Quarterly Financial Data (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
Consolidated Results
Total revenues	$ 9,944		$ 9,880		$ 9,960		$ 9,884		$ 9,528		$ 9,389		$ 9,492		$ 9,467	$ 39,668	$ 37,876	$ 34,914
Income before income taxes	605		742		813		819		659		878		936		854	2,979	3,327	3,304
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
Shareholders' Net Income Per Share:
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, diluted	1.47		1.76		1.97		2.00		1.64		2.10		2.26		2.04	7.19	8.04	7.83
Stock and Dividend Data
Price range of common stock - high	142.00		148.99		142.91		147.93		148.51		166.19		170.63		131.13
Price range of common stock - low	115.03		123.53		121.87		123.54		127.51		125.61		124.30		100.68
Dividends Declared Per Share	$ 0		$ 0		$ 0		$ 0.04		$ 0		$ 0		$ 0		$ 0.04	$ 0.04	$ 0.04	$ 0.04
Special items
Merger-related transaction costs	$ 39		$ 46		$ 26		$ 36		$ 28		$ 29					$ 147	$ 57
Charges associated with litigation matters			$ 25													25
Debt extinguishment costs													$ 65				$ 65
Risk corridor allowance	$ 80															$ 80

[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.