Fair Value Measurements and Derivative Instruments - Fair Value of Company's Derivative by Contract Type (Detail) (USD $) In Thousands, unless otherwise specified	Feb. 28, 2013	Aug. 31, 2012
Derivatives, Fair Value [Line Items]
Assets Fair Value	$ 173	$ 12
Liabilities Fair Value	592	1,434
Corn Futures [Member] | Cash Flow Hedging [Member] | Designated as Hedging Instrument [Member] | Other Current Assets [Member] | Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Assets Fair Value		12
Liabilities Fair Value	451	126
Corn Futures [Member] | Fair Value Hedging [Member] | Designated as Hedging Instrument [Member] | Other Current Assets [Member] | Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Assets Fair Value	173
Liabilities Fair Value	44	602
Ethanol Futures [Member] | Cash Flow Hedging [Member] | Designated as Hedging Instrument [Member] | Other Current Assets [Member] | Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Assets Fair Value
Liabilities Fair Value	4	706
Natural Gas Futures [Member] | Not Designated as Hedging Instrument [Member] | Other Current Assets [Member] | Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Assets Fair Value
Liabilities Fair Value	$ 93