Financial Information for Reportable Segments (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Aug. 31, 2014	Aug. 31, 2013	May 31, 2014	May 31, 2013
Segment Reporting Information [Line Items]
Net sales	$ 862,414	$ 692,291
Operating income (loss)	52,202	38,638
Restructuring and other expense (income)	(7)	(3,997)
Impairment of long-lived assets	1,950	4,641
Joint venture transactions	107	142
Total assets	2,354,696		2,296,381	2,296,381
Steel Processing
Segment Reporting Information [Line Items]
Net sales	552,331	402,441
Operating income (loss)	35,869	22,663
Restructuring and other expense (income)	(30)	(4,762)
Impairment of long-lived assets	1,950	4,641
Total assets	896,069			850,748
Pressure Cylinders
Segment Reporting Information [Line Items]
Net sales	248,959	216,900
Operating income (loss)	19,606	19,454
Restructuring and other expense (income)	23	402
Total assets	849,168			818,720
Engineered Cabs
Segment Reporting Information [Line Items]
Net sales	49,554	48,461
Operating income (loss)	(2,145)	(304)
Total assets	181,876			181,251
Other
Segment Reporting Information [Line Items]
Net sales	11,570	24,489
Operating income (loss)	(1,128)	(3,175)
Restructuring and other expense (income)		363
Joint venture transactions	107	142
Total assets	$ 427,583			$ 445,662