Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details) (Fair Value, Inputs, Level 3 [Member], USD $)	9 Months Ended
	May 31, 2014 Accrued liability for contingent crack spread payments related to purchase of noncontroling interests Minimum [Member]		May 31, 2014 Accrued liability for contingent crack spread payments related to purchase of noncontroling interests Maximum [Member]		May 31, 2014 Accrued liability for contingent crack spread payments related to purchase of noncontroling interests Average		May 31, 2014 National Cooperative Refinery Association [Member] Accrued liability for contingent crack spread payments related to purchase of noncontroling interests		May 31, 2014 National Cooperative Refinery Association [Member] Fair Value, Measurements, Recurring [Member]	Aug. 31, 2013 National Cooperative Refinery Association [Member] Fair Value, Measurements, Recurring [Member]	May 31, 2013 National Cooperative Refinery Association [Member] Fair Value, Measurements, Recurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Accrued liability for contingent crack spread payments related to purchase of noncontrolling interest									$ 130,159,000	$ 134,134,000	$ 146,245,000
Forward crack spread margin	$ 14.34	[1]	$ 17.72	[1]	$ 16.55	[1]
Contractual target crack spread margin (in dollars per share)							$ 17.50	[2]
Expected volatility							128.59%	[3]
Own credit risk	1.80%	[4]	2.60%	[4]	2.23%	[4]
Expected life (years)	0 years 3 months	[5]	3 years 3 months	[5]	1 year 10 months 25 days	[5]

[1] Represents forward crack spread margin quotes and management estimates based on future settlement dates

[2] Represents the minimum contractual threshold that would require settlement with the counterparties

[3] Represents quarterly adjusted volatility estimates derived from daily historical market data

[4] Represents yield curves for U.S. Treasury securities

[5] Represents the range in the number of years remaining related to each contingent payment