MPLX (MPLX) 10-Q 9 May 19 2019 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2019	May 03, 2019
Document Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
Trading Symbol	MPLX
Entity Registrant Name	MPLX LP
Entity Central Index Key	0001552000
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Emerging Growth Company	false
Entity Small Business	false
Common Units [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		794,349,225
General Partner Units
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		0

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) shares in Millions, $ in Millions		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018
Revenues and other income:
Total revenues from contracts with customers		$ 1,263	$ 1,107
Rental income		94	79
Rental income - related parties		193	145
Income from equity method investments	[1]	70	61
Other income		0	4
Other income - related parties		26	23
Total revenues and other income		1,646	1,420
Costs and expenses:
Rental cost of sales		37	29
Rental cost of sales - related parties		3	1
Purchases - related parties		212	177
Depreciation and amortization	[2]	211	176
General and administrative expenses		82	69
Other taxes		19	18
Total costs and expenses		968	863
Income from operations		678	557
Related party interest and other financial costs		1	1
Interest expense (net of amounts capitalized of $7 million and $9 million, respectively)		156	112
Other financial costs		14	17
Income before income taxes		507	427
(Benefit)/provision for income taxes		(2)	4
Net income		509	423
Less: Net income (loss) attributable to noncontrolling interest		6	2
Net income (loss) attributable to MPLX LP	[3]	503	421
Less: Preferred unit distributions		20	16
Limited partners' interest in net income (loss) attributable to MPLX LP		$ 483	$ 405
Weighted average limited partner units outstanding:
Common - basic (in shares)		825	692
Common - diluted (in shares)		826	692
Limited Partners Common Units
Costs and expenses:
Net income (loss) attributable to MPLX LP	[3]	$ 483	$ 405
Net income (loss) attributable to MPLX LP per limited partner unit:
Common - basic (in USD per unit)		$ 0.61	$ 0.61
Common - diluted (in USD per unit)		$ 0.61	$ 0.61
Weighted average limited partner units outstanding:
Common - basic (in shares)		794	661
Common - diluted (in shares)		795	661
Service [Member]
Revenues and other income:
Total revenues from contracts with customers		$ 438	$ 382
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		578	471
Service, Other [Member]
Revenues and other income:
Total revenues from contracts with customers		34	44
Product [Member]
Revenues and other income:
Total revenues from contracts with customers		202	207
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties		11	4
Oil and Gas, Refining and Marketing [Member]
Costs and expenses:
Cost of Goods and Services Sold		210	206
Natural Gas, Midstream [Member]
Costs and expenses:
Cost of Goods and Services Sold		$ 194	$ 187
[1]	“Income from equity method investments” includes the impact of any basis differential amortization or accretion.
[2]	Depreciation and amortization attributable to L&S was $70 million and $48 million for the three months ended March 31, 2019 and 2018, respectively. Depreciation and amortization attributable to G&P was $141 million and $128 million for the three months ended March 31, 2019 and 2018, respectively.
[3]	Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.

Consolidated Statements of In_2

Consolidated Statements of Income (Parenthetical) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Statement [Abstract]
Interest costs capitalized	$ 7	$ 9

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 509	$ 423
Other comprehensive income/(loss), net of tax:
Remeasurements of pension and other postretirement benefits related to equity method investments, net of tax	1	(2)
Comprehensive income	510	421
Less comprehensive income attributable to:
Noncontrolling interests	6	2
Comprehensive income attributable to MPLX LP	$ 504	$ 419

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions		Mar. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents		$ 93	$ 68
Receivables, net		365	417
Current Assets, Related Parties		386	290
Inventories		74	77
Other current assets		34	45
Total current assets		952	897
Equity method investments		4,270	4,174
Property, plant and equipment, net		14,816	14,639
Intangibles, net		414	424
Goodwill		2,581	2,586
Operating Lease, Right-of-Use Asset		262	0
Other Assets, Related Parties, Noncurrent		256	24
Other noncurrent assets		33	35
Total assets		23,584	22,779
Current liabilities:
Accounts payable		110	162
Accrued Liabilities, Current		189	250
Current Liabilities, Related Parties		204	254
Accrued Property Plant and Equipment Current		249	294
Accrued interest payable		156	143
Operating Lease, Liability, Current		46	0
Other current liabilities		75	83
Total current liabilities		1,029	1,186
Long-term deferred revenue		94	80
Liabilities, Related Parties, Noncurrent		273	43
Long-term debt		13,832	13,392
Deferred income taxes		12	13
Operating Lease, Liability, Noncurrent		216	0
Deferred credits and other liabilities		195	197
Total liabilities		15,651	14,911
Commitments and contingencies (see Note 20)
Redeemable preferred units		1,004	1,004
Equity
Total MPLX LP partners’ capital		6,679	6,708
Accumulated other comprehensive loss	[1]	(15)	(16)
Noncontrolling interests		250	156
Total equity		6,929	6,864
Total liabilities, preferred units and equity		23,584	22,779
Debt, Current		1	1
MPC
Current assets:
Operating Lease, Right-of-Use Asset		232	0
Current liabilities:
Operating Lease, Liability, Current		1	0
Operating Lease, Liability, Noncurrent		231	0
Limited Partners Common Units | Public
Equity
Total MPLX LP partners’ capital		8,326	8,336
Total equity		8,326	8,336
Limited Partners Common Units | MPC
Equity
Total MPLX LP partners’ capital		(1,632)	(1,612)
Total equity		(1,632)	(1,612)
LOOP and Explorer
Equity
Accumulated other comprehensive loss		$ (15)	$ (16)
[1]	These components of “Accumulated other comprehensive loss” are included in the computation of net periodic benefit cost by LOOP and Explorer and are therefore included on the Consolidated Statements of Income under the caption “Income/(loss) from equity method investments.”

