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
	Nov. 30, 2012		Nov. 30, 2011
Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in OCI	$ 498		$ (441)
Amount of Gain (Loss) Reclassified from AOCI into Income	3,525		968
Amount of Gain (Loss) Recognized in Income	186		(311)
Cash Flow Hedging [Member] | Corn Futures [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in OCI	(255)	[1]	(1,609)	[1]
Amount of Gain (Loss) Reclassified from AOCI into Income	3,432	[1]	2,074	[1]
Amount of Gain (Loss) Recognized in Income	197	[1]	(311)	[1]
Cash Flow Hedging [Member] | Natural Gas Futures [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in OCI		[1]		[1]
Amount of Gain (Loss) Reclassified from AOCI into Income		[1]	(272)	[1]
Amount of Gain (Loss) Recognized in Income		[1]		[1]
Cash Flow Hedging [Member] | Ethanol Futures [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in OCI	753	[1]	1,168	[1]
Amount of Gain (Loss) Reclassified from AOCI into Income	93	[1]	(834)	[1]
Amount of Gain (Loss) Recognized in Income	(11)	[1]		[1]
Fair Value Hedging [Member] | Corn Futures [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income	15	[1],[2]	(99)	[1],[2]
Fair Value Hedging [Member] | Natural Gas Futures [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Income	$ (171)	[1]	$ (712)	[1]

[1] Gains and losses reported in cost of sales

[2] Hedged items are firm commitments and inventory