Stock-Based Compensation (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	May 31, 2012 Y increament	May 31, 2011 Y	May 31, 2010 Y
Share-Based Compensation
Total number of shares granted since inception	8,809,000
Shares available for future grant	2,322,527
Stock options
Share-Based Compensation
Expiration term of awards (in years)	P10Y
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 (in years)	1
Weighted average fair value of stock options granted (in dollars per share)	$ 11.42	$ 8.06	$ 7.45
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)	1.50%	1.80%	2.30%
Expected volatility of common stock (as a percent)	46.10%	47.00%	49.10%
Dividend yield (as a percent)	1.10%	0.00%	0.00%
Expected option term (in years)	5.7	5.8	6.0
Shares
Outstanding at beginning of year (in shares)	1,994,000	1,543,000	1,225,000
Granted (in shares)	169,281	720,970	694,500
Exercised (in shares)	(339,000)	(128,000)	(217,000)
Cancelled (in shares)	(121,000)	(142,000)	(160,000)
Outstanding at end of year (in shares)	1,703,000	1,994,000	1,543,000
Options exercisable at end of year (in shares)	880,000	935,000	663,000
Weighted Average Exercise Price
Outstanding at beginning of year (in dollars per share)	$ 18.56	$ 19.28	$ 21.18
Granted (in dollars per share)	$ 28.45	$ 17.54	$ 15.21
Exercised (in dollars per share)	$ 19.95	$ 15.85	$ 12.47
Cancelled (in dollars per share)	$ 19.73	$ 21.79	$ 22.60
Outstanding at end of year (in dollars per share)	$ 17.96	$ 18.56	$ 19.28
Options exercisable at end of year (in dollars per share)	$ 17.10	$ 19.40	$ 20.92
Stock options, additional disclosures
Total fair value of stock options vested	$ 3,953	$ 4,006	$ 690
Total intrinsic value of stock options exercised	3,349	1,344	2,353
Aggregate intrinsic value of options outstanding	327
Tax benefit realized from stock options exercised	482	247	817
Compensation expense	4,162	4,152	2,265
Unearned compensation not yet recognized	3,659
Average remaining amortization period of unearned compensation (in years)	0.8
Restricted stock
Share-Based Compensation
Vesting period, minimum (in years)	P3Y
Vesting period, maximum (in years)	P7Y
Stock options, additional disclosures
Compensation expense	8,384	8,701	7,070
Unearned compensation not yet recognized	$ 17,167
Average remaining amortization period of unearned compensation (in years)	2.1
Performance-based restricted stock
Share-Based Compensation
Vesting period, minimum (in years)	P1Y
Vesting period, maximum (in years)	P5Y