Financing Arrangements (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended							6 Months Ended			6 Months Ended									3 Months Ended		6 Months Ended			6 Months Ended		6 Months Ended			6 Months Ended							3 Months Ended
	Nov. 30, 2011	Nov. 30, 2010	May 31, 2011	Nov. 30, 2011 Recourse Debt	Nov. 30, 2011 Revolving credit facility expiring April 12, 2016 with interest payable monthly (see Note 7)	Oct. 13, 2011 Revolving credit facility expiring April 12, 2016 with interest payable monthly (see Note 7)	May 31, 2011 Revolving credit facility expiring April 12, 2016 with interest payable monthly (see Note 7)	Nov. 30, 2011 Revolving credit facility expiring April 12, 2016 with interest payable monthly (see Note 7) Eurodollar rate	Nov. 30, 2011 Revolving credit facility expiring April 12, 2016 with interest payable monthly (see Note 7) Eurodollar rate Minimum	Nov. 30, 2011 Revolving credit facility expiring April 12, 2016 with interest payable monthly (see Note 7) Eurodollar rate Maximum	Nov. 30, 2011 Revolving credit facility expiring April 12, 2016 with interest payable monthly (see Note 7) Base rate	Nov. 30, 2011 Revolving credit facility expiring April 12, 2016 with interest payable monthly (see Note 7) Base rate Minimum	Nov. 30, 2011 Revolving credit facility expiring April 12, 2016 with interest payable monthly (see Note 7) Base rate Maximum	Nov. 30, 2011 Note payable due July 19, 2012 with interest at 7.22%, payable monthly	May 31, 2011 Note payable due July 19, 2012 with interest at 7.22%, payable monthly	Nov. 30, 2011 Revolving credit facility (secured by aircraft and related engines and components) due May 1, 2015 with floating interest rate, payable monthly	May 31, 2011 Revolving credit facility (secured by aircraft and related engines and components) due May 1, 2015 with floating interest rate, payable monthly	Nov. 30, 2011 Mortgage loan (secured by Wood Dale, Illinois facility) due August 1, 2015 with interest at 5.01%	May 31, 2011 Mortgage loan (secured by Wood Dale, Illinois facility) due August 1, 2015 with interest at 5.01%	Nov. 30, 2011 Convertible notes payable	Nov. 30, 2010 Convertible notes payable	Nov. 30, 2011 Convertible notes payable	Nov. 30, 2010 Convertible notes payable	May 31, 2011 Convertible notes payable	Nov. 30, 2011 Convertible notes payable due March 1, 2014 with interest at 1.625% payable semi-annually on March 1 and September 1	May 31, 2011 Convertible notes payable due March 1, 2014 with interest at 1.625% payable semi-annually on March 1 and September 1	Nov. 30, 2011 Convertible notes payable due March 1, 2016 with interest at 2.25% payable semi-annually on March 1 and September 1	Nov. 30, 2010 Convertible notes payable due March 1, 2016 with interest at 2.25% payable semi-annually on March 1 and September 1	May 31, 2011 Convertible notes payable due March 1, 2016 with interest at 2.25% payable semi-annually on March 1 and September 1	Nov. 30, 2011 Convertible notes payable due February 1, 2026 with interest at 1.75% payable semi-annually on February 1 and August 1	May 31, 2011 Convertible notes payable due February 1, 2026 with interest at 1.75% payable semi-annually on February 1 and August 1	Nov. 30, 2011 Industrial revenue bond (secured by trust indenture on property, plant and equipment) due August 1, 2018 with floating interest rate, payable monthly	May 31, 2011 Industrial revenue bond (secured by trust indenture on property, plant and equipment) due August 1, 2018 with floating interest rate, payable monthly	Nov. 30, 2011 Non-recourse note payable due December 8, 2011 with interest at 13.0%	Nov. 30, 2011 Non-recourse note payable due July 19, 2012 with interest at 7.22%	May 31, 2011 Non-recourse note payable due July 19, 2012 with interest at 7.22%	Nov. 30, 2011 Non-recourse note payable due April 3, 2015 with interest at 8.38%	May 31, 2011 Non-recourse note payable due April 3, 2015 with interest at 8.38%
Financing Arrangements
Interest rate (as a percent)														7.22%				5.01%							1.625%		2.25%			1.75%				13.00%	7.22%		8.38%
Total recourse debt	$ 497,366		$ 425,304		$ 150,000		$ 100,000							$ 9,421	$ 2,217	$ 63,296	$ 54,940	$ 11,000	$ 11,000						$ 75,237	$ 73,418	$ 52,485		$ 51,309	$ 110,927	$ 107,420	$ 25,000	$ 25,000
Current maturities of recourse debt	(70,586)		(111,323)
Long-term recourse debt	426,780		313,981
Total non-recourse debt	6,545		11,855																															6,545		8,201		3,654
Current maturities of non-recourse debt	(6,545)		(823)
Long-term non-recourse debt			11,032
Maximum borrowing capacity						580,000
Maximum borrowing capacity under the credit agreement before amendment						400,000
Increase in the revolving commitment subject to the agreement						100,000
Maximum borrowing capacity optional expansion						680,000
Description of variable rate basis								Eurodollar Rate			Base Rate
Basis spread on variable rate (as a percent)									1.25%	2.25%		0.25%	1.25%
Debt associated with aircraft sold, assumed by acquirer																																					3,252
Non-recourse debt assumed in connection with purchase of joint venture partner's interest in a narrow-body aircraft																																		6,545
Repurchased of convertible notes																												6,000
Repurchased of convertible notes in cash																												4,667
Gain on extinguishment of debt		97																										97
Face value of long-term recourse debt				456,511																268,380		268,380		268,380
Estimated fair value of long-term recourse debt				438,000
Long-term debt:
Principal amount				456,511																268,380		268,380		268,380
Unamortized discount																				(29,731)		(29,731)		(36,233)
Net carrying amount																				238,649		238,649		232,147
Equity component, net of tax																				74,966		74,966		74,966
Effective interest rate (as a percent)																									6.82%		7.41%			8.48%
Interest expense
Coupon interest																				1,228	1,228	2,456	2,475
Amortization of deferred financing fees																				188	188	376	377
Amortization of discount	6,502	6,051																		3,282	3,040	6,502	6,051
Interest expense related to convertible notes																				$ 4,698	$ 4,456	$ 9,334	$ 8,903