Financing Arrangements (Details) (USD $) In Millions, unless otherwise specified	0 Months Ended	12 Months Ended						12 Months Ended									0 Months Ended	12 Months Ended			0 Months Ended		12 Months Ended		12 Months Ended						0 Months Ended			12 Months Ended			3 Months Ended	12 Months Ended													12 Months Ended
	Feb. 14, 2013	May 31, 2014	May 31, 2013	May 31, 2012	May 31, 2014 Revolving credit facility expiring April 24, 2018 with interest payable monthly	May 31, 2013 Revolving credit facility expiring April 24, 2018 with interest payable monthly	Apr. 24, 2013 Revolving credit facility expiring April 24, 2018 with interest payable monthly	May 31, 2014 Revolving credit facility expiring April 24, 2018 with interest payable monthly Eurodollar rate	May 31, 2014 Revolving credit facility expiring April 24, 2018 with interest payable monthly Eurodollar rate Minimum	May 31, 2014 Revolving credit facility expiring April 24, 2018 with interest payable monthly Eurodollar rate Maximum	May 31, 2014 Revolving credit facility expiring April 24, 2018 with interest payable monthly Base rate	May 31, 2014 Revolving credit facility expiring April 24, 2018 with interest payable monthly Base rate Minimum	May 31, 2014 Revolving credit facility expiring April 24, 2018 with interest payable monthly Base rate Maximum	May 31, 2014 Secured credit facility (secured by aircraft and related engines and components) due April 23, 2015 with floating interest rate, payable monthly	May 31, 2013 Secured credit facility (secured by aircraft and related engines and components) due April 23, 2015 with floating interest rate, payable monthly	Mar. 28, 2013 Secured credit facility (secured by aircraft and related engines and components) due April 23, 2015 with floating interest rate, payable monthly	Mar. 28, 2013 Secured credit facility (secured by aircraft and related engines and components) due April 23, 2015 with floating interest rate, payable monthly LIBOR	May 31, 2014 Secured credit facility (secured by aircraft and related engines and components) due April 23, 2015 with floating interest rate, payable monthly LIBOR	May 31, 2014 Note payable due March 9, 2017 with floating interest rate, payable semi-annually on June 1 and December 1	May 31, 2013 Note payable due March 9, 2017 with floating interest rate, payable semi-annually on June 1 and December 1	Apr. 15, 2013 Notes payable due January 15, 2022 with interest at 7.25% payable semi-annually on January 15 and July 15	Jan. 23, 2013 Notes payable due January 15, 2022 with interest at 7.25% payable semi-annually on January 15 and July 15	May 31, 2014 Notes payable due January 15, 2022 with interest at 7.25% payable semi-annually on January 15 and July 15	May 31, 2013 Notes payable due January 15, 2022 with interest at 7.25% payable semi-annually on January 15 and July 15	May 31, 2014 Convertible notes payable	May 31, 2013 Convertible notes payable	May 31, 2012 Convertible notes payable	May 31, 2013 Convertible notes payable due March 1, 2014 with interest at 1.625% payable semi-annually on March 1 and September 1	May 31, 2012 Convertible notes payable due March 1, 2014 with interest at 1.625% payable semi-annually on March 1 and September 1	May 31, 2014 Convertible notes payable due March 1, 2014 with interest at 1.625% payable semi-annually on March 1 and September 1	Feb. 14, 2013 Convertible notes payable due February 1, 2015 with interest at 1.75% payable semi-annually on February 1 and August 1	May 31, 2014 Convertible notes payable due February 1, 2015 with interest at 1.75% payable semi-annually on February 1 and August 1	May 31, 2013 Convertible notes payable due February 1, 2015 with interest at 1.75% payable semi-annually on February 1 and August 1	May 31, 2013 Convertible notes payable due March 1, 2016 with interest at 2.25% payable semi-annually on March 1 and September 1	May 31, 2012 Convertible notes payable due March 1, 2016 with interest at 2.25% payable semi-annually on March 1 and September 1	May 31, 2014 Convertible notes payable due March 1, 2016 with interest at 2.25% payable semi-annually on March 1 and September 1	May 31, 2013 Convertible notes payable due February 1, 2026 with interest at 1.75% payable semi-annually on February 1 and August 1	May 31, 2013 Convertible notes payable due February 1, 2026 with interest at 1.75% payable semi-annually on February 1 and August 1	May 31, 2012 Convertible notes payable due February 1, 2026 