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Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details) (Fair Value, Inputs, Level 3, USD $)
 3 Months Ended
Nov. 30, 2013
Contingent Liability [Member]
Nov. 30, 2013
Contingent Liability [Member]
Minimum [Member]
Nov. 30, 2013
Contingent Liability [Member]
Maximum [Member]
Nov. 30, 2013
Contingent Liability [Member]
Average
Nov. 30, 2013
Fair Value, Measurements, Recurring
Aug. 31, 2013
Fair Value, Measurements, Recurring
Nov. 30, 2012
Fair Value, Measurements, Recurring
Nov. 30, 2013
Fair Value, Measurements, Recurring
Contingent Liability [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]                
Contingent Liability, Fair Value DIsclosure         $ 150,991,000 $ 134,134,000 $ 170,621,000 $ 150,991,000
Forward crack spread margin   $ 16.32 [1] $ 25.30 [1] $ 21.10 [1]        
Contractual target crack spread margin (in dollars per share) $ 17.50 [2]              
Expected volatility 105.25% [3]              
Own credit risk   1.80% [4] 2.60% [4] 2.23% [4]        
Expected life (years)   9 months [5] 3 years 9 months [5] 2 years 4 months 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
[5] Represents the range in the number of years remaining related to each contingent payment