Treasury Stock - Schedule of repurchase of common stock (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	19 Months Ended
	Feb. 28, 2017	Feb. 28, 2017	Feb. 28, 2017	Jul. 22, 2015
Stockholders' Equity Attributable to Parent [Abstract]
Additional authorization July 2015				$ 50.0
Less repurchases made under the authorization as of February 28, 2017	$ 0.0	$ (6.0)	$ (10.5)
Remaining Board authorization at February 28, 2017	$ 39.5	$ 39.5	$ 39.5