Equity Method Investments (Schedule Of Equity Method Investments) (Details) - USD ($) $ in Millions		Dec. 31, 2024	Dec. 31, 2023	Sep. 30, 2023
Schedule of Equity Method Investments [Line Items]
Equity method investments		$ 6,857	$ 6,260
Martinez Renewables LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments				$ 971
Refining & Marketing
Schedule of Equity Method Investments [Line Items]
Equity method investments		$ 282	302
Refining & Marketing | The Andersons Marathon Holdings LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments, ownership percentage		50.00%
Equity method investments		$ 190	227
Refining & Marketing | Other equity method investees
Schedule of Equity Method Investments [Line Items]
Equity method investments	[1]	92	75
Midstream
Schedule of Equity Method Investments [Line Items]
Equity method investments		5,301	4,599
Renewable Diesel
Schedule of Equity Method Investments [Line Items]
Equity method investments		$ 1,274	1,359
Renewable Diesel | Martinez Renewables LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments, ownership percentage		50.00%
Equity method investments		$ 1,184	1,266
Renewable Diesel | Other equity method investees
Schedule of Equity Method Investments [Line Items]
Equity method investments	[1]	90	93
MPLX | Midstream
Schedule of Equity Method Investments [Line Items]
Equity method investments		$ 4,531	3,743
MPLX | Midstream | BANGL
Schedule of Equity Method Investments [Line Items]
Equity method investments, ownership percentage		45.00%
Equity method investments		$ 281	63
MPLX | Midstream | Illinois Extension Pipeline
Schedule of Equity Method Investments [Line Items]
Equity method investments, ownership percentage		35.00%
Equity method investments		$ 218	228
MPLX | Midstream | LOOP
Schedule of Equity Method Investments [Line Items]
Equity method investments, ownership percentage		41.00%
Equity method investments		$ 310	314
MPLX | Midstream | MarEn Bakken Company LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments, ownership percentage		25.00%
Equity method investments		$ 526	449
MPLX | Midstream | MarkWest EMG Jefferson Dry Gas
Schedule of Equity Method Investments [Line Items]
Equity method investments, ownership percentage		67.00%
Equity method investments		$ 329	336
MPLX | Midstream | MarkWest Utica EMG
Schedule of Equity Method Investments [Line Items]
Equity method investments, ownership percentage		59.00%
Equity method investments		$ 742	676
MPLX | Midstream | Ohio Gathering Company
Schedule of Equity Method Investments [Line Items]
Equity method investments, ownership percentage		35.00%
Equity method investments		$ 470	0
MPLX | Midstream | Sherwood Midstream
Schedule of Equity Method Investments [Line Items]
Equity method investments, ownership percentage		50.00%
Equity method investments		$ 488	500
MPLX | Midstream | WPC Parent, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments, ownership percentage		30.00%
Equity method investments		$ 208	214
MPLX | Midstream | Other equity method investees
Schedule of Equity Method Investments [Line Items]
Equity method investments	[1]	959	963
Marathon Petroleum Corporation | Midstream
Schedule of Equity Method Investments [Line Items]
Equity method investments		$ 770	856
Marathon Petroleum Corporation | Midstream | Capline LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments, ownership percentage		33.00%
Equity method investments		$ 382	402
Marathon Petroleum Corporation | Midstream | Gray Oak Pipeline LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments, ownership percentage		25.00%
Equity method investments		$ 274	284
Marathon Petroleum Corporation | Midstream | Other equity method investees
Schedule of Equity Method Investments [Line Items]
Equity method investments	[1]	$ 114	$ 170

[1] Some investments included within “Other” have been deemed to be VIEs.