Financial Information for Reportable Segments (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Aug. 31, 2012	Aug. 31, 2011	May 31, 2012
Segment Reporting Information [Line Items]
Net sales	$ 666,035	$ 602,387
Operating income (loss)	33,418	21,183
Restructuring and other expense	403	1,703
Impairment of long-lived assets	1,570
Joint venture transactions	(1,162)	3,215
Total assets	1,805,506		1,877,797
Steel Processing
Segment Reporting Information [Line Items]
Net sales	379,972	408,174
Operating income (loss)	16,019	16,276
Total assets	661,141		703,336
Pressure Cylinders
Segment Reporting Information [Line Items]
Net sales	194,236	168,829
Operating income (loss)	15,026	11,915
Restructuring and other expense	6
Impairment of long-lived assets	1,570
Total assets	552,524		575,250
Engineered Cabs
Segment Reporting Information [Line Items]
Net sales	64,495
Operating income (loss)	4,694
Total assets	200,466		199,594
Other
Segment Reporting Information [Line Items]
Net sales	27,332	25,384
Operating income (loss)	(2,321)	(7,008)
Restructuring and other expense	397	1,703
Joint venture transactions	(1,162)	3,215
Total assets	$ 391,375		$ 399,617