Segment Information - Segment Financial Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	$ 9,599	$ 9,174	$ 19,243	$ 18,292
Net investment income	294	297	566	573
Total operating revenues	9,893	9,471	19,809	18,865
Total revenues	9,960	9,492	19,844	18,959
Operating Segments [Member] | Global Health Care [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	7,759	7,416	15,571	14,787
Net investment income	81	91	153	166
Total operating revenues	7,840	7,507	15,724	14,953
Total revenues	7,868	7,515	15,735	15,010
Operating Segments [Member] | Global Supplemental Benefits [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	808	756	1,588	1,499
Net investment income	27	26	53	52
Total operating revenues	835	782	1,641	1,551
Total revenues	835	780	1,639	1,552
Operating Segments [Member] | Group Disability And Life [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	1,010	978	2,039	1,954
Net investment income	88	85	168	168
Total operating revenues	1,098	1,063	2,207	2,122
Total revenues	1,122	1,071	2,228	2,152
Operating Segments [Member] | Other Operations Segment [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	28	30	55	62
Net investment income	93	94	183	186
Total operating revenues	121	124	238	248
Total revenues	136	131	243	254
Corporate [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	(6)	(6)	(10)	(10)
Net investment income	5	1	9	1
Total operating revenues	(1)	(5)	(1)	(9)
Total revenues	$ (1)	$ (5)	$ (1)	$ (9)