Significant Accounting Policies - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Net investment income	$ 184	$ 293	$ 346	$ 542
Total revenues	65,341	63,431	132,096	125,077
Pharmacy Services
Disaggregation of Revenue [Line Items]
Total revenues	34,889	34,842	69,872	68,400
Retail/ LTC
Disaggregation of Revenue [Line Items]
Total revenues	21,662	21,447	44,411	42,562
Health Care Benefits
Disaggregation of Revenue [Line Items]
Total revenues	18,468	17,403	37,666	35,273
Corporate/ Other
Disaggregation of Revenue [Line Items]
Net investment income	57	145	126	230
Total revenues	86	161	176	271
Pharmacy
Disaggregation of Revenue [Line Items]
Revenues	41,774	40,674	83,646	79,204
Pharmacy | Corporate/ Other
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Front Store
Disaggregation of Revenue [Line Items]
Revenues	4,653	4,875	9,861	9,674
Front Store | Corporate/ Other
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Premiums
Disaggregation of Revenue [Line Items]
Revenues	16,927	15,791	34,567	32,073
Premiums | Corporate/ Other
Disaggregation of Revenue [Line Items]
Revenues	14	14	33	37
Other
Disaggregation of Revenue [Line Items]
Revenues	1,803	1,798	3,676	3,584
Other | Corporate/ Other
Disaggregation of Revenue [Line Items]
Revenues	15	2	17	4
Operating Segments | Pharmacy Services
Disaggregation of Revenue [Line Items]
Net investment income	0	0	0	0
Total revenues	34,889	34,842	69,872	68,400
Operating Segments | Retail/ LTC
Disaggregation of Revenue [Line Items]
Net investment income	0	0	0	0
Total revenues	21,662	21,447	44,411	42,562
Operating Segments | Health Care Benefits
Disaggregation of Revenue [Line Items]
Net investment income	127	148	220	312
Total revenues	18,468	17,403	37,666	35,273
Operating Segments | Pharmacy | Pharmacy Services
Disaggregation of Revenue [Line Items]
Revenues	34,645	34,698	69,419	68,111
Operating Segments | Pharmacy | Retail/ LTC
Disaggregation of Revenue [Line Items]
Revenues	16,870	16,392	34,225	32,510
Operating Segments | Pharmacy | Health Care Benefits
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Front Store | Pharmacy Services
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Front Store | Retail/ LTC
Disaggregation of Revenue [Line Items]
Revenues	4,653	4,875	9,861	9,674
Operating Segments | Front Store | Health Care Benefits
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Premiums | Pharmacy Services
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Premiums | Retail/ LTC
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Premiums | Health Care Benefits
Disaggregation of Revenue [Line Items]
Revenues	16,913	15,777	34,534	32,036
Operating Segments | Other | Pharmacy Services
Disaggregation of Revenue [Line Items]
Revenues	244	144	453	289
Operating Segments | Other | Retail/ LTC
Disaggregation of Revenue [Line Items]
Revenues	139	180	325	378
Operating Segments | Other | Health Care Benefits
Disaggregation of Revenue [Line Items]
Revenues	1,428	1,478	2,912	2,925
Operating Segments | Pharmacy network | Pharmacy Services
Disaggregation of Revenue [Line Items]
Total revenues	20,536	21,974	41,636	43,506
Operating Segments | Mail choice | Pharmacy Services
Disaggregation of Revenue [Line Items]
Total revenues	14,109	12,724	27,783	24,605
Operating Segments | Other | Pharmacy Services
Disaggregation of Revenue [Line Items]
Total revenues	244	144	453	289
Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Revenues	(9,764)	(10,422)	(20,029)	(21,429)
Net investment income	0	0	0	0
Total revenues	(9,764)	(10,422)	(20,029)	(21,429)
Intersegment Eliminations | Pharmacy Services
Disaggregation of Revenue [Line Items]
Revenues	2,266	2,854	5,131	6,586
Intersegment Eliminations | Retail/ LTC
Disaggregation of Revenue [Line Items]
Revenues	7,475	7,562	14,867	14,831
Intersegment Eliminations | Health Care Benefits
Disaggregation of Revenue [Line Items]
Revenues	23	6	31	12
Intersegment Eliminations | Corporate/ Other
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations | Pharmacy
Disaggregation of Revenue [Line Items]
Revenues	(9,741)	(10,416)	(19,998)	(21,417)
Intersegment Eliminations | Front Store
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations | Premiums
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations | Other
Disaggregation of Revenue [Line Items]
Revenues	$ (23)	$ (6)	$ (31)	$ (12)