Income taxes (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	May 31, 2018	May 31, 2017	May 31, 2018	May 31, 2017
Income Tax Disclosure [Abstract]
Effective tax rate	7.60%	12.40%	19.40%	16.20%
Tax Cuts and Jobs Act of 2017, provisional income tax expense (benefit)	$ (140)		$ 44
Cash paid for income taxes			428	$ 839
Transition tax, provisional discrete tax expense			679
Tax Cuts and Jobs Act of 2017, discrete tax benefit			$ 635