Segment Information - Revenue from External Customers (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue from External Customer [Line Items]
Mail order pharmacy revenues	$ 758	$ 757	$ 1,475	$ 1,467
Other	19	32	55	68
Revenues from external customers	11,128	10,066	22,212	20,191
Operating Segments [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	8,985	8,032	17,958	16,159
Total fees from ongoing operations	1,366	1,245	2,724	2,497
Operating Segments [Member] | Global Health Care [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	6,924	6,128	13,853	12,387
Total fees from ongoing operations	1,333	1,213	2,659	2,433
Revenues from external customers	9,025	8,100	17,999	16,295
Operating Segments [Member] | Global Supplemental Benefits Segment [Member]
Revenue from External Customer [Line Items]
Total fees from ongoing operations	8	7	15	12
Revenues from external customers	1,050	924	2,116	1,805
Operating Segments [Member] | Group Disability And Life Segment [Member]
Revenue from External Customer [Line Items]
Total fees from ongoing operations	25	25	50	52
Revenues from external customers	1,031	1,023	2,057	2,055
Guaranteed Cost [Member] | Operating Segments [Member] | Global Health Care [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	2,046	1,522	4,045	3,065
Experience Rated [Member] | Operating Segments [Member] | Global Health Care [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	736	727	1,475	1,454
Stop Loss [Member] | Operating Segments [Member] | Global Health Care [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	993	856	1,973	1,708
International Health Care [Member] | Operating Segments [Member] | Global Health Care [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	515	479	1,028	954
Dental [Member] | Operating Segments [Member] | Global Health Care [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	482	441	961	882
Other Medical [Member] | Operating Segments [Member] | Global Health Care [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	259	243	517	489
Medicare [Member] | Operating Segments [Member] | Global Health Care [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	1,461	1,389	2,959	2,850
Medicaid [Member] | Operating Segments [Member] | Global Health Care [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	243	273	478	553
Medicare Part D [Member] | Operating Segments [Member] | Global Health Care [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	189	198	417	432
Disability [Member] | Operating Segments [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	510	491	1,015	992
Life, Accident and Supplemental Health [Member] | Operating Segments [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	$ 1,551	$ 1,413	$ 3,090	$ 2,780