Exit and disposal activities - restructuring costs (Details) - USD ($) $ in Millions	12 Months Ended
	Aug. 31, 2018	Aug. 31, 2017	Aug. 31, 2016
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs		$ 835	$ 424
Operating Segments | Retail Pharmacy USA
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs		731	374
Operating Segments | Retail Pharmacy International
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs		67	29
Operating Segments | Pharmaceutical Wholesale
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs		37	21
Employee severance and other exit costs
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs		168	108
Employee severance and other exit costs | Operating Segments | Retail Pharmacy USA
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs		87	70
Employee severance and other exit costs | Operating Segments | Retail Pharmacy International
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs		46	18
Employee severance and other exit costs | Operating Segments | Pharmaceutical Wholesale
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs		35	20
Asset impairments
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs		295	225
Asset impairments | Operating Segments | Retail Pharmacy USA
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs		272	215
Asset impairments | Operating Segments | Retail Pharmacy International
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs		21	10
Asset impairments | Operating Segments | Pharmaceutical Wholesale
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs		2	0
Real estate costs
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs		372	91
Real estate costs | Operating Segments | Retail Pharmacy USA
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs		372	89
Real estate costs | Operating Segments | Retail Pharmacy International
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs		0	1
Real estate costs | Operating Segments | Pharmaceutical Wholesale
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs		$ 0	$ 1
Store Optimization Program
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	$ 100
Store Optimization Program | Lease obligations and other real estate costs
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	19
Store Optimization Program | Employee severance and other exit costs
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	$ 81