Redeemable Preferred Units (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
	May 13, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Redeemable Noncontrolling Interest [Line Items]
Issuance of preferred units			14,129,589	26,885,597
Issuance of redeemable preferred units		$ 0	$ 0	$ 984
Series A Convertible Preferred Units
Redeemable Noncontrolling Interest [Line Items]
Issuance of preferred units	30,800,000
Preferred units, 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 preferred units into common units at any time after the third anniversary of the issuance date or prior to liquidation, dissolution or winding up of the Partnership, in full or in part, subject to minimum conversion amounts and conditions. After the fourth anniversary of the issuance date, MPLX may convert the 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 LP common units is greater than $48.75 for the 20-day trading period immediately preceding the conversion notice date. The conversion rate for the 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 preferred units are entitled to vote on an as-converted basis with the common unitholders and will have certain other class voting rights with respect to any amendment to the 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 preferred units to common units at the then applicable change of control conversion rate.