Summary of major accounting policies - additional information (Details) shares in Millions, $ in Millions	12 Months Ended
	Aug. 31, 2018 USD ($) segment shares	Aug. 31, 2017 USD ($) shares	Aug. 31, 2016 USD ($) shares
Accounting Policies [Abstract]
Number of reportable segments | segment	3
Cash and Cash Equivalents [Abstract]
Minimum number of days for settlement of credit and debit charges	2 days
Maximum number of days for settlement of credit and debit charges	7 days
Credit and debit card receivables	$ 127	$ 98
Restricted Cash [Abstract]
Restricted cash	$ 190	202
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interval period of renewal options	5 years
Allowance for doubtful accounts	$ 75	158
Advertising Costs [Abstract]
Advertising Expense	$ 665	$ 571	$ 598
Net earnings per common share:
Outstanding options to purchase common shares excluded from earnings per share calculations (in shares) | shares	10.1	3.9	2.5
Trade Accounts Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	$ 5,400	$ 5,500
Other Accounts Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	$ 1,200	$ 1,100
Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Initial term of operating lease	15 years
Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Initial term of operating lease	25 years