Stock-Based Compensation (Details) $ / shares in Units, $ in Millions	12 Months Ended
	May. 31, 2015 USD ($) item $ / shares shares	May. 31, 2014 USD ($) $ / shares shares	May. 31, 2013 USD ($) $ / shares shares
Share-Based Compensation
Total number of shares granted since inception	11,149,000
2013 Stock Plan
Share-Based Compensation
Shares available for future grant	2,180,000
Stock options
Share-Based Compensation
Expiration term	10 years
First number of equal annual increments in which stock options are exercisable | item	3
First number of equal annual increments in which stock options are exercisable | item	4
First number of equal annual increments in which stock options are exercisable | item	5
Commencement period after the date of grant for annual increments of stock options becoming exercisable	1 year
Granted (in shares)	0
Weighted average fair value of stock options granted (in dollars per share) | $ / shares		$ 10.24	$ 4.85
Assumptions used in the Black-Scholes option pricing models to estimate the fair value of each stock option grant
Risk-free interest rate		1.40%	0.60%
Expected volatility of common stock		49.10%	51.40%
Dividend yield		1.20%	2.30%
Expected option term in years		5 years 2 months 12 days	5 years 4 months 24 days
Shares
Outstanding at beginning of year (in shares)	2,753,000	2,300,000	1,703,000
Granted (in shares)		1,033,015	972,180
Exercised (in shares)	(793,000)	(473,000)	(124,000)
Cancelled (in shares)	(103,000)	(107,000)	(251,000)
Outstanding at end of year (in shares)	1,857,000	2,753,000	2,300,000
Options exercisable at end of year (in shares)	627,000	1,019,000	1,089,000
Weighted Average Exercise Price
Outstanding at beginning of year (in dollars per share) | $ / shares	$ 19.59	$ 16.22	$ 17.96
Granted (in dollars per share) | $ / shares		25.43	12.95
Exercised (in dollars per share) | $ / shares	15.98	16.20	13.72
Cancelled (in dollars per share) | $ / shares	21.03	18.48	15.82
Outstanding at end of year (in dollars per share) | $ / shares	21.05	19.59	16.22
Options exercisable at end of year (in dollars per share) | $ / shares	$ 21.78	$ 17.88	$ 17.73
Stock options, additional disclosures
Total fair value of stock options vested | $	$ 3.1	$ 2.0	$ 3.6
Total intrinsic value of stock options exercised | $	11.1	4.3	0.5
Aggregate intrinsic value of options outstanding | $	16.0	15.2
Tax benefit realized from stock options exercised | $	2.4	1.0	0.0
Compensation expense | $	2.9	3.8	3.5
Unearned compensation not yet recognized | $	$ 6.7
Average remaining amortization period of unearned compensation	1 year 4 months 24 days
Restricted stock
Share-Based Compensation
Granted (in shares)	331,000
Stock options, additional disclosures
Compensation expense | $	$ 4.9	$ 4.9	$ 7.6
Unearned compensation not yet recognized | $	$ 8.5
Average remaining amortization period of unearned compensation	1 year 3 months 18 days
Restricted stock | 2013 Stock Plan
Share-Based Compensation
Granted (in shares)	331,000
Restricted stock | Minimum
Share-Based Compensation
Vesting period	1 year
Restricted stock | Maximum
Share-Based Compensation
Vesting period	5 years
Performance-based restricted stock | Minimum
Share-Based Compensation
Vesting period	3 years
Performance-based restricted stock | Maximum
Share-Based Compensation
Vesting period	6 years
Time-based restricted stock | Board of Directors
Share-Based Compensation
Granted (in shares)	45,000	45,000	48,333