Segment Information - Revenues by Product Type (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	$ 8,973	$ 8,127
Mail order pharmacy revenues	717	710
Other	36	36
Revenues from external customers	11,084	10,125
Operating Segments [Member]
Revenue from External Customer [Line Items]
Total fees from ongoing operations	1,358	1,252
Operating Segments [Member] | Global Health Care [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	6,929	6,259
Total fees from ongoing operations	1,326	1,220
Revenues from external customers	8,974	8,195
Operating Segments [Member] | Global Supplemental Benefits Segment [Member]
Revenue from External Customer [Line Items]
Total fees from ongoing operations	7	5
Revenues from external customers	1,066	881
Operating Segments [Member] | Group Disability And Life Segment [Member]
Revenue from External Customer [Line Items]
Total fees from ongoing operations	25	27
Revenues from external customers	1,026	1,032
Guaranteed Cost [Member] | Operating Segments [Member] | Global Health Care [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	1,999	1,543
Experience Rated [Member] | Operating Segments [Member] | Global Health Care [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	739	727
Stop Loss [Member] | Operating Segments [Member] | Global Health Care [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	980	852
International Health Care [Member] | Operating Segments [Member] | Global Health Care [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	513	475
Dental [Member] | Operating Segments [Member] | Global Health Care [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	479	441
Other Medical [Member] | Operating Segments [Member] | Global Health Care [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	258	246
Medicare [Member] | Operating Segments [Member] | Global Health Care [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	1,498	1,461
Medicaid [Member] | Operating Segments [Member] | Global Health Care [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	235	280
Medicare Part D [Member] | Operating Segments [Member] | Global Health Care [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	228	234
Disability [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	505	501
Life, Accident and Supplemental Health [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	$ 1,539	$ 1,367