Variable Interest Entities	9 Months Ended
Sep. 30, 2018
Variable Interest Entity, Not Primary Beneficiary, Disclosures [Abstract]
Variable Interest Entities	VARIABLE INTEREST ENTITIES In addition to MPLX, as described in Note 4, the following entities are also VIEs. Crowley Coastal Partners In May 2016, Crowley Coastal Partners LLC (“Crowley Coastal Partners”) was formed to own an interest in both Crowley Ocean Partners LLC (“Crowley Ocean Partners”) and Crowley Blue Water Partners LLC (“Crowley Blue Water Partners”). We have determined that Crowley Coastal Partners is a VIE based on the terms of the existing financing arrangements for Crowley Blue Water Partners and Crowley Ocean Partners and the associated debt guarantees by MPC and Crowley. Our maximum exposure to loss at September 30, 2018 was $483 million, which includes our equity method investment in Crowley Coastal Partners and the debt guarantees provided to each of the lenders to Crowley Blue Water Partners and Crowley Ocean Partners. We are not the primary beneficiary of this VIE because we do not have the power to control the activities that significantly influence the economic outcomes of the entity and, therefore, do not consolidate the entity. MarkWest Utica EMG On January 1, 2012, MarkWest Utica Operating Company, LLC (“Utica Operating”), a wholly-owned and consolidated subsidiary of MarkWest, and EMG Utica, LLC (“EMG Utica”), executed agreements to form a joint venture, MarkWest Utica EMG LLC (“MarkWest Utica EMG”), to develop significant natural gas gathering, processing and NGL fractionation, transportation and marketing infrastructure in eastern Ohio. As of September 30, 2018, MPLX had a 56 percent ownership interest in MarkWest Utica EMG. MarkWest Utica EMG's inability to fund its planned activities without subordinated financial support qualify it as a VIE. Utica Operating is not deemed to be the primary beneficiary due to EMG Utica’s voting rights on significant matters. We account for our ownership interest in MarkWest Utica EMG as an equity method investment. Our maximum exposure to loss as a result of our involvement with MarkWest Utica EMG includes our equity investment, any additional capital contribution commitments and any operating expenses incurred by the subsidiary operator in excess of compensation received for the performance of the operating services. Our equity investment in MarkWest Utica EMG at September 30, 2018 was $2.1 billion. Ohio Gathering Ohio Gathering Company, L.L.C. (“Ohio Gathering”) is a subsidiary of MarkWest Utica EMG and is engaged in providing natural gas gathering services in the Utica Shale in eastern Ohio. Ohio Gathering is a joint venture between MarkWest Utica EMG and Summit Midstream Partners, LLC. As of September 30, 2018, MPLX had a 34 percent indirect ownership interest in Ohio Gathering. As this entity is a subsidiary of MarkWest Utica EMG, which is accounted for as an equity method investment, MPLX reports its portion of Ohio Gathering’s net assets as a component of its investment in MarkWest Utica EMG. Sherwood Midstream As described in Note 5, MPLX and Antero Midstream formed a joint venture, Sherwood Midstream, to support the development of Antero Resources Corporation’s Marcellus Shale acreage in West Virginia. As of September 30, 2018, MPLX had a 50 percent ownership interest in Sherwood Midstream. Sherwood Midstream’s inability to fund its planned activities without additional subordinated financial support qualify it as a VIE. MPLX is not deemed to be the primary beneficiary, due to Antero Midstream’s voting rights on significant matters. We account for our ownership interest in Sherwood Midstream using the equity method of accounting. Our maximum exposure to loss as a result of our involvement with Sherwood Midstream includes our equity investment, any additional capital contribution commitments and any operating expenses incurred by the subsidiary operator in excess of compensation received for the performance of the operating services. Our equity investment in Sherwood Midstream at September 30, 2018 was $325 million. Ohio Fractionation As described in Note 5, MPLX converted all of its ownership interests in Ohio Fractionation to Class A Interests and amended its LLC Agreement to create Class B-3 Interests, which were sold to Sherwood Midstream, providing it with the right to fractionation revenue and the obligation to pay expenses related to 20 mbpd of capacity in the Hopedale 3 fractionator. Ohio Fractionation’s inability to fund its operations without additional subordinated financial support qualify it as a VIE. MPLX has been deemed to be the primary beneficiary of Ohio Fractionation because it has control over decisions that could significantly impact its financial performance, and as a result, consolidates Ohio Fractionation. Sherwood Midstream Holdings As described in Note 5, MPLX and Sherwood Midstream entered into a joint venture, Sherwood Midstream Holdings, for the purpose of owning, operating and maintaining all of the shared assets for the benefit of and use in the operation of the gas plants and other assets owned by Sherwood Midstream and the gas plants and deethanization facilities owned by MPLX. MPLX had an initial 79 percent direct ownership in Sherwood Midstream Holdings, in addition to an initial 10.5 percent indirect interest through its ownership in Sherwood Midstream. Sherwood Midstream Holdings’ inability to fund its operations without additional subordinated financial support qualify it as a VIE. We account for our ownership interest in Sherwood Midstream Holdings using the equity method of accounting as Sherwood Midstream is considered to be the general partner and controls all decisions related to Sherwood Midstream Holdings. Our maximum exposure to loss as a result of our involvement with Sherwood Midstream Holdings includes our equity investment, any additional capital contribution commitments and any operating expenses incurred by the subsidiary operator in excess of compensation received for the performance of the operating services. Our equity investment in Sherwood Midstream Holdings at September 30, 2018 was $155 million.

