Accounting Policies - Revenue Disaggregation (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Revenue Disaggregation
Net revenue	$ 45,693
Pharmacy
Revenue Disaggregation
Net revenue	39,305
Front Store
Revenue Disaggregation
Net revenue	4,726
Other
Revenue Disaggregation
Net revenue	1,662
Operating Segments | Pharmacy Services Segment
Revenue Disaggregation
Net revenue	32,218
Operating Segments | Pharmacy Services Segment | Pharmacy
Revenue Disaggregation
Net revenue	30,762
Operating Segments | Pharmacy Services Segment | Mail choice
Revenue Disaggregation
Net revenue	11,208
Operating Segments | Pharmacy Services Segment | Retail network
Revenue Disaggregation
Net revenue	19,554
Operating Segments | Pharmacy Services Segment | Other
Revenue Disaggregation
Net revenue	1,456
Operating Segments | Retail/LTC Segment
Revenue Disaggregation
Net revenue	20,432
Operating Segments | Retail/LTC Segment | Pharmacy
Revenue Disaggregation
Net revenue	15,500
Operating Segments | Retail/LTC Segment | Front Store
Revenue Disaggregation
Net revenue	4,726
Operating Segments | Retail/LTC Segment | Other
Revenue Disaggregation
Net revenue	206
Intersegment Eliminations
Revenue Disaggregation
Net revenue	(6,957)
Intersegment Eliminations | Pharmacy
Revenue Disaggregation
Net revenue	$ (6,957)