Segment Information - Segment Financial Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	$ 9,523	$ 9,094	$ 28,766	$ 27,386
Net investment income	282	285	848	858
Total operating revenues	9,805	9,379	29,614	28,244
Total revenues	9,880	9,389	29,724	28,348
Operating Segments [Member] | Global Health Care [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	7,636	7,323	23,207	22,110
Net investment income	77	85	230	251
Total operating revenues	7,713	7,408	23,437	22,361
Total revenues	7,775	7,427	23,510	22,437
Operating Segments [Member] | Global Supplemental Benefits [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	837	767	2,425	2,266
Net investment income	29	26	82	78
Total operating revenues	866	793	2,507	2,344
Total revenues	868	792	2,507	2,344
Operating Segments [Member] | Group Disability And Life [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	1,026	980	3,065	2,934
Net investment income	85	84	253	252
Total operating revenues	1,111	1,064	3,318	3,186
Total revenues	1,128	1,055	3,356	3,207
Operating Segments [Member] | Other Operations Segment [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	28	28	83	90
Net investment income	87	90	270	276
Total operating revenues	115	118	353	366
Total revenues	109	119	352	373
Corporate [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	(4)	(4)	(14)	(14)
Net investment income	4	0	13	1
Total operating revenues	0	(4)	(1)	(13)
Total revenues	$ 0	$ (4)	$ (1)	$ (13)