Segment Information - Revenue from External Customers (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenue from External Customer [Line Items]
Premiums	$ 10,682	$ 9,935	$ 31,928	$ 29,709
Total revenues from external customers	40,658	38,207	117,816	114,286
U.S. Medical
Revenue from External Customer [Line Items]
Total revenues from external customers	9,047	8,648	26,999	26,226
International Markets
Revenue from External Customer [Line Items]
Premiums	1,360	1,309	4,079	3,914
Total revenues from external customers	1,439	1,368	4,325	4,095
Pharmacy revenues
Revenue from External Customer [Line Items]
Revenues	27,802	25,987	79,464	77,454
Network revenues
Revenue from External Customer [Line Items]
Revenues	13,968	12,923	39,200	37,954
Home delivery and specialty revenues
Revenue from External Customer [Line Items]
Revenues	12,422	11,837	36,319	35,893
Other
Revenue from External Customer [Line Items]
Revenues	1,412	1,227	3,945	3,607
U.S. Medical premiums | U.S. Medical
Revenue from External Customer [Line Items]
Premiums	8,196	7,544	24,428	22,523
Health Insurance | U.S. Medical
Revenue from External Customer [Line Items]
Premiums	3,397	3,174	9,948	9,278
Stop loss | U.S. Medical
Revenue from External Customer [Line Items]
Premiums	1,146	1,085	3,459	3,221
Other | U.S. Medical
Revenue from External Customer [Line Items]
Premiums	281	260	853	779
Medicare Advantage | U.S. Medical
Revenue from External Customer [Line Items]
Premiums	1,895	1,562	5,680	4,779
Medicare Part D | U.S. Medical
Revenue from External Customer [Line Items]
Premiums	360	404	1,242	1,329
Other | U.S. Medical
Revenue from External Customer [Line Items]
Premiums	1,117	1,059	3,246	3,137
Domestic disability, life and accident premiums
Revenue from External Customer [Line Items]
Premiums	1,106	1,049	3,346	3,165
Other premiums
Revenue from External Customer [Line Items]
Premiums	20	33	75	107
Services
Revenue from External Customer [Line Items]
Revenues	2,174	2,285	6,424	7,123
Fees
Revenue from External Customer [Line Items]
Revenues	2,120	2,249	6,253	7,015
Other external revenues
Revenue from External Customer [Line Items]
Revenues	$ 54	$ 36	$ 171	$ 108