Exit and disposal activities - narrative (Details) $ in Millions				1 Months Ended				12 Months Ended
	Oct. 15, 2020 USD ($)	Dec. 20, 2018 USD ($)	Oct. 24, 2017 store	Jul. 31, 2020 store	Oct. 31, 2019 USD ($)	Apr. 30, 2019 USD ($)	Mar. 31, 2018 store	Aug. 31, 2020 USD ($) store	Sep. 01, 2019 USD ($)	Aug. 31, 2019 USD ($)	Aug. 31, 2018 USD ($)	Aug. 31, 2017 USD ($)
Restructuring Cost and Reserve [Line Items]
Cumulative effect adjustment to decrease retained earnings								$ (21,136)		$ (24,152)	$ (26,689)	$ (28,274)
Cumulative effect, period of adoption, adjustment
Restructuring Cost and Reserve [Line Items]
Cumulative effect adjustment to decrease retained earnings										442	88
Retained earnings
Restructuring Cost and Reserve [Line Items]
Cumulative effect adjustment to decrease retained earnings								(34,210)		(35,815)	(33,551)	$ (30,137)
Retained earnings | Cumulative effect, period of adoption, adjustment
Restructuring Cost and Reserve [Line Items]
Cumulative effect adjustment to decrease retained earnings									$ 400	$ 442	$ 88
Transformational cost management program
Restructuring Cost and Reserve [Line Items]
Costs incurred								$ 1,200
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				250
Transformational cost management program | Retained earnings | Cumulative effect, period of adoption, adjustment
Restructuring Cost and Reserve [Line Items]
Cumulative effect adjustment to decrease retained earnings									$ 508
Store optimization program
Restructuring Cost and Reserve [Line Items]
Number of stores closed | store								769
Number of stores expected to close | store			600				750
Costs incurred								$ 349
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								242
Lease obligations and other real estate costs | Store optimization program
Restructuring Cost and Reserve [Line Items]
Costs incurred								160
Asset impairments | Transformational cost management program
Restructuring Cost and Reserve [Line Items]
Costs incurred								350
Employee severance and business transition costs | Transformational cost management program
Restructuring Cost and Reserve [Line Items]
Costs incurred								420
Information technology transformation and other exit costs | Transformational cost management program
Restructuring Cost and Reserve [Line Items]
Costs incurred								140
Employee severance and other exit costs | Store optimization program
Restructuring Cost and Reserve [Line Items]
Costs incurred								189
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 restructuring costs								2,100
Minimum | Transformational cost management program | Forecast
Restructuring Cost and Reserve [Line Items]
Expected annual cost savings of restructuring plan	$ 2,000
Minimum | Exit and disposal costs | Transformational cost management program
Restructuring Cost and Reserve [Line Items]
Expected restructuring costs								1,800
Maximum | Transformational cost management program
Restructuring Cost and Reserve [Line Items]
Expected restructuring costs								2,400
Maximum | Exit and disposal costs | Transformational cost management program
Restructuring Cost and Reserve [Line Items]
Expected restructuring costs								$ 2,100