Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	3 Months Ended								12 Months Ended
	Apr. 30, 2015	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Consolidated Results
Total revenues		$ 10,531	$ 10,382	$ 10,318	$ 10,385	$ 9,944	$ 9,880	$ 9,960	$ 9,884	$ 41,616	$ 39,668	$ 37,876
Income (loss) before income taxes		758	824	1,134	890	605	742	813	819	3,606	2,979	3,327
Shareholders' net income		$ 266	$ 560	$ 813	$ 598	$ 382	$ 456	$ 510	$ 519	$ 2,237	$ 1,867	$ 2,094
Shareholders' Net Income Per Share:
EPS, basic		$ 1.09	$ 2.25	$ 3.2	$ 2.34	$ 1.49	$ 1.79	$ 2	$ 2.04	$ 8.92	$ 7.31	$ 8.17
EPS, diluted		1.07	2.21	3.15	2.3	1.47	1.76	1.97	2	8.77	7.19	8.04
Stock and Dividend Data
Price range of common stock - high		212.46	188.36	173.21	154.83	142	148.99	142.91	147.93
Price range of common stock - low		183.08	166.81	146.7	133.52	115.03	123.53	121.87	123.54
Dividends Declared Per Share		$ 0	$ 0	$ 0	$ 0.04	$ 0	$ 0	$ 0	$ 0.04	$ 0.04	$ 0.04	$ 0.04
Special items
U.S. tax reform		$ 196	$ 0	$ 0	$ 0					$ 196
Debt extinguishment costs	$ 65	0	209	0	0					209		$ 65
Long-term care guaranty fund assessment		0	0	0	83					83
Transaction-related costs		25	6	(47)	49	$ 39	$ 46	$ 26	$ 36	$ 33	$ 147	$ 57
Risk corridor allowance						80	0	0	0		80
Charges associated with litigation matters						0	25	0	0		$ 25
Total impact of special items, after-tax		$ 221	$ 215	$ (47)	$ 132	$ 119	$ 71	$ 26	$ 36