Segment Information - Summarized Segment Financial Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	$ 9,967	$ 9,523	$ 29,962	$ 28,766
Net investment income	298	282	909	848
Total operating revenues	10,265	9,805	30,871	29,614
Total revenues	10,382	9,880	31,085	29,724
Shareholders' Net Income	560	456	1,971	1,485
After-tax adjustments to reconcile to adjusted income from operations:
Realized investment (gains) losses	(75)	(48)	(140)	(71)
Amortization of other acquired intangible assets	16	24	54	72
Special items:
Debt extinguishment costs	209	0	209	0
Merger-related transaction costs	6	46	8	108
Long-term care guaranty fund assessment			83	0
Charges associated with litigation matters	0	25	0	25
Total impact of special items, after-tax	215	71	300	133
Adjusted income from operations	716	503	2,185	1,619
Operating Segments [Member] | Global Health Benefits Segment [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	7,980	7,636	24,079	23,207
Net investment income	89	77	273	230
Total operating revenues	8,069	7,713	24,352	23,437
Total revenues	8,144	7,775	24,485	23,510
Shareholders' Net Income	610	413	1,753	1,414
After-tax adjustments to reconcile to adjusted income from operations:
Realized investment (gains) losses	(47)	(42)	(85)	(49)
Amortization of other acquired intangible assets	12	20	40	56
Special items:
Debt extinguishment costs	0		0
Merger-related transaction costs	0	0	0	0
Long-term care guaranty fund assessment			68
Charges associated with litigation matters		25		25
Total impact of special items, after-tax	0	25	68	25
Adjusted income from operations	575	416	1,776	1,446
Operating Segments [Member] | Global Supplemental Benefits Segment [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	954	837	2,759	2,425
Net investment income	31	29	90	82
Total operating revenues	985	866	2,849	2,507
Total revenues	985	868	2,861	2,507
Shareholders' Net Income	105	77	283	214
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	4	14	16
Special items:
Debt extinguishment costs	0		0
Merger-related transaction costs	0	0	0	0
Long-term care guaranty fund assessment			0
Charges associated with litigation matters		0		0
Total impact of special items, after-tax	0	0	0	0
Adjusted income from operations	109	81	288	231
Operating Segments [Member] | Group Disability And Life Segment [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	1,015	1,026	3,070	3,065
Net investment income	84	85	262	253
Total operating revenues	1,099	1,111	3,332	3,318
Total revenues	1,136	1,128	3,399	3,356
Shareholders' Net Income	97	65	253	81
After-tax adjustments to reconcile to adjusted income from operations:
Realized investment (gains) losses	(24)	(12)	(44)	(25)
Amortization of other acquired intangible assets	0	0	0	0
Special items:
Debt extinguishment costs	0		0
Merger-related transaction costs	0	0	0	0
Long-term care guaranty fund assessment			15
Charges associated with litigation matters		0		0
Total impact of special items, after-tax	0	0	15	0
Adjusted income from operations	73	53	224	56
Operating Segments [Member] | Other Operations Segment [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	29	28	89	83
Net investment income	86	87	260	270
Total operating revenues	115	115	349	353
Total revenues	120	109	351	352
Shareholders' Net Income	18	9	55	53
After-tax adjustments to reconcile to adjusted income from operations:
Realized investment (gains) losses	(4)	5	(2)	1
Amortization of other acquired intangible assets	0	0	0	0
Special items:
Debt extinguishment costs	0		0
Merger-related transaction costs	0	0	0	0
Long-term care guaranty fund assessment			0
Charges associated with litigation matters		0		0
Total impact of special items, after-tax	0	0	0	0
Adjusted income from operations	14	14	53	54
Corporate [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	(11)	(4)	(35)	(14)
Net investment income	8	4	24	13
Total operating revenues	(3)	0	(11)	(1)
Total revenues	(3)	0	(11)	(1)
Shareholders' Net Income	(270)	(108)	(373)	(277)
After-tax adjustments to reconcile to adjusted income from operations:
Realized investment (gains) losses	0	1	0	1
Amortization of other acquired intangible assets	0	0	0	0
Special items:
Debt extinguishment costs	209		209
Merger-related transaction costs	6	46	8	108
Long-term care guaranty fund assessment			0
Charges associated with litigation matters		0		0
Total impact of special items, after-tax	215	46	217	108
Adjusted income from operations	$ (55)	$ (61)	$ (156)	$ (168)