Impairments (Long-Lived Assets) (Details) - USD ($) $ in Millions	3 Months Ended			12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Mar. 31, 2020	Dec. 31, 2020	[1]	Dec. 31, 2019	Jun. 30, 2020
Impairment expense
Finite-Lived Intangible Assets [Line Items]
Impairment of long-lived assets held-for-use				$ 1,032		$ 0
Minimum
Finite-Lived Intangible Assets [Line Items]
Reporting unit, percentage of fair value in excess of carrying amount							17.00%
Refining & Marketing | Impairment expense
Finite-Lived Intangible Assets [Line Items]
Impairment of long-lived assets held-for-use	$ 44	$ 342	$ 142
Midstream | Impairment expense | MPLX
Finite-Lived Intangible Assets [Line Items]
Impairment of long-lived assets held-for-use	$ 67	$ 27	$ 350

[1] The remaining difference not described in the narrative below is related to certain immaterial Midstream assets.