Financial Information for Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended
	Aug. 31, 2017	Aug. 31, 2016	May 31, 2017
Segment Reporting Information [Line Items]
Net sales	$ 848,237	$ 737,549
Operating income (loss)	42,225	64,898
Restructuring and other expense	2,304	1,328
Total assets	2,630,554		$ 2,325,344
Steel Processing
Segment Reporting Information [Line Items]
Net sales	543,491	505,674
Operating income (loss)	32,872	54,782
Restructuring and other expense	279	966
Total assets	882,161		882,863
Pressure Cylinders
Segment Reporting Information [Line Items]
Net sales	269,811	205,209
Operating income (loss)	10,458	14,105
Restructuring and other expense	1,877	146
Total assets	1,176,298		766,611
Engineered Cabs
Segment Reporting Information [Line Items]
Net sales	31,946	25,581
Operating income (loss)	(361)	(1,843)
Restructuring and other expense	4	206
Total assets	68,738		62,141
Other
Segment Reporting Information [Line Items]
Net sales	2,989	1,085
Operating income (loss)	(744)	(2,146)
Restructuring and other expense	144	$ 10
Total assets	$ 503,357		$ 613,729