Stock-Based Compensation (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	May 31, 2014 item	May 31, 2013	May 31, 2012
Share-Based Compensation
Total number of shares granted since inception	11,149,000
Shares available for grant	2,500,000
Stock options
Share-Based Compensation
Expiration term	10 years
First number of equal annual increments in which stock options are exercisable	3
First number of equal annual increments in which stock options are exercisable	4
First number of equal annual increments in which stock options are exercisable	5
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)	$ 10.24	$ 4.85	$ 11.42
Assumptions used in the Black-Scholes option pricing models to estimate the fair value of each stock option grant
Risk-free interest rate (as a percent)	1.40%	0.60%	1.50%
Expected volatility of common stock (as a percent)	49.10%	51.40%	46.10%
Dividend yield (as a percent)	1.20%	2.30%	1.10%
Expected option term	5 years 2 months 12 days	5 years 4 months 24 days	5 years 8 months 12 days
Shares
Outstanding at beginning of year (in shares)	2,300,000	1,703,000	1,994,000
Granted (in shares)	1,033,015	972,180	169,281
Exercised (in shares)	(473,000)	(124,000)	(339,000)
Cancelled (in shares)	(107,000)	(251,000)	(121,000)
Outstanding at end of year (in shares)	2,753,000	2,300,000	1,703,000
Options exercisable at end of year (in shares)	1,019,000	1,089,000	880,000
Weighted Average Exercise Price
Outstanding at beginning of year (in dollars per share)	$ 16.22	$ 17.96	$ 18.56
Granted (in dollars per share)	$ 25.43	$ 12.95	$ 28.45
Exercised (in dollars per share)	$ 16.20	$ 13.72	$ 19.95
Cancelled (in dollars per share)	$ 18.48	$ 15.82	$ 19.73
Outstanding at end of year (in dollars per share)	$ 19.59	$ 16.22	$ 17.96
Options exercisable at end of year (in dollars per share)	$ 17.88	$ 17.73	$ 17.10
Stock options, additional disclosures
Total fair value of stock options vested	$ 2.0	$ 3.6	$ 4.0
Total intrinsic value of stock options exercised	4.3	0.5	3.3
Aggregate intrinsic value of options outstanding	15.2	10.9
Tax benefit realized from stock options exercised	1.0	0	0.5
Compensation expense	3.8	3.5	4.2
Unearned compensation not yet recognized	10.4
Average remaining amortization period of unearned compensation	2 years 6 months
Restricted stock
Stock options, additional disclosures
Compensation expense	4.9	7.6	8.4
Unearned compensation not yet recognized	$ 6.5
Average remaining amortization period of unearned compensation	1 year 4 months 24 days
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