Exit and disposal activities - Restructuring costs and reserves (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended	16 Months Ended
	Feb. 28, 2019	Feb. 28, 2019	Feb. 28, 2019
Transformational Cost Management Program
Restructuring Cost and Reserve [Line Items]
Restructuring costs	$ 150	$ 179
Transformational Cost Management Program | Severance and other business transition and exit costs
Restructuring Cost and Reserve [Line Items]
Restructuring costs	39	62
Transformational Cost Management Program | Asset impairments
Restructuring Cost and Reserve [Line Items]
Restructuring costs	111	117
Transformational Cost Management Program | Retail Pharmacy USA
Restructuring Cost and Reserve [Line Items]
Restructuring costs	14	16
Transformational Cost Management Program | Retail Pharmacy USA | Severance and other business transition and exit costs
Restructuring Cost and Reserve [Line Items]
Restructuring costs	14	16
Transformational Cost Management Program | Retail Pharmacy USA | Asset impairments
Restructuring Cost and Reserve [Line Items]
Restructuring costs	0	0
Transformational Cost Management Program | Retail Pharmacy International
Restructuring Cost and Reserve [Line Items]
Restructuring costs	40	67
Transformational Cost Management Program | Retail Pharmacy International | Severance and other business transition and exit costs
Restructuring Cost and Reserve [Line Items]
Restructuring costs	14	35
Transformational Cost Management Program | Retail Pharmacy International | Asset impairments
Restructuring Cost and Reserve [Line Items]
Restructuring costs	26	32
Transformational Cost Management Program | Pharmaceutical Wholesale
Restructuring Cost and Reserve [Line Items]
Restructuring costs	96	96
Transformational Cost Management Program | Pharmaceutical Wholesale | Severance and other business transition and exit costs
Restructuring Cost and Reserve [Line Items]
Restructuring costs	11	11
Transformational Cost Management Program | Pharmaceutical Wholesale | Asset impairments
Restructuring Cost and Reserve [Line Items]
Restructuring costs	85	85
Store Optimization Program
Restructuring Cost and Reserve [Line Items]
Restructuring costs	31	51	$ 150
Store Optimization Program | Severance and other business transition and exit costs
Restructuring Cost and Reserve [Line Items]
Restructuring costs	22	49	130
Store Optimization Program | Lease obligations and other real estate costs
Restructuring Cost and Reserve [Line Items]
Restructuring costs	$ 9	$ 2	$ 21