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Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details) (Fair Value, Inputs, Level 3 [Member], USD $)
 6 Months Ended 6 Months Ended
Feb. 28, 2014
Accrued liability for contingent crack spread payments related to purchase of noncontroling interests
Minimum [Member]
Feb. 28, 2014
Accrued liability for contingent crack spread payments related to purchase of noncontroling interests
Maximum [Member]
Feb. 28, 2014
Accrued liability for contingent crack spread payments related to purchase of noncontroling interests
Average
Feb. 28, 2014
National Cooperative Refinery Association [Member]
Accrued liability for contingent crack spread payments related to purchase of noncontroling interests
Feb. 28, 2014
National Cooperative Refinery Association [Member]
Fair Value, Measurements, Recurring [Member]
Aug. 31, 2013
National Cooperative Refinery Association [Member]
Fair Value, Measurements, Recurring [Member]
Feb. 28, 2013
National Cooperative Refinery Association [Member]
Fair Value, Measurements, Recurring [Member]
Feb. 28, 2014
Crack Spread Contingent Payment [Member]
National Cooperative Refinery Association [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]                
Accrued liability for contingent crack spread payments related to purchase of noncontrolling interest         $ 140,631,000 $ 134,134,000 $ 140,987,000  
Forward crack spread margin $ 14.81 [1] $ 22.65 [1] $ 18.71 [1]          
Contractual target crack spread margin (in dollars per share)       $ 17.50 [2]        
Expected volatility               121.03% [3]
Own credit risk 1.80% [4] 2.60% [4] 2.23% [4]          
Expected life (years) 0 years 6 months [5] 3 years 6 months [5] 2 years 1 month 13 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