Segment Information - Revenue from External Customers (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue from External Customer [Line Items]
Premiums	$ 39,714	$ 36,113	$ 32,491
Revenues from external customers	152,176	47,170	40,580
Integrated Medical [Member]
Revenue from External Customer [Line Items]
Premiums	30,089	26,848	23,631
Revenues from external customers	34,861	31,759	28,193
International Markets [Member]
Revenue from External Customer [Line Items]
Premiums	5,266	5,043	4,619
Revenues from external customers	5,500	5,174	4,774
Health Services [Member]
Revenue from External Customer [Line Items]
Revenues from external customers	107,354	5,902	3,250
Group Disability and Other [Member]
Revenue from External Customer [Line Items]
Revenues from external customers	4,461	4,335	4,363
Product [Member]
Revenue from External Customer [Line Items]
Pharmacy revenues, fees and other revenues	103,099	5,479	2,979
Network Pharmacy [Member]
Revenue from External Customer [Line Items]
Pharmacy revenues, fees and other revenues	50,431	1,415	0
Home Delivery And Specialty [Member]
Revenue from External Customer [Line Items]
Pharmacy revenues, fees and other revenues	47,768	3,997	2,979
Other Pharmacy [Member]
Revenue from External Customer [Line Items]
Pharmacy revenues, fees and other revenues	4,900	67	0
Service [Member]
Revenue from External Customer [Line Items]
Fees	9,229	5,558	5,053
Other external revenues	134	20	57
Pharmacy revenues, fees and other revenues	9,363	5,578	5,110
Medical Risk Products [Member] | Integrated Medical [Member]
Revenue from External Customer [Line Items]
Premiums	12,523	10,710	9,439
Stop Loss [Member] | Integrated Medical [Member]
Revenue from External Customer [Line Items]
Premiums	4,328	4,008	3,483
Other Commercial Medical Products [Member] | Integrated Medical [Member]
Revenue from External Customer [Line Items]
Premiums	1,040	1,038	917
Medicare Advantage [Member] | Integrated Medical [Member]
Revenue from External Customer [Line Items]
Premiums	6,314	5,832	5,534
Medicare Part D [Member] | Integrated Medical [Member]
Revenue from External Customer [Line Items]
Premiums	1,699	764	764
Other Government Products [Member] | Integrated Medical [Member]
Revenue from External Customer [Line Items]
Premiums	4,185	4,496	3,494
Disability Life Accident [Member] | Group Disability and Other [Member]
Revenue from External Customer [Line Items]
Premiums	4,225	4,000	3,973
Other [Member] | Group Disability and Other [Member]
Revenue from External Customer [Line Items]
Premiums	$ 134	$ 222	$ 268