Summary of Transactions Involving Stock Options (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	May 31, 2013	May 31, 2012	May 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares under option, beginning of period, No. of Options	1,562	1,856	2,213
Granted, No. of Options
Exercised, No. of Options	(509)	(278)	(329)
Cancelled, No. of Options	(1)	(2)	(13)
Expired, No. of Options		(14)	(15)
Shares under option, end of period, No. of Options	1,052	1,562	1,856
Exercisable at end of period, No. of Options	1,052	1,562	1,711
Shares under option, beginning of period, Weighted Average Exercise Price	$ 18.95	$ 17.81	$ 16.50
Granted, Weighted Average Exercise Price
Exercised, Weighted Average Exercise Price	$ 16.25	$ 10.77	$ 8.08
Cancelled, Weighted Average Exercise Price	$ 30.33	$ 30.33	$ 30.33
Expired, Weighted Average Exercise Price		$ 30.33	$ 26.56
Shares under option, end of period, Weighted Average Exercise Price	$ 20.24	$ 18.95	$ 17.81
Exercisable at end of period, Weighted Average Exercise Price	$ 20.24	$ 18.95	$ 16.75