Income taxes - reconciliation to effective tax rate (Details)	12 Months Ended
	Aug. 31, 2018	Aug. 31, 2017	Aug. 31, 2016
Income Tax Disclosure [Abstract]
Federal statutory rate	25.70%	35.00%	35.00%
State income taxes, net of federal benefit	2.30%	0.30%	0.80%
Foreign income taxed at non-U.S. rates	(12.20%)	(11.80%)	(7.80%)
Non-taxable income	(5.20%)	(5.30%)	(4.40%)
Non-deductible expenses	2.10%	1.50%	1.10%
Transition tax	12.40%	0.00%	0.00%
Tax law changes	(10.90%)	(1.60%)	(3.50%)
Change in valuation allowance	8.70%	0.70%	1.70%
Tax credits	(6.90%)	(2.90%)	(1.50%)
Other	0.70%	(0.20%)	(2.00%)
Effective income tax rate	16.70%	15.70%	19.40%