Income taxes - reconciliation to effective tax rate (Details)	12 Months Ended
	Aug. 31, 2019	Aug. 31, 2018	Aug. 31, 2017
Income Tax Disclosure [Abstract]
Federal statutory rate	21.00%	25.70%	35.00%
State income taxes, net of federal benefit	0.90%	2.30%	0.30%
Foreign income taxed at non-U.S. rates	(2.10%)	(12.20%)	(11.80%)
Non-taxable income	(3.50%)	(5.20%)	(5.30%)
Non-deductible expenses	0.50%	2.10%	1.50%
Transition tax	0.00%	12.40%	0.00%
Tax law changes	(0.40%)	(10.90%)	(1.60%)
Change in valuation allowance1	1.90%	8.70%	0.70%
Tax credits	(4.80%)	(6.90%)	(2.90%)
Other	(0.50%)	0.70%	(0.20%)
Effective income tax rate	13.00%	16.70%	15.70%