Accounting for Stock-Based Compensation (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Nov. 30, 2011 Y	Nov. 30, 2010	Nov. 30, 2011 Y	Nov. 30, 2010 Y
Stock options
Accounting for Stock-Based Compensation
Expiration term of awards (in years)			ten years
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	$ 7.98
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	708,970
Exercised (in shares)			(332,000)
Cancelled (in shares)			(45,000)
Outstanding at the end of the period (in shares)	1,779,000		1,779,000
Exercisable at the end of the period (in shares)	871,000		871,000
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)	$ 20.03		$ 20.03
Cancelled (in dollars per share)	$ 17.81		$ 17.81
Outstanding at the end of the period (in dollars per share)	$ 19.65		$ 19.65
Exercisable at the end of the period (in dollars per share)	$ 18.27		$ 18.27
Weighted Average Remaining Contractual Life (in years)
Outstanding at the end of the period (in years)	6.5		6.5
Exercisable at the end of the period (in years)			4.2
Aggregate Intrinsic Value
Outstanding at the end of the period	$ 3,408		$ 3,408
Exercisable at the end of the period	2,301		2,301
Stock options, additional disclosures
Total fair value of stock options vested			3,776	2,275
Total intrinsic value of stock options exercised			3,323	87
Tax benefit realized from stock options exercised			595	15
Share-based compensation costs
Compensation expense	1,102	1,157	2,201	2,045
Unearned compensation not yet recognized	6,057		6,057
Average remaining amortization period of unearned compensation (in years)			1.2
Restricted shares
Share-based compensation costs
Compensation expense	2,406	2,048	3,924	3,783
Unearned compensation not yet recognized	$ 22,229		$ 22,229
Average remaining amortization period of unearned compensation (in years)			2.5
Number of Shares
Unvested at the beginning of the period (in shares)			1,374,000
Granted (in shares)			672,000
Vested (in shares)			(324,000)
Forfeited (in shares)			(84,000)
Unvested at the end of the period (in shares)	1,638,000		1,638,000
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)			$ 24.61
Vested (in dollars per share)			$ 26.71
Forfeited (in dollars per share)	$ 24.01		$ 24.01
Unvested at the end of the period (in dollars per share)	$ 22.91		$ 22.91
Restricted shares | Board of Directors
Number of Shares
Granted (in shares)			45,000