Financial Information for Reportable Segments (Detail) - Entity [Domain] - USD ($) $ in Thousands	1 Months Ended	3 Months Ended															12 Months Ended
	Oct. 31, 2012	May. 31, 2015		Feb. 28, 2015		Nov. 30, 2014		Aug. 31, 2014		May. 31, 2014		Feb. 28, 2014	Nov. 30, 2013		Aug. 31, 2013		May. 31, 2015	May. 31, 2014	May. 31, 2013
Segment Reporting Information [Line Items]
Net sales		$ 846,023		$ 804,785		$ 871,012		$ 862,414		$ 891,005		$ 773,230	$ 769,900		$ 692,291		$ 3,384,234	$ 3,126,426	$ 2,612,244
Operating income (loss)																	60,557	135,753	129,142
Depreciation and amortization																	85,089	79,730	66,469
Impairment of goodwill and long-lived assets		2,344	[1]	$ 81,600	[1]	$ 14,235	[1]	$ 1,950	[1]	22,871	[1]		$ 30,734	[1]	4,641	[1]	100,129	58,246	6,488
Restructuring and other expense (income)																	6,514	(2,912)	3,293
Joint venture transactions																	413	1,036	(604)
Total assets		2,085,142								2,296,381							2,085,142	2,296,381	1,950,857
Total capital expenditures																	96,255	71,338	44,588
Steel Processing
Segment Reporting Information [Line Items]
Net sales																	2,145,744	1,936,073	1,462,630
Operating income (loss)																	108,707	119,025	68,156
Depreciation and amortization																	34,526	32,882	25,918
Impairment of goodwill and long-lived assets																	3,050	7,141
Restructuring and other expense (income)																	72	(3,382)
Total assets		829,116								850,748							829,116	850,748	610,464
Total capital expenditures																	34,546	16,682	8,371
Pressure Cylinders
Segment Reporting Information [Line Items]
Net sales																	1,001,402	928,396	859,264
Operating income (loss)																	58,113	55,004	66,367
Depreciation and amortization																	34,953	31,984	26,919
Impairment of goodwill and long-lived assets	$ 1,520														$ 1,570		11,911	32,005	6,488
Restructuring and other expense (income)																	6,408	(745)	2,665
Total assets		804,799								818,720							804,799	818,720	742,686
Total capital expenditures																	35,872	32,364	13,368
Engineered Cabs
Segment Reporting Information [Line Items]
Net sales																	192,953	200,528	226,002
Operating income (loss)																	(97,260)	(26,516)	4,158
Depreciation and amortization																	10,184	10,027	9,096
Impairment of goodwill and long-lived assets																	83,989	19,100
Restructuring and other expense (income)																	(332)
Total assets		94,506								181,251							94,506	181,251	201,048
Total capital expenditures																	8,951	10,351	6,265
Other
Segment Reporting Information [Line Items]
Net sales																	44,135	61,429	64,348
Operating income (loss)																	(9,003)	(11,760)	(9,539)
Depreciation and amortization																	5,426	4,837	4,536
Impairment of goodwill and long-lived assets																	1,179
Restructuring and other expense (income)																	366	1,215	628
Joint venture transactions																	413	1,036	(604)
Total assets		$ 356,721								$ 445,662							356,721	445,662	396,659
Total capital expenditures																	$ 16,886	$ 11,941	$ 16,584

[1] For additional information regarding the Company's impairment charges, refer to "Note C - Goodwill and Other Long-Lived Assets."