Summary of Significant Accounting Policies - Derivative Assets and Liabilities Not Designated As Hedging Instruments (Details) (Not Designated as Hedging Instrument, USD $) In Thousands, unless otherwise specified	Feb. 28, 2013	Aug. 31, 2012	Feb. 29, 2012
Derivatives, Fair Value [Line Items]
Derivative Assets	$ 500,719	$ 1,071,778	$ 456,153
Derivative Liabilities	429,975	730,878	544,128
Commodity and freight derivatives
Derivatives, Fair Value [Line Items]
Derivative Assets	485,219	1,070,800	456,153
Derivative Liabilities	428,866	727,946	543,470
Foreign exchange derivatives
Derivatives, Fair Value [Line Items]
Derivative Assets	15,500	978
Derivative Liabilities	691	2,388	72
Interest rate derivatives
Derivatives, Fair Value [Line Items]
Derivative Liabilities	$ 418	$ 544	$ 586