Financing Arrangements (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Nov. 30, 2012	Nov. 30, 2011	Nov. 30, 2012	Nov. 30, 2011	May 31, 2012
Financing Arrangements
Total debt	$ 778.9		$ 778.9		$ 792.2
Current maturities of debt	(106.6)		(106.6)		(122.8)
Long-term debt	672.3		672.3		669.4
Loss on extinguishment of debt	0.1		0.3
Face value of long-term debt	794.2		794.2
Estimated fair value of long-term debt	793.1		793.1
Long-term debt:
Principal amount	794.2		794.2
Net carrying amount	778.9		778.9		792.2
Interest expense
Amortization of discount			5.7	6.5
Revolving credit facility expiring April 12, 2016 with interest payable monthly
Financing Arrangements
Total debt	300.0		300.0		280.0
Long-term debt:
Net carrying amount	300.0		300.0		280.0
Note payable due July 19, 2012 with interest at 7.22%, payable monthly
Financing Arrangements
Interest rate (as a percent)	7.22%		7.22%
Total debt					8.4
Debt repayment			8.4
Long-term debt:
Net carrying amount					8.4
Note payable due March 15, 2014 with floating interest rate, payable monthly
Financing Arrangements
Total debt	1.9		1.9		2.6
Long-term debt:
Net carrying amount	1.9		1.9		2.6
Note payable due March 9, 2017 with floating interest rate, payable quarterly
Financing Arrangements
Total debt	45.0		45.0		50.0
Long-term debt:
Net carrying amount	45.0		45.0		50.0
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	172.1		172.1		172.1
Long-term debt:
Net carrying amount	172.1		172.1		172.1
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%		5.01%
Total debt	11.0		11.0		11.0
Long-term debt:
Net carrying amount	11.0		11.0		11.0
Convertible notes payable
Financing Arrangements
Total debt	194.0		194.0		209.5
Face value of long-term debt	206.4		206.4		229.3
Long-term debt:
Principal amount	206.4		206.4		229.3
Unamortized discount	(12.4)		(12.4)		(19.8)
Net carrying amount	194.0		194.0		209.5
Equity component, net of tax	74.8		74.8		74.8
Interest expense
Coupon interest	1.0	1.2	2.0	2.4
Amortization of deferred financing fees	0.1	0.2	0.3	0.4
Amortization of discount	2.8	3.3	5.6	6.5
Interest expense related to convertible notes	3.9	4.7	7.9	9.3
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%
Total debt	87.2		87.2		94.9
Amount of convertible notes repurchased			11.0
Cash paid for convertible notes repurchased			11.0
Long-term debt:
Net carrying amount	87.2		87.2		94.9
Effective interest rate (as a percent)			8.48%
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%
Total debt	64.3		64.3		68.5
Amount of convertible notes repurchased			6.4
Cash paid for convertible notes repurchased			6.1
Long-term debt:
Net carrying amount	64.3		64.3		68.5
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%		2.25%
Total debt	42.5		42.5		46.1
Amount of convertible notes repurchased			5.5
Cash paid for convertible notes repurchased			4.9
Long-term debt:
Net carrying amount	42.5		42.5		46.1
Effective interest rate (as a percent)			7.41%
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 debt	25.0		25.0		25.0
Long-term debt:
Net carrying amount	25.0		25.0		25.0
Revolving credit facility (secured by aircraft and related engines and components) due April 23, 2015 with floating interest rate, payable monthly
Financing Arrangements
Total debt	29.3		29.3		33.0
Long-term debt:
Net carrying amount	29.3		29.3		33.0
Revolving credit facility subject to annual review in March with interest payable quarterly
Financing Arrangements
Total debt	0.6		0.6		0.6
Long-term debt:
Net carrying amount	$ 0.6		$ 0.6		$ 0.6