Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Earnings Per Share [Abstract]
Shareholders' net income	$ 382	[1],[2]	$ 456	[1],[3]	$ 510	[1]	$ 519	[1]	$ 426	[1]	$ 547	[1]	$ 588	[4]	$ 533	$ 1,867	$ 2,094	$ 2,102
Shares:
Weighted average																255,360	256,149	263,889
Common stock equivalents																4,287	4,443	4,714
Total shares																259,647	260,592	268,603
EPS, basic	$ 1.49		$ 1.79		$ 2.00		$ 2.04		$ 1.66		$ 2.14		$ 2.30		$ 2.08	$ 7.31	$ 8.17	$ 7.97
EPS, effect of dilution																(0.12)	(0.13)	(0.14)
EPS, diluted	$ 1.47		$ 1.76		$ 1.97		$ 2.00		$ 1.64		$ 2.10		$ 2.26		$ 2.04	$ 7.19	$ 8.04	$ 7.83
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive options																2,300	400	1,000
Common shares held in Treasury	39,276								39,601							39,276	39,601	36,869

[1] Shareholders’ net income includes after-tax charges related to the Company’s proposed merger with Anthem as follows: $29 million in the third quarter of 2015, $28 million in the fourth quarter of 2015, $36 million in the first quarter of 2016, $26 million in the second quarter of 2016, $46 million in the third quarter of 2016 and $39 million in the fourth quarter of 2016. See Note 3 to the Consolidated Financial Statements for additional details.

[2] Shareholders’ net income includes an after-tax charge of $80 million for an allowance for the risk corridor receivable balance in the fourth quarter of 2016. See Note 22 to the Consolidated Financial Statements for additional details.

[3] Shareholders’ net income includes an after-tax charge of $25 million for charges related to litigation matters in the third quarter of 2016. See Note 21 to the Consolidated Financial Statements for a description of litigation matters.

[4] 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 5 to the Consolidated Financial Statements for additional details.