Exit and disposal activities - Narrative (Details) $ in Millions				1 Months Ended			9 Months Ended
	Jul. 09, 2020 USD ($)	Dec. 20, 2018 USD ($)	Oct. 24, 2017 store	Oct. 31, 2019 USD ($)	Apr. 30, 2019 USD ($)	Mar. 31, 2018 USD ($)	May 31, 2020 USD ($) store	Apr. 30, 2020 USD ($)	Feb. 29, 2020 USD ($)	Sep. 01, 2019 USD ($)	Aug. 31, 2019 USD ($)	May 31, 2019 USD ($)	Feb. 28, 2019 USD ($)	Aug. 31, 2018 USD ($)
Restructuring Cost and Reserve [Line Items]
Number of stores expected to close | store			600				750
Cumulative effect adjustment							$ 21,323		$ 24,334		$ 24,152	$ 24,771	$ 25,413	$ 26,689
Cumulative Effect, Period of Adoption, Adjustment
Restructuring Cost and Reserve [Line Items]
Cumulative effect adjustment									0		(442)			(88)
Retained earnings
Restructuring Cost and Reserve [Line Items]
Cumulative effect adjustment							34,244		36,351		35,815	$ 35,547	$ 34,928	33,551
Retained earnings | Cumulative Effect, Period of Adoption, Adjustment
Restructuring Cost and Reserve [Line Items]
Cumulative effect adjustment									$ 0	$ (400)	$ (442)			$ (88)
Transformational Cost Management Program
Restructuring Cost and Reserve [Line Items]
Costs incurred							$ 899
Transformational Cost Management Program | UNITED KINGDOM
Restructuring Cost and Reserve [Line Items]
Number of stores expected to close | store							200
Transformational Cost Management Program | UNITED STATES
Restructuring Cost and Reserve [Line Items]
Number of stores expected to close | store							200
Transformational Cost Management Program | Retained earnings | Cumulative Effect, Period of Adoption, Adjustment
Restructuring Cost and Reserve [Line Items]
Cumulative effect adjustment										$ (508)
Store Optimization Program
Restructuring Cost and Reserve [Line Items]
Expected cost							$ 375	$ 400
Costs incurred							345
Cost Transformation Program
Restructuring Cost and Reserve [Line Items]
Decrease in restructuring reserve							382
Lease obligations and other real estate costs | Transformational Cost Management Program
Restructuring Cost and Reserve [Line Items]
Costs incurred							209
Lease obligations and other real estate costs | Store Optimization Program
Restructuring Cost and Reserve [Line Items]
Expected cost							185
Costs incurred							162
Asset impairments | Transformational Cost Management Program
Restructuring Cost and Reserve [Line Items]
Costs incurred							305
Employee severance and business transition costs | Transformational Cost Management Program
Restructuring Cost and Reserve [Line Items]
Costs incurred							293
Information technology transformation and other exit costs | Transformational Cost Management Program
Restructuring Cost and Reserve [Line Items]
Costs incurred							92
Employee severance and other exit costs | Store Optimization Program
Restructuring Cost and Reserve [Line Items]
Expected cost							190
Costs incurred							183
Minimum | Transformational Cost Management Program
Restructuring Cost and Reserve [Line Items]
Expected annual cost savings of restructuring plan		$ 1,000		$ 1,800	$ 1,500
Expected cost							2,100
Minimum | Transformational Cost Management Program | Forecast
Restructuring Cost and Reserve [Line Items]
Expected annual cost savings of restructuring plan	$ 2,000
Minimum | Store Optimization Program
Restructuring Cost and Reserve [Line Items]
Expected annual cost savings of restructuring plan						$ 350
Minimum | Exit and disposal costs | Transformational Cost Management Program
Restructuring Cost and Reserve [Line Items]
Expected cost							1,800
Maximum | Transformational Cost Management Program
Restructuring Cost and Reserve [Line Items]
Expected cost							2,400
Maximum | Exit and disposal costs | Transformational Cost Management Program
Restructuring Cost and Reserve [Line Items]
Expected cost							$ 2,100