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Income Taxes - Reconciliation of the Statutory Tax Rates to the Effective Tax Rates (Details)
12 Months Ended
Aug. 31, 2017
Aug. 31, 2016
Aug. 31, 2015
Effective Income Tax Rate Reconciliation, Percent [Abstract]      
Statutory federal income tax rate 35.00% 35.00% 35.00%
State and local income taxes, net of federal income tax benefit 24.70% 0.40% (0.50%)
Patronage earnings 192.00% (23.20%) (29.00%)
Domestic production activities deduction 64.70% (13.20%) (5.60%)
Export activities at rates other than the U.S. statutory rate 145.30% 1.50% (0.20%)
Valuation allowance (182.30%) 19.60% (0.10%)
Tax credits 45.80% (11.80%) (0.80%)
Crack spread contingency 9.60% (5.00%) (1.70%)
Other (2.90%) (4.30%) 1.30%
Effective tax rate 331.90% (1.00%) (1.60%)