Accounting for Stock-Based Compensation (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Feb. 29, 2012 Y	Feb. 28, 2011	Feb. 29, 2012 Y	Feb. 28, 2011 Y
Stock options
Accounting for Stock-Based Compensation
Expiration term of awards (in years)			P10Y
Number of equal annual increments in which stock options are exercisable			three, four or five
Commencement period after the date of grant for the stock options to be exercisable (in years)			1
Weighted average fair value of stock options granted (in dollars per share)			$ 11.64	$ 8.06
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%
Expected volatility of common stock (as a percent)			45.80%	47.00%
Dividend yield (as a percent)			1.00%	0.00%
Expected option term (in years)			5.7	5.8
Number of Options
Outstanding at the beginning of the period (in shares)			1,994,000
Granted (in shares)			162,281	720,970
Exercised (in shares)			(335,000)
Cancelled (in shares)			(78,000)
Outstanding at the end of the period (in shares)	1,743,000		1,743,000
Exercisable at the end of the period (in shares)	862,000		862,000
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)			$ 18.56
Granted (in dollars per share)	$ 28.96		$ 28.96
Exercised (in dollars per share)	$ 20.00		$ 20.00
Cancelled (in dollars per share)	$ 18.69		$ 18.69
Outstanding at the end of the period (in dollars per share)	$ 19.61		$ 19.61
Exercisable at the end of the period (in dollars per share)	$ 18.20		$ 18.20
Weighted Average Remaining Contractual Life (in years)
Outstanding at the end of the period (in years)	5.9		5.9
Exercisable at the end of the period (in years)			4.0
Aggregate Intrinsic Value
Outstanding at the end of the period	$ 8,835		$ 8,835
Exercisable at the end of the period	5,186		5,186
Stock options, additional disclosures
Total fair value of stock options vested			3,827	2,275
Total intrinsic value of stock options exercised			3,339	898
Tax benefit realized from stock options exercised			533	146
Share-based compensation costs
Compensation expense	1,078	1,177	3,279	3,222
Unearned compensation not yet recognized	4,924		4,924
Average remaining amortization period of unearned compensation (in years)			1.0
Restricted shares
Share-based compensation costs
Compensation expense	2,711	2,462	6,635	6,245
Unearned compensation not yet recognized	$ 19,577		$ 19,577
Average remaining amortization period of unearned compensation (in years)			2.3
Number of Shares
Unvested at the beginning of the period (in shares)			1,374,000
Granted (in shares)			677,000
Vested (in shares)			(325,000)
Forfeited (in shares)			(107,000)
Unvested at the end of the period (in shares)	1,619,000		1,619,000
Weighted Average Fair Value on Grant Date
Unvested at the beginning of the period (in dollars per share)			$ 23.06
Granted (in dollars per share)			$ 24.64
Vested (in dollars per share)			$ 26.71
Forfeited (in dollars per share)	$ 23.90		$ 23.90
Unvested at the end of the period (in dollars per share)	$ 22.92		$ 22.92
Restricted shares | Board of Directors
Number of Shares
Granted (in shares)			45,000