Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details) (Fair Value, Inputs, Level 3, USD $)	12 Months Ended
	Aug. 31, 2013 Accrued liability for contingent crack spread payments related to purchase of noncontroling interests		Aug. 31, 2013 Accrued liability for contingent crack spread payments related to purchase of noncontroling interests Minimum		Aug. 31, 2013 Accrued liability for contingent crack spread payments related to purchase of noncontroling interests Average		Aug. 31, 2013 Accrued liability for contingent crack spread payments related to purchase of noncontroling interests Maximum		Aug. 31, 2013 Mandatorily redeemable noncontrolling interests Minimum		Aug. 31, 2013 Mandatorily redeemable noncontrolling interests Average		Aug. 31, 2013 Mandatorily redeemable noncontrolling interests Maximum		Aug. 31, 2013 Fair Value, Measurements, Recurring	Aug. 31, 2012 Fair Value, Measurements, Recurring	Aug. 31, 2013 Fair Value, Measurements, Recurring Accrued liability for contingent crack spread payments related to purchase of noncontroling interests
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Forward Crack Spread Margin Quotes and Management Estimates Based on Future Settlement			$ 13.77	[1]	$ 16.15	[1]	$ 21.62	[1]
Accrued liability for contingent crack spread payments related to purchase of noncontrolling interest															$ 134,134,000	$ 127,516,000	$ 134,134,000
Contractual target crack spread margin (in dollars per share)	$ 17.50	[2]
Expected volatility	80.31%	[3]
Risk-free interest rate			0.16%	[4]	0.38%	[4]	0.59%	[4]
Expected life (years)			1 year 0 months	[5]	3 years 4 months 24 days	[5]	5 years 0 months	[5]
Own credit risk									2.16%	[6]	2.40%	[6]	2.56%	[6]

[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

[6]