New accounting pronouncements - Narrative (Details) - USD ($) $ in Millions	Sep. 01, 2019	May 31, 2020	Feb. 29, 2020	Aug. 31, 2019	May 31, 2019	Feb. 28, 2019	Aug. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201602Member
Operating lease liability	$ 24,000	$ 24,231
Operating lease right-of-use assets	22,000	21,609		$ 0
Cumulative effect adjustment		$ 21,323	$ 24,334	24,152	$ 24,771	$ 25,413	$ 26,689
Impaired long-lived assets	800
De-recognition of deferred gains on historical sale and leaseback transactions	400
Term of renewal contract		5 years
Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment			0	(442)			(88)
Retained earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment		$ 34,244	36,351	35,815	$ 35,547	$ 34,928	33,551
Retained earnings | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment	$ (400)		$ 0	$ (442)			$ (88)
Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Term of lease		15 years
Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Term of lease		25 years