Fair Value Measurements and Derivative Instruments - Fair Value of Company's Derivatives (Detail) (Other Current Assets [Member], Commodity Contract [Member], USD $) In Thousands, unless otherwise specified	May 31, 2013	Aug. 31, 2012
Derivative [Line Items]
Commodity derivatives	$ (636)	$ (1,422)
Level 1 [Member]
Derivative [Line Items]
Commodity derivatives	(636)	(1,422)
Level 2 [Member]
Derivative [Line Items]
Commodity derivatives
Level 3 [Member]
Derivative [Line Items]
Commodity derivatives