Significant Accounting Policies - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Revenues from customers	$ 63,138	$ 46,708	$ 124,535	$ 92,401
Net investment income	293	214	542	264
Total revenues	63,431	46,922	125,077	92,665
Pharmacy
Disaggregation of Revenue [Line Items]
Revenues from customers	40,668	40,947	79,192	80,252
Front Store
Disaggregation of Revenue [Line Items]
Revenues from customers	4,875	4,707	9,674	9,433
Premiums
Disaggregation of Revenue [Line Items]
Revenues from customers	15,791	751	32,073	2,057
Other
Disaggregation of Revenue [Line Items]
Revenues from customers	1,804	303	3,596	659
Operating Segments | Pharmacy Services Segment
Disaggregation of Revenue [Line Items]
Revenues from customers	34,842	33,427	68,400	65,973
Net investment income	0	0	0	0
Total revenues	34,842	33,427	68,400	65,973
Operating Segments | Retail Long-Term Care Segment
Disaggregation of Revenue [Line Items]
Revenues from customers	21,447	20,672	42,562	41,104
Net investment income	0	0	0	0
Total revenues	21,447	20,672	42,562	41,104
Operating Segments | Health Care Benefits Segment
Disaggregation of Revenue [Line Items]
Revenues from customers	17,255	760	34,961	2,076
Net investment income	148	4	312	6
Total revenues	17,403	764	35,273	2,082
Operating Segments | Corporate / Other
Disaggregation of Revenue [Line Items]
Revenues from customers	16	0	41	0
Net investment income	145	210	230	258
Total revenues	161	210	271	258
Operating Segments | Pharmacy | Pharmacy Services Segment
Disaggregation of Revenue [Line Items]
Revenues from customers	34,698	33,293	68,111	65,699
Operating Segments | Pharmacy | Retail Long-Term Care Segment
Disaggregation of Revenue [Line Items]
Revenues from customers	16,392	15,805	32,510	31,305
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	4,875	4,707	9,674	9,433
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	15,777	751	32,036	2,057
Operating Segments | Premiums | Corporate / Other
Disaggregation of Revenue [Line Items]
Revenues from customers	14	0	37	0
Operating Segments | Other | Pharmacy Services Segment
Disaggregation of Revenue [Line Items]
Revenues from customers	144	134	289	274
Operating Segments | Other | Retail Long-Term Care Segment
Disaggregation of Revenue [Line Items]
Revenues from customers	180	160	378	366
Operating Segments | Other | Health Care Benefits Segment
Disaggregation of Revenue [Line Items]
Revenues from customers	1,478	9	2,925	19
Operating Segments | Other | Corporate / Other
Disaggregation of Revenue [Line Items]
Revenues from customers	2	0	4	0
Operating Segments | Pharmacy network | Pharmacy Services Segment
Disaggregation of Revenue [Line Items]
Revenues from customers	22,028	21,506	43,602	42,704
Operating Segments | Mail choice | Pharmacy Services Segment
Disaggregation of Revenue [Line Items]
Revenues from customers	12,670	11,787	24,509	22,995
Operating Segments | Other | Pharmacy Services Segment
Disaggregation of Revenue [Line Items]
Revenues from customers	144	134	289	274
Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Revenues from customers	(10,422)	(8,151)	(21,429)	(16,752)
Net investment income	0	0	0	0
Total revenues	(10,422)	(8,151)	(21,429)	(16,752)
Intersegment Eliminations | Pharmacy
Disaggregation of Revenue [Line Items]
Revenues from customers	(10,422)	(8,151)	(21,429)	(16,752)
Total revenues	(10,422)	(8,151)	(21,429)	(16,752)
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