Investments (Contractual Maturity Dates For Fixed-Maturity And Short-Term Investments) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Amortized cost
Less than 1 year	$ 588
Years 1 - 5	2,927
Years 6 - 10	3,511
Due after ten years	1,196
Total	8,222	8,084
Fair value
Less than 1 year	598
Years 1 - 5	3,149
Years 6 - 10	4,002
Due after ten years	1,344
Total	$ 9,093	$ 8,779
% of fair value
Less than 1 year	6.60%
Years 1 - 5	34.60%
Years 6 - 10	44.00%
Due after ten years	14.80%
Total	100.00%