Segment Data - Financial Information for Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended											12 Months Ended
	May 31, 2018		Feb. 28, 2018	Nov. 30, 2017		Aug. 31, 2017	May 31, 2017	Feb. 28, 2017	Nov. 30, 2016	Aug. 31, 2016	Aug. 31, 2015	May 31, 2018	May 31, 2017	May 31, 2016
Segment Reporting Information [Line Items]
Net sales	$ 1,020,460		$ 841,657	$ 871,266		$ 848,237	$ 845,343	$ 703,436	$ 727,780	$ 737,549		$ 3,581,620	$ 3,014,108	$ 2,819,714
Operating income (loss)												141,610	213,121	122,052
Depreciation and amortization												103,359	86,793	84,699
Impairment of goodwill and long-lived assets	52,919	[1]		$ 8,289	[1]							61,208		25,962
Restructuring and other expense (income), net												(7,421)	6,411	7,177
Total assets	2,621,787						2,325,344					2,621,787	2,325,344	2,061,264
Total capital expenditures												76,088	68,386	97,036
Steel Processing
Segment Reporting Information [Line Items]
Net sales												2,252,771	2,074,869	1,843,661
Operating income (loss)												152,690	170,481	112,001
Depreciation and amortization												43,331	42,861	38,523
Restructuring and other expense (income), net												(10,087)	1,828	4,110
Total assets	999,238						882,863					999,238	882,863	819,853
Total capital expenditures												31,966	40,775	42,063
Pressure Cylinders
Segment Reporting Information [Line Items]
Net sales												1,206,183	829,846	844,898
Operating income (loss)												23,396	54,098	28,375
Depreciation and amortization												46,691	31,052	32,403
Impairment of goodwill and long-lived assets												53,883		22,962
Restructuring and other expense (income), net												2,365	3,411	392
Total assets	1,147,268						766,611					1,147,268	766,611	787,786
Total capital expenditures												32,697	24,798	29,916
Engineered Cabs
Segment Reporting Information [Line Items]
Net sales												116,631	101,388	121,946
Operating income (loss)												(11,305)	(7,685)	(19,331)
Depreciation and amortization												5,415	5,197	6,205
Impairment of goodwill and long-lived assets											$ 3,000			3,000
Restructuring and other expense (income), net												(78)	1,219	3,570
Total assets	66,456						62,141					66,456	62,141	75,124
Total capital expenditures												2,067	755	6,945
Other
Segment Reporting Information [Line Items]
Net sales												6,035	8,005	9,209
Operating income (loss)												(23,171)	(3,773)	1,007
Depreciation and amortization												7,922	7,683	7,568
Impairment of goodwill and long-lived assets												7,325
Restructuring and other expense (income), net												379	(47)	(895)
Total assets	$ 408,825						$ 613,729					408,825	613,729	378,501
Total capital expenditures												$ 9,358	$ 2,058	$ 18,112

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