STOCK-BASED COMPENSATION - Unrecognized Compensation Expense (Details) $ in Millions	12 Months Ended
	May 31, 2020 USD ($)
Unrecognized compensation expense, net of estimated forfeitures
Unrecognized compensation expense	$ 23.6
Remaining weighted average recognition period	1 year 7 months 6 days
Stock-settled restricted stock units
Unrecognized compensation expense, net of estimated forfeitures
Unrecognized compensation expense	$ 15.9
Remaining weighted average recognition period	1 year 8 months 12 days
Performance shares
Unrecognized compensation expense, net of estimated forfeitures
Unrecognized compensation expense	$ 7.7
Remaining weighted average recognition period	1 year 6 months