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Debt (Summary of Long-Term Debt) (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018	Mar. 01, 2018
Summary of long term debt
Debt at face value	$ 11,576,000,000	$ 11,076,000,000
Finance leases	277,000,000
Finance leases		184,000,000
Net unamortized discounts and debt issuance costs	(100,000,000)	(100,000,000)
Total debt	11,763,000,000	11,160,000,000
Short-term debt	(547,000,000)	(67,000,000)
Long-term debt	$ 11,216,000,000	$ 11,093,000,000
Floating-rate notes due April 2020 at 2.751% and 3.186% at year-end 2019 and 2018, respectively
Summary of long term debt
Long-term debt, percentage	2.751%	3.186%
Term loan due April 2020 at 2.699% and 3.422% at year-end 2019 and 2018, respectively
Summary of long term debt
Long-term debt, percentage	2.699%	3.422%
Floating-rate Advance Term Loan due December 2034 at 2.392%—related party
Summary of long term debt
Long-term debt, percentage	2.392%
Software obligations
Summary of long term debt
Finance leases	$ 10,000,000
Finance leases		$ 0
Senior Notes \| 4.300% Senior Notes due April 2022
Summary of long term debt
Stated interest rate of debt, percent	4.30%
Debt	$ 2,000,000,000	2,000,000,000
Senior Notes \| 3.900% Senior Notes due March 2028
Summary of long term debt
Stated interest rate of debt, percent	3.90%		3.90%
Debt	$ 800,000,000	800,000,000
Senior Notes \| 4.650% Senior Notes due November 2034
Summary of long term debt
Stated interest rate of debt, percent	4.65%
Debt	$ 1,000,000,000	1,000,000,000
Senior Notes \| 5.875% Senior Notes due May 2042
Summary of long term debt
Stated interest rate of debt, percent	5.875%
Debt	$ 1,500,000,000	1,500,000,000
Senior Notes \| 4.875% Senior Notes due November 2044
Summary of long term debt
Stated interest rate of debt, percent	4.875%		4.875%
Debt	$ 1,700,000,000	$ 1,700,000,000
Senior Notes \| Floating-rate Senior Notes due February 2021 at 2.517% and 3.289% at year-end 2019 and 2018, respectively
Summary of long term debt
Long-term debt, percentage	2.517%	3.289%
Debt	$ 500,000,000	$ 500,000,000
Loans Payable \| Floating-rate notes due April 2020 at 2.751% and 3.186% at year-end 2019 and 2018, respectively
Summary of long term debt
Debt at face value	300,000,000	300,000,000
Loans Payable \| Term loan due April 2020 at 2.699% and 3.422% at year-end 2019 and 2018, respectively
Summary of long term debt
Debt at face value	200,000,000	200,000,000
WRB Refining LP \| Other
Summary of long term debt
Debt	1,000,000	1,000,000
Revolving credit facility due January 2019 and October 2021 at weighted-average rate of 3.669% at year-end 2018
Summary of long term debt
Revolving credit facility due January 2019 and October 2021 at weighted-average rate of 3.669% at year-end 2018	$ 0	0
Phillips 66 Partners \| Senior Notes \| Phillips 66 Partners LP \| 2.646% Senior Notes due February 2020
Summary of long term debt
Stated interest rate of debt, percent	2.646%
Debt	$ 0	300,000,000
Phillips 66 Partners \| Senior Notes \| Phillips 66 Partners LP \| 2.450% Senior Notes due December 2024
Summary of long term debt
Stated interest rate of debt, percent	2.45%
Debt	$ 300,000,000	0
Phillips 66 Partners \| Senior Notes \| Phillips 66 Partners LP \| 3.605% Senior Notes due February 2025
Summary of long term debt
Stated interest rate of debt, percent	3.605%
Debt	$ 500,000,000	500,000,000
Phillips 66 Partners \| Senior Notes \| Phillips 66 Partners LP \| 3.550% Senior Notes due October 2026
Summary of long term debt
Stated interest rate of debt, percent	3.55%
Debt	$ 500,000,000	500,000,000
Phillips 66 Partners \| Senior Notes \| Phillips 66 Partners LP \| 3.750% Senior Notes due March 2028
Summary of long term debt
Stated interest rate of debt, percent	3.75%
Debt	$ 500,000,000	500,000,000
Phillips 66 Partners \| Senior Notes \| Phillips 66 Partners LP \| 3.150% Senior Notes due December 2029
Summary of long term debt
Stated interest rate of debt, percent	3.15%
Debt	$ 600,000,000	0
Phillips 66 Partners \| Senior Notes \| Phillips 66 Partners LP \| 4.680% Senior Notes due February 2045
Summary of long term debt
Stated interest rate of debt, percent	4.68%
Debt	$ 450,000,000	450,000,000
Phillips 66 Partners \| Senior Notes \| Phillips 66 Partners LP \| 4.900% Senior Notes due October 2046
Summary of long term debt
Stated interest rate of debt, percent	4.90%
Debt	$ 625,000,000	$ 625,000,000
Phillips 66 Partners \| Tax-Exempt Bonds \| Phillips 66 Partners LP \| Tax-exempt bonds due April 2020 and April 2021 at 1.850% and 1.885% at year-end 2019 and 2018, respectively
Summary of long term debt
Long-term debt, percentage	1.85%	1.885%
Debt at face value	$ 75,000,000	$ 75,000,000
Phillips 66 Partners \| Revolving credit facility due January 2019 and October 2021 at weighted-average rate of 3.669% at year-end 2018 \| Phillips 66 Partners LP
Summary of long term debt
Weighted-average interest rate		3.669%
Revolving credit facility due January 2019 and October 2021 at weighted-average rate of 3.669% at year-end 2018	0
Debt at face value	0	$ 125,000,000
Equity Method Investee \| Loans Payable \| Floating-rate Advance Term Loan due December 2034 at 2.392%—related party
Summary of long term debt
Debt	$ 25,000,000	$ 0