Operating leases (Details) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Feb. 28, 2018 USD ($) lease	Feb. 28, 2017 USD ($)	Feb. 28, 2018 USD ($) lease	Feb. 28, 2017 USD ($)	Aug. 31, 2017 USD ($)
Operating Leased Assets [Line Items]
Interval period of renewal options			5 years
Proceeds from sale-leaseback transactions			$ 0	$ 436
Charges related to facilities that were closed or relocated	$ 28	$ 247	$ 67	264
Number of leases | lease	72		72
Potential undiscounted future payments	$ 315		$ 315
Changes in reserve for facility closings and related lease termination charges [Roll Forward]
Balance at beginning of period			718	$ 466	$ 466
Provision for present value of non-cancellable lease payments on closed facilities			32		344
Changes in assumptions			(2)		13
Accretion expense			37		37
Cash payments, net of sublease income			(109)		(142)
Balance at end of period	$ 676		$ 676		$ 718
Minimum
Operating Leased Assets [Line Items]
Initial term of operating lease			15 years
Maximum
Operating Leased Assets [Line Items]
Initial term of operating lease			25 years