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - shares shares in Millions	Mar. 31, 2019	Dec. 31, 2018
Limited Partners Common Units | Public
Units issued	290	289
Units outstanding	290	289
Limited Partners Common Units | MPC
Units issued	505	505
Units outstanding	505	505
General Partner | MPC
GP units issued		0
GP units outstanding		0

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2019		Mar. 31, 2018
Operating activities:
Net income		$ 509		$ 423
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization of deferred financing costs		13		16
Depreciation and amortization	[1]	211		176
Deferred income taxes		(2)		4
Asset retirement expenditures		0		(1)
Loss on disposal of assets		1		0
Income from equity method investments	[2]	(70)		(61)
Distributions from unconsolidated affiliates		101		68
Changes in:
Current receivables		57		(8)
Inventories		3		2
Fair value of derivatives		7		(9)
Current accounts payable and accrued liabilities		(78)		(44)
Current assets/current liabilities - related parties		(147)		(126)
Increase (Decrease) in Other Operating Assets and Liabilities, Net		3		0
Deferred revenue		14		7
All other, net		(4)		3
Net cash provided by operating activities		618		450
Investing activities:
Additions to property, plant and equipment		(457)		(455)
Acquisitions, net of cash acquired		1		0
Disposal of assets		7		2
Investments in unconsolidated affiliates		(128)		(38)
Distributions from unconsolidated affiliates - return of capital		2		0
All other, net		0		1
Net cash used in investing activities		(575)		(490)
Financing activities:
Long-term debt - borrowings		825		9,610
Long-term debt - repayments		(400)		(4,655)
Related party debt - borrowings		851		452
Related party debt - repayments		(851)		(838)
Debt issuance costs		0		(53)
Distributions to MPC for acquisitions		0		(4,111)
Distributions to noncontrolling interests		(6)		(3)
Distributions to preferred unitholders		(20)		(16)
Distributions to unitholders and general partner		(515)		(347)
Contributions from noncontrolling interests		94		1
All other, net		(4)		(3)
Net cash (used in)/provided by financing activities		(26)		37
Net increase/(decrease) in cash, cash equivalents and restricted cash		17		(3)
Cash, cash equivalents and restricted cash at beginning of period		76	[3]	9
Cash, cash equivalents and restricted cash at end of period		$ 93	[3]	$ 6
[1]	Depreciation and amortization attributable to L&S was $70 million and $48 million for the three months ended March 31, 2019 and 2018, respectively. Depreciation and amortization attributable to G&P was $141 million and $128 million for the three months ended March 31, 2019 and 2018, respectively.
[2]	“Income from equity method investments” includes the impact of any basis differential amortization or accretion.
[3]	As a result of the adoption of ASU 2016-18, Statement of Cash Flows - Restricted Cash, the Consolidated Statements of Cash Flows now explain the change during the period of both “Cash and cash equivalents” and “Restricted cash.”

Consolidated Statements of Equi

Consolidated Statements of Equity - USD ($) $ in Millions	Total	PublicLimited Partners Common Units	MPCLimited Partners Common Units	MPCGeneral Partner	Accumulated Other Comprehensive Loss	Noncontrolling Interests	Equity of PredecessorMPC
Beginning Balance at Dec. 31, 2017	$ 9,973	$ 8,379	$ 2,099	$ (637)	$ (14)	$ 146	$ 0
Distributions to:	(407)	180	225			(2)
Unitholders	1,046						1,046
Noncontrolling interests	0		5,172	(4,126)			(1,046)
Contributions from:			(936)	(3,164)
Noncontrolling interests	(347)	(176)	(171)
Conversion of Series A Preferred Units	(3)					(3)
Contributions from noncontrolling interests	1					1
Partners' Capital Account, Exchanges and Conversions	0		(7,926)	7,926
Other	1	2		1	(2)
Ending Balance at Mar. 31, 2018	6,978	8,385	(1,537)	0	(16)	146	0
Beginning Balance at Dec. 31, 2017	9,973	8,379	2,099	(637)	(14)	146	0
Contributions from:	(4,100)
Ending Balance at Dec. 31, 2018	6,864	8,336	(1,612)	0	(16)	156	0
Distributions to:	(489)	176	307			(6)
Noncontrolling interests	(515)	(188)	(327)	0
Conversion of Series A Preferred Units	(6)					(6)
Contributions from noncontrolling interests	94					94
Other	3	2			1
Ending Balance at Mar. 31, 2019	$ 6,929	$ 8,326	$ (1,632)	$ 0	$ (15)	$ 250	$ 0

Description of the Business and

Description of the Business and Basis of Presentation	3 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
Business Description and Basis of Presentation [Text Block]	Description of the Business and Basis of Presentation Description of the Business – MPLX LP is a diversified, large-cap master limited partnership formed by Marathon Petroleum Corporation that owns and operates midstream energy infrastructure and logistics assets, and provides fuels distribution services. References in this report to “MPLX LP,” “MPLX,” “the Partnership,” “we,” “ours,” “us,” or like terms refer to MPLX LP and its subsidiaries. References to “MPC” refer collectively to Marathon Petroleum Corporation as our sponsor and its subsidiaries, other than the Partnership. We are engaged in the transportation, storage and distribution of crude oil and refined petroleum products; the gathering, processing and transportation of natural gas; and the gathering, transportation, fractionation, storage and marketing of NGLs. MPLX’s principal executive office is located in Findlay, Ohio. MPLX’s business consists of two segments based on the nature of services it offers: Logistics and Storage (“L&S”), which relates primarily to crude oil and refined petroleum products; and Gathering and Processing (“G&P”), which relates primarily to natural gas and NGLs. See Note 9 for additional information regarding the operations and results of these segments. Basis of Presentation – The accompanying interim consolidated financial statements are unaudited; however, in the opinion of MPLX’s management, these statements reflect all adjustments necessary for a fair statement of the results for the periods reported. All such adjustments are of a normal, recurring nature unless otherwise disclosed. These interim consolidated financial statements, including the notes, have been prepared in accordance with the rules and regulations of the SEC applicable to interim period financial statements and do not include all of the information and disclosures required by GAAP for complete financial statements. Certain amounts in prior years have been reclassified to conform to current year presentation. These interim consolidated financial statements should be read in conjunction with the audited consolidated financial statements and notes thereto included in the Annual Report on Form 10-K for the year ended December 31, 2018 . The results of operations for the three months ended March 31, 2019 are not necessarily indicative of the results to be expected for the full year. MPLX’s consolidated financial statements include all majority-owned and controlled subsidiaries. For non-wholly-owned consolidated subsidiaries, the interests owned by third parties have been recorded as “Noncontrolling interests” on the accompanying Consolidated Balance Sheets. Intercompany investments, accounts and transactions have been eliminated. MPLX’s investments in which MPLX exercises significant influence but does not control and does not have a controlling financial interest are accounted for using the equity method. MPLX’s investments in a VIE in which MPLX exercises significant influence but does not control and is not the primary beneficiary are also accounted for using the equity method. In preparing the Consolidated Statements of Equity, net income attributable to MPLX LP is allocated to preferred unitholders based on a fixed distribution schedule. Distributions, although earned, are not accrued until declared. The allocation of net income attributable to MPLX LP for purposes of calculating net income per limited partner unit is described in Note 6 .

