Summary of Changes in Carrying Amount of Goodwill (Detail) - USD ($) $ in Thousands	12 Months Ended
	May 31, 2017	May 31, 2016
Goodwill [Line Items]
Goodwill, beginning balance	$ 412,594	$ 405,526
Accumulated impairment losses, beginning balance	(166,527)	(166,527)
Goodwill net, beginning balance	246,067	238,999
Acquisitions and purchase accounting adjustments	854	7,171
Translation adjustments	752	(103)
Goodwill, Period Increase (Decrease)	1,606	7,068
Goodwill, ending balance	414,200	412,594
Accumulated impairment losses, ending balance	(166,527)	(166,527)
Goodwill net, ending balance	247,673	246,067
Steel Processing
Goodwill [Line Items]
Goodwill, beginning balance	7,045	6,587
Goodwill net, beginning balance	7,045	6,587
Acquisitions and purchase accounting adjustments	854	458
Goodwill, Period Increase (Decrease)	854	458
Goodwill, ending balance	7,899	7,045
Goodwill net, ending balance	7,899	7,045
Pressure Cylinders
Goodwill [Line Items]
Goodwill, beginning balance	233,371	226,761
Goodwill net, beginning balance	233,371	226,761
Acquisitions and purchase accounting adjustments		6,713
Translation adjustments	752	(103)
Goodwill, Period Increase (Decrease)	752	6,610
Goodwill, ending balance	234,123	233,371
Goodwill net, ending balance	234,123	233,371
Engineered Cabs
Goodwill [Line Items]
Goodwill, beginning balance	44,933	44,933
Accumulated impairment losses, beginning balance	(44,933)	(44,933)
Goodwill, ending balance	44,933	44,933
Accumulated impairment losses, ending balance	(44,933)	(44,933)
Other
Goodwill [Line Items]
Goodwill, beginning balance	127,245	127,245
Accumulated impairment losses, beginning balance	(121,594)	(121,594)
Goodwill net, beginning balance	5,651	5,651
Goodwill, ending balance	127,245	127,245
Accumulated impairment losses, ending balance	(121,594)	(121,594)
Goodwill net, ending balance	$ 5,651	$ 5,651