Financing Arrangements (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Nov. 30, 2014	Nov. 30, 2013	Nov. 30, 2014	Nov. 30, 2013	May 31, 2014
Financing Arrangements
Total debt	$ 635.3		$ 635.3		$ 634.0
Current maturities of debt	(46.3)		(46.3)		(69.7)
Long-term debt	589.0		589.0		564.3
Amount of convertible notes repurchased	30.0		30.0
Cash paid for convertible notes repurchased	31.1		31.1
Accrued interest	1.1		1.1
Interest expense
Amortization of discount			1.4	3.0
Revolving credit facility expiring April 24, 2018 with interest payable monthly
Financing Arrangements
Total debt	170.0		170.0		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	25.3		25.3		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		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%		7.25%
Total debt	332.2		332.2		332.6
Convertible notes payable
Financing Arrangements
Carrying value of long-term debt	49.8		49.8		49.8
Long-term debt:
Principal amount	49.8		49.8		49.8
Unamortized discount	(3.0)		(3.0)		(4.1)
Net carrying amount	46.8		46.8		45.7
Equity component, net of tax	20.6		20.6		20.6
Interest expense
Coupon interest	0.4	0.9	0.9	1.7
Amortization of deferred financing fees		0.1	0.1	0.2
Amortization of discount	0.6	1.5	1.3	2.9
Interest expense related to convertible notes	1.0	2.5	2.3	4.8
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%		1.75%
Long-term debt:
Net carrying amount	0		0		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%		2.25%
Total debt	46.8		46.8		45.7
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
Long-term debt:
Effective interest rate (as a Percent)	7.41%		7.41%
7.25% bonds, 1.75% and 2.25% convertible notes
Financing Arrangements
Carrying value of long-term debt	379.0		379.0
Long-term debt:
Principal amount	379.0		379.0
7.25% bonds, 1.75% and 2.25% convertible notes | Significant other observable inputs (Level 2)
Financing Arrangements
Estimated fair value of long-term debt	402.2		402.2
Remaining variable rate and fixed rate debt | Significant other unobservable inputs (Level 3)
Financing Arrangements
Estimated fair value of long-term debt	256.3		256.3
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	$ 36.0		$ 36.0		$ 65.8