Financing Arrangements (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Aug. 31, 2014	Aug. 31, 2013	May 31, 2014
Financing Arrangements
Total debt	$ 627.1		$ 634.0
Current maturities of debt	(78.5)		(69.7)
Long-term debt	548.6		564.3
Interest expense
Amortization of discount	0.7	1.5
Revolving credit facility expiring April 24, 2018 with interest payable monthly
Financing Arrangements
Total debt	130.0		130.0
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	27.6		29.9
Note payable due March 9, 2017 with floating interest rate, payable semi-annually on June 1 and December 1
Financing Arrangements
Total debt	25.0		30.0
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%
Total debt	332.4		332.6
Convertible notes payable
Financing Arrangements
Carrying value of long-term debt	79.8		79.8
Long-term debt:
Principal amount	79.8		79.8
Unamortized discount	(3.7)		(4.4)
Net carrying amount	76.1		75.4
Equity component, net of tax	75.3		75.3
Interest expense
Coupon interest	0.5	0.8
Amortization of deferred financing fees	0.1	0.1
Amortization of discount	0.7	1.4
Interest expense related to convertible notes	1.3	2.3
Convertible notes payable due February 1, 2015 with interest at 1.75% payable semi-annually on February 1 and August 1
Financing Arrangements
Interest rate (as a percent)	1.75%
Effective interest rate (as a Percent)	5.00%
Redemption price of principal amount plus accrued and unpaid interest (as a percent)	103.358%
Long-term debt:
Net carrying amount	29.9		29.8
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 debt	46.2		45.7
Effective interest rate (as a Percent)	7.41%
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
Industrial revenue bond (secured by property, plant and equipment) due August 1, 2018
Long-term debt:
Net carrying amount	25.0		25.0
Other
Financing Arrangements
Total debt	65.9		65.8
7.25% bonds, 1.75% and 2.25% convertible notes
Financing Arrangements
Carrying value of long-term debt	408.5
Long-term debt:
Principal amount	408.5
7.25% bonds, 1.75% and 2.25% convertible notes | Level 2
Financing Arrangements
Estimated fair value of long-term debt	435.4
Remaining variable rate and fixed rate debt | Level 3
Financing Arrangements
Estimated fair value of long-term debt	$ 218.6