Summary of Significant Accounting Policies - Derivative Instruments Designated as Hedging Instruments (Details) (USD $) In Thousands, unless otherwise specified	Feb. 28, 2014	Aug. 31, 2013	Feb. 28, 2013
Purchase and Sales Contracts [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 621,523	$ 499,890	$ 501,178
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	566,429	445,823	442,970
Derivative Asset, Fair Value, Gross Amount Not Offset on Balance Sheet	55,094	54,067	45,653
Derivative Liability, Fair Value, Gross Liability	553,693	465,066	429,975
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	472,271	409,408	375,455
Derivative Liability, Fair Value, Gross Amount Not Offset on Balance Sheet	55,093	54,067	45,653
Interest Rate Swap [Member] | Designated as Hedging Instrument [Member]
Purchase and Sales Contracts [Line Items]
Derivative Asset, Fair Value, Gross Asset		24,135	459
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		24,135	459
Derivative Asset, Fair Value, Gross Amount Not Offset on Balance Sheet
Interest Rate Swap [Member] | Not Designated as Hedging Instrument [Member]
Purchase and Sales Contracts [Line Items]
Derivative Asset, Fair Value, Gross Asset		3
Derivative Asset, Fair Value, Amount Not Offset Against Collateral
Derivative Asset, Fair Value, Gross Amount Not Offset on Balance Sheet		3
Derivative Liability, Fair Value, Gross Liability	194	248	418
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	194	245	418
Derivative Liability, Fair Value, Gross Amount Not Offset on Balance Sheet		3
Commodity and Freight Derivatives [Member]
Purchase and Sales Contracts [Line Items]
Derivative Asset, Fair Value, Gross Asset	615,178	468,673	485,222
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	562,025	415,566	427,014
Derivative Asset, Fair Value, Gross Amount Not Offset on Balance Sheet	53,153	53,107	45,653
Derivative Liability, Fair Value, Gross Liability	550,870	458,893	428,866
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	471,389	404,195	374,346
Derivative Liability, Fair Value, Gross Amount Not Offset on Balance Sheet	53,152	53,107	45,653
Foreign Exchange Contract [Member]
Purchase and Sales Contracts [Line Items]
Derivative Asset, Fair Value, Gross Asset	6,345	7,079	15,497
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	4,404	6,122	15,497
Derivative Asset, Fair Value, Gross Amount Not Offset on Balance Sheet	1,941	957
Derivative Liability, Fair Value, Gross Liability	2,629	5,925	691
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	688	4,968	691
Derivative Liability, Fair Value, Gross Amount Not Offset on Balance Sheet	$ 1,941	$ 957