Summary of Significant Accounting Policies - Treasury Stock (Details) - Treasury Stock - USD ($) $ in Millions	12 Months Ended
	May 31, 2021	May 31, 2020	May 31, 2019
Total increase (decrease) in treasury stock	$ 8.6	$ (5.0)	$ 7.0
Repurchase of treasury shares	5.6	4.1	10.3
Restricted stock activity	5.6	0.8	(0.8)
Re-issuance of treasury stock upon exercise of stock options, net of shares withheld to satisfy statutory tax obligations	$ 3.0	$ 8.3	$ 4.1