Treasury Stock - Schedule of repurchase of common stock (Details) - USD ($) $ in Millions	3 Months Ended	25 Months Ended
	Aug. 31, 2017	Aug. 31, 2017	Jul. 22, 2015
Stockholders' Equity Attributable to Parent [Abstract]
Additional authorization July 2015			$ 50.0
Less repurchases made under the authorization as of August 31, 2017	$ (4.7)	$ (16.1)
Remaining Board authorization at August 31, 2017	$ 33.9	$ 33.9