Exit and disposal activities - Restructuring Costs (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Feb. 29, 2020	Feb. 28, 2019	Feb. 29, 2020	Feb. 28, 2019
Transformational Cost Management Program
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	$ 109	$ 150	$ 173	$ 179
Transformational Cost Management Program | Reportable Segments | Retail Pharmacy USA
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	60	14	110	16
Transformational Cost Management Program | Reportable Segments | Retail Pharmacy International
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	43	40	52	67
Transformational Cost Management Program | Reportable Segments | Pharmaceutical Wholesale
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	5	96	12	96
Transformational Cost Management Program | Lease obligations and other real estate costs
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	10		11
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	9		10
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	1		1
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		0
Transformational Cost Management Program | Asset impairment
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	3	111	15	117
Transformational Cost Management Program | Asset impairment | Reportable Segments | Retail Pharmacy USA
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	3	0	11	0
Transformational Cost Management Program | Asset impairment | Reportable Segments | Retail Pharmacy International
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	0	26	3	32
Transformational Cost Management Program | Asset impairment | Reportable Segments | Pharmaceutical Wholesale
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	0	85	0	85
Transformational Cost Management Program | Employee severance and business transition costs
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	72	39	111	62
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	39	14	72	16
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	29	14	30	35
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	4	11	9	11
Transformational Cost Management Program | Information technology transformation and other exit costs
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	24		36
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	9		16
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		17
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	1		2
Store Optimization Program
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	30	31	39	51
Store Optimization Program | Lease obligations and other real estate costs
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	18	9	21	2
Store Optimization Program | Employee severance and other exit costs
Restructuring Cost and Reserve [Line Items]
Total pre-tax exit and disposal costs	$ 12	$ 22	$ 18	$ 49