Significant Accounting Policies - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Net investment income	$ 162	$ 249
Total revenues	66,755	61,646
Pharmacy Services
Disaggregation of Revenue [Line Items]
Total revenues	34,983	33,558
Retail/ LTC
Disaggregation of Revenue [Line Items]
Total revenues	22,749	21,115
Health Care Benefits
Disaggregation of Revenue [Line Items]
Total revenues	19,198	17,870
Corporate/ Other
Disaggregation of Revenue [Line Items]
Net investment income	69	85
Total revenues	90	110
Pharmacy
Disaggregation of Revenue [Line Items]
Revenues	41,872	38,530
Pharmacy | Corporate/ Other
Disaggregation of Revenue [Line Items]
Revenues	0	0
Front Store
Disaggregation of Revenue [Line Items]
Revenues	5,208	4,799
Front Store | Corporate/ Other
Disaggregation of Revenue [Line Items]
Revenues	0	0
Premiums
Disaggregation of Revenue [Line Items]
Revenues	17,640	16,282
Premiums | Corporate/ Other
Disaggregation of Revenue [Line Items]
Revenues	19	23
Other
Disaggregation of Revenue [Line Items]
Revenues	1,873	1,786
Other | Corporate/ Other
Disaggregation of Revenue [Line Items]
Revenues	2	2
Operating Segments | Pharmacy Services
Disaggregation of Revenue [Line Items]
Net investment income	0	0
Total revenues	34,983	33,558
Operating Segments | Retail/ LTC
Disaggregation of Revenue [Line Items]
Net investment income	0	0
Total revenues	22,749	21,115
Operating Segments | Health Care Benefits
Disaggregation of Revenue [Line Items]
Net investment income	93	164
Total revenues	19,198	17,870
Operating Segments | Pharmacy | Pharmacy Services
Disaggregation of Revenue [Line Items]
Revenues	34,774	33,413
Operating Segments | Pharmacy | Retail/ LTC
Disaggregation of Revenue [Line Items]
Revenues	17,355	16,118
Operating Segments | Pharmacy | Health Care Benefits
Disaggregation of Revenue [Line Items]
Revenues	0	0
Operating Segments | Front Store | Pharmacy Services
Disaggregation of Revenue [Line Items]
Revenues	0	0
Operating Segments | Front Store | Retail/ LTC
Disaggregation of Revenue [Line Items]
Revenues	5,208	4,799
Operating Segments | Front Store | Health Care Benefits
Disaggregation of Revenue [Line Items]
Revenues	0	0
Operating Segments | Premiums | Pharmacy Services
Disaggregation of Revenue [Line Items]
Revenues	0	0
Operating Segments | Premiums | Retail/ LTC
Disaggregation of Revenue [Line Items]
Revenues	0	0
Operating Segments | Premiums | Health Care Benefits
Disaggregation of Revenue [Line Items]
Revenues	17,621	16,259
Operating Segments | Other | Pharmacy Services
Disaggregation of Revenue [Line Items]
Revenues	209	145
Operating Segments | Other | Retail/ LTC
Disaggregation of Revenue [Line Items]
Revenues	186	198
Operating Segments | Other | Health Care Benefits
Disaggregation of Revenue [Line Items]
Revenues	1,484	1,447
Operating Segments | Pharmacy network | Pharmacy Services
Disaggregation of Revenue [Line Items]
Total revenues	13,674	11,881
Operating Segments | Mail choice | Pharmacy Services
Disaggregation of Revenue [Line Items]
Total revenues	21,100	21,532
Operating Segments | Other | Pharmacy Services
Disaggregation of Revenue [Line Items]
Total revenues	209	145
Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Revenues	(10,265)	(11,007)
Net investment income	0	0
Total revenues	(10,265)	(11,007)
Intersegment Eliminations | Pharmacy Services
Disaggregation of Revenue [Line Items]
Revenues	2,865	3,732
Intersegment Eliminations | Retail/ LTC
Disaggregation of Revenue [Line Items]
Revenues	7,392	7,269
Intersegment Eliminations | Health Care Benefits
Disaggregation of Revenue [Line Items]
Revenues	8	6
Intersegment Eliminations | Corporate/ Other
Disaggregation of Revenue [Line Items]
Revenues	0	0
Intersegment Eliminations | Pharmacy
Disaggregation of Revenue [Line Items]
Revenues	(10,257)	(11,001)
Intersegment Eliminations | Front Store
Disaggregation of Revenue [Line Items]
Revenues	0	0
Intersegment Eliminations | Premiums
Disaggregation of Revenue [Line Items]
Revenues	0	0
Intersegment Eliminations | Other
Disaggregation of Revenue [Line Items]
Revenues	$ (8)	$ (6)