Income taxes (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Feb. 28, 2018	Feb. 28, 2017	Feb. 28, 2018	Feb. 28, 2017
Income Tax Disclosure [Abstract]
Effective tax rate	27.40%	19.00%	25.40%	18.20%
Tax Cuts And Jobs Act Of 2017, Incomplete Accounting, Provisional Income Tax Expense (Benefit)			$ 184
Tax Cuts and Jobs Act of 2017, discrete tax benefit	$ (184)		610
Cash paid for income taxes			301	$ 537
Transition tax, provisional discrete tax expense			$ 794
Federal corporate tax rate			25.70%