Segment Operations - Financial Information for Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended
	Aug. 31, 2018	Aug. 31, 2017	May 31, 2018
Segment Reporting Information [Line Items]
Net sales	$ 988,107	$ 848,237
Operating income (loss)	50,911	42,225
Impairment of long-lived assets	2,381
Restructuring and other expense (income), net	(936)	2,304
Total assets	2,610,175		$ 2,621,787
Steel Processing
Segment Reporting Information [Line Items]
Net sales	660,487	543,491
Operating income (loss)	39,660	32,872
Restructuring and other expense (income), net	(9)	279
Total assets	1,029,852		999,238
Pressure Cylinders
Segment Reporting Information [Line Items]
Net sales	300,353	269,811
Operating income (loss)	14,733	10,458
Impairment of long-lived assets	2,381
Restructuring and other expense (income), net	(927)	1,877
Total assets	1,131,120		1,147,268
Engineered Cabs
Segment Reporting Information [Line Items]
Net sales	27,252	31,946
Operating income (loss)	(4,311)	(361)
Restructuring and other expense (income), net		4
Total assets	64,333		66,456
Other
Segment Reporting Information [Line Items]
Net sales	15	2,989
Operating income (loss)	829	(744)
Restructuring and other expense (income), net		$ 144
Total assets	$ 384,870		$ 408,825