Comprehensive Income (Details) (USD $)	3 Months Ended
Aug. 31, 2011	Aug. 31, 2010
Comprehensive Income
Net income	$ 68,638,000	$ 61,277,000
Other comprehensive income:
Foreign currency translation adjustment	218,000	3,349,000
Change in fair value of derivatives	153,000	(606,000)
Amortization of interest rate lock agreements	377,000	192,000
Change in fair value of available- for-sale securities	18,000	27,000
Comprehensive income	69,404,000	64,239,000
Tax expense (benefit) for change in the fair value of derivatives	100,000	(400,000)
Tax expense for change in fair value of available-for-sale securities	$ 100,000	$ 100,000

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