Income taxes - reconciliation to effective tax rate (Details)	12 Months Ended
	Aug. 31, 2020	Aug. 31, 2019	Aug. 31, 2018
Income Tax Disclosure [Abstract]
Federal statutory rate	21.00%	21.00%	25.70%
State income taxes, net of federal benefit	5.30%	0.90%	2.30%
Foreign income taxed at non-U.S. rates	(12.00%)	(2.10%)	(12.20%)
Non-taxable income	(28.50%)	(3.50%)	(5.20%)
Non-deductible expenses	5.90%	0.50%	2.10%
Transition tax	0.00%	0.00%	12.40%
Tax law changes	18.80%	(0.40%)	(10.90%)
Change in valuation allowance	1.20%	1.90%	8.70%
Goodwill impairment	43.50%	0.00%	0.00%
Tax credits	(9.80%)	(4.80%)	(6.90%)
Other	3.10%	(0.50%)	0.70%
Effective income tax rate	48.50%	13.00%	16.70%