Accounting Standards

Accounting Standards	3 Months Ended
	Mar. 31, 2019
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Accounting Standards	Recently Adopted ASU 2016-02, Leases We adopted ASU No. 2016-02, Leases (Topic 842), as of January 1, 2019, electing the transition method which permits entities to adopt the provisions of the standard using the modified retrospective approach without adjusting comparative periods. We also elected the package of practical expedients permitted under the transition guidance within the new standard, which among other things allowed us to grandfather the historical accounting conclusions until a reassessment event is present. We have also elected the practical expedient to not recognize short-term leases on the balance sheet, the practical expedient related to right of way permits and land easements which allows us to carry forward our accounting treatment for those existing agreements, and the practical expedient to combine lease and non-lease components for the majority of our underlying classes of assets except for our third-party contractor service and equipment agreements and boat and barge equipment agreements in which we are the lessee. We did not elect the practical expedient to combine lease and non-lease components for arrangements in which we are the lessor. In instances where the practical expedient was not elected, lease and non-lease consideration is allocated based on relative standalone selling price. Right of use (“ROU”) assets represent our right to use an underlying asset in which we obtain substantially all of the economic benefits and the right to direct the use of the asset during the lease term while lease liabilities represent our obligation to make lease payments arising from the lease. Operating lease ROU assets and lease liabilities are recognized at commencement date based on the present value of lease payments over the lease term. We recognize ROU assets and lease liabilities on the balance sheet for leases with a lease term of greater than one year. Payments that are not fixed at the commencement of the lease are considered variable and are excluded from the ROU asset and lease liability calculations. In the measurement of our ROU assets and lease liabilities, the fixed lease payments in the agreement are discounted using a secured incremental borrowing rate for a term similar to the duration of the lease, as our leases do not provide implicit rates. Operating lease expense is recognized on a straight-line basis over the lease term. Adoption of the new standard resulted in the recording of ROU assets and lease liabilities of approximately $505 million and $502 million , respectively, as of January 1, 2019. The standard did not materially impact our consolidated statements of income, cash flows or equity as a result of adoption. As a lessor under ASC 842, MPLX may be required to re-classify existing operating leases to sales-type leases upon modification and related reassessment of the leases. If such a modification were to occur, it may result in the de-recognition of existing assets, recognition of a receivable in the amount of the present value of fixed payments expected to be received by MPLX under the lease, and recognition of a corresponding gain or loss in the period of change. MPLX will evaluate the impacts of lease reassessments as modifications occur. We also adopted the following standard during the first quarter of 2019 , which did not have a material impact to our financial statements or financial statement disclosures: ASU Effective Date 2017-12 Derivatives and Hedging - Targeted Improvements to Accounting for Hedging Activities January 1, 2019
Description of New Accounting Pronouncements Not yet Adopted [Text Block]	Not Yet Adopted ASU 2017-04, Intangibles - Goodwill and Other - Simplifying the Test for Goodwill Impairment In January 2017, the FASB issued an ASU which simplifies the subsequent measurement of goodwill by eliminating Step 2 from the goodwill impairment test. Under the new guidance, the recognition of an impairment charge is calculated based on the amount by which the carrying amount exceeds the reporting unit’s fair value, which could be different from the amount calculated under the current method using the implied fair value of the goodwill; however, the loss recognized should not exceed the total amount of goodwill allocated to that reporting unit. The guidance should be applied on a prospective basis, and is effective for annual or any interim goodwill impairment tests in fiscal years beginning after December 15, 2019. Early adoption is permitted for interim or annual goodwill impairment tests performed on testing dates after January 1, 2017. ASU 2016-13, Credit Losses - Measurement of Credit Losses on Financial Instruments In June 2016, the FASB issued an ASU related to the accounting for credit losses on certain financial instruments. The guidance requires that for most financial assets, losses be based on an expected loss approach which includes estimates of losses over the life of exposure that considers historical, current and forecasted information. Expanded disclosures related to the methods used to estimate the losses as well as a specific disaggregation of balances for financial assets are also required. The change is effective for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years, with early adoption permitted for fiscal years beginning after December 15, 2018, and interim periods within those fiscal years. We do not expect application of this ASU to have a material impact on our consolidated financial statements.

