Impairment of Long-Lived Assets - Additional Information (Detail)	3 Months Ended
	Aug. 31, 2015 USD ($) Facility		Aug. 31, 2014 USD ($)	May. 31, 2015 USD ($)
Impaired Long-Lived Assets Held and Used [Line Items]
Asset measured at fair value on nonrecurring basis	$ 1,059,000			$ 12,403,000
Impairment of long-lived assets	$ 3,000,000		$ 1,950,000
Number of equipment facilities | Facility	5
Long-lived assets total	$ 36,687,000
Maximum
Impaired Long-Lived Assets Held and Used [Line Items]
Estimated undiscounted future cash flows over book value	0.05
Long-lived Assets Held and Used
Impaired Long-Lived Assets Held and Used [Line Items]
Carrying value long-lived assets	4,059,000
Asset measured at fair value on nonrecurring basis	$ 1,059,000	[1]		$ 12,403,000	[2]

[1] During the first quarter of fiscal 2016, management reviewed certain long-lived assets of its Engineered Cabs facility in Florence, South Carolina, for impairment. In accordance with the applicable accounting guidance, long-lived assets with a carrying value of $4,059,000 were written down to their estimated fair value of $1,059,000 resulting in an impairment charge of $3,000,000 during the three months ended August 31, 2015. Comparable market transactions were used to measure fair value. Refer to "NOTE C - Impairment of Long-Lived Assets" for additional information.

[2] During the fourth quarter of fiscal 2015, management reviewed certain intangible assets related to our CNG fuel systems joint venture, dHybrid, for impairment. In accordance with the applicable accounting guidance, the intangible assets were written down to their fair value of $600,000, resulting in an impairment charge of $2,344,000. The key assumptions that drove the fair value calculation were projected cash flows and the discount rate. During the third quarter of fiscal 2015, the Company concluded that an interim impairment test of the goodwill of its Engineered Cabs operating segment was necessary. Prior to conducting the goodwill impairment test, the Company first evaluated the other long-lived assets of the Engineered Cabs operating segment for recoverability. Recoverability was tested using future cash flow projections based on management's long-range estimates of market conditions. The sum of the undiscounted future cash flows for the customer relationship intangible asset and the property, plant and equipment of the Florence, South Carolina facility were less than their respective carrying values. As a result, these assets were written down to their respective fair values of $2,000,000 and $9,803,000. The fair value measurements were based on Level 3 inputs not observable in the market. The key assumptions that drove the fair value calculations were projected cash flows and the discount rate.