Investments - Schedule of Contractual Maturities of Investments (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Amortized cost
Due in one year or less	$ 481
Due after one year through five years	3,017
Due after five years through ten years	3,676
Due after ten years	2,268
Total	9,442	$ 9,324
Fair value
Due in one year or less	488
Due after one year through five years	3,202
Due after five years through ten years	3,797
Due after ten years	2,397
Total	$ 9,884	$ 9,650
% of fair value
Due in one year or less	4.90%
Due after one year through five years	32.40%
Due after five years through ten years	38.40%
Due after ten years	24.30%
Total	100.00%