Equity (Details) $ / shares in Units, $ in Millions	12 Months Ended
	May 31, 2021 USD ($) item $ / shares shares	May 31, 2020 USD ($) $ / shares shares	May 31, 2019 USD ($) $ / shares shares
Stock-Based Compensation
Total number of shares granted since inception	11,149,000
2013 Stock Plan
Stock-Based Compensation
Shares available for future grant	1,850,828
Stock options
Stock-Based Compensation
Expiration term	10 years
First number of equal annual increments in which stock options are exercisable | item	3
Commencement period after the date of grant for annual increments of stock options becoming exercisable	1 year
Weighted average fair value of stock options granted (in dollars per share) | $ / shares	$ 5.89	$ 10.30	$ 13.60
Assumptions used in the Black-Scholes option pricing models to estimate the fair value of each stock option grant
Risk-free interest rate	0.40%	1.90%	2.70%
Expected volatility of common stock	40.20%	32.00%	30.80%
Dividend yield	1.60%	0.80%	0.60%
Expected option term in years	4 years 9 months 18 days	4 years 6 months	4 years 6 months
Shares
Outstanding at beginning of year (in shares)	1,851,000	1,777,000	2,082,000
Granted (in shares)	936,170	414,460	300,240
Exercised (in shares)	(110,000)	(300,000)	(571,000)
Cancelled (in shares)	(65,000)	(40,000)	(34,000)
Outstanding at end of year (in shares)	2,612,000	1,851,000	1,777,000
Options exercisable at end of year (in shares)	1,371,000	1,133,000	981,000
Weighted Average Exercise Price
Outstanding at beginning of year (in dollars per share) | $ / shares	$ 32.74	$ 30.37	$ 26.72
Granted (in dollars per share) | $ / shares	18.94	37.66	47.84
Exercised (in dollars per share) | $ / shares	23.93	24.99	25.56
Cancelled (in dollars per share) | $ / shares	25.59	36.72	42.02
Outstanding at end of year (in dollars per share) | $ / shares	28.34	32.74	30.37
Options exercisable at end of year (in dollars per share) | $ / shares	$ 31.56	$ 28.32	$ 25.28
Stock options, additional disclosures
Total fair value of stock options vested | $	$ 3.9	$ 3.7	$ 5.6
Total intrinsic value of stock options exercised | $	1.5	6.2	12.2
Aggregate intrinsic value of options outstanding | $	36.6	0.5
Tax expense (benefit) realized from stock options exercised | $	0.7	(2.1)	(2.7)
Unearned compensation not yet recognized | $	$ 5.4
Average remaining expensed period of unearned compensation	1 year 4 months 24 days
Stock options | Selling, general and administrative expenses
Stock options, additional disclosures
Compensation expense | $	$ 4.0	3.9	4.1
Restricted stock
Stock-Based Compensation
Granted (in shares)	290,000
Stock options, additional disclosures
Unearned compensation not yet recognized | $	$ 2.5
Average remaining expensed period of unearned compensation	2 years
Restricted stock | Selling, general and administrative expenses
Stock options, additional disclosures
Compensation expense | $	$ 5.2	$ 3.4	$ 9.4
Restricted stock | 2013 Stock Plan
Stock-Based Compensation
Granted (in shares)	5,058,230
Restricted stock | Minimum
Stock-Based Compensation
Vesting period	1 year
Restricted stock | Maximum
Stock-Based Compensation
Vesting period	3 years
Performance-based restricted stock
Stock-Based Compensation
Vesting period	3 years
Performance Based Restricted Stock | Executives and other key employees
Stock-Based Compensation
Granted (in shares)	0	52,475	43,680