Income Taxes - Provision for Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Aug. 31, 2018	Aug. 31, 2017	Aug. 31, 2016
Income Tax Disclosure [Abstract]
Provision for current federal taxes	$ 88.9	$ 151.2	$ 139.6
Provision for current state taxes	16.4	20.4	17.6
Provision for current foreign taxes	9.2	7.0	5.1
Benefit for deferred taxes	(38.2)	(7.7)	(8.5)
Total provision for income taxes	$ 76.3	$ 170.9	$ 153.8