Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details) (Fair Value, Inputs, Level 3, USD $)	6 Months Ended
	Feb. 28, 2013 Accrued liability for contingent crack spread payments related to purchase of noncontroling interests		Feb. 28, 2013 Accrued liability for contingent crack spread payments related to purchase of noncontroling interests Minimum		Feb. 28, 2013 Accrued liability for contingent crack spread payments related to purchase of noncontroling interests Maximum		Feb. 28, 2013 Accrued liability for contingent crack spread payments related to purchase of noncontroling interests Average		Feb. 28, 2013 Fair Value, Measurements, Recurring	Aug. 31, 2012 Fair Value, Measurements, Recurring	Feb. 29, 2012 Fair Value, Measurements, Recurring	Feb. 28, 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]
Accrued liability for contingent crack spread payments related to purchase of noncontrolling interest									$ 140,987,000	$ 127,516,000	$ 101,003,000	$ 140,987,000
Forward crack spread margin			$ 24.74	[1]	$ 39.23	[1]	$ 30.24	[1]
Contractual target crack spread margin (in dollars per share)	$ 17.50	[2]
Expected volatility	71.36%	[3]
Own credit risk			1.80%	[4]	2.80%	[4]	2.42%	[4]
Expected life (years)			0 years 6 months	[5]	4 years 6 months	[5]	2 years 10 months 24 days	[5]

[1] Represents adjusted 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 the range of company-specific risk adjustments commensurate with typical long-term borrowing rates available to us at measurement date

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