Financial Information for Reportable Segments (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Aug. 31, 2013	Aug. 31, 2012	May 31, 2013
Segment Reporting Information [Line Items]
Net sales	$ 692,291	$ 666,035
Operating income (loss)	38,638	33,418
Restructuring and other expense (income)	(3,997)	403
Impairment of long-lived assets	4,641	1,570
Joint venture transactions	142	(1,162)
Total assets	2,093,060	1,950,857	1,950,857
Steel Processing
Segment Reporting Information [Line Items]
Net sales	402,441	385,013
Operating income (loss)	22,663	16,659
Restructuring and other expense (income)	(4,762)
Impairment of long-lived assets	4,641
Total assets	843,385	610,464
Pressure Cylinders
Segment Reporting Information [Line Items]
Net sales	216,900	194,236
Operating income (loss)	19,454	15,026
Restructuring and other expense (income)	402	6
Impairment of long-lived assets		1,570
Total assets	718,791	742,686
Engineered Cabs
Segment Reporting Information [Line Items]
Net sales	48,461	64,495
Operating income (loss)	(304)	4,694
Total assets	200,815	201,048
Other
Segment Reporting Information [Line Items]
Net sales	24,489	22,291
Operating income (loss)	(3,175)	(2,961)
Restructuring and other expense (income)	363	397
Joint venture transactions	142	(1,162)
Total assets	$ 330,069	$ 396,659