Description of the Business and Basis of Presentation (Detail)				12 Months Ended
	Jul. 30, 2019 shares	Oct. 31, 2012	Mar. 27, 2012	Dec. 31, 2019
Description Of Business And Basis Of Presentation [Line Items]
Date of partnership formation			Mar. 27, 2012
Date for initial public offering completed		Oct. 31, 2012
Number of reportable segments				2
ANDX LP [Member] | Nonpublic [Member]
Description Of Business And Basis Of Presentation [Line Items]
Common Units Conversion Ratio - ANDX to MPLX	1.0328
ANDX LP [Member] | Public
Description Of Business And Basis Of Presentation [Line Items]
Common Units Conversion Ratio - ANDX to MPLX	1.135