Segment Operations - Financial Information for Reportable Segments (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Feb. 28, 2021	Feb. 29, 2020	Feb. 28, 2021	Feb. 29, 2020	May 31, 2020
Segment Reporting Information [Line Items]
Net sales		$ 759,109	$ 763,996	$ 2,193,110	$ 2,447,492
Operating income (loss)		49,772	(1,386)	57,015	16,144
Segment operating income		47,344	(2,628)	109,651	18,931
Incremental expenses related to Nikola gains		781	0	(53,300)	0
Impairment of goodwill and long-lived assets		0	34,627	13,739	75,228
Restructuring and other expense (income), net		28,212	1,376	37,656	1,781
Total assets		3,137,873		3,137,873		$ 2,331,515
Steel Processing
Segment Reporting Information [Line Items]
Net sales		504,477	491,136	1,404,220	1,531,448
Operating income (loss)		62,874	19,021	114,315	42,361
Impairment of goodwill and long-lived assets		0	1,274	0	1,274
Restructuring and other expense (income), net		(42)	728	1,804	702
Total assets		1,041,246		1,041,246		821,657
Pressure Cylinders
Segment Reporting Information [Line Items]
Net sales		254,643	270,995	787,831	865,527
Operating income (loss)		(15,641)	(19,865)	(3,694)	25,405
Impairment of goodwill and long-lived assets		0	33,353	13,739	33,353
Restructuring and other expense (income), net		28,435	747	36,006	747
Total assets		1,160,884		1,160,884		1,104,603
Other
Segment Reporting Information [Line Items]
Net sales		(11)	1,865	1,059	50,517
Operating income (loss)		111	(1,784)	(970)	(48,835)
Impairment of goodwill and long-lived assets		0	0	0	40,601
Restructuring and other expense (income), net		(181)	(99)	(154)	332
Total assets	[1]	935,743		935,743		$ 405,255
Unallocated Corporate Other
Segment Reporting Information [Line Items]
Operating income (loss)		$ 1,647	$ 1,242	$ 664	$ (2,787)

[1] The increase in other assets primarily relates to cash proceeds the Company received from the sale of its Nikola shares of common stock.  For additional information, refer to “NOTE C – Investment in Nikola”.