Summary of Significant Accounting Policies - Purchase and Sales Contracts Outstanding (Details)	May 31, 2013 t Barrels Bushels	Aug. 31, 2012 T Barrels Bushels	May 31, 2012 T Bushels Barrels
Purchase Contracts
Purchase and Sales Contracts [Line Items]
Grain and oilseed - bushels	519,293,000	722,895,000	590,021,000
Energy products - barrels	13,833,000	9,047,000	9,271,000
Soy products - tons	70,000	15,000	10,000
Crop nutrients - tons	675,000	600,000	699,000
Ocean and barge freight - metric tons	888,000	1,018,000	1,215,000
Sales Contracts
Purchase and Sales Contracts [Line Items]
Grain and oilseed - bushels	806,244,000	1,074,535,000	843,934,000
Energy products - barrels	20,125,000	19,561,000	14,012,000
Soy products - tons	325,000	215,000	250,000
Crop nutrients - tons	778,000	725,000	894,000
Ocean and barge freight - metric tons	130,000	183,000	234,000