Segment Information - Revenue from External Customers (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Revenue from External Customer [Line Items]
Premiums	$ 10,840	$ 9,971
Total revenues from external customers	38,116	37,600
International Markets
Revenue from External Customer [Line Items]
Premiums	1,375	1,304
Pharmacy revenues
Revenue from External Customer [Line Items]
Revenues	25,098	25,179
Network revenues
Revenue from External Customer [Line Items]
Revenues	12,142	12,273
Home delivery and specialty revenues
Revenue from External Customer [Line Items]
Revenues	11,714	11,784
Other
Revenue from External Customer [Line Items]
Revenues	1,242	1,122
Integrated Medical premiums | Integrated Medical
Revenue from External Customer [Line Items]
Premiums	8,320	7,583
Health Insurance | Integrated Medical
Revenue from External Customer [Line Items]
Premiums	3,461	3,039
Stop loss | Integrated Medical
Revenue from External Customer [Line Items]
Premiums	1,161	1,069
Other | Integrated Medical
Revenue from External Customer [Line Items]
Premiums	289	278
Medicare Advantage | Integrated Medical
Revenue from External Customer [Line Items]
Premiums	1,881	1,607
Medicare Part D | Integrated Medical
Revenue from External Customer [Line Items]
Premiums	462	525
Other | Integrated Medical
Revenue from External Customer [Line Items]
Premiums	1,066	1,065
Domestic disability, life and accident premiums
Revenue from External Customer [Line Items]
Premiums	1,116	1,047
Other premiums
Revenue from External Customer [Line Items]
Premiums	29	37
Services
Revenue from External Customer [Line Items]
Revenues	2,178	2,450
Fees
Revenue from External Customer [Line Items]
Revenues	2,154	2,390
Other external revenues
Revenue from External Customer [Line Items]
Revenues	$ 24	$ 60