Significant Accounting Policies - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Revenues from customers	$ 64,547	$ 47,269	$ 189,082	$ 139,670
Net investment income	263	221	805	485
Total revenues	64,810	47,490	189,887	140,155
Pharmacy
Disaggregation of Revenue [Line Items]
Revenues from customers	42,552	41,768	121,744	122,020
Front Store
Disaggregation of Revenue [Line Items]
Revenues from customers	4,614	4,557	14,288	13,990
Premiums
Disaggregation of Revenue [Line Items]
Revenues from customers	15,539	627	47,612	2,684
Other
Disaggregation of Revenue [Line Items]
Revenues from customers	1,842	317	5,438	976
Operating Segments | Pharmacy Services Segment
Disaggregation of Revenue [Line Items]
Revenues from customers	36,018	33,864	104,418	99,837
Net investment income	0	0	0	0
Total revenues	36,018	33,864	104,418	99,837
Operating Segments | Retail Long-Term Care Segment
Disaggregation of Revenue [Line Items]
Revenues from customers	21,466	20,856	64,028	61,960
Net investment income	0	0	0	0
Total revenues	21,466	20,856	64,028	61,960
Operating Segments | Health Care Benefits Segment
Disaggregation of Revenue [Line Items]
Revenues from customers	17,035	637	51,996	2,713
Net investment income	146	4	458	10
Total revenues	17,181	641	52,454	2,723
Operating Segments | Corporate / Other
Disaggregation of Revenue [Line Items]
Revenues from customers	35	0	76	0
Net investment income	117	217	347	475
Total revenues	152	217	423	475
Operating Segments | Pharmacy | Pharmacy Services Segment
Disaggregation of Revenue [Line Items]
Revenues from customers	35,872	33,733	103,983	99,432
Operating Segments | Pharmacy | Retail Long-Term Care Segment
Disaggregation of Revenue [Line Items]
Revenues from customers	16,687	16,123	49,197	47,428
Operating Segments | Pharmacy | Health Care Benefits Segment
Disaggregation of Revenue [Line Items]
Revenues from customers	0	0	0	0
Operating Segments | Pharmacy | Corporate / Other
Disaggregation of Revenue [Line Items]
Revenues from customers	0	0	0	0
Operating Segments | Front Store | Pharmacy Services Segment
Disaggregation of Revenue [Line Items]
Revenues from customers	0	0	0	0
Operating Segments | Front Store | Retail Long-Term Care Segment
Disaggregation of Revenue [Line Items]
Revenues from customers	4,614	4,557	14,288	13,990
Operating Segments | Front Store | Health Care Benefits Segment
Disaggregation of Revenue [Line Items]
Revenues from customers	0	0	0	0
Operating Segments | Front Store | Corporate / Other
Disaggregation of Revenue [Line Items]
Revenues from customers	0	0	0	0
Operating Segments | Premiums | Pharmacy Services Segment
Disaggregation of Revenue [Line Items]
Revenues from customers	0	0	0	0
Operating Segments | Premiums | Retail Long-Term Care Segment
Disaggregation of Revenue [Line Items]
Revenues from customers	0	0	0	0
Operating Segments | Premiums | Health Care Benefits Segment
Disaggregation of Revenue [Line Items]
Revenues from customers	15,507	627	47,543	2,684
Operating Segments | Premiums | Corporate / Other
Disaggregation of Revenue [Line Items]
Revenues from customers	32	0	69	0
Operating Segments | Other | Pharmacy Services Segment
Disaggregation of Revenue [Line Items]
Revenues from customers	146	131	435	405
Operating Segments | Other | Retail Long-Term Care Segment
Disaggregation of Revenue [Line Items]
Revenues from customers	165	176	543	542
Operating Segments | Other | Health Care Benefits Segment
Disaggregation of Revenue [Line Items]
Revenues from customers	1,528	10	4,453	29
Operating Segments | Other | Corporate / Other
Disaggregation of Revenue [Line Items]
Revenues from customers	3	0	7	0
Operating Segments | Pharmacy network | Pharmacy Services Segment
Disaggregation of Revenue [Line Items]
Revenues from customers	22,469	21,921	66,071	64,625
Operating Segments | Mail choice | Pharmacy Services Segment
Disaggregation of Revenue [Line Items]
Revenues from customers	13,403	11,812	37,912	34,807
Operating Segments | Other | Pharmacy Services Segment
Disaggregation of Revenue [Line Items]
Revenues from customers	146	131	435	405
Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Revenues from customers	(10,007)	(8,088)	(31,436)	(24,840)
Net investment income	0	0	0	0
Total revenues	(10,007)	(8,088)	(31,436)	(24,840)
Intersegment Eliminations | Pharmacy
Disaggregation of Revenue [Line Items]
Revenues from customers	(10,007)	(8,088)	(31,436)	(24,840)
Total revenues	(10,007)	(8,088)	(31,436)	(24,840)
Intersegment Eliminations | Front Store
Disaggregation of Revenue [Line Items]
Revenues from customers	0	0	0	0
Intersegment Eliminations | Premiums
Disaggregation of Revenue [Line Items]
Revenues from customers	0	0	0	0
Intersegment Eliminations | Other
Disaggregation of Revenue [Line Items]
Revenues from customers	$ 0	$ 0	$ 0	$ 0