Acquisitions

Acquisitions	3 Months Ended
	Mar. 31, 2019
Business Combinations [Abstract]
Acquisitions	Acquisitions Mt. Airy Terminal On September 26, 2018, MPLX acquired an eastern U.S. Gulf Coast export terminal (the “Mt. Airy Terminal”) from Pin Oak Holdings, LLC for total consideration of $451 million . At the time of the acquisition, the terminal included tanks with 4 million barrels of third-party leased storage capacity and a dock with 120 mbpd of capacity. The Mt. Airy Terminal is located on the Mississippi River between New Orleans and Baton Rouge, is in close proximity to several Gulf Coast refineries including MPC’s Garyville Refinery and is near numerous rail lines and pipelines. The Mt. Airy Terminal is accounted for within the L&S segment. In the first quarter of 2019, an adjustment to the initial purchase price was made for approximately $5 million related to the final settlement of the acquisition. This reduced the total purchase price to $446 million and resulted in $336 million of property, plant and equipment, $121 million of goodwill and the remainder being attributable to net liabilities assumed. Goodwill represents the significant growth potential of the terminal due to the multiple pipelines and rail lines which cross the property, the terminal’s position as an aggregation point for liquids growth in the region for both ocean-going vessels and inland barges, the proximity of the terminal to MPC’s Garyville refinery and other refineries in the region as well as the opportunity to construct an additional dock at the site. Refining Logistics and Fuels Distribution Acquisition On February 1, 2018 , MPC and MPLX LP closed on an agreement for the dropdown of refining logistics assets and fuels distribution services to MPLX LP. MPC contributed these assets and services in exchange for $4.1 billion in cash and a fixed number of MPLX LP common units and general partner units of 111,611,111 and 2,277,778 , respectively. The fair value of the common and general partner units issued as of the acquisition date was $4.3 billion based on the closing common unit price as of February 1, 2018 , as recorded on the Consolidated Statements of Equity, for a total purchase price of $8.4 billion . The equity issued consisted of: (i) 85,610,278 common units to MPLX GP LLC (“MPLX GP”), (ii) 18,176,666 common units to MPLX Logistics Holdings LLC and (iii) 7,824,167 common units to MPLX Holdings Inc. MPLX also issued 2,277,778 general partner units to MPLX GP in order to maintain its two percent general partner interest (“GP Interest”) in MPLX. MPC agreed to waive approximately one-third of the first quarter 2018 distributions on the common units issued in connection with this transaction. As a result of this waiver, MPC did not receive $23.7 million of the distributions that would have otherwise accrued on such common units with respect to the first quarter of 2018. Immediately following this transaction, the GP Interest was converted into a non-economic general partner interest. MPLX recorded this transaction on a historical basis as required for transactions between entities under common control. No effect was given to the prior periods as these entities were not considered businesses prior to the February 1, 2018 dropdown. In connection with the dropdown, approximately $830 million of net property, plant and equipment was recorded in addition to $85 million and $130 million of goodwill allocated to MPLX Refining Logistics LLC (“Refining Logistics”) and MPLX Fuels Distribution LLC (“Fuels Distribution”), respectively. Both the refining logistics assets and the fuels distribution services are accounted for within the L&S segment. As of the transaction date, the Refining Logistics assets included 619 tanks with approximately 56 million barrels of storage capacity (crude, finished products and intermediates), 32 rail and truck racks, 18 docks, and gasoline blenders. These assets generate revenue through storage services agreements with MPC. Refining Logistics provides certain services to MPC related to the receipt, storage, throughput, custody and delivery of petroleum products in and through certain storage and logistical facilities and assets associated with MPC’s refineries. Fuels Distribution, which is a wholly-owned subsidiary of MPLXT, generates revenue through a fuels distribution services agreement with MPC. Fuels Distribution is structured to provide a broad range of scheduling and marketing services as MPC’s agent.

