Assets Measured at Fair Value on Non-recurring Basis (Detail) - USD ($) $ in Thousands		Aug. 31, 2015		May. 31, 2015
Fair Value [Line Items]
Asset measured at fair value on nonrecurring basis		$ 1,059		$ 12,403
Long-lived Assets Held and Used
Fair Value [Line Items]
Asset measured at fair value on nonrecurring basis		1,059	[1]	12,403	[2]
Significant Other Observable Inputs (Level 2)
Fair Value [Line Items]
Asset measured at fair value on nonrecurring basis		1,059
Significant Other Observable Inputs (Level 2) | Long-lived Assets Held and Used
Fair Value [Line Items]
Asset measured at fair value on nonrecurring basis	[1]	$ 1,059
Significant Unobservable Inputs (Level 3)
Fair Value [Line Items]
Asset measured at fair value on nonrecurring basis				12,403
Significant Unobservable Inputs (Level 3) | Long-lived Assets Held and Used
Fair Value [Line Items]
Asset measured at fair value on nonrecurring basis	[2]			$ 12,403

[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.