Schedule of Major Classifications of Inventories (Detail) (USD $) In Thousands, unless otherwise specified	Apr. 30, 2013	Jul. 31, 2012
Schedule of Inventory [Line Items]
Raw materials	$ 125,702	$ 104,446
Chassis	47,347	39,044
Work in process	66,723	59,803
Finished goods	31,613	16,570
Total	271,385	219,863
Excess of FIFO costs over LIFO costs	(35,205)	(33,780)
Total inventories	$ 236,180	$ 186,083