Equity Method Investments

Equity Method Investments	3 Months Ended
	Mar. 31, 2019
Equity Method Investments and Joint Ventures [Abstract]
Investments and Noncontrolling Interests [Text Block]	Investments and Noncontrolling Interests The following table presents MPLX’s equity method investments at the dates indicated: Ownership as of Carrying value at March 31, March 31, December 31, (In millions, except ownership percentages) 2019 2019 2018 Explorer Pipeline Company 25% $ 85 $ 90 Illinois Extension Pipeline Company, L.L.C. 35% 280 275 LOCAP LLC 59% 27 27 LOOP LLC 41% 232 226 MarEn Bakken Bakken Company LLC 25% 492 498 Centrahoma Processing LLC 40% 158 160 MarkWest EMG Jefferson Dry Gas Gathering Company, L.L.C. 67% 247 236 MarkWest Utica EMG, L.L.C. 56% 2,017 2,039 Sherwood Midstream LLC 50% 455 366 Sherwood Midstream Holdings LLC 56% 163 157 Other 114 100 Total $ 4,270 $ 4,174 Summarized financial information for MPLX’s equity method investments for the three months ended March 31, 2019 and 2018 is as follows: Three Months Ended March 31, 2019 (In millions) VIEs Non-VIEs Total Revenues and other income $ 143 $ 340 $ 483 Costs and expenses 70 172 242 Income from operations 73 168 241 Net income 66 156 222 Income from equity method investments (1) $ 24 $ 46 $ 70 Three Months Ended March 31, 2018 (In millions) VIEs Non-VIEs Total Revenues and other income $ 106 $ 297 $ 403 Costs and expenses 62 155 217 Income from operations 44 142 186 Net income 44 129 173 Income from equity method investments (1) $ 15 $ 46 $ 61 (1) “ Income from equity method investments ” includes the impact of any basis differential amortization or accretion. Summarized balance sheet information for MPLX’s equity method investments as of March 31, 2019 and December 31, 2018 is as follows: March 31, 2019 (In millions) VIEs Non-VIEs Total Current assets $ 162 $ 327 $ 489 Noncurrent assets 4,438 4,693 9,131 Current liabilities 129 221 350 Noncurrent liabilities $ 193 $ 843 $ 1,036 December 31, 2018 (In millions) VIEs Non-VIEs Total Current assets $ 235 $ 379 $ 614 Noncurrent assets 3,535 4,715 8,250 Current liabilities 155 246 401 Noncurrent liabilities $ 189 $ 841 $ 1,030 As of March 31, 2019 and December 31, 2018 , the carrying value of MPLX’s equity method investments exceeded the underlying net assets of its investees by $1.0 billion for the G&P segment. As of March 31, 2019 and December 31, 2018 , the carrying value of MPLX’s equity method investments in the L&S segment exceeded the underlying net assets of its investees by $112 million and $114 million , respectively. This basis difference is being amortized into net income over the remaining estimated useful lives of the underlying assets, except for $459 million and $39 million of excess related to goodwill for the G&P and L&S segments, respectively. MarkWest Utica EMG MarkWest Utica EMG, L.L.C.’s (“MarkWest Utica EMG”) is deemed to be a VIE. Neither MPLX nor any of its subsidiaries are deemed to be the primary beneficiary due to EMG Utica, LLC’s voting rights on significant matters. MPLX’s maximum exposure to loss as a result of its involvement with MarkWest Utica EMG includes its equity investment, any additional capital contribution commitments and any operating expenses incurred by the subsidiary operator in excess of its compensation received for the performance of the operating services. MarkWest Utica EMG holds an investment in its subsidiary, Ohio Gathering Company, L.L.C. (“Ohio Gathering”), which does not appear elsewhere in the tables above. The investment was $766 million and $750 million as of March 31, 2019 and December 31, 2018 , respectively. MPLX did not provide any financial support to MarkWest Utica EMG that it was not contractually obligated to provide during the three months ended March 31, 2019 . Ohio Gathering 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 March 31, 2019 , MPLX has an approximate 34 percent indirect ownership interest in Ohio Gathering. As Ohio Gathering 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 Sherwood Midstream LLC (“Sherwood Midstream”) is deemed to be a VIE. Neither MPLX nor any of its subsidiaries are deemed to be the primary beneficiary of Sherwood Midstream due to Antero Midstream Partners, LP’s voting rights on significant matters. MPLX’s maximum exposure to loss as a result of its involvement with Sherwood Midstream includes its equity investment, any additional capital contribution commitments and any operating expenses incurred by the subsidiary operator in excess of its compensation received for the performance of the operating services. MPLX did not provide any financial support to Sherwood Midstream that it was not contractually obligated to provide during the three months ended March 31, 2019 . Sherwood Midstream also has an investment in MarkWest Ohio Fractionation Company, L.L.C. (“Ohio Fractionation”), which is a VIE, that it accounts for as an equity method investment as Sherwood Midstream does not control Ohio Fractionation. During the three months ended March 31, 2019 , Sherwood Midstream acquired the right to fractionation revenue and the obligation to pay expenses related to 20 mbpd of capacity in the Hopedale 4 fractionator; this transaction is shown as “Contributions from noncontrolling interests” on the Consolidated Statements of Cash Flows. MarkWest Liberty Midstream & Resources, L.L.C (“MarkWest Liberty Midstream”), a wholly-owned and consolidated subsidiary, has been deemed to be the primary beneficiary of Ohio Fractionation because it has control over the decisions that could significantly impact its financial performance, and as a result, consolidates Ohio Fractionation. The creditors of Ohio Fractionation do not have recourse to MPLX LP’s general credit through guarantees or other financial arrangements. The assets of Ohio Fractionation are the property of Ohio Fractionation and cannot be used to satisfy the obligations of MPLX LP. Sherwood Midstream’s interests are reflected in “Net income attributable to noncontrolling interests” on the Consolidated Statements of Income and “Noncontrolling interests” on the Consolidated Balance Sheets. Sherwood Midstream Holdings MPLX accounts for Sherwood Midstream Holdings LLC (“Sherwood Midstream Holdings”), which is a VIE, as an equity method investment as Sherwood Midstream is considered to be the general partner and controls all decisions. During the three months ended March 31, 2018, MarkWest Liberty Midstream sold to Sherwood Midstream six percent of its equity ownership in Sherwood Midstream Holdings for $15 million . MPLX’s maximum exposure to loss as a result of its involvement with Sherwood Midstream Holdings includes its equity investment, any additional capital contribution commitments and any operating expenses incurred by the subsidiary operator in excess of its compensation received for the performance of the operating services. MPLX did not provide any financial support to Sherwood Midstream Holdings that it was not contractually obligated to provide during the three months ended March 31, 2019 . Sherwood Midstream has been deemed the primary beneficiary of Sherwood Midstream Holdings due to its controlling financial interest through its authority to manage the joint venture. As a result, Sherwood Midstream consolidates Sherwood Midstream Holdings. Therefore, MPLX also reports its portion of Sherwood Midstream Holdings’ net assets as a component of its investment in Sherwood Midstream. As of March 31, 2019 , MPLX has a 21.8 percent indirect ownership interest in Sherwood Midstream Holdings through Sherwood Midstream.

