Description of Business and Basis of Presentation - Additional Information (Detail) $ in Millions			3 Months Ended	9 Months Ended	12 Months Ended
	Jul. 31, 2020 shares	Jul. 30, 2019 shares	Mar. 31, 2020 USD ($)	Sep. 30, 2020 USD ($) shares	Dec. 31, 2019 USD ($)
Description Of Business And Basis Of Presentation [Line Items]
Goodwill, Impairment Loss				$ 1,814	$ 1,197
Equity Method Investment, Other than Temporary Impairment			$ 1,264	$ 1,264
Impairment of Intangible Assets, Finite-lived			177
Number of reportable segments				2
Impairment of Long-Lived Assets Held-for-use			174
Impairment Charges			3,429
G&P
Description Of Business And Basis Of Presentation [Line Items]
Goodwill, Impairment Loss			$ 1,814	$ 1,814	$ 1,197
ANDX LP [Member] | Public
Description Of Business And Basis Of Presentation [Line Items]
Common Units Conversion Ratio - ANDX to MPLX | shares		1.135
ANDX LP [Member] | Nonpublic [Member]
Description Of Business And Basis Of Presentation [Line Items]
Common Units Conversion Ratio - ANDX to MPLX | shares		1.0328
Western Refining Wholesale & Western Refining Product Transport [Member]
Description Of Business And Basis Of Presentation [Line Items]
Partners' Capital Account, Units, Redeemed | shares	18,582,088			18,582,088