Treasury Stock - Schedule of repurchase of common stock (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended	43 Months Ended
	Feb. 28, 2019	Nov. 30, 2018	Aug. 31, 2018	Feb. 28, 2018	Nov. 30, 2017	Aug. 31, 2017	Feb. 28, 2019	Feb. 28, 2019	Mar. 21, 2018	Jul. 22, 2015
Stockholders' Equity Attributable to Parent [Abstract]
Authorized amount of stock to be repurchased	$ 100.0						$ 100.0	$ 100.0	$ 50.0	$ 50.0
Total current Board authorizations	(2.0)	$ 0.0	$ 0.0	$ (11.9)	$ (8.6)	$ (4.7)	0.0	(40.6)
Less repurchases made under these authorizations	$ 59.4						$ 59.4	$ 59.4