VARIABLE INTEREST ENTITIES

In addition to MPLX, as described in Note 4, the following entities are also VIEs.

Crowley Coastal Partners

In May 2016, Crowley Coastal Partners LLC (“Crowley Coastal Partners”) was formed to own an interest in both Crowley Ocean Partners LLC (“Crowley Ocean Partners”) and Crowley Blue Water Partners LLC (“Crowley Blue Water Partners”). We have determined that Crowley Coastal Partners is a VIE based on the terms of the existing financing arrangements for Crowley Blue Water Partners and Crowley Ocean Partners and the associated debt guarantees by MPC and Crowley. Our maximum exposure to loss at September 30, 2018 was $483 million, which includes our equity method investment in Crowley Coastal Partners and the debt guarantees provided to each of the lenders to Crowley Blue Water Partners and Crowley Ocean Partners. We are not the primary beneficiary of this VIE because we do not have the power to control the activities that significantly influence the economic outcomes of the entity and, therefore, do not consolidate the entity.

MarkWest Utica EMG

On January 1, 2012, MarkWest Utica Operating Company, LLC (“Utica Operating”), a wholly-owned and consolidated subsidiary of MarkWest, and EMG Utica, LLC (“EMG Utica”), executed agreements to form a joint venture, MarkWest Utica EMG LLC (“MarkWest Utica EMG”), to develop significant natural gas gathering, processing and NGL fractionation, transportation and marketing infrastructure in eastern Ohio.

As of September 30, 2018, MPLX had a 56 percent ownership interest in MarkWest Utica EMG. MarkWest Utica EMG's inability to fund its planned activities without subordinated financial support qualify it as a VIE. Utica Operating is not deemed to be the primary beneficiary due to EMG Utica’s voting rights on significant matters. We account for our ownership interest in MarkWest Utica EMG as an equity method investment. Our maximum exposure to loss as a result of our involvement with MarkWest Utica EMG includes our equity investment, any additional capital contribution commitments and any operating expenses incurred by the subsidiary operator in excess of compensation received for the performance of the operating services. Our equity investment in MarkWest Utica EMG at September 30, 2018 was $2.1 billion.

Ohio Gathering

Ohio Gathering Company, L.L.C. (“Ohio Gathering”) is a subsidiary of MarkWest Utica EMG and is engaged in providing natural gas gathering services in the Utica Shale in eastern Ohio. Ohio Gathering is a joint venture between MarkWest Utica EMG and Summit Midstream Partners, LLC. As of September 30, 2018, MPLX had a 34 percent indirect ownership interest in Ohio Gathering. As this entity is a subsidiary of MarkWest Utica EMG, which is accounted for as an equity method investment, MPLX reports its portion of Ohio Gathering’s net assets as a component of its investment in MarkWest Utica EMG.

Sherwood Midstream

As described in Note 5, MPLX and Antero Midstream formed a joint venture, Sherwood Midstream, to support the development of Antero Resources Corporation’s Marcellus Shale acreage in West Virginia. As of September 30, 2018, MPLX had a 50 percent ownership interest in Sherwood Midstream. Sherwood Midstream’s inability to fund its planned activities without additional subordinated financial support qualify it as a VIE. MPLX is not deemed to be the primary beneficiary, due to Antero Midstream’s voting rights on significant matters. We account for our ownership interest in Sherwood Midstream using the equity method of accounting. Our maximum exposure to loss as a result of our involvement with Sherwood Midstream includes our equity investment, any additional capital contribution commitments and any operating expenses incurred by the subsidiary operator in excess of compensation received for the performance of the operating services. Our equity investment in Sherwood Midstream at September 30, 2018 was $325 million.

Ohio Fractionation

As described in Note 5, MPLX converted all of its ownership interests in Ohio Fractionation to Class A Interests and amended its LLC Agreement to create Class B-3 Interests, which were sold to Sherwood Midstream, providing it with the right to fractionation revenue and the obligation to pay expenses related to 20 mbpd of capacity in the Hopedale 3 fractionator. Ohio Fractionation’s inability to fund its operations without additional subordinated financial support qualify it as a VIE. MPLX has been deemed to be the primary beneficiary of Ohio Fractionation because it has control over decisions that could significantly impact its financial performance, and as a result, consolidates Ohio Fractionation.

Sherwood Midstream Holdings

As described in Note 5, MPLX and Sherwood Midstream entered into a joint venture, Sherwood Midstream Holdings, for the purpose of owning, operating and maintaining all of the shared assets for the benefit of and use in the operation of the gas plants and other assets owned by Sherwood Midstream and the gas plants and deethanization facilities owned by MPLX. MPLX had an initial 79 percent direct ownership in Sherwood Midstream Holdings, in addition to an initial 10.5 percent indirect interest through its ownership in Sherwood Midstream. Sherwood Midstream Holdings’ inability to fund its operations without additional subordinated financial support qualify it as a VIE. We account for our ownership interest in Sherwood Midstream Holdings using the equity method of accounting as Sherwood Midstream is considered to be the general partner and controls all decisions related to Sherwood Midstream Holdings. Our maximum exposure to loss as a result of our involvement with Sherwood Midstream Holdings includes our equity investment, any additional capital contribution commitments and any operating expenses incurred by the subsidiary operator in excess of compensation received for the performance of the operating services. Our equity investment in Sherwood Midstream Holdings at September 30, 2018 was $155 million.