Segment Operations - Financial Information for Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Nov. 30, 2019	Nov. 30, 2018	Nov. 30, 2019	Nov. 30, 2018	May 31, 2019
Segment Reporting Information [Line Items]
Net sales	$ 827,637	$ 958,226	$ 1,683,496	$ 1,946,333
Operating income (loss)	32,118	35,864	17,530	86,775
Impairment of long-lived assets	0	0	40,601	2,381
Restructuring and other expense (income), net	(50)	402	405	(534)
Total assets	2,408,961		2,408,961		$ 2,510,796
Steel Processing
Segment Reporting Information [Line Items]
Net sales	516,937	635,043	1,040,312	1,295,530
Operating income (loss)	17,172	25,016	23,340	64,676
Restructuring and other expense (income), net			(26)	(9)
Total assets	868,597		868,597		924,966
Pressure Cylinders
Segment Reporting Information [Line Items]
Net sales	290,136	294,447	594,532	594,800
Operating income (loss)	15,647	14,758	45,270	29,491
Impairment of long-lived assets				2,381
Restructuring and other expense (income), net		402		(525)
Total assets	1,143,256		1,143,256		1,123,115
Other
Segment Reporting Information [Line Items]
Net sales	20,564	28,736	48,652	56,003
Operating income (loss)	(701)	$ (3,910)	(51,080)	$ (7,392)
Impairment of long-lived assets			40,601
Restructuring and other expense (income), net	(50)		431
Total assets	$ 397,108		$ 397,108		$ 462,715