Investments - Schedule of Contractual Maturities of Investments (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Amortized cost
Due in one year or less	$ 446
Due after one year through five years	3,046
Due after five years through ten years	3,708
Due after ten years	2,318
Total	9,518	$ 9,324
Fair value
Due in one year or less	454
Due after one year through five years	3,273
Due after five years through ten years	3,896
Due after ten years	2,515
Total	$ 10,138	$ 9,650
% of fair value
Due in one year or less	4.50%
Due after one year through five years	32.30%
Due after five years through ten years	38.40%
Due after ten years	24.80%
Total	100.00%