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Investments, Loans and Long-Term Receivables (Gray Oak Pipeline, LLC) (Details) - USD ($)
Jun. 30, 2020
Jan. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Phillips 66 Partners LP | Variable Interest Entity, Primary Beneficiary        
Schedule of Equity Method Investments [Line Items]        
Equity investments $ 3,340,000,000   $ 2,961,000,000  
Third Party | Gray Oak Holdings LLC        
Schedule of Equity Method Investments [Line Items]        
Percentage of ownership 35.00%     35.00%
Gray Oak Pipeline LLC        
Schedule of Equity Method Investments [Line Items]        
Maximum loan   $ 1,379,000,000    
Gray Oak Pipeline LLC | Phillips 66 Partners LP | Variable Interest Entity, Primary Beneficiary        
Schedule of Equity Method Investments [Line Items]        
Effective ownership percent 42.25%      
Maximum potential amount of future payments under the guarantees $ 583,000,000      
Equity investments $ 896,000,000      
Gray Oak Pipeline LLC | Phillips 66 Partners LP | Variable Interest Entity, Primary Beneficiary | Gray Oak Holdings LLC        
Schedule of Equity Method Investments [Line Items]        
Percentage of ownership 65.00%