Exit and disposal activities - restructuring costs (Details) $ in Millions	12 Months Ended
	Aug. 31, 2017 USD ($) store	Aug. 31, 2016 USD ($)	Aug. 31, 2015 USD ($)
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	$ 835	$ 424	$ 559
Operating Segments | Retail Pharmacy USA
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	731	374	540
Operating Segments | Retail Pharmacy International
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	67	29	19
Operating Segments | Pharmaceutical Wholesale
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	37	21	0
Asset impairments
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	295	225	223
Asset impairments | Operating Segments | Retail Pharmacy USA
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	272	215	216
Asset impairments | Operating Segments | Retail Pharmacy International
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	21	10	7
Asset impairments | Operating Segments | Pharmaceutical Wholesale
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	2	0	0
Real estate costs
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	372	91	219
Real estate costs | Operating Segments | Retail Pharmacy USA
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	372	89	219
Real estate costs | Operating Segments | Retail Pharmacy International
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	0	1	0
Real estate costs | Operating Segments | Pharmaceutical Wholesale
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	0	1	0
Severance and other business transition and exit costs
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	168	108	117
Severance and other business transition and exit costs | Operating Segments | Retail Pharmacy USA
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	87	70	105
Severance and other business transition and exit costs | Operating Segments | Retail Pharmacy International
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	46	18	12
Severance and other business transition and exit costs | Operating Segments | Pharmaceutical Wholesale
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	$ 35	20	0
Cost Transformation Program
Restructuring Reserve Disclosures [Abstract]
Number of stores closed | store	255
Cumulative pre-tax charges	$ 1,800
Pre-tax charges incurred during the period	835	$ 424	$ 542
Cost Transformation Program | Asset impairments
Restructuring Reserve Disclosures [Abstract]
Cumulative pre-tax charges	743
Cost Transformation Program | Real estate costs
Restructuring Reserve Disclosures [Abstract]
Cumulative pre-tax charges	665
Cost Transformation Program | Severance and other business transition and exit costs
Restructuring Reserve Disclosures [Abstract]
Cumulative pre-tax charges	$ 393