Related Party Agreements and Tr

Related Party Agreements and Transactions	3 Months Ended
	Mar. 31, 2019
Related Party Transactions [Abstract]
Related Party Agreements and Transactions	Related Party Agreements and Transactions MPLX’s material related parties are: • MPC, which refines, markets and transports crude oil and petroleum products. • MarkWest Utica EMG, in which MPLX LP has a 56 percent interest as of March 31, 2019 . MarkWest Utica EMG is engaged in natural gas processing and NGL fractionation, transportation and marketing in Ohio. • Ohio Gathering, in which MPLX LP has a 34 percent indirect interest as of March 31, 2019 . Ohio Gathering is a subsidiary of MarkWest Utica EMG providing natural gas gathering service in the Utica Shale region of eastern Ohio. • Sherwood Midstream, in which MPLX LP has a 50 percent interest as of March 31, 2019 . Sherwood Midstream supports the development of Antero Resources Corporation’s Marcellus Shale acreage in the rich-gas corridor of West Virginia. • Sherwood Midstream Holdings, in which MPLX LP has a 78 percent total direct and indirect interest as of March 31, 2019 . Sherwood Midstream Holdings owns certain infrastructure at the Sherwood Complex that is shared by and supports the operation of both the Sherwood Midstream and MarkWest gas processing plants and de-ethanization facilities. • MarkWest EMG Jefferson Dry Gas Gathering Company, L.L.C. (“Jefferson Dry Gas”), in which MPLX LP has a 67 percent interest as of March 31, 2019 . Jefferson Dry Gas provides natural dry gas gathering and related services in the Utica Shale region of Ohio. Related Party Agreements MPLX has various long-term, fee-based commercial agreements with MPC. Under these agreements, MPLX provides transportation, terminal, fuels distribution, marketing, storage, management, operational and other services to MPC. MPC has committed to provide MPLX with minimum throughput volumes on crude oil and refined products systems; fees for storage capacity; a fixed fee for substantially all available capacity for boats and barges under the marine transportation services agreement; operating and management fees; as well as reimbursements for certain direct and indirect costs. In addition, MPLX has obligations to MPC for services provided to MPLX by MPC under omnibus and employee services agreements as well as other various agreements. MPLX is also party to a loan agreement with MPC Investment LLC (“MPC Investment”) (the “MPC Loan Agreement”). Under the terms of the MPC Loan Agreement, MPC Investment makes a loan or loans to MPLX on a revolving basis as requested by MPLX and as agreed to by MPC Investment. On April 27, 2018, MPLX and MPC Investment entered into an amendment to the MPC Loan Agreement to increase the borrowing capacity under the MPC Loan Agreement from $500 million to $1 billion in aggregate principal amount of all loans outstanding at any one time. The entire unpaid principal amount of the loan, together with all accrued and unpaid interest and other amounts (if any), shall become due and payable on December 4, 2020 . MPC Investment may demand payment of all or any portion of the outstanding principal amount of the loan, together with all accrued and unpaid interest and other amounts (if any), at any time prior to December 4, 2020 . Borrowings under the loan will bear interest at LIBOR plus 1.50 percent . Activity on the MPC Loan Agreement was as follows: (In millions) Three Months Ended March 31, 2019 Year Ended December 31, 2018 Borrowings $ 851 $ 3,962 Average interest rate of borrowings 3.988 % 3.473 % Repayments $ 851 $ 4,347 Outstanding balance at end of period (1) $ — $ — (1) Included in “Current liabilities - related parties” on the Consolidated Balance Sheets. Related Party Revenue Related party sales to MPC consist of crude oil and refined products pipeline transportation services based on tariff rates; storage, terminal and fuels distribution services based on contracted rates; and marine transportation services. Related party sales to MPC also consist of revenue related to volume deficiency credits. MPLX also has operating agreements with MPC under which it receives a fee for operating MPC’s retained pipeline assets and a fixed annual fee for providing oversight and management services required to run the marine business. MPLX also receives management fee revenue for engineering, construction and administrative services for operating certain of its equity method investments. Revenue received from related parties included on the Consolidated Statements of Income was as follows: Three Months Ended March 31, (In millions) 2019 2018 Service revenues - related parties MPC $ 578 $ 471 Rental income - related parties MPC 193 145 Product sales - related parties (1) MPC 11 4 Other income - related parties MPC 10 10 MarkWest Utica EMG 4 4 Ohio Gathering 4 4 Sherwood Midstream 4 3 Jefferson Dry Gas 2 1 Other 2 1 Total Other income - related parties $ 26 $ 23 (1) There were additional product sales to MPC that net to zero within the consolidated financial statements as the transactions are recorded net due to the terms of the agreements under which such product was sold. For the three months ended March 31, 2019 and March 31, 2018 , these sales totaled $86 million and $79 million , respectively. Related Party Expenses MPC provides executive management services and certain general and administrative services to MPLX under the terms of an omnibus agreement (“Omnibus charges”). Omnibus charges included in “Rental cost of sales - related parties” primarily relate to services that support MPLX’s rental operations and maintenance of assets available for rent. Omnibus charges included in “Purchases - related parties” primarily relate to services that support MPLX’s operations and maintenance activities, as well as compensation expenses. Omnibus charges included in “General and administrative expenses” primarily relate to services that support MPLX’s executive management, accounting and human resources activities. MPLX LP also obtains employee services from MPC under employee services agreements (“ESA charges”). ESA charges for personnel directly involved in or supporting operations and maintenance activities related to rental services are classified as “Rental cost of sales - related parties.” ESA charges for personnel directly involved in or supporting operations and maintenance activities related to other services are classified as “Purchases - related parties.” ESA charges for personnel involved in executive management, accounting and human resources activities are classified as “General and administrative expenses.” In addition to these agreements, MPLX purchases products from MPC, makes payments to MPC in its capacity as general contractor to MPLX, and has certain rent and lease agreements with MPC. Expenses incurred from MPC under the omnibus and employee services agreements as well as other purchases from MPC included on the Consolidated Statements of Income are as follows: Three Months Ended March 31, (In millions) 2019 2018 Rental cost of sales - related parties $ 3 $ 1 Purchases - related parties 212 177 General and administrative expenses 50 39 Total $ 265 $ 217 Some charges incurred under the omnibus and ESA agreements are related to engineering services and are associated with assets under construction. These charges are added to “Property, plant and equipment, net” on the Consolidated Balance Sheets. For the three months ended March 31, 2019 and March 31, 2018 , these charges totaled $41 million and $22 million , respectively. Related Party Assets and Liabilities Assets and liabilities with related parties appearing on the Consolidated Balance Sheets are detailed in the table below. This table identifies the various components of related party assets and liabilities, including those associated with leases (see Note 19 for additional information) and deferred revenue on minimum volume commitments. During the three months ended March 31, 2019 and the year ended December 31, 2018 , MPC did not ship its minimum committed volumes on certain pipelines. Under MPLX’s pipeline transportation services agreements, if MPC fails to transport its minimum throughput volumes during any quarter, then MPC will pay MPLX a deficiency payment equal to the volume of the deficiency multiplied by the tariff rate then in effect. The deficiency amounts are recorded as “Current liabilities - related parties.” MPC may then apply the amount of any such deficiency payments as a credit for volumes transported on the applicable pipeline in excess of its minimum volume commitment in future periods under the terms of the applicable transportation services agreement. MPLX recognizes related party revenues for the deficiency payments when credits are used for volumes transported in excess of minimum quarterly volume commitments, when it becomes impossible to physically transport volumes necessary to utilize the credits or upon the expiration of the credits. The use or expiration of the credits is a decrease in “Current liabilities - related parties.” In addition, capital projects MPLX is undertaking at the request of MPC are reimbursed in cash and recognized in income over the remaining term of the applicable agreements. (In millions) March 31, 2019 December 31, 2018 Current assets - related parties Receivables - MPC $ 372 $ 281 Receivables - Other 7 8 Prepaid - MPC 7 1 Total 386 290 Noncurrent assets - related parties Long-term receivables - MPC 24 24 Right of use assets - MPC 232 — Total 256 24 Current liabilities - related parties Payables - MPC 119 131 Payables - MarkWest Utica EMG 16 51 Payables - Sherwood Midstream 18 16 Payables - Other 5 5 Operating lease liabilities - MPC 1 — Deferred revenue - Minimum volume deficiencies - MPC 38 44 Deferred revenue - Project reimbursements - MPC 7 7 Total 204 254 Long-term liabilities - related parties Long-term operating lease liabilities - MPC 231 — Long-term deferred revenue - Project reimbursements - MPC 42 43 Total $ 273 $ 43 Other Related Party Transactions From time to time, MPLX may also sell to or purchase from related parties, assets and inventory at the lesser of average unit cost or net realizable value. Sales to and purchases from related parties for the three months ended March 31, 2019 and 2018 were less than $1 million , respectively.

