Treasury Stock - Schedule of repurchase of common stock (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended	28 Months Ended
	Nov. 30, 2017	Nov. 30, 2017	Nov. 30, 2017	Jul. 22, 2015
Stockholders' Equity Attributable to Parent [Abstract]
Additional authorization July 2015				$ 50.0
Less repurchases made under the authorization as of November 30, 2017	$ (8.6)	$ (13.3)	$ (24.7)
Remaining Board authorization at November 30, 2017	$ 25.3	$ 25.3	$ 25.3