Schedule of Major Classifications of Inventories (Detail) (USD $) In Thousands, unless otherwise specified	Oct. 31, 2012	Jul. 31, 2012
Schedule of Inventory [Line Items]
Raw materials	$ 119,516	$ 104,446
Chassis	45,640	39,044
Work in process	70,252	59,803
Finished goods	24,192	16,570
Total	259,600	219,863
Excess of FIFO costs over LIFO costs	(33,780)	(33,780)
Total inventories	$ 225,820	$ 186,083