Segment Operations - Financial Information for Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended
	Aug. 31, 2019	Aug. 31, 2018	May 31, 2019
Segment Reporting Information [Line Items]
Net sales	$ 855,859	$ 988,107
Operating income (loss)	(14,588)	50,911
Impairment of long-lived assets	40,601	2,381
Restructuring and other expense (income), net	455	(936)
Total assets	2,382,632		$ 2,510,796
Steel Processing
Segment Reporting Information [Line Items]
Net sales	523,375	660,487
Operating income (loss)	6,168	39,660
Restructuring and other expense (income), net	(26)	(9)
Total assets	882,839		924,966
Pressure Cylinders
Segment Reporting Information [Line Items]
Net sales	304,396	300,353
Operating income (loss)	29,623	14,733
Impairment of long-lived assets		2,381
Restructuring and other expense (income), net		(927)
Total assets	1,130,095		1,123,115
Engineered Cabs
Segment Reporting Information [Line Items]
Net sales	28,066	27,252
Operating income (loss)	(45,128)	(4,311)
Impairment of long-lived assets	40,601
Total assets	20,477		66,226
Other
Segment Reporting Information [Line Items]
Net sales	22	15
Operating income (loss)	(5,251)	$ 829
Restructuring and other expense (income), net	481
Total assets	$ 349,221		$ 396,489