Segment Operations - Financial Information for Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Nov. 30, 2018	Nov. 30, 2017	Nov. 30, 2018	Nov. 30, 2017	May 31, 2018
Segment Reporting Information [Line Items]
Net sales	$ 958,226	$ 871,266	$ 1,946,333	$ 1,719,503
Operating income (loss)	35,864	52,059	86,775	94,284
Impairment of goodwill and long-lived assets		8,289	2,381	8,289
Restructuring and other expense (income), net	402	(9,694)	(534)	(7,390)
Total assets	2,562,437		2,562,437		$ 2,621,787
Steel Processing
Segment Reporting Information [Line Items]
Net sales	635,043	538,390	1,295,530	1,081,881
Operating income (loss)	25,016	41,130	64,676	74,002
Restructuring and other expense (income), net		(10,335)	(9)	(10,056)
Total assets	963,717		963,717		999,238
Pressure Cylinders
Segment Reporting Information [Line Items]
Net sales	294,447	300,862	594,800	570,673
Operating income (loss)	14,758	24,675	29,491	35,133
Impairment of goodwill and long-lived assets		964	2,381	964
Restructuring and other expense (income), net	402	488	(525)	2,365
Total assets	1,128,656		1,128,656		1,147,268
Engineered Cabs
Segment Reporting Information [Line Items]
Net sales	28,729	30,404	55,981	62,350
Operating income (loss)	(3,371)	(1,587)	(7,682)	(1,948)
Restructuring and other expense (income), net		(82)		(78)
Total assets	66,590		66,590		66,456
Other
Segment Reporting Information [Line Items]
Net sales	7	1,610	22	4,599
Operating income (loss)	(539)	(12,159)	290	(12,903)
Impairment of goodwill and long-lived assets		7,325		7,325
Restructuring and other expense (income), net		$ 235		$ 379
Total assets	$ 403,474		$ 403,474		$ 408,825