Segment Information - Revenue from External Customers (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue from External Customer [Line Items]
Mail order pharmacy revenues	$ 747	$ 733	$ 2,222	$ 2,200
Other	34	39	89	107
Revenues from external customers	11,102	10,074	33,314	30,265
Operating Segments [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	8,965	8,052	26,923	24,211
Total fees from ongoing operations	1,356	1,250	4,080	3,747
Operating Segments [Member] | Global Health Care [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	6,919	6,131	20,772	18,518
Total fees from ongoing operations	1,325	1,219	3,984	3,652
Revenues from external customers	8,995	8,087	26,994	24,382
Operating Segments [Member] | Global Supplemental Benefits Segment [Member]
Revenue from External Customer [Line Items]
Total fees from ongoing operations	7	6	22	18
Revenues from external customers	1,055	954	3,171	2,759
Operating Segments [Member] | Group Disability And Life Segment [Member]
Revenue from External Customer [Line Items]
Total fees from ongoing operations	24	25	74	77
Revenues from external customers	1,032	1,015	3,089	3,070
Guaranteed Cost [Member] | Operating Segments [Member] | Global Health Care [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	2,080	1,591	6,125	4,656
Experience Rated [Member] | Operating Segments [Member] | Global Health Care [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	711	737	2,186	2,191
Stop Loss [Member] | Operating Segments [Member] | Global Health Care [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	1,010	874	2,983	2,582
International Health Care [Member] | Operating Segments [Member] | Global Health Care [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	513	484	1,541	1,438
Dental [Member] | Operating Segments [Member] | Global Health Care [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	486	452	1,447	1,334
Other Medical [Member] | Operating Segments [Member] | Global Health Care [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	260	247	777	736
Medicare [Member] | Operating Segments [Member] | Global Health Care [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	1,455	1,324	4,414	4,174
Medicaid [Member] | Operating Segments [Member] | Global Health Care [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	247	253	725	806
Medicare Part D [Member] | Operating Segments [Member] | Global Health Care [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	157	169	574	601
Disability [Member] | Operating Segments [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	507	493	1,522	1,485
Life, Accident and Supplemental Health [Member] | Operating Segments [Member]
Revenue from External Customer [Line Items]
Total premiums from ongoing operations	$ 1,539	$ 1,428	$ 4,629	$ 4,208