Financial Information for Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended
	Aug. 31, 2016	Aug. 31, 2015	May 31, 2016
Segment Reporting Information [Line Items]
Net sales	$ 737,549	$ 758,147
Operating income (loss)	64,898	30,996
Impairment of long-lived assets	0	3,000
Restructuring and other expense (income)	1,328	3,069
Total assets	2,181,953		$ 2,061,264
Steel Processing
Segment Reporting Information [Line Items]
Net sales	505,674	490,800
Operating income (loss)	54,782	23,638
Restructuring and other expense (income)	966	462
Total assets	871,588		819,853
Pressure Cylinders
Segment Reporting Information [Line Items]
Net sales	205,209	224,394
Operating income (loss)	14,105	16,819
Restructuring and other expense (income)	146	731
Total assets	766,070		787,786
Engineered Cabs
Segment Reporting Information [Line Items]
Net sales	25,581	38,617
Operating income (loss)	(1,843)	(9,291)
Impairment of long-lived assets		3,000
Restructuring and other expense (income)	206	1,878
Total assets	70,471		75,124
Other
Segment Reporting Information [Line Items]
Net sales	1,085	4,336
Operating income (loss)	(2,146)	(170)
Restructuring and other expense (income)	10	$ (2)
Total assets	$ 473,824		$ 378,501