year or less	452
Due after one year through five years	3,265
Due after five years through ten years	4,067
Due after ten years	2,473
Total	$ 10,257	$ 9,650
% of fair value
Due in one year or less	4.40%
Due after one year through five years	31.80%
Due after five years through ten years	39.70%
Due after ten years	24.10%
Total	100.00%