Significant Accounting Policies - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Net investment income	$ 204	$ 263	$ 550	$ 805
Total revenues	67,056	64,810	199,152	189,887
Pharmacy Services
Disaggregation of Revenue [Line Items]
Total revenues	35,711	36,018	105,583	104,418
Retail/ LTC
Disaggregation of Revenue [Line Items]
Total revenues	22,725	21,466	67,136	64,028
Health Care Benefits
Disaggregation of Revenue [Line Items]
Total revenues	18,698	17,181	56,364	52,454
Corporate/ Other
Disaggregation of Revenue [Line Items]
Net investment income	83	117	209	347
Total revenues	116	152	292	423
Pharmacy
Disaggregation of Revenue [Line Items]
Revenues	43,062	42,560	126,725	121,764
Pharmacy | Corporate/ Other
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Front Store
Disaggregation of Revenue [Line Items]
Revenues	4,740	4,614	14,601	14,288
Front Store | Corporate/ Other
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Premiums
Disaggregation of Revenue [Line Items]
Revenues	17,182	15,539	51,749	47,612
Premiums | Corporate/ Other
Disaggregation of Revenue [Line Items]
Revenues	17	32	50	69
Other
Disaggregation of Revenue [Line Items]
Revenues	1,868	1,834	5,527	5,418
Other | Corporate/ Other
Disaggregation of Revenue [Line Items]
Revenues	16	3	33	7
Operating Segments | Pharmacy Services
Disaggregation of Revenue [Line Items]
Net investment income	0	0	0	0
Total revenues	35,711	36,018	105,583	104,418
Operating Segments | Retail/ LTC
Disaggregation of Revenue [Line Items]
Net investment income	0	0	0	0
Total revenues	22,725	21,466	67,136	64,028
Operating Segments | Health Care Benefits
Disaggregation of Revenue [Line Items]
Net investment income	121	146	341	458
Total revenues	18,698	17,181	56,364	52,454
Operating Segments | Pharmacy | Pharmacy Services
Disaggregation of Revenue [Line Items]
Revenues	35,505	35,872	104,924	103,983
Operating Segments | Pharmacy | Retail/ LTC
Disaggregation of Revenue [Line Items]
Revenues	17,608	16,687	51,833	49,197
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	4,740	4,614	14,601	14,288
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	17,165	15,507	51,699	47,543
Operating Segments | Other | Pharmacy Services
Disaggregation of Revenue [Line Items]
Revenues	206	146	659	435
Operating Segments | Other | Retail/ LTC
Disaggregation of Revenue [Line Items]
Revenues	377	165	702	543
Operating Segments | Other | Health Care Benefits
Disaggregation of Revenue [Line Items]
Revenues	1,412	1,528	4,324	4,453
Operating Segments | Pharmacy network | Pharmacy Services
Disaggregation of Revenue [Line Items]
Total revenues	21,473	22,411	63,109	65,917
Operating Segments | Mail choice | Pharmacy Services
Disaggregation of Revenue [Line Items]
Total revenues	14,032	13,461	41,815	38,066
Operating Segments | Other | Pharmacy Services
Disaggregation of Revenue [Line Items]
Total revenues	206	146	659	435
Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Revenues	(10,194)	(10,007)	(30,223)	(31,436)
Net investment income	0	0	0	0
Total revenues	(10,194)	(10,007)	(30,223)	(31,436)
Intersegment Eliminations | Pharmacy Services
Disaggregation of Revenue [Line Items]
Revenues	2,219	2,338	7,350	8,924
Intersegment Eliminations | Retail/ LTC
Disaggregation of Revenue [Line Items]
Revenues	7,955	7,661	22,822	22,492
Intersegment Eliminations | Health Care Benefits
Disaggregation of Revenue [Line Items]
Revenues	20	8	51	20
Intersegment Eliminations | Corporate/ Other
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations | Pharmacy
Disaggregation of Revenue [Line Items]
Revenues	(10,051)	(9,999)	(30,032)	(31,416)
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	$ (143)	$ (8)	$ (191)	$ (20)