Segment Information - Summarized Segment Financial Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	$ 11,102	$ 10,074	$ 33,314	$ 30,265
Net investment income	355	298	1,036	909
Less net realized investment gains (losses) on equity method subsidiaries	(1)		(23)
Less special item (transaction related costs)	13		13
Total operating revenues	11,445	10,372	34,360	31,174
Total revenues	11,457	10,489	34,314	31,388
Shareholders' net income	772	560	2,493	1,971
After-tax adjustments to reconcile to adjusted income from operations:
Net realized investment (gains) losses	(1)	(75)	46	(140)
Amortization of other acquired intangible assets, net	36	16	74	54
Special items:
Transaction-related costs	108	6	267	8
Charges associated with litigation matters	35	0	35	0
U.S. tax reform	(5)	0	(5)	0
Debt extinguishment costs	0	209	0	209
Long-term care guaranty fund assessment			0	83
Total special items	138	215	297	300
Adjusted income (loss) from operations	945	716	2,910	2,185
Operating Segments [Member] | Global Health Care [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	8,995	8,087	26,994	24,382
Net investment income	122	89	362	273
Less net realized investment gains (losses) on equity method subsidiaries	0		0
Less special item (transaction related costs)	0		0
Total operating revenues	9,117	8,176	27,356	24,655
Total revenues	9,121	8,251	27,345	24,788
Shareholders' net income	750	610	2,372	1,753
After-tax adjustments to reconcile to adjusted income from operations:
Net realized investment (gains) losses	(4)	(47)	7	(85)
Amortization of other acquired intangible assets, net	32	12	59	40
Special items:
Transaction-related costs	0	0	0	0
Charges associated with litigation matters	35		35
U.S. tax reform	(9)		(9)
Debt extinguishment costs		0		0
Long-term care guaranty fund assessment				68
Total special items	26	0	26	68
Adjusted income (loss) from operations	804	575	2,464	1,776
Operating Segments [Member] | Global Supplemental Benefits Segment [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	1,055	954	3,171	2,759
Net investment income	37	31	107	90
Less net realized investment gains (losses) on equity method subsidiaries	(1)		(23)
Less special item (transaction related costs)	0		0
Total operating revenues	1,093	985	3,301	2,849
Total revenues	1,093	985	3,268	2,861
Shareholders' net income	80	105	271	283
After-tax adjustments to reconcile to adjusted income from operations:
Net realized investment (gains) losses	0	0	28	(9)
Amortization of other acquired intangible assets, net	4	4	15	14
Special items:
Transaction-related costs	0	0	0	0
Charges associated with litigation matters	0		0
U.S. tax reform	9		9
Debt extinguishment costs		0		0
Long-term care guaranty fund assessment				0
Total special items	9	0	9	0
Adjusted income (loss) from operations	93	109	323	288
Operating Segments [Member] | Group Disability And Life Segment [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	1,032	1,015	3,089	3,070
Net investment income	89	84	275	262
Less net realized investment gains (losses) on equity method subsidiaries	0		0
Less special item (transaction related costs)	0		0
Total operating revenues	1,121	1,099	3,364	3,332
Total revenues	1,124	1,136	3,356	3,399
Shareholders' net income	106	97	267	253
After-tax adjustments to reconcile to adjusted income from operations:
Net realized investment (gains) losses	(4)	(24)	5	(44)
Amortization of other acquired intangible assets, net	0	0	0	0
Special items:
Transaction-related costs	0	0	0	0
Charges associated with litigation matters	0		0
U.S. tax reform	(2)		(2)
Debt extinguishment costs		0		0
Long-term care guaranty fund assessment				15
Total special items	(2)	0	(2)	15
Adjusted income (loss) from operations	100	73	270	224
Operating Segments [Member] | Other Operations Segment [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	33	29	97	89
Net investment income	87	86	260	260
Less net realized investment gains (losses) on equity method subsidiaries	0		0
Less special item (transaction related costs)	0		0
Total operating revenues	120	115	357	349
Total revenues	112	120	351	351
Shareholders' net income	9	18	57	55
After-tax adjustments to reconcile to adjusted income from operations:
Net realized investment (gains) losses	8	(4)	5	(2)
Amortization of other acquired intangible assets, net	0	0	0	0
Special items:
Transaction-related costs	0	0	0	0
Charges associated with litigation matters	0		0
U.S. tax reform	(1)		(1)
Debt extinguishment costs		0		0
Long-term care guaranty fund assessment				0
Total special items	(1)	0	(1)	0
Adjusted income (loss) from operations	16	14	61	53
Corporate [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	(13)	(11)	(37)	(35)
Net investment income	20	8	32	24
Less net realized investment gains (losses) on equity method subsidiaries	0		0
Less special item (transaction related costs)	13		13
Total operating revenues	(6)	(3)	(18)	(11)
Total revenues	7	(3)	(6)	(11)
Shareholders' net income	(173)	(270)	(474)	(373)
After-tax adjustments to reconcile to adjusted income from operations:
Net realized investment (gains) losses	(1)	0	1	0
Amortization of other acquired intangible assets, net	0	0	0	0
Special items:
Transaction-related costs	108	6	267	8
Charges associated with litigation matters	0		0
U.S. tax reform	(2)		(2)
Debt extinguishment costs		209		209
Long-term care guaranty fund assessment				0
Total special items	106	215	265	217
Adjusted income (loss) from operations	$ (68)	$ (55)	$ (208)	$ (156)