Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended											12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016		Dec. 31, 2015		Sep. 30, 2015	Jun. 30, 2015		Mar. 31, 2015	Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Quarterly Financial Data [Abstract]
Revenues	$ 2,390	$ 2,433	$ 2,348	$ 2,195		$ 2,269		$ 2,153	$ 2,327		$ 2,352	$ 9,366		$ 9,101		$ 9,692
Net income (loss)	$ 241	$ 343	$ 209	$ 66	[1]	$ (70)	[2]	$ 178	$ 138	[1]	$ 233	$ 859	[1]	$ 479	[1],[2]	$ 691
Basic earnings (loss) per share	$ 0.89	$ 1.27	$ 0.77	$ 0.25		$ (0.26)		$ 0.66	$ 0.51		$ 0.86	$ 3.18		$ 1.77		$ 2.56
Diluted earnings (loss) per share	$ 0.89	$ 1.26	$ 0.77	$ 0.24		$ (0.26)		$ 0.66	$ 0.51		$ 0.86	$ 3.17		$ 1.77		$ 2.55

[1] Net income (loss) in the first quarter of 2016 and the second quarter of 2015 included a charge related to the application of retroactive reinsurance accounting to adverse reserve development ceded under the 2010 A&EP Loss Portfolio Transfer in our Corporate & Other Non-Core segment.

[2] Net income (loss) in the fourth quarter of 2015 included a charge related to recognition of a premium deficiency in our long term care business.