with interest at 1.75% payable semi-annually on February 1 and August 1	Feb. 28, 2013 Convertible notes payable due February 1, 2026 with interest at 1.75% payable semi-annually on February 1 and August 1	May 31, 2014 Mortgage loan (secured by Wood Dale, Illinois facility) due August 1, 2015 with interest at 5.01%	May 31, 2013 Mortgage loan (secured by Wood Dale, Illinois facility) due August 1, 2015 with interest at 5.01%	May 31, 2014 Industrial revenue bond (secured by property, plant and equipment) due August 1, 2018	May 31, 2013 Industrial revenue bond (secured by property, plant and equipment) due August 1, 2018	May 31, 2014 Foreign line of credit	May 31, 2014 Other	May 31, 2013 Other	May 31, 2014 7.25% bonds, 1.75% and 2.25% convertible notes	May 31, 2014 7.25% bonds, 1.75% and 2.25% convertible notes Level 2	May 31, 2014 Remaining variable rate and fixed rate debt Level 3	May 31, 2014 Credit Agreement and its Predecessor	May 31, 2013 Credit Agreement and its Predecessor	May 31, 2012 Credit Agreement and its Predecessor
Financing Arrangements
Interest rate (as a percent)																					7.25%	7.25%	7.25%							1.625%		1.75%				2.25%	1.75%	1.75%
Total debt		$ 634.0	$ 708.6		$ 130.0	$ 120.0								$ 29.9	$ 39.2				$ 30.0	$ 40.0			$ 332.6	$ 333.4				$ 65.9						$ 43.5		$ 45.7										$ 65.8	$ 66.6
Current maturities of debt		(69.7)	(86.4)
Long-term debt		564.3	622.2
Aggregate par value of debt maturing in next five fiscal years
2015		69.9
2016		70.8
2017		10.0
2018		130.0
2019		25.0
Carrying value of long-term debt																									79.8	148.3					30.0																	408.1
Estimated fair value of long-term debt																																																	434.8	225.9
Maximum borrowing capacity under the credit agreement before amendment							580.0									65.0
Extended maturity period					2 years
Maximum borrowing capacity					475.0											40.0																													5.4
Description of variable rate basis								Eurodollar Rate			Base Rate						LIBOR	LIBOR
Basis spread on variable rate (as a percent)									1.25%	2.25%		0.25%	1.25%				1.75%	1.75%
Basis spread on variable rate under the credit agreement before amendment (as a percent)																	3.25%
Amount outstanding					130.0									29.9	39.2
Outstanding letter of credit		14.8			14.8
Remaining borrowing capacity					330.2
Short-term borrowing activity
Maximum amount borrowed																																																			190.0	395.0	525.0
Average daily borrowings																																																			135.8	312.3	288.3
Average interest rate during the year (as a percent)																																																			1.77%	2.03%	1.90%
Aggregate principal amount of notes sold																					150.0	175.0
Debt instrument issuance price as a percentage of the principal amount																					107.50%
Effective interest rate (as a Percent)																					6.128%										3.75%	5.00%	5.00%	7.41%		7.41%	1.75%	1.75%
Aggregate principal amount of securities exchanged	22.7
Net proceeds from offering of notes			181.8	236.8																			157.6
Cash portion received in new debt issuance	7.3
Notice period for redemption of debt instrument																															30 days
Call price at maturity (as a percent)																															104.038%
Retirement of Principal Amount			22.9	39.1																								6.4	9.4					5.5	9.1			11.0	20.6
Cash Paid			22.0	37.3																								6.1	8.9					4.9	8.1			11.0	20.3
Loss on Repurchase			0.3	0.7																								0.1	0.2					0.2	0.3		0		0.2
Redemption price of debt instrument as a percentage of the principal amount																																								100.00%
Aggregate purchase price including principal, accrued and unpaid interest pursuant to Put Option																																					66.2	66.2
Long-term debt:
Principal amount																									79.8	148.3					30.0																	408.1
Unamortized discount																									(4.4)	(9.5)
Net carrying amount																									75.4	138.8						29.8	29.4								11.0	11.0	25.0	25.0
Equity component, net of tax																									75.3	75.3
Interest expense
Coupon interest																									3.1	3.7	4.9
Amortization of deferred financing fees																									0.3	0.5	0.7
Amortization of discount		5.3	9.6	13.2																					5.1	9.3	13.1
Interest expense related to convertible notes																									$ 8.5	$ 13.5	$ 18.7