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Commitments and Contingencies - RFS (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Petroleum Segment | Environmental, Health, and Safety Matters        
Loss Contingencies [Line Items]        
Expense recognized for compliance with RFS $ 16 $ 21 $ 35 $ 33