Financing Arrangements (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Nov. 30, 2013	Nov. 30, 2012	Nov. 30, 2013	Nov. 30, 2012	May 31, 2013
Financing Arrangements
Total debt	$ 680.8		$ 680.8		$ 708.6
Current maturities of debt	(87.3)		(87.3)		(86.4)
Long-term debt	593.5		593.5		622.2
Loss on extinguishment of debt		0.1		0.3
Face value of long-term debt	679.3		679.3
Estimated fair value of long-term debt	705.8		705.8
Long-term debt:
Principal amount	679.3		679.3
Interest expense
Amortization of discount			3.0	5.7
Revolving credit facility expiring April 24, 2018 with interest payable monthly
Financing Arrangements
Total debt	100.0		100.0		120.0
Secured credit facility (secured by aircraft and related engines and components) due April 23, 2015 with floating interest rate, payable monthly
Financing Arrangements
Total debt	34.6		34.6		39.2
Note payable due March 9, 2017 with floating interest rate, payable semi-annually on June 1 and December 1
Financing Arrangements
Total debt	35.0		35.0		40.0
Notes payable due January 15, 2022 with interest at 7.25% payable semi-annually on January 15 and July 15
Financing Arrangements
Interest rate (as a percent)	7.25%		7.25%
Total debt	333.0		333.0		333.4
Convertible notes payable
Financing Arrangements
Face value of long-term debt	148.3		148.3		148.3
Long-term debt:
Principal amount	148.3		148.3		148.3
Unamortized discount	(6.6)		(6.6)		(9.5)
Net carrying amount	141.7		141.7		138.8
Equity component, net of tax	75.3		75.3		75.3
Interest expense
Coupon interest	0.9	1.0	1.7	2.0
Amortization of deferred financing fees	0.1	0.1	0.2	0.3
Amortization of discount	1.5	2.8	2.9	5.6
Interest expense related to convertible notes	2.5	3.9	4.8	7.9
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%	1.625%	1.625%	1.625%
Total debt	67.6		67.6		65.9
Amount of convertible notes repurchased		6.4		6.4
Cash paid for convertible notes repurchased		6.1		6.1
Long-term debt:
Effective interest rate (as a Percent)	6.82%		6.82%
Convertible notes payable due February 1, 2015 with interest at 1.75% payable semi-annually on February 1 and August 1
Long-term debt:
Net carrying amount	29.5		29.5		29.4
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%	2.25%	2.25%	2.25%
Total debt	44.6		44.6		43.5
Amount of convertible notes repurchased		5.5		5.5
Cash paid for convertible notes repurchased		4.9		4.9
Long-term debt:
Effective interest rate (as a Percent)	7.41%		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%		1.75%
Amount of convertible notes repurchased		11.0		11.0
Cash paid for convertible notes repurchased		11.0		11.0
Long-term debt:
Effective interest rate (as a Percent)	5.00%		5.00%
Note payable due March 15, 2014
Long-term debt:
Net carrying amount	0.5		0.5		1.2
Mortgage loan (secured by Wood Dale, Illinois facility) due August 1, 2015 with interest at 5.01%
Long-term debt:
Net carrying amount	11.0		11.0		11.0
Industrial revenue bond (secured by property, plant and equipment) due August 1, 2018
Long-term debt:
Net carrying amount	25.0		25.0		25.0
Other
Financing Arrangements
Total debt	$ 66.0		$ 66.0		$ 66.6