Financial Information for Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Feb. 29, 2016	Nov. 30, 2015	Aug. 31, 2015	Feb. 28, 2015	Feb. 29, 2016	Feb. 28, 2015	May. 31, 2015
Segment Reporting Information [Line Items]
Net sales	$ 647,080			$ 804,785	$ 2,105,043	$ 2,538,211
Operating income (loss)	25,072			(52,050)	68,040	33,344
Impairment of goodwill and long-lived assets		$ 22,962		81,600	25,962	97,785
Restructuring and other expense (income)	702			2,177	5,294	2,765
Total assets	1,959,928				1,959,928		$ 2,085,142
Steel Processing
Segment Reporting Information [Line Items]
Net sales	419,026			500,703	1,377,638	1,605,790
Operating income (loss)	21,294			16,406	71,574	86,152
Impairment of goodwill and long-lived assets						3,050
Restructuring and other expense (income)	1,068			(28)	3,788	(58)
Total assets	734,182				734,182		829,116
Pressure Cylinders
Segment Reporting Information [Line Items]
Net sales	200,721			248,086	626,288	749,789
Operating income (loss)	8,969			18,611	15,479	47,797
Impairment of goodwill and long-lived assets					22,962	9,567
Restructuring and other expense (income)	(1,031)			2,498	(316)	2,926
Total assets	784,880				784,880		804,799
Engineered Cabs
Segment Reporting Information [Line Items]
Net sales	25,553			45,390	92,869	146,484
Operating income (loss)	(4,053)			(85,780)	(17,634)	(93,534)
Impairment of goodwill and long-lived assets			$ 1,059	81,600	3,000	83,989
Restructuring and other expense (income)	416			(313)	3,059	(313)
Total assets	75,603				75,603		94,506
Other
Segment Reporting Information [Line Items]
Net sales	1,780			10,606	8,248	36,148
Operating income (loss)	(1,138)			(1,287)	(1,379)	(7,071)
Impairment of goodwill and long-lived assets						1,179
Restructuring and other expense (income)	249			$ 20	(1,237)	$ 210
Total assets	$ 365,263				$ 365,263		$ 356,721