Schedule of Major Classifications of Inventories (Detail) (USD $) In Thousands, unless otherwise specified	Jan. 31, 2013	Jul. 31, 2012
Schedule of Inventory [Line Items]
Raw materials	$ 122,455	$ 104,446
Chassis	50,066	39,044
Work in process	72,189	59,803
Finished goods	24,786	16,570
Total	269,496	219,863
Excess of FIFO costs over LIFO costs	(34,380)	(33,780)
Total inventories	$ 235,116	$ 186,083