Fair Value Measurements and Derivative Instruments - Effect of Derivative Instruments on Company's Financial Performance (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	May 31, 2013	May 31, 2012	May 31, 2013	May 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income	$ 274	$ (51)	$ (36)	$ (1,078)
Cash Flow Hedges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in OCI	(27)	(1,075)	386	(716)
Amount of Gain (Loss) Reclassified from AOCI into Income	241	36	4,016	1,543
Amount of Gain (Loss) Recognized in Income	(474)	(138)	(600)	(317)
Cash Flow Hedges [Member] | Corn Futures [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in OCI	530	(1,075)	(43)	(1,825)
Amount of Gain (Loss) Reclassified from AOCI into Income	596	91	4,087	2,035
Amount of Gain (Loss) Recognized in Income	(474)	(138)	(589)	(317)
Cash Flow Hedges [Member] | Natural Gas Futures [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in OCI				(41)
Amount of Gain (Loss) Reclassified from AOCI into Income		(37)		(531)
Cash Flow Hedges [Member] | Ethanol Futures [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in OCI	(557)		429	1,150
Amount of Gain (Loss) Reclassified from AOCI into Income	(355)	(18)	(71)	39
Amount of Gain (Loss) Recognized in Income			(11)
Fair Value Hedges [Member] | Corn Futures [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income	143	(21)	127	89
Fair Value Hedges [Member] | Natural Gas Futures [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income	294	(34)	(16)	(1,082)
Fair Value Hedges [Member] | FX Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income		(3)		6
Fair Value Hedges [Member] | Soybean Oil Futures [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income	(20)
Fair Value Hedges [Member] | Soybean Meal Futures [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income		(14)		(14)
Fair Value Hedges [Member] | Natural Gas Options [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income			$ (20)	$ 12