Acquisitions Acquisitions (Refining Logistics and Fuels Distribution) (Details) bbl in Millions, $ in Millions					3 Months Ended	11 Months Ended	12 Months Ended
		Feb. 01, 2018 USD ($) Tank shares bbl	Sep. 01, 2017 USD ($) shares	Mar. 01, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Limited Liability Company (LLC) or Limited Partnership (LP), Managing Member or General Partner, Ownership Interest		2.00%		2.00%			2.00%
Contribution from MPC							$ 346.0	$ 11,980.0	$ 675.0
Property, plant and equipment, net					$ 21,525.0	$ 21,525.0	22,145.0	21,525.0
Goodwill					10,016.0	10,016.0	9,536.0	$ 10,016.0	2,245.0
Number of storage tanks | Tank		619
Number of rail and truck racks		32
Number of docks and gasoline blenders		18
Refining Logistics & Fuels Distribution [Member]
Business Acquisition [Line Items]
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual						1,359.0
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual						$ 874.0
Payments to Acquire Businesses, Gross		$ 4,100.0
Partners' Capital Account, Units, Acquisitions | shares								113,888,889
Equity interest issued or issuable, fair value assigned		4,300.0
Business Acquisition, Total Consideration, Fair Value Assigned		8,400.0
Property, plant and equipment, net		830.0
Refining Logistics [Member]
Business Acquisition [Line Items]
Goodwill		85.0
Fuels Distribution [Member]
Business Acquisition [Line Items]
Goodwill		$ 130.0
Limited Partners Common Units | Refining Logistics & Fuels Distribution [Member]
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares								111,611,111
Limited Partners Common Units | MPLX LP [Member] | Refining Logistics & Fuels Distribution [Member]
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares		111,611,111
Limited Partners Common Units | MPLX Logistics LLC | Refining Logistics & Fuels Distribution [Member]
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares		18,176,666
Limited Partners Common Units | MPLX Holdings Inc | Refining Logistics & Fuels Distribution [Member]
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares		7,824,167
General Partner Units | Refining Logistics & Fuels Distribution [Member]
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares	[1]							2,277,778
General Partner Units | MPLX LP [Member] | Refining Logistics & Fuels Distribution [Member]
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares		2,277,778
General Partner Common Units | Refining Logistics & Fuels Distribution [Member]
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares		85,610,278
LOOP LOCAP SAX and Explorer
Business Acquisition [Line Items]
Equity interest issued or issuable, fair value assigned			$ 653.0
Business Acquisition, Total Consideration, Fair Value Assigned			$ 1,070.0
Limited Liability Company (LLC) or Limited Partnership (LP), Managing Member or General Partner, Ownership Interest			2.00%
LOOP LOCAP SAX and Explorer | Limited Partners Common Units | MPLX Logistics LLC
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares			3,350,893
LOOP LOCAP SAX and Explorer | Limited Partners Common Units | MPLX Holdings Inc
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares			1,441,224
LOOP LOCAP SAX and Explorer | General Partner Units
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares			377,778
LOOP LOCAP SAX and Explorer | General Partner Common Units
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares			13,719,017
Marathon Petroleum Corporation [Member]
Business Acquisition [Line Items]
Contribution from MPC				$ 6.0
Marathon Petroleum Corporation [Member] | Refining Logistics & Fuels Distribution [Member]
Business Acquisition [Line Items]
Contribution from MPC		$ 23.7
Marathon Petroleum Corporation [Member] | Limited Partners Common Units
Business Acquisition [Line Items]
Contribution from MPC							$ 315.0	$ 0.0	$ 0.0
Marathon Petroleum Corporation [Member] | LOOP LOCAP SAX and Explorer
Business Acquisition [Line Items]
Contribution from MPC					$ 10.0
Crude Oil
Business Acquisition [Line Items]
Storage Capacity | bbl		56

[1] Changes to the number of general partner units outstanding, other than changes due to contributions made to MPC for the acquisitions of HSM, HST, WHC, MPLXT, the Joint-Interest Acquisition and Refining Logistics and Fuels Distribution, are the result of cash contributions made by the general partner in order to maintain its two percent GP Interest.