Exit and disposal activities - Restructuring Costs (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	May 31, 2020	May 31, 2019	May 31, 2020	May 31, 2019
Transformational Cost Management Program
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	$ 294	$ 56	$ 467	$ 235
Transformational Cost Management Program | Reportable Segments | Retail Pharmacy USA
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	261	13	371	29
Transformational Cost Management Program | Reportable Segments | Retail Pharmacy International
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	27	21	80	88
Transformational Cost Management Program | Reportable Segments | Pharmaceutical Wholesale
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	5	22	17	119
Transformational Cost Management Program | Lease obligations and other real estate costs
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	173		184
Transformational Cost Management Program | Lease obligations and other real estate costs | Reportable Segments | Retail Pharmacy USA
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	170		179
Transformational Cost Management Program | Lease obligations and other real estate costs | Reportable Segments | Retail Pharmacy International
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	3		4
Transformational Cost Management Program | Lease obligations and other real estate costs | Reportable Segments | Pharmaceutical Wholesale
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	0		1
Transformational Cost Management Program | Asset impairments
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	30	32	45	149
Transformational Cost Management Program | Asset impairments | Reportable Segments | Retail Pharmacy USA
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	19	5	31	5
Transformational Cost Management Program | Asset impairments | Reportable Segments | Retail Pharmacy International
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	10	16	13	48
Transformational Cost Management Program | Asset impairments | Reportable Segments | Pharmaceutical Wholesale
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	1	11	1	96
Transformational Cost Management Program | Employee severance and business transition costs
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	57	24	168	86
Transformational Cost Management Program | Employee severance and business transition costs | Reportable Segments | Retail Pharmacy USA
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	51	7	124	24
Transformational Cost Management Program | Employee severance and business transition costs | Reportable Segments | Retail Pharmacy International
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	2	5	32	39
Transformational Cost Management Program | Employee severance and business transition costs | Reportable Segments | Pharmaceutical Wholesale
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	3	11	12	22
Transformational Cost Management Program | Information technology transformation and other exit costs
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	34		70
Transformational Cost Management Program | Information technology transformation and other exit costs | Reportable Segments | Retail Pharmacy USA
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	21		37
Transformational Cost Management Program | Information technology transformation and other exit costs | Reportable Segments | Retail Pharmacy International
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	13		30
Transformational Cost Management Program | Information technology transformation and other exit costs | Reportable Segments | Pharmaceutical Wholesale
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	0		2
Store Optimization Program
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	10	49	49	99
Store Optimization Program | Lease obligations and other real estate costs
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	3	44	24	45
Store Optimization Program | Employee severance and other exit costs
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	$ 7	$ 5	$ 25	$ 54