Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended											12 Months Ended
	Dec. 31, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Consolidated Quarterly Results [Abstract]
Total revenues	$ 9,528		$ 9,389		$ 9,492		$ 9,467	$ 8,928	$ 8,757	$ 8,733	$ 8,496	$ 37,876	$ 34,914	$ 32,380
Income before income taxes	659		878		936		854	732	818	901	853	3,327	3,304	2,176
Shareholders' net income	$ 426	[1]	$ 547	[1]	$ 588	[2]	$ 533	$ 467	$ 534	$ 573	$ 528	$ 2,094	$ 2,102	$ 1,476
Shareholders' Net Income Per Share:
EPS, basic	$ 1.66		$ 2.14		$ 2.30		$ 2.08	$ 1.80	$ 2.04	$ 2.16	$ 1.96	$ 8.17	$ 7.97	$ 5.28
EPS, diluted	1.64		2.10		2.26		2.04	1.77	2.01	2.12	1.92	8.04	7.83	5.18
Stock And Dividend Data [Abstract]
Price Range Common Stock High	148.51		166.19		170.63		131.13	105.73	97.28	93.20	90.63
Price Range Common Stock Low	127.51		125.61		124.30		100.68	85.75	87.33	73.47	75.37
Dividends Declared Per Share	$ 0		$ 0		$ 0		$ 0.04	$ 0	$ 0	$ 0	$ 0.04	$ 0.04	$ 0.04	$ 0.04
Quarterly Financial Data Text Details [Abstract]
Debt extinguishment costs					$ 65							$ 65
Merger-related transaction costs	$ 28		$ 29									$ 57

[1] Shareholders’ net income includes after-tax charges of $29 million in the third quarter of 2015 and $28 million in the fourth quarter of 2015 for advisory, legal and other transactions costs directly related to the Company’s proposed merger with Anthem. See Note 3 to the Consolidated Financial Statements for additional details.

[2] Shareholders’ net income includes an after-tax charge of $65 million for the early extinguishment of debt in the second quarter of 2015. See Note 15 to the Consolidated Financial Statements for additional details.