Income taxes - reconciliation to effective tax rate (Details) - USD ($) $ in Millions	12 Months Ended
	Aug. 31, 2017	Aug. 31, 2016	Aug. 31, 2015
Income Tax Disclosure [Abstract]
Federal statutory rate	35.00%	35.00%	35.00%
State income taxes, net of federal benefit	0.30%	0.80%	1.30%
Deferred tax asset recognition	(0.00%)	(0.00%)	(4.10%)
Gain on previously held equity interest	0.00%	0.00%	(5.80%)
Foreign income taxed at non-U.S. rates	(11.80%)	(7.80%)	(6.20%)
Non-taxable income	(5.30%)	(4.40%)	(2.60%)
Non-deductible expenses	1.50%	1.10%	2.30%
Tax law changes	(1.60%)	(3.50%)	0.00%
Tax credits	(2.90%)	(1.50%)	(0.00%)
Other	0.50%	(0.30%)	0.00%
Effective income tax rate	15.70%	19.40%	19.90%
Remaining ownership subject to call option			55.00%
Nondeductible loss on exercise of call option			$ 866