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Commitments and Contingencies - Crude Oil Supply Agreement (Details) - New Vitol Agreement
3 Months Ended
Aug. 31, 2012
Mar. 31, 2019
Mar. 31, 2018
Loss Contingencies [Line Items]      
Renewal term of agreement 1 year    
Notice of nonrenewal period prior to expiration 180 days    
Petroleum Segment | Contracted Volume | Supplier Concentration Risk      
Loss Contingencies [Line Items]      
Volume contracted throughout Vitol as percentage of total crude oil purchases   39.00% 35.00%