Exit and disposal activities - narrative (Details) $ in Millions			1 Months Ended	12 Months Ended
	Oct. 12, 2021 USD ($) store	Dec. 20, 2018 USD ($)	Sep. 30, 2023 market clinic	Aug. 31, 2023 USD ($) store	Aug. 31, 2022 USD ($)	Aug. 31, 2021 USD ($)	Aug. 31, 2020 USD ($)	Sep. 01, 2019 USD ($)
Restructuring Cost and Reserve [Line Items]
Cumulative effect adjustment to decrease retained earnings				$ (28,322)	$ (29,366)	$ (23,822)	$ (21,136)
Adoption of new accounting standards
Restructuring Cost and Reserve [Line Items]
Cumulative effect adjustment to decrease retained earnings							6
Retained earnings
Restructuring Cost and Reserve [Line Items]
Cumulative effect adjustment to decrease retained earnings				(33,058)	(37,801)	(35,121)	(34,210)
Retained earnings | Adoption of new accounting standards
Restructuring Cost and Reserve [Line Items]
Cumulative effect adjustment to decrease retained earnings							$ 3
Transformational cost management program
Restructuring Cost and Reserve [Line Items]
Expected annual cost savings of restructuring plan				4,500	3,500
Costs incurred				3,100
Restructuring costs				$ 1,158	690	335
Transformational cost management program | Subsequent event
Restructuring Cost and Reserve [Line Items]
Number of clinics closed | clinic			60
Number of markets exited | market			5
Transformational cost management program | United Kingdom
Restructuring Cost and Reserve [Line Items]
Number of stores expected to close | store	350			300
Number of stores closed | store				291
Transformational cost management program | United States
Restructuring Cost and Reserve [Line Items]
Number of stores expected to close | store				200
Number of stores closed | store				466
Transformational cost management program | Retained earnings | Adoption of new accounting standards
Restructuring Cost and Reserve [Line Items]
Cumulative effect adjustment to decrease retained earnings								$ 508
Lease obligations and other real estate costs | Transformational cost management program
Restructuring Cost and Reserve [Line Items]
Costs incurred				$ 1,100
Restructuring costs				516	249	108
Asset impairments | Transformational cost management program
Restructuring Cost and Reserve [Line Items]
Costs incurred				883
Restructuring costs				441	190	24
Employee severance and business transition costs | Transformational cost management program
Restructuring Cost and Reserve [Line Items]
Costs incurred				873
Restructuring costs				150	210	165
Information technology transformation and other exit costs | Transformational cost management program
Restructuring Cost and Reserve [Line Items]
Costs incurred				253
Restructuring costs				51	40	$ 38
Minimum | Transformational cost management program
Restructuring Cost and Reserve [Line Items]
Expected annual cost savings of restructuring plan	$ 3,300	$ 2,000
Expected restructuring costs				4,100	3,600
Minimum | Transformational cost management program | United States
Restructuring Cost and Reserve [Line Items]
Number of stores expected to close | store	450
Minimum | Exit and disposal costs | Transformational cost management program
Restructuring Cost and Reserve [Line Items]
Expected restructuring costs				3,800	3,300
Maximum | Transformational cost management program
Restructuring Cost and Reserve [Line Items]
Expected annual cost savings of restructuring plan					3,500
Expected restructuring costs				4,400	3,900
Maximum | Transformational cost management program | United States
Restructuring Cost and Reserve [Line Items]
Number of stores expected to close | store	500
Maximum | Exit and disposal costs | Transformational cost management program
Restructuring Cost and Reserve [Line Items]
Expected restructuring costs				$ 4,100	$ 3,600