Segment Operations - Financial Information for Reportable Segments (Detail) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Nov. 30, 2020	Nov. 30, 2019	Nov. 30, 2020	Nov. 30, 2019	May 31, 2020
Segment Reporting Information [Line Items]
Net sales		$ 731,092	$ 827,637	$ 1,434,001	$ 1,683,496
Operating income (loss)		37,364	32,118	7,243	17,530
Segment operating income		40,807	30,900	62,307	21,559
Impairment of long-lived assets		3,815	0	13,739	40,601
Restructuring and other expense (income), net		7,596	(50)	9,444	405
Total assets		3,027,385		3,027,385		$ 2,331,515
Steel Processing
Segment Reporting Information [Line Items]
Net sales		468,723	516,937	899,743	1,040,312
Operating income (loss)		37,824	17,172	51,441	23,340
Restructuring and other expense (income), net		375		1,846	(26)
Total assets		886,578		886,578		821,657
Pressure Cylinders
Segment Reporting Information [Line Items]
Net sales		262,284	290,136	533,188	594,532
Operating income (loss)		3,305	15,647	11,947	45,270
Impairment of long-lived assets		3,815		13,739
Restructuring and other expense (income), net		7,257		7,571
Total assets		1,016,404		1,016,404		1,104,603
Other
Segment Reporting Information [Line Items]
Net sales		85	20,564	1,070	48,652
Operating income (loss)		(322)	(1,919)	(1,081)	(47,051)
Impairment of long-lived assets					40,601
Restructuring and other expense (income), net		(36)	(50)	27	431
Total assets	[1]	1,124,403		1,124,403		$ 405,255
Unallocated Corporate Other
Segment Reporting Information [Line Items]
Operating income (loss)		1,127	$ 1,218	(983)	$ (4,029)
Incremental Expenses Related To Nikola Gains
Segment Reporting Information [Line Items]
Operating income (loss)		$ (4,570)		$ (54,081)

[1] The material changes in total assets primarily relate to the Company’s investment in Nikola.  For additional information, refer to “NOTE C – Investment in Nikola”.