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Redeemable Preferred Units (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
 6 Months Ended
May 13, 2016
Jun. 30, 2017
Jun. 30, 2016
Redeemable Noncontrolling Interest [Line Items]      
Issuance of redeemable preferred units   $ 0 $ 984
Series A Convertible Preferred Units [Member]      
Redeemable Noncontrolling Interest [Line Items]      
Issuance of preferred units 30.8    
Preferred Stock, Dividend Rate, Percentage 6.50%    
Shares Issued, Price Per Share $ 32.50    
Issuance of redeemable preferred units $ 984    
Preferred Stock, Dividend Rate, Per-Dollar-Amount $ 0.528125    
Preferred Units, Description The purchasers 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, the Partnership 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. 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 in control the holders of Preferred units may elect, among other potential elections, to convert their Preferred units to common units at the then-change of control conversion rate.