Income taxes - unrecognized tax benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Aug. 31, 2020	Aug. 31, 2019	Aug. 31, 2018
Reconciliation of Unrecognized Tax Benefits
Balance at beginning of year	$ 455	$ 456	$ 409
Gross increases related to tax positions in a prior period	60	33	123
Gross decreases related to tax positions in a prior period	(23)	(53)	(15)
Gross increases related to tax positions in the current period	9	26	29
Settlements with taxing authorities	(4)	(2)	(87)
Lapse of statute of limitations	(3)	(5)	(3)
Balance at end of year	$ 494	$ 455	$ 456