STOCK-BASED COMPENSATION - Unrecognized Compensation Expense (Details) $ in Millions	9 Months Ended
	Feb. 28, 2021 USD ($)
Unrecognized compensation expense, net of estimated forfeitures
Unrecognized compensation expense	$ 32.9
Remaining weighted average recognition period	1 year 9 months 18 days
Stock-settled restricted stock units
Unrecognized compensation expense, net of estimated forfeitures
Unrecognized compensation expense	$ 21.5
Remaining weighted average recognition period	1 year 9 months 18 days
Performance shares
Unrecognized compensation expense, net of estimated forfeitures
Unrecognized compensation expense	$ 11.4
Remaining weighted average recognition period	1 year 9 months 18 days