Summary of Significant Accounting Policies - Treasury Stock (Details) - USD ($) $ in Millions	12 Months Ended
	May 31, 2020	May 31, 2019	May 31, 2018
Total increase (decrease) in treasury stock	$ (5.0)
Repurchase of treasury shares	4.1
Re-issuance of treasury stock upon exercise of stock options, net of shares withheld to satisfy statutory tax obligations	8.3
Treasury Stock
Total increase (decrease) in treasury stock		$ 7.0	$ 0.9
Repurchase of treasury shares	4.1	10.3	13.1
Restricted stock activity	0.8	(0.8)	1.0
Re-issuance of treasury stock upon exercise of stock options, net of shares withheld to satisfy statutory tax obligations	$ 8.3	$ 4.1	$ 11.2