Redeemable Preferred Units (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions				12 Months Ended
	Jan. 23, 2020	Sep. 20, 2019	May 13, 2016	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Redeemable Noncontrolling Interest [Line Items]
Issuance of preferred units						14,129,589
Partners' Capital Account, Units, Converted				1,148,330	264,413,309	366,509
Partners' Capital Account, Units				1,058,355,471	794,089,518	415,438,793	368,555,271
Series A Convertible Preferred Units
Redeemable Noncontrolling Interest [Line Items]
Issuance of preferred units			30,800,000
Preferred Stock, Dividend Rate, Percentage			6.50%
Units issued, price per unit			$ 32.50
Issuance of redeemable preferred units			$ 984
Preferred units, dividend rate, per-dollar-amount			$ 0.528125
Preferred units, description			The holders may convert their Series A preferred units into common units at any time, in full or in part, subject to minimum conversion amounts and conditions. After the fourth anniversary of the issuance date, MPLX may convert the Series A preferred units into common units at any time, in whole or in part, subject to certain minimum conversion amounts and conditions, if the closing price of MPLX common units is greater than $48.75 for the 20-day trading period immediately preceding the conversion notice date. The conversion rate for the Series A preferred units shall be the quotient of (a) the sum of (i) $32.50, plus (ii) any unpaid cash distributions on the applicable preferred unit, divided by (b) $32.50, subject to adjustment for unit distributions, unit splits and similar transactions. The holders of the Series A preferred units are entitled to vote on an as-converted basis with the common unitholders and have certain other class voting rights with respect to any amendment to the MPLX partnership agreement that would adversely affect any rights, preferences or privileges of the preferred units. In addition, upon certain events involving a change of control, the holders of preferred units may elect, among other potential elections, to convert their Series A preferred units to common units at the then applicable change of control conversion rate.
Partners' Capital Account, Units, Converted		1,200,000
Partners' Capital Account, Units				29,600,000
Subsequent Event
Redeemable Noncontrolling Interest [Line Items]
Distribution made to limited partner, declaration date	Jan. 23, 2020
Cash distributions declared per limited partner common unit	$ 0.6875