New accounting pronouncements - Narrative (Details) - USD ($) $ in Millions	Sep. 01, 2019	Feb. 29, 2020	Dec. 01, 2019	Aug. 31, 2019	Sep. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease liability		$ 24,237
Operating lease right-of-use assets		$ 21,755		$ 0
Cumulative effect of new accounting principle in period of adoption	$ (442)		$ 0		$ (88)
Term of renewal contract		5 years
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease liability	24,000
Operating lease right-of-use assets	22,000
Impaired long-lived assets	800
De-recognition of deferred gains on historical sale and leaseback transactions	400
Retained earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption	(442)		$ 0		$ (88)
Retained earnings | Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption	$ (400)
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