Net Income Per Limited Partner

Net Income Per Limited Partner Unit	3 Months Ended
	Mar. 31, 2019
Earnings Per Share [Abstract]
Net Income Per Limited Partner Unit	Net Income/(Loss) Per Limited Partner Unit Net income/(loss) per unit applicable to common limited partner units is computed by dividing net income/(loss) attributable to MPLX LP less income/(loss) allocated to participating securities by the weighted average number of common units outstanding. The classes of participating securities include common units, certain equity-based compensation awards and Series A Convertible preferred units. For the three months ended March 31, 2019 and 2018 , MPLX had dilutive potential common units consisting of certain equity-based compensation awards. Potential common units omitted from the diluted earnings per unit calculation for the three months ended March 31, 2019 and 2018 were less than 1 million . Three Months Ended March 31, (In millions) 2019 2018 Net income attributable to MPLX LP $ 503 $ 421 Less: Limited partners’ distributions declared on preferred units (1) 20 16 Limited partners’ distributions declared on common units (including common units of general partner) (1) 523 467 Undistributed net loss attributable to MPLX LP $ (40 ) $ (62 ) (1) See Note 7 for distribution information. Three Months Ended March 31, 2019 (In millions, except per unit data) Limited Partners’ Common Units Redeemable Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit Net income attributable to MPLX LP: Distributions declared $ 523 $ 20 $ 543 Undistributed net loss attributable to MPLX LP (40 ) — (40 ) Net income attributable to MPLX LP (1) $ 483 $ 20 $ 503 Weighted average units outstanding: Basic 794 31 825 Diluted 795 31 826 Net income attributable to MPLX LP per limited partner unit: Basic $ 0.61 Diluted $ 0.61 Three Months Ended March 31, 2018 (In millions, except per unit data) Limited Partners’ Common Units Redeemable Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit Net income attributable to MPLX LP: Distributions declared (including IDRs) $ 467 $ 16 $ 483 Undistributed net loss attributable to MPLX LP (62 ) — (62 ) Net income attributable to MPLX LP (1) $ 405 $ 16 $ 421 Weighted average units outstanding: Basic 661 31 692 Diluted 661 31 692 Net income attributable to MPLX LP per limited partner unit: Basic $ 0.61 Diluted $ 0.61 (1) Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.

Equity

Equity	3 Months Ended
	Mar. 31, 2019
Equity [Abstract]
Equity	Equity The changes in the number of common units outstanding during the three months ended March 31, 2019 are summarized below: (In units) Common Balance at December 31, 2018 794,089,518 Unit-based compensation awards 148,379 Balance at March 31, 2019 794,237,897 Cash distributions – In accordance with the Partnership Agreement, on April 29, 2019 , MPLX declared a quarterly cash distribution, based on the results of the first quarter of 2019 , totaling $523 million , or $0.6575 per common unit; this rate will also be received by preferred unitholders. These distributions will be paid on May 15, 2019 to common unitholders of record on May 9, 2019 . Distributions for the first quarter of 2018 were $0.6175 per common unit. The allocation of total quarterly cash distributions to limited and preferred unitholders is as follows for the three months ended March 31, 2019 and 2018 . MPLX’s distributions are declared subsequent to quarter end; therefore, the following table represents total cash distributions applicable to the period in which the distributions were earned. Three Months Ended March 31, (In millions) 2019 2018 Common and preferred unit distributions: Common unitholders, includes common units of general partner $ 523 $ 467 Preferred unit distributions 20 16 Total cash distributions declared $ 543 $ 483

