Fair Value Measurements and Derivative Instruments - Effect of Derivative Instruments on Company's Financial Performance (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Aug. 31, 2013	Aug. 31, 2012	Aug. 31, 2011
Cash Flow Hedges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in AOCI	$ 1,204	$ 3,734	$ (9,708)
Amount of Gain (Loss) Reclassified from AOCI into Income	5,228	2,271	(11,825)
Amount of Gain (Loss) Recognized in Income	(1,021)	36	(247)
Cash Flow Hedges [Member] | Corn Futures [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in AOCI	(407)	2,067	(4,949)
Amount of Gain (Loss) Reclassified from AOCI into Income	5,351	1,500	(7,418)
Amount of Gain (Loss) Recognized in Income	(870)	36	(162)
Cash Flow Hedges [Member] | Natural Gas Futures [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in AOCI			(579)
Amount of Gain (Loss) Reclassified from AOCI into Income		(492)	(1,332)
Amount of Gain (Loss) Recognized in Income			(85)
Cash Flow Hedges [Member] | Ethanol Futures [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in AOCI	1,611	1,667	(4,180)
Amount of Gain (Loss) Reclassified from AOCI into Income	(123)	1,263	(3,075)
Amount of Gain (Loss) Recognized in Income	(151)
Fair Value Hedges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income	94	(1,078)
Fair Value Hedges [Member] | FX Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income		6
Fair Value Hedges [Member] | Soybean Oil Futures [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income		12
Fair Value Hedges [Member] | Soybean Meal Futures [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income		(14)
Fair Value Hedges [Member] | Corn Futures [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income	363	98	28
Fair Value Hedges [Member] | Natural Gas Futures [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income	$ 94	$ (1,082)