Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details) (Fair Value, Inputs, Level 3, USD $)	12 Months Ended
	Aug. 31, 2012
Accrued liability for contingent crack spread payments related to purchase of noncontroling interests
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Contractual target crack spread margin (in dollars per share)	$ 17.50	[1]
Expected volatility	86.11%	[2]
Accrued liability for contingent crack spread payments related to purchase of noncontroling interests | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Forward crack spread margin on May 31	$ 13.77	[3]
Risk-free interest rate	0.16%	[4]
Expected life (years)	1 year 0 months	[5]
Accrued liability for contingent crack spread payments related to purchase of noncontroling interests | Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Forward crack spread margin on May 31	16.15	[3]
Risk-free interest rate	0.38%	[4]
Expected life (years)	3 years 4 months 24 days	[5]
Accrued liability for contingent crack spread payments related to purchase of noncontroling interests | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Forward crack spread margin on May 31	21.62	[3]
Risk-free interest rate	0.59%	[4]
Expected life (years)	5 years 0 months	[5]
Mandatorily redeemable noncontrolling interests | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Own credit risk	2.16%	[6]
Mandatorily redeemable noncontrolling interests | Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Own credit risk	2.40%	[6]
Mandatorily redeemable noncontrolling interests | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Own credit risk	2.56%	[6]
Fair Value, Measurements, Recurring
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Accrued liability for contingent crack spread payments related to purchase of noncontrolling interest	127,516,000
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]
Accrued liability for contingent crack spread payments related to purchase of noncontrolling interest	127,516,000
Fair Value, Measurements, Nonrecurring | Mandatorily redeemable noncontrolling interests
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Mandatorily redeemable noncontrolling interests	$ 334,707,000

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

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

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

[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] Represents the range of company-specific risk adjustments commensurate with typical long-term borrowing rates available to the Company at inception of the contract