Financial Information for Reportable Segments (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended			9 Months Ended
	Feb. 28, 2013	Aug. 31, 2012	Feb. 29, 2012	Feb. 28, 2013	Feb. 29, 2012	May 31, 2012
Segment Reporting Information [Line Items]
Net sales	$ 619,527		$ 611,255	$ 1,908,184	$ 1,779,294
Operating income (loss)	33,406		18,075	95,646	42,107
Restructuring and other expense (income)	146		956	1,811	4,707
Impairment of long-lived assets				1,520
Joint venture transactions	253		1,812	(1,188)	3,835
Total assets	1,885,182			1,885,182		1,877,797
Steel Processing
Segment Reporting Information [Line Items]
Net sales	349,569		367,259	1,068,854	1,148,894
Operating income (loss)	17,504		15,405	46,837	39,069
Total assets	624,494			624,494		703,336
Pressure Cylinders
Segment Reporting Information [Line Items]
Net sales	205,206		187,737	606,936	533,283
Operating income (loss)	17,860		10,887	49,965	23,333
Restructuring and other expense (income)	177			183
Impairment of long-lived assets		1,570		1,520
Total assets	639,286			639,286		575,250
Engineered Cabs
Segment Reporting Information [Line Items]
Net sales	48,628		40,173	170,927	40,173
Operating income (loss)	108		(1,447)	5,367	(1,447)
Total assets	202,489			202,489		199,594
Other
Segment Reporting Information [Line Items]
Net sales	16,124		16,086	61,467	56,944
Operating income (loss)	(2,066)		(6,770)	(6,523)	(18,848)
Restructuring and other expense (income)	(31)		956	1,628	4,707
Joint venture transactions	253		1,812	(1,188)	3,835
Total assets	$ 418,913			$ 418,913		$ 399,617