Summary of Significant Accounting Policies - Purchase and Sales Contracts Outstanding (Details)	Feb. 28, 2013 t Barrels Bushels	Aug. 31, 2012 T Barrels Bushels	Feb. 29, 2012 T Bushels Barrels
Purchase Contracts
Purchase and Sales Contracts [Line Items]
Grain and oilseed - bushels	605,309,000	722,895,000	550,564,000
Energy products - barrels	12,103,000	9,047,000	12,913,000
Soy products - tons	32,000	15,000	14,000
Crop nutrients - tons	1,553,000	600,000	1,461,000
Ocean and barge freight - metric tons	933,000	1,018,000	1,211,000
Sales Contracts
Purchase and Sales Contracts [Line Items]
Grain and oilseed - bushels	906,758,000	1,074,535,000	803,266,000
Energy products - barrels	21,575,000	19,561,000	15,348,000
Soy products - tons	333,000	215,000	252,000
Crop nutrients - tons	2,128,000	725,000	1,703,000
Ocean and barge freight - metric tons	125,000	183,000	235,000