Summary of major accounting policies - narrative (Details) shares in Millions, $ in Millions		12 Months Ended
	Sep. 01, 2019 USD ($)	Aug. 31, 2020 USD ($) segment shares	Aug. 31, 2019 USD ($) shares	Aug. 31, 2018 USD ($) shares	Aug. 31, 2017 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of reportable segments | segment		3
Minimum number of days for settlement of credit and debit charges		1 day
Maximum number of days for settlement of credit and debit charges		7 days
Credit and debit card receivables		$ 101	$ 90
Restricted cash (included in other current assets)		230	184
Other cash flows from operating activities		(464)	(302)	$ (296)
Asset impairment charges		462	328	240
Gains on sale-leaseback transactions		308
Allowance for doubtful accounts		61	95
Inventories		9,451	9,333
Depreciation and amortization expense		$ 1,500	1,500	1,400
Term of renewal contract		5 years
Advertising expense		$ 534	585	665
Impairment of definite-lived assets		$ 2,016	$ 0	$ 0
Expected performance goal achievement rate		100.00%
Outstanding options to purchase common shares excluded from earnings per share calculations (in shares) | shares		19.0	14.9	10.1
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201602Member			us-gaap:AccountingStandardsUpdate201707Member
Operating lease liability	$ 24,000	$ 24,399
Operating lease right-of-use asset	22,000	21,724	$ 0
Cumulative effect adjustment to decrease retained earnings		(21,136)	(24,152)	$ (26,689)	$ (28,274)
Impaired long-lived assets	800
De-recognition of deferred gains on historical sale and leaseback transactions	400
Trade Accounts Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable		6,000	6,000
Other Accounts Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable		$ 1,200	1,200
Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Term of lease		15 years
Highly effective hedging percentage		80.00%
Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Term of lease		25 years
Highly effective hedging percentage		125.00%
Capitalized system development costs and software
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amortization of capitalized system development costs and software		$ 314	273	254
Unamortized capitalized software costs		$ 1,700	1,500
Capitalized system development costs and software | Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Estimated useful life of assets		3 years
Capitalized system development costs and software | Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Estimated useful life of assets		10 years
Selling, general and administrative expense
Accounts, Notes, Loans and Financing Receivable [Line Items]
Impairment of definite-lived assets		$ 422	260	57
Retail Pharmacy USA | Reportable Segments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Inventories		6,400	6,600
LIFO reserve		3,300	3,200
Retail Pharmacy International and Pharmaceutical Wholesale | Reportable Segments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Inventories		3,000	2,700
Retained earnings
Accounts, Notes, Loans and Financing Receivable [Line Items]
Cumulative effect adjustment to decrease retained earnings		$ (34,210)	(35,815)	(33,551)	$ (30,137)
Cumulative effect, period of adoption, adjustment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Cumulative effect adjustment to decrease retained earnings			442	88
Cumulative effect, period of adoption, adjustment | Retained earnings
Accounts, Notes, Loans and Financing Receivable [Line Items]
Cumulative effect adjustment to decrease retained earnings	$ 400		$ 442	$ 88