Segment Information - Summarized Segment Financial Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	$ 9,959	$ 9,599	$ 19,995	$ 19,243
Net investment income	308	294	611	566
Total operating revenues	10,267	9,893	20,606	19,809
Total revenues	10,318	9,960	20,703	19,844
Shareholders' Net Income	813	510	1,411	1,029
After-tax adjustments to reconcile to adjusted income from operations:
Realized investment (gains) losses	(34)	(44)	(65)	(23)
Amortization of other acquired intangible assets	18	23	38	48
Special items:
Long-term care guaranty fund assessment			83	0
Merger-related transaction costs	(47)	26	2	62
Adjusted income from operations	750	515	1,469	1,116
Operating Segments [Member] | Global Health Benefits Segment [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	7,993	7,759	16,099	15,571
Net investment income	92	81	184	153
Total operating revenues	8,085	7,840	16,283	15,724
Total revenues	8,117	7,868	16,341	15,735
Shareholders' Net Income	599	487	1,143	1,001
After-tax adjustments to reconcile to adjusted income from operations:
Realized investment (gains) losses	(22)	(19)	(38)	(7)
Amortization of other acquired intangible assets	14	18	28	36
Special items:
Long-term care guaranty fund assessment			68
Merger-related transaction costs	0	0	0	0
Adjusted income from operations	591	486	1,201	1,030
Operating Segments [Member] | Global Supplemental Benefits Segment [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	924	808	1,805	1,588
Net investment income	31	27	59	53
Total operating revenues	955	835	1,864	1,641
Total revenues	954	835	1,876	1,639
Shareholders' Net Income	101	78	178	137
After-tax adjustments to reconcile to adjusted income from operations:
Realized investment (gains) losses	0	0	(9)	1
Amortization of other acquired intangible assets	4	5	10	12
Special items:
Long-term care guaranty fund assessment			0
Merger-related transaction costs	0	0	0	0
Adjusted income from operations	105	83	179	150
Operating Segments [Member] | Group Disability And Life Segment [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	1,023	1,010	2,055	2,039
Net investment income	89	88	178	168
Total operating revenues	1,112	1,098	2,233	2,207
Total revenues	1,134	1,122	2,263	2,228
Shareholders' Net Income	97	3	156	16
After-tax adjustments to reconcile to adjusted income from operations:
Realized investment (gains) losses	(14)	(15)	(20)	(13)
Amortization of other acquired intangible assets	0	0	0	0
Special items:
Long-term care guaranty fund assessment			15
Merger-related transaction costs	0	0	0	0
Adjusted income from operations	83	(12)	151	3
Operating Segments [Member] | Other Operations Segment [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	30	28	60	55
Net investment income	88	93	174	183
Total operating revenues	118	121	234	238
Total revenues	116	136	231	243
Shareholders' Net Income	17	30	37	44
After-tax adjustments to reconcile to adjusted income from operations:
Realized investment (gains) losses	2	(10)	2	(4)
Amortization of other acquired intangible assets	0	0	0	0
Special items:
Long-term care guaranty fund assessment			0
Merger-related transaction costs	0	0	0	0
Adjusted income from operations	19	20	39	40
Corporate [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	(11)	(6)	(24)	(10)
Net investment income	8	5	16	9
Total operating revenues	(3)	(1)	(8)	(1)
Total revenues	(3)	(1)	(8)	(1)
Shareholders' Net Income	(1)	(88)	(103)	(169)
After-tax adjustments to reconcile to adjusted income from operations:
Realized investment (gains) losses	0	0	0	0
Amortization of other acquired intangible assets	0	0	0	0
Special items:
Long-term care guaranty fund assessment			0
Merger-related transaction costs	(47)	26	2	62
Adjusted income from operations	$ (48)	$ (62)	$ (101)	$ (107)