Earnings per Share (Details) - USD ($)	3 Months Ended								12 Months Ended
	May. 31, 2015	Feb. 28, 2015	Nov. 30, 2014	Aug. 31, 2014	May. 31, 2014	Feb. 28, 2014	Nov. 30, 2013	Aug. 31, 2013	May. 31, 2015	May. 31, 2014	May. 31, 2013
Earnings per Share
Effect to the participating securities as result of net loss									$ 0
Antidilutive stock options excluded from the computation of diluted earnings per share (in shares)									168,200	180,000	872,200
Basic EPS:
Income (Loss) from continuing operations	$ (82,100,000)	$ 1,900,000	$ 13,900,000	$ 11,800,000	$ 14,900,000	$ 16,900,000	$ 18,700,000	$ 16,700,000	$ (54,500,000)	$ 67,200,000	$ 48,800,000
Less income attributable to participating shares									(300,000)	(1,500,000)	(2,000,000)
Income (loss) from continuing operations attributable to common shareholders									(54,800,000)	65,700,000	46,800,000
Income from discontinued operations attributable to common shareholders	97.20	(36.40)	1.30	2.60	2,200,000	1,000,000	1,300,000	1,200,000	64,700,000	5,700,000	6,200,000
Net income attributable to common shareholders for earnings per share - basic									9,900,000	71,400,000	53,000,000
Diluted EPS:
Income (Loss) from continuing operations	(82,100,000)	1,900,000	13,900,000	11,800,000	14,900,000	16,900,000	18,700,000	16,700,000	(54,500,000)	67,200,000	48,800,000
Add after-tax interest on convertible debt											3,200,000
Less income attributable to participating shares									(300,000)	(1,500,000)	(1,900,000)
Income (loss) from continuing operations attributable to common shareholders									(54,800,000)	65,700,000	50,100,000
Income from discontinued operations attributable to common shareholders	$ 97.20	$ (36.40)	$ 1.30	$ 2.60	$ 2,200,000	$ 1,000,000	$ 1,300,000	$ 1,200,000	64,700,000	5,700,000	6,200,000
Net income attributable to common shareholders for earnings per share - diluted									$ 9,900,000	$ 71,400,000	$ 56,300,000
Weighted average common shares outstanding - basic									39,100,000	38,600,000	38,300,000
Additional shares from assumed conversion of debt											2,200,000
Additional shares from the assumed exercise of stock options									300,000	500,000	100,000
Weighted average common shares outstanding - diluted									39,400,000	39,100,000	40,600,000
Earnings (Loss) per share - basic:
Earnings (Loss) from continuing operations	$ (2.12)	$ 0.05	$ 0.35	$ 0.30	$ 0.38	$ 0.43	$ 0.48	$ 0.42	$ (1.40)	$ 1.70	$ 1.23
Earnings from discontinued operations	2.50	(0.94)	0.03	0.07	0.06	0.02	0.03	0.03	1.66	0.15	0.15
Earnings per share - basic	0.38	(0.89)	0.38	0.37	0.44	0.45	0.51	0.45	0.26	1.85	1.38
Earnings per share - diluted:
Earnings (Loss) from continuing operations	(2.12)	0.05	0.35	0.30	0.37	0.43	0.47	0.42	(1.40)	1.68	1.23
Earnings from discontinued operations	2.48	(0.94)	0.03	0.07	0.06	0.02	0.03	0.03	1.64	0.15	0.15
Earnings per share-diluted	$ 0.36	$ (0.89)	$ 0.38	$ 0.37	$ 0.43	$ 0.45	$ 0.50	$ 0.45	$ 0.24	$ 1.83	$ 1.38