Segment Information - Summarized Segment Financial Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	$ 11,128	$ 10,066	$ 22,212	$ 20,191
Net investment income	352	308	681	611
Less net realized investment gains (losses) on equity method subsidiaries	(20)		(22)
Total operating revenues	11,500	10,374	22,915	20,802
Total revenues	11,477	10,425	22,857	20,899
Shareholders' net income	806	813	1,721	1,411
After-tax adjustments to reconcile to adjusted income from operations:
Net realized investment (gains) losses	22	(34)	47	(65)
Amortization of other acquired intangible assets, net	18	18	38	38
Special items:
Transaction-related costs	109	(47)	159	2
Long-term care guaranty fund assessment			0	83
Total special items	109	(47)	159	85
Adjusted income (loss) from operations	955	750	1,965	1,469
Operating Segments [Member] | Global Health Care [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	9,025	8,100	17,999	16,295
Net investment income	125	92	240	184
Less net realized investment gains (losses) on equity method subsidiaries	0		0
Total operating revenues	9,150	8,192	18,239	16,479
Total revenues	9,155	8,224	18,224	16,537
Shareholders' net income	780	599	1,622	1,143
After-tax adjustments to reconcile to adjusted income from operations:
Net realized investment (gains) losses	(4)	(22)	11	(38)
Amortization of other acquired intangible assets, net	13	14	27	28
Special items:
Transaction-related costs	0	0	0	0
Long-term care guaranty fund assessment				68
Total special items				68
Adjusted income (loss) from operations	789	591	1,660	1,201
Operating Segments [Member] | Global Supplemental Benefits Segment [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	1,050	924	2,116	1,805
Net investment income	36	31	70	59
Less net realized investment gains (losses) on equity method subsidiaries	(20)		(22)
Total operating revenues	1,106	955	2,208	1,864
Total revenues	1,075	954	2,175	1,876
Shareholders' net income	86	101	191	178
After-tax adjustments to reconcile to adjusted income from operations:
Net realized investment (gains) losses	27	0	28	(9)
Amortization of other acquired intangible assets, net	5	4	11	10
Special items:
Transaction-related costs	0	0	0	0
Long-term care guaranty fund assessment				0
Total special items				0
Adjusted income (loss) from operations	118	105	230	179
Operating Segments [Member] | Group Disability And Life Segment [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	1,031	1,023	2,057	2,055
Net investment income	96	89	186	178
Less net realized investment gains (losses) on equity method subsidiaries	0		0
Total operating revenues	1,127	1,112	2,243	2,233
Total revenues	1,132	1,134	2,232	2,263
Shareholders' net income	107	97	161	156
After-tax adjustments to reconcile to adjusted income from operations:
Net realized investment (gains) losses	(4)	(14)	9	(20)
Amortization of other acquired intangible assets, net	0	0	0	0
Special items:
Transaction-related costs	0	0	0	0
Long-term care guaranty fund assessment				15
Total special items				15
Adjusted income (loss) from operations	103	83	170	151
Operating Segments [Member] | Other Operations Segment [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	33	30	64	60
Net investment income	88	88	173	174
Less net realized investment gains (losses) on equity method subsidiaries	0		0
Total operating revenues	121	118	237	234
Total revenues	119	116	239	231
Shareholders' net income	19	17	48	37
After-tax adjustments to reconcile to adjusted income from operations:
Net realized investment (gains) losses	2	2	(3)	2
Amortization of other acquired intangible assets, net	0	0	0	0
Special items:
Transaction-related costs	0	0	0	0
Long-term care guaranty fund assessment				0
Total special items				0
Adjusted income (loss) from operations	21	19	45	39
Corporate [Member]
Segment Reporting Information [Line Items]
Premiums, fees and other revenues and mail order pharmacy revenues	(11)	(11)	(24)	(24)
Net investment income	7	8	12	16
Less net realized investment gains (losses) on equity method subsidiaries	0		0
Total operating revenues	(4)	(3)	(12)	(8)
Total revenues	(4)	(3)	(13)	(8)
Shareholders' net income	(186)	(1)	(301)	(103)
After-tax adjustments to reconcile to adjusted income from operations:
Net realized investment (gains) losses	1	0	2	0
Amortization of other acquired intangible assets, net	0	0	0	0
Special items:
Transaction-related costs	109	(47)	159	2
Long-term care guaranty fund assessment				0
Total special items				2
Adjusted income (loss) from operations	$ (76)	$ (48)	$ (140)	$ (101)