Business Segment Information (Details 4) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	May 31, 2012	Feb. 29, 2012	Nov. 30, 2011	Aug. 31, 2011	May 31, 2011	Feb. 28, 2011	Nov. 30, 2010	Aug. 31, 2010	May 31, 2012	May 31, 2011	May 31, 2010
Business Segment Information
Long-lived assets	$ 1,132,381				$ 789,742				$ 1,132,381	$ 789,742
Sales	563,346	534,195	481,922	485,535	487,826	458,035	447,054	412,197	2,064,998	1,805,112	1,352,151
United States
Business Segment Information
Long-lived assets	882,400				777,791				882,400	777,791
Europe
Business Segment Information
Long-lived assets	248,370				11,830				248,370	11,830
Other
Business Segment Information
Long-lived assets	1,611				121				1,611	121
Foreign countries
Business Segment Information
Sales									$ 435,663	$ 330,213	$ 289,435
Foreign countries | Total sales | Geographic concentration
Business Segment Information
Percentage of total sales	21.10%				18.30%				21.10%	18.30%	21.40%