Financing Arrangements (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Aug. 31, 2011	Aug. 31, 2010	May 31, 2011
Financing Arrangements
Total recourse debt	$ 483,908		$ 425,304
Current maturities of recourse debt	(69,208)		(111,323)
Long-term recourse debt	414,700		313,981
Total non-recourse debt	3,455		11,855
Current maturities of non-recourse debt	(840)		(823)
Long-term non-recourse debt	2,615		11,032
Gain on extinguishment of debt		97
Interest expense
Amortization of discount	3,220	3,011
Recourse Debt
Financing Arrangements
Face value of long-term recourse debt	447,713
Estimated fair value of long-term recourse debt	442,000
Long-term debt:
Principal amount	447,713
Revolving credit facility expiring April 12, 2016 with interest payable monthly (see Note 7)
Financing Arrangements
Total recourse debt	150,000		100,000
Note payable due July 19, 2012 with interest at 7.22%, payable monthly
Financing Arrangements
Interest rate (as a percent)	7.22%
Total recourse debt	9,922		2,217
Note payable due May 1, 2015 with interest at 3.44%, payable monthly
Financing Arrangements
Interest rate (as a percent)	3.44%
Total recourse debt	52,619		54,940
Mortgage loan (secured by Wood Dale, Illinois facility) due August 1, 2015 with interest at 5.01%
Financing Arrangements
Interest rate (as a percent)	5.01%
Total recourse debt	11,000		11,000
Convertible notes payable
Financing Arrangements
Face value of long-term recourse debt	268,380		268,380
Long-term debt:
Principal amount	268,380		268,380
Unamortized discount	(33,013)		(36,233)
Net carrying amount	235,367		232,147
Equity component, net of tax	74,966		74,966
Interest expense
Coupon interest	1,228	1,247
Amortization of deferred financing fees	188	189
Amortization of discount	3,220	3,011
Interest expense related to convertible notes	4,636	4,447
Convertible notes payable due March 1, 2014 with interest at 1.625% payable semi-annually on March 1 and September 1
Financing Arrangements
Interest rate (as a percent)	1.625%
Total recourse debt	74,320		73,418
Long-term debt:
Effective interest rate (as a percent)	6.82%
Convertible notes payable due March 1, 2016 with interest at 2.25% payable semi-annually on March 1 and September 1
Financing Arrangements
Interest rate (as a percent)	2.25%
Total recourse debt	51,892		51,309
Retirement of convertible notes		6,000
Retirement of convertible notes in cash		4,667
Gain on extinguishment of debt		97
Long-term debt:
Effective interest rate (as a percent)	7.41%
Convertible notes payable due February 1, 2026 with interest at 1.75% payable semi-annually on February 1 and August 1
Financing Arrangements
Interest rate (as a percent)	1.75%
Total recourse debt	109,155		107,420
Long-term debt:
Effective interest rate (as a percent)	8.48%
Industrial revenue bond (secured by trust indenture on property, plant and equipment) due August 1, 2018 with floating interest rate, payable monthly
Financing Arrangements
Total recourse debt	25,000		25,000
Non-recourse note payable due July 19, 2012 with interest at 7.22%
Financing Arrangements
Interest rate (as a percent)	7.22%
Total non-recourse debt			8,201
Non-recourse note payable due April 3, 2015 with interest at 8.38%
Financing Arrangements
Interest rate (as a percent)	8.38%
Total non-recourse debt	$ 3,455		$ 3,654