Financing Arrangements (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Nov. 30, 2015	Aug. 31, 2015	Nov. 30, 2014	Nov. 30, 2015	Nov. 30, 2014	May. 31, 2015
Financing Arrangements
Total debt	$ 170.3			$ 170.3		$ 154.0
Current maturities of debt	(35.3)			(35.3)		(69.0)
Long-term debt	135.0			135.0		85.0
Loss on extinguishment of debt	0.1			0.4
Interest expense
Amortization of discount				1.0	$ 1.4
Revolving Credit Facility expiring March 24, 2020 with interest payable monthly
Financing Arrangements
Total debt	105.0			105.0		50.0
Industrial revenue bond (secured by property, plant and equipment)
Financing Arrangements
Total debt	25.0			25.0		25.0
Note payable due March 9, 2017 with floating interest rate, payable semi-annually on June 1 and December 1
Financing Arrangements
Total debt	15.0			15.0		$ 20.0
Convertible notes payable
Interest expense
Coupon interest	0.1		$ 0.4	0.4	0.9
Amortization of deferred financing fees					0.1
Amortization of discount	0.4		0.6	1.0	1.3
Interest expense related to convertible notes	$ 0.5		$ 1.0	$ 1.4	$ 2.3
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%
Total debt	$ 25.3			$ 25.3		$ 48.0
Amount of convertible notes repurchased	9.7	$ 14.4		9.7
Cash paid for convertible notes repurchased	9.7	14.6		9.7
Accrued interest		0.2
Loss on extinguishment of debt	0.1	$ 0.3
Mortgage loan (secured by Wood Dale, Illinois facility) due August 1, 2015
Financing Arrangements
Total debt						$ 11.0
Remaining variable rate and fixed rate debt | Significant other unobservable inputs (Level 3)
Financing Arrangements
Estimated fair value of long-term debt	$ 170.3			$ 170.3