Investments (Contractual Maturity Dates For Fixed-Maturity And Short-Term Investments) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Amortized cost
Due in one year or less	$ 511
Due after one year through five years	2,736
Due after five years through ten years	3,921
Due after ten years	3,146
Total	10,314	$ 9,799
Fair value
Due in one year or less	519
Due after one year through five years	2,839
Due after five years through ten years	4,058
Due after ten years	3,283
Total	$ 10,699	$ 10,085
% of fair value
Due in one year or less	4.90%
Due after one year through five years	26.50%
Due after five years through ten years	37.90%
Due after ten years	30.70%
Total	100.00%