Exit and disposal activities - Narrative (Details) $ in Millions			1 Months Ended		6 Months Ended
	Dec. 20, 2018 USD ($)	Oct. 24, 2017 store	Oct. 31, 2019 USD ($)	Apr. 30, 2019 USD ($)	Feb. 29, 2020 USD ($) store	Dec. 01, 2019 USD ($)	Sep. 01, 2019 USD ($)	Sep. 01, 2018 USD ($)
Restructuring Cost and Reserve [Line Items]
Number of stores expected to close | store		600			750
Adoption of new accounting standards						$ 0	$ (442)	$ (88)
Retained earnings
Restructuring Cost and Reserve [Line Items]
Adoption of new accounting standards						$ 0	(442)	$ (88)
Transformational Cost Management Program
Restructuring Cost and Reserve [Line Items]
Costs incurred					$ 605
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
Restructuring Cost and Reserve [Line Items]
Adoption of new accounting standards							$ (508)
Store Optimization Program
Restructuring Cost and Reserve [Line Items]
Expected cost					$ 400
Costs incurred					335
Store Optimization Program | Previously Reported
Restructuring Cost and Reserve [Line Items]
Expected cost					350
Cost Transformation Program
Restructuring Cost and Reserve [Line Items]
Increase (decrease) in restructuring reserve					382
Lease obligations and other real estate costs | Transformational Cost Management Program
Restructuring Cost and Reserve [Line Items]
Costs incurred					36
Lease obligations and other real estate costs | Store Optimization Program
Restructuring Cost and Reserve [Line Items]
Expected cost					190
Costs incurred					159
Asset impairment | Transformational Cost Management Program
Restructuring Cost and Reserve [Line Items]
Costs incurred					275
Employee severance and business transition costs | Transformational Cost Management Program
Restructuring Cost and Reserve [Line Items]
Costs incurred					236
Information technology transformation and other exit costs | Transformational Cost Management Program
Restructuring Cost and Reserve [Line Items]
Costs incurred					58
Employee severance and other exit costs | Store Optimization Program
Restructuring Cost and Reserve [Line Items]
Expected cost					210
Costs incurred					176
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					1,900
Minimum | Exit and disposal costs | Transformational Cost Management Program
Restructuring Cost and Reserve [Line Items]
Expected cost					1,600
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					$ 2,000