Segment Information - Revenue from External Customers (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue from External Customer [Line Items]
Premiums	$ 36,113	$ 32,491	$ 30,824
Revenues from external customers	47,170	40,580	38,691
Integrated Medical [Member]
Revenue from External Customer [Line Items]
Premiums	26,848	23,631	22,262
Revenues from external customers	31,759	28,193	26,695
International Markets [Member]
Revenue from External Customer [Line Items]
Premiums	5,043	4,619	4,273
Revenues from external customers	5,174	4,774	4,424
Health Services [Member]
Revenue from External Customer [Line Items]
Revenues from external customers	5,902	3,250	3,169
Group Disability and Other [Member]
Revenue from External Customer [Line Items]
Revenues from external customers	4,335	4,363	4,403
Product [Member]
Revenue from External Customer [Line Items]
Pharmacy revenues, fees and other revenues	5,479	2,979	2,966
Home Delivery And Specialty [Member]
Revenue from External Customer [Line Items]
Pharmacy revenues, fees and other revenues	3,997	2,979	2,966
Network Pharmacy [Member]
Revenue from External Customer [Line Items]
Pharmacy revenues, fees and other revenues	1,415	0	0
Other Pharmacy [Member]
Revenue from External Customer [Line Items]
Pharmacy revenues, fees and other revenues	67	0	0
Service [Member]
Revenue from External Customer [Line Items]
Fees	5,558	5,053	4,844
Other external revenues	20	57	57
Pharmacy revenues, fees and other revenues	5,578	5,110	4,901
Medical Risk Products [Member] | Integrated Medical [Member]
Revenue from External Customer [Line Items]
Premiums	10,710	9,439	7,911
Stop Loss [Member] | Integrated Medical [Member]
Revenue from External Customer [Line Items]
Premiums	4,008	3,483	3,082
Other Commercial Medical Products [Member] | Integrated Medical [Member]
Revenue from External Customer [Line Items]
Premiums	1,038	917	886
Medicare Advantage [Member] | Integrated Medical [Member]
Revenue from External Customer [Line Items]
Premiums	5,832	5,534	6,621
Medicare Part D [Member] | Integrated Medical [Member]
Revenue from External Customer [Line Items]
Premiums	764	764	1,122
Other Government Products [Member] | Integrated Medical [Member]
Revenue from External Customer [Line Items]
Premiums	4,496	3,494	2,640
Disability Life Accident [Member] | Group Disability and Other [Member]
Revenue from External Customer [Line Items]
Premiums	4,000	3,973	4,002
Other [Member] | Group Disability and Other [Member]
Revenue from External Customer [Line Items]
Premiums	$ 222	$ 268	$ 287