Fair Value Measurements and Derivative Instruments - Fair Value of Company's Derivative by Contract Type (Detail) (USD $) In Thousands, unless otherwise specified	May 31, 2013	Aug. 31, 2012
Derivatives, Fair Value [Line Items]
Assets Fair Value	$ 254	$ 12
Liabilities Fair Value	890	1,434
Corn Futures [Member] | Cash Flow Hedges [Member] | Designated as Hedging Instrument [Member] | Other Current Assets [Member] | Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Assets Fair Value	50	12
Liabilities Fair Value	323	126
Corn Futures [Member] | Fair Value Hedges [Member] | Designated as Hedging Instrument [Member] | Other Current Assets [Member] | Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Assets Fair Value	125
Liabilities Fair Value		602
Ethanol Futures [Member] | Cash Flow Hedges [Member] | Designated as Hedging Instrument [Member] | Other Current Assets [Member] | Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Liabilities Fair Value	567	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	56
Natural Gas Options [Member] | Not Designated as Hedging Instrument [Member] | Other Current Assets [Member] | Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Assets Fair Value	$ 23