Financial Information for Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended
	Aug. 31, 2015	Aug. 31, 2014	May. 31, 2015
Segment Reporting Information [Line Items]
Net sales	$ 758,147	$ 862,414
Operating income (loss)	30,996	52,202
Impairment of long-lived assets	3,000	1,950
Restructuring and other expense (income)	3,069	100
Total assets	2,045,910		$ 2,085,142
Steel Processing
Segment Reporting Information [Line Items]
Net sales	490,800	552,331
Operating income (loss)	23,638	35,869
Impairment of long-lived assets		1,950
Restructuring and other expense (income)	462	(30)
Total assets	818,834		829,116
Pressure Cylinders
Segment Reporting Information [Line Items]
Net sales	224,394	248,959
Operating income (loss)	16,819	19,606
Restructuring and other expense (income)	731	23
Total assets	791,752		804,799
Engineered Cabs
Segment Reporting Information [Line Items]
Net sales	38,617	49,554
Operating income (loss)	(9,291)	(2,145)
Impairment of long-lived assets	3,000
Restructuring and other expense (income)	1,878
Total assets	89,018		94,506
Other
Segment Reporting Information [Line Items]
Net sales	4,336	11,570
Operating income (loss)	(170)	(1,128)
Restructuring and other expense (income)	(2)	$ 107
Total assets	$ 346,306		$ 356,721