Exit and disposal activities - restructuring costs (Details) - USD ($) $ in Millions	12 Months Ended
	Aug. 31, 2019	Aug. 31, 2018
Transformational Cost Management Program
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	$ 432
Transformational Cost Management Program | Operating Segments | Retail Pharmacy USA
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	147
Transformational Cost Management Program | Operating Segments | Retail Pharmacy International
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	130
Transformational Cost Management Program | Operating Segments | Pharmaceutical Wholesale
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	154
Transformational Cost Management Program | Lease obligations and other real estate costs
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	25
Transformational Cost Management Program | Lease obligations and other real estate costs | Operating Segments | Retail Pharmacy USA
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	5
Transformational Cost Management Program | Lease obligations and other real estate costs | Operating Segments | Retail Pharmacy International
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	19
Transformational Cost Management Program | Lease obligations and other real estate costs | Operating Segments | Pharmaceutical Wholesale
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	1
Transformational Cost Management Program | Asset impairments
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	260
Transformational Cost Management Program | Asset impairments | Operating Segments | Retail Pharmacy USA
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	95
Transformational Cost Management Program | Asset impairments | Operating Segments | Retail Pharmacy International
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	67
Transformational Cost Management Program | Asset impairments | Operating Segments | Pharmaceutical Wholesale
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	98
Transformational Cost Management Program | Employee severance and business transition costs
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	125
Transformational Cost Management Program | Employee severance and business transition costs | Operating Segments | Retail Pharmacy USA
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	42
Transformational Cost Management Program | Employee severance and business transition costs | Operating Segments | Retail Pharmacy International
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	34
Transformational Cost Management Program | Employee severance and business transition costs | Operating Segments | Pharmaceutical Wholesale
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	49
Transformational Cost Management Program | Information technology transformation and other exit costs
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	22
Transformational Cost Management Program | Information technology transformation and other exit costs | Operating Segments | Retail Pharmacy USA
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	5
Transformational Cost Management Program | Information technology transformation and other exit costs | Operating Segments | Retail Pharmacy International
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	10
Transformational Cost Management Program | Information technology transformation and other exit costs | Operating Segments | Pharmaceutical Wholesale
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	7
Store Optimization Program
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	196	$ 100
Store Optimization Program | Lease obligations and other real estate costs
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	119	19
Store Optimization Program | Employee severance and business transition costs
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs	$ 77	81
Cost Transformation Program
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs		835
Cost Transformation Program | Operating Segments | Retail Pharmacy USA
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs		731
Cost Transformation Program | Operating Segments | Retail Pharmacy International
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs		67
Cost Transformation Program | Operating Segments | Pharmaceutical Wholesale
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs		37
Cost Transformation Program | Asset impairments
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs		295
Cost Transformation Program | Asset impairments | Operating Segments | Retail Pharmacy USA
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs		272
Cost Transformation Program | Asset impairments | Operating Segments | Retail Pharmacy International
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs		21
Cost Transformation Program | Asset impairments | Operating Segments | Pharmaceutical Wholesale
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs		2
Cost Transformation Program | Employee severance and business transition costs
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs		168
Cost Transformation Program | Employee severance and business transition costs | Operating Segments | Retail Pharmacy USA
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs		87
Cost Transformation Program | Employee severance and business transition costs | Operating Segments | Retail Pharmacy International
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs		46
Cost Transformation Program | Employee severance and business transition costs | Operating Segments | Pharmaceutical Wholesale
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs		35
Cost Transformation Program | Real estate costs
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs		372
Cost Transformation Program | Real estate costs | Operating Segments | Retail Pharmacy USA
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs		372
Cost Transformation Program | Real estate costs | Operating Segments | Retail Pharmacy International
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs		0
Cost Transformation Program | Real estate costs | Operating Segments | Pharmaceutical Wholesale
Restructuring Reserve Disclosures [Abstract]
Total restructuring costs		$ 0