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Investments, Loans and Long-Term Receivables (Gray Oak Pipeline, LLC) (Details) - USD ($)
Mar. 31, 2020
Jan. 31, 2020
Dec. 31, 2019
Gray Oak Pipeline LLC      
Schedule of Equity Method Investments [Line Items]      
Maximum loan   $ 1,379,000,000  
Variable Interest Entity, Primary Beneficiary | Phillips 66 Partners LP      
Schedule of Equity Method Investments [Line Items]      
Equity investments $ 3,136,000,000   $ 2,961,000,000
Variable Interest Entity, Primary Beneficiary | Phillips 66 Partners LP | Gray Oak Pipeline LLC      
Schedule of Equity Method Investments [Line Items]      
Effective ownership percent 42.25%    
Maximum potential amount of future payments under the guarantees $ 583,000,000    
Maximum loss exposure 1,382,000,000    
Equity investments 799,000,000    
Guarantor obligations, current carrying value $ 583,000,000