Financial Information for Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Nov. 30, 2016	Nov. 30, 2015	Nov. 30, 2016	Nov. 30, 2015	May 31, 2016
Segment Reporting Information [Line Items]
Net sales	$ 727,780	$ 699,816	$ 1,465,329	$ 1,457,963
Operating income (loss)	43,044	11,972	107,942	42,968
Impairment of long-lived assets		22,962	0	25,962
Restructuring and other expense (income)	3,272	1,523	4,600	4,592
Total assets	2,142,724		2,142,724		$ 2,061,264
Steel Processing
Segment Reporting Information [Line Items]
Net sales	508,806	467,812	1,014,480	958,612
Operating income (loss)	35,448	26,642	90,230	50,280
Restructuring and other expense (income)	318	2,258	1,284	2,720
Total assets	836,265		836,265		819,853
Pressure Cylinders
Segment Reporting Information [Line Items]
Net sales	194,661	201,173	399,870	425,567
Operating income (loss)	11,304	(10,309)	25,409	6,510
Impairment of long-lived assets		22,962		22,962
Restructuring and other expense (income)	1,963	(16)	2,109	715
Total assets	763,716		763,716		787,786
Engineered Cabs
Segment Reporting Information [Line Items]
Net sales	22,463	28,699	48,044	67,316
Operating income (loss)	(3,381)	(4,290)	(5,224)	(13,581)
Impairment of long-lived assets				3,000
Restructuring and other expense (income)	1,004	765	1,210	2,643
Total assets	65,273		65,273		75,124
Other
Segment Reporting Information [Line Items]
Net sales	1,850	2,132	2,935	6,468
Operating income (loss)	(327)	(71)	(2,473)	(241)
Restructuring and other expense (income)	(13)	$ (1,484)	(3)	$ (1,486)
Total assets	$ 477,470		$ 477,470		$ 378,501