Summary of Significant Accounting Policies - Treasury Stock (Details) - Treasury Stock - USD ($) $ in Millions	12 Months Ended
	May 31, 2018	May 31, 2017	May 31, 2016
Total increase in treasury stock	$ 0.9	$ 12.2	$ 21.3
Repurchase of treasury shares	13.1	19.8	18.8
Change in treasury stock due to restricted stock grants	(1.0)	(1.3)	(4.8)
Change in treasury stock due to exercise of stock options, net of shares withheld to satisfy statutory tax obligations	$ 11.2	$ 8.9	$ 2.3