Schedule of Major Classifications of Inventories (Detail) (USD $) In Thousands, unless otherwise specified	Jul. 31, 2012	Jul. 31, 2011
Schedule of Inventory [Line Items]
Finished products	$ 16,570	$ 14,179
Work in process	59,803	48,575
Raw materials	104,446	104,360
Chassis	39,044	46,548
Subtotal	219,863	213,662
Excess of FIFO costs over LIFO costs	(33,780)	(29,164)
Total inventories	$ 186,083	$ 184,498