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Major Customers and Concentration of Credit Risk (Details) - Revenues - Customer Concentration Risk
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Marathon Petroleum Corporation      
Major Customers and Concentration of Credit Risk      
Major customer percentage of total revenue 12.00% 14.00% 19.00%
ExxonMobil Corporation      
Major Customers and Concentration of Credit Risk      
Major customer percentage of total revenue 12.00% 14.00% 11.00%
Phillips 66      
Major Customers and Concentration of Credit Risk      
Major customer percentage of total revenue 11.00%   11.00%