Financial Information for Reportable Segments (Detail) (USD $)	3 Months Ended								12 Months Ended
	May 31, 2014	Feb. 28, 2014	Nov. 30, 2013	Aug. 31, 2013	May 31, 2013	Feb. 28, 2013	Nov. 30, 2012	Aug. 31, 2012	May 31, 2014	May 31, 2013	May 31, 2012
Segment Reporting Information [Line Items]
Net sales	$ 891,005,000	$ 773,230,000	$ 769,900,000	$ 692,291,000	$ 704,060,000	$ 619,527,000	$ 622,622,000	$ 666,035,000	$ 3,126,426,000	$ 2,612,244,000	$ 2,534,701,000
Operating income (loss)									135,753,000	129,142,000	101,610,000
Depreciation and amortization									79,730,000	66,469,000	55,873,000
Impairment of long-lived assets									58,246,000	6,488,000	355,000
Restructuring and other expense (income)									(2,912,000)	3,293,000	5,984,000
Joint venture transactions									1,036,000	(604,000)	(150,000)
Total assets	2,296,381,000				1,950,857,000				2,296,381,000	1,950,857,000	1,877,797,000
Total capital expenditures									71,338,000	44,588,000	31,713,000
Steel Processing
Segment Reporting Information [Line Items]
Net sales									1,936,073,000	1,462,630,000	1,598,812,000
Operating income (loss)									119,025,000	68,156,000	73,626,000
Depreciation and amortization									32,882,000	25,918,000	27,093,000
Impairment of long-lived assets									7,141,000
Restructuring and other expense (income)									(3,382,000)
Joint venture transactions											(2,102,000)
Total assets	850,748,000				610,464,000				850,748,000	610,464,000	709,030,000
Total capital expenditures									16,682,000	8,371,000	9,705,000
Pressure Cylinders
Segment Reporting Information [Line Items]
Net sales									928,396,000	859,264,000	770,101,000
Operating income (loss)									55,004,000	66,367,000	45,108,000
Depreciation and amortization									31,984,000	26,919,000	20,407,000
Impairment of long-lived assets									32,005,000	6,488,000
Restructuring and other expense (income)									(745,000)	2,665,000	52,000
Total assets	818,720,000				742,686,000				818,720,000	742,686,000	575,250,000
Total capital expenditures									32,364,000	13,368,000	8,456,000
Engineered Cabs
Segment Reporting Information [Line Items]
Net sales									200,528,000	226,002,000	104,272,000
Operating income (loss)									(26,516,000)	4,158,000	4,878,000
Depreciation and amortization									10,027,000	9,096,000	3,540,000
Impairment of long-lived assets									19,100,000
Total assets	181,251,000				201,048,000				181,251,000	201,048,000	199,594,000
Total capital expenditures									10,351,000	6,265,000	4,641,000
Other
Segment Reporting Information [Line Items]
Net sales									61,429,000	64,348,000	61,516,000
Operating income (loss)									(11,760,000)	(9,539,000)	(22,002,000)
Depreciation and amortization									4,837,000	4,536,000	4,833,000
Impairment of long-lived assets											355,000
Restructuring and other expense (income)									1,215,000	628,000	5,932,000
Joint venture transactions									1,036,000	(604,000)	1,952,000
Total assets	445,662,000				396,659,000				445,662,000	396,659,000	393,923,000
Total capital expenditures									$ 11,941,000	$ 16,584,000	$ 8,911,000