Segment Data - Financial Information for Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	May 31, 2019		Feb. 28, 2019	Nov. 30, 2018	Aug. 31, 2018		May 31, 2018		Feb. 28, 2018	Nov. 30, 2017		Aug. 31, 2017	May 31, 2019	May 31, 2018	May 31, 2017
Segment Reporting Information [Line Items]
Net sales	$ 938,842		$ 874,381	$ 958,226	$ 988,107		$ 1,020,460		$ 841,657	$ 871,266		$ 848,237	$ 3,759,556	$ 3,581,620	$ 3,014,108
Operating income (loss)													144,764	141,610	213,121
Depreciation and amortization													95,602	103,359	86,793
Impairment of goodwill and long-lived assets	5,436	[1]			$ 2,381	[1]	52,919	[1]		$ 8,289	[1]		7,817	61,208
Restructuring and other expense (income), net													(11,018)	(7,421)	6,411
Total assets	2,510,796						2,621,787						2,510,796	2,621,787	2,325,344
Total capital expenditures													84,499	76,088	68,386
Steel Processing
Segment Reporting Information [Line Items]
Net sales													2,435,818	2,252,771	2,074,869
Operating income (loss)													89,761	152,690	170,481
Depreciation and amortization													40,374	43,331	42,861
Impairment of goodwill and long-lived assets													3,269
Restructuring and other expense (income), net													(9)	(10,087)	1,828
Total assets	924,966						999,238						924,966	999,238	882,863
Total capital expenditures													39,114	31,966	40,775
Pressure Cylinders
Segment Reporting Information [Line Items]
Net sales													1,207,798	1,206,183	829,846
Operating income (loss)													69,872	23,396	54,098
Depreciation and amortization													42,403	46,691	31,052
Impairment of goodwill and long-lived assets													4,548	53,883
Restructuring and other expense (income), net													(11,009)	2,365	3,411
Total assets	1,123,115						1,147,268						1,123,115	1,147,268	766,611
Total capital expenditures													37,558	32,697	24,798
Engineered Cabs
Segment Reporting Information [Line Items]
Net sales													115,902	116,631	101,388
Operating income (loss)													(14,664)	(11,305)	(7,685)
Depreciation and amortization													5,687	5,415	5,197
Restructuring and other expense (income), net														(78)	1,219
Total assets	66,226						66,456						66,226	66,456	62,141
Total capital expenditures													1,131	2,067	755
Other
Segment Reporting Information [Line Items]
Net sales													38	6,035	8,005
Operating income (loss)													(205)	(23,171)	(3,773)
Depreciation and amortization													7,138	7,922	7,683
Impairment of goodwill and long-lived assets														7,325
Restructuring and other expense (income), net														379	(47)
Total assets	$ 396,489						$ 408,825						396,489	408,825	613,729
Total capital expenditures													$ 6,696	$ 9,358	$ 2,058

[1] For additional information regarding the Company’s impairment activity, refer to “Note D – Goodwill and Other Long-Lived Assets.”