Financial Information for Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Nov. 30, 2017	Nov. 30, 2016	Nov. 30, 2017	Nov. 30, 2016	May 31, 2017
Segment Reporting Information [Line Items]
Net sales	$ 871,266	$ 727,780	$ 1,719,503	$ 1,465,329
Operating income (loss)	52,059	43,044	94,284	107,942
Impairment of goodwill and long-lived assets	8,289		8,289
Restructuring and other expense (income), net	(9,694)	3,272	(7,390)	4,600
Total assets	2,571,713		2,571,713		$ 2,325,344
Steel Processing
Segment Reporting Information [Line Items]
Net sales	538,390	508,806	1,081,881	1,014,480
Operating income (loss)	41,130	35,448	74,002	90,230
Restructuring and other expense (income), net	(10,335)	318	(10,056)	1,284
Total assets	882,710		882,710		882,863
Pressure Cylinders
Segment Reporting Information [Line Items]
Net sales	300,862	194,661	570,673	399,870
Operating income (loss)	24,675	11,304	35,133	25,409
Impairment of goodwill and long-lived assets	964		964
Restructuring and other expense (income), net	488	1,963	2,365	2,109
Total assets	1,200,361		1,200,361		766,611
Engineered Cabs
Segment Reporting Information [Line Items]
Net sales	30,404	22,463	62,350	48,044
Operating income (loss)	(1,587)	(3,381)	(1,948)	(5,224)
Restructuring and other expense (income), net	(82)	1,004	(78)	1,210
Total assets	66,957		66,957		62,141
Other
Segment Reporting Information [Line Items]
Net sales	1,610	1,850	4,599	2,935
Operating income (loss)	(12,159)	(327)	(12,903)	(2,473)
Impairment of goodwill and long-lived assets	7,325		7,325
Restructuring and other expense (income), net	235	$ (13)	379	$ (3)
Total assets	$ 421,685		$ 421,685		$ 613,729