Share-Based Compensation (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Aug. 31, 2012	Aug. 31, 2011
Transactions involving stock options
Shares under option, beginning of period, No. of Options	1,562	1,856
Shares under option, beginning of period, Weighted Average Exercise Price	$ 18.95	$ 17.81
Changes during the period:
Granted, No. of Options
Granted, Weighted Average Exercise Price
Exercised, No. of Options	(57)	(37)
Exercised, Weighted Average Exercise Price	$ 8.00	$ 3.52
Cancelled, No. of Options		(4)
Cancelled, Weighted Average Exercise Price		$ 30.33
Expired, No. of Options
Expired, Weighted Average Exercise Price
Shares under option, end of period, No. of Options	1,505	1,815
Shares under option, end of period, Weighted Average Exercise Price	$ 19.36	$ 18.08
Exercisable at end of period, No. of Options	1,505	1,670
Exercisable at end of period, Weighted Average Exercise Price	$ 19.36	$ 17.01