Redeemable Preferred Units

Redeemable Preferred Units	3 Months Ended
	Mar. 31, 2019
Temporary Equity Disclosure [Abstract]
Redeemable Preferred Units	Redeemable Preferred Units Private Placement of Preferred Units – On May 13, 2016, MPLX LP completed the private placement of approximately 30.8 million 6.5 percent Series A Convertible preferred units for a cash purchase price of $32.50 per unit. The aggregate net proceeds of approximately $984 million from the sale of the preferred units were used for capital expenditures, repayment of debt and general business purposes. The preferred units rank senior to all common units with respect to distributions and rights upon liquidation. The holders of the preferred units received cumulative quarterly distributions equal to $0.528125 per unit for each quarter prior to the second quarter of 2018. Beginning with the second quarter of 2018, the holders of the preferred units are entitled to receive a quarterly distribution equal to the greater of $0.528125 per unit or the amount of distributions they would have received on an as converted basis. On April 29, 2019 , MPLX declared a quarterly cash distribution of $0.6575 per common unit representing the distribution of income earned during the first quarter of 2019 . The preferred units will receive the common unit rate in lieu of the lower $0.528125 base amount. The changes in the redeemable preferred balance from December 31, 2018 through March 31, 2019 are summarized below: (In millions) Redeemable Preferred Units Balance at December 31, 2018 $ 1,004 Net income allocated 20 Distributions received by preferred unitholders (20 ) Balance at March 31, 2019 $ 1,004 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 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 change of control conversion rate. The preferred units are considered redeemable securities under GAAP due to the existence of redemption provisions upon a deemed liquidation event which is outside MPLX’s control. Therefore, they are presented as temporary equity in the mezzanine section of the Consolidated Balance Sheets. The preferred units have been recorded at their issuance date fair value, net of issuance costs. Income allocations increase the carrying value and declared distributions decrease the carrying value of the preferred units. As the preferred units are not currently redeemable and not probable of becoming redeemable, adjustment to the initial carrying amount is not necessary and would only be required if it becomes probable that the preferred units would become redeemable.

Segment Information

Segment Information	3 Months Ended
	Mar. 31, 2019
Segment Reporting [Abstract]
Segment Reporting Disclosure [Text Block]	Segment Information MPLX’s chief operating decision maker is the chief executive officer (“CEO”) of its general partner. The CEO reviews MPLX’s discrete financial information, makes operating decisions, assesses financial performance and allocates resources on a type of service basis. MPLX has two reportable segments: L&S and G&P. Each of these segments is organized and managed based upon the nature of the products and services it offers. • L&S – transports, stores, distributes and markets crude oil and refined petroleum products. • G&P – gathers, processes and transports natural gas; and gathers, transports, fractionates, stores and markets NGLs. During the second quarter of 2018, our CEO began to evaluate the performance of our segments using Segment Adjusted EBITDA. We have modified our presentation of segment performance metrics to be consistent with this change, including prior periods presented for consistent and comparable presentation. Amounts included in net income and excluded from Segment Adjusted EBITDA include: (i) depreciation and amortization; (ii) provision/(benefit) for income taxes; (iii) amortization of deferred financing costs; (iv) extinguishment of debt; (v) non-cash equity-based compensation; (vi) impairment expense; (vii) net interest and other financial costs; (viii) income/(loss) from equity method investments; (ix) distributions and adjustments related to equity method investments; (x) unrealized derivative gains/(losses); (xi) acquisition costs; (xii) noncontrolling interest; and (xiii) other adjustments as deemed necessary. These items are either: (i) believed to be non-recurring in nature; (ii) not believed to be allocable or controlled by the segment; or (iii) are not tied to the operational performance of the segment. The tables below present information about revenues and other income, capital expenditures and total assets for our reportable segments: Three Months Ended March 31, (In millions) 2019 2018 L&S Service revenue $ 612 $ 499 Rental income 199 145 Product related revenue 3 2 Income from equity method investments 41 44 Other income 11 12 Total segment revenues and other income (1) 866 702 Segment Adjusted EBITDA (2) 559 437 Maintenance capital expenditures 13 22 Growth capital expenditures 103 154 G&P Service revenue 404 354 Rental income 88 79 Product related revenue 244 253 Income from equity method investments 29 17 Other income 15 15 Total segment revenues and other income (1) 780 718 Segment Adjusted EBITDA (2) 371 323 Maintenance capital expenditures 6 3 Growth capital expenditures $ 261 $ 271 (1) Within the total segment revenues and other income amounts presented above, third party revenues for the L&S segment were $82 million and $74 million for the three months ended March 31, 2019 and 2018 , respectively. Third party revenues for the G&P segment were $757 million and $703 million for the three months ended March 31, 2019 and 2018 , respectively. (2) See below for the reconciliation from Segment Adjusted EBITDA to “Net income.” (In millions) March 31, 2019 December 31, 2018 Segment assets Cash and cash equivalents $ 93 $ 68 L&S (1) 7,040 6,566 G&P (1) 16,451 16,145 Total assets $ 23,584 $ 22,779 (1) Equity method investments included in L&S assets were $1.12 billion at March 31, 2019 and December 31, 2018 , respectively. Equity method investments included in G&P assets were $3.15 billion at March 31, 2019 and $3.05 billion at December 31, 2018 . The table below provides a reconciliation between “Net income” and Segment Adjusted EBITDA. Three Months Ended March 31, (In millions) 2019 2018 Reconciliation to Net income: L&S Segment Adjusted EBITDA $ 559 $ 437 G&P Segment Adjusted EBITDA 371 323 Total reportable segments 930 760 Depreciation and amortization (1) (211 ) (176 ) Benefit/(provision) for income taxes 2 (4 ) Amortization of deferred financing costs (13 ) (16 ) Non-cash equity-based compensation (6 ) (4 ) Net interest and other financial costs (158 ) (114 ) Income from equity method investments 70 61 Distributions/adjustments related to equity method investments (108 ) (90 ) Unrealized derivative (losses)/gains (2) (4 ) 7 Acquisition costs — (3 ) Adjusted EBITDA attributable to noncontrolling interests 7 2 Net income $ 509 $ 423 (1) Depreciation and amortization attributable to L&S was $70 million and $48 million for the three months ended March 31, 2019 and 2018 , respectively. Depreciation and amortization attributable to G&P was $141 million and $128 million for the three months ended March 31, 2019 and 2018 , respectively. (2) MPLX makes a distinction between realized and unrealized gains and losses on derivatives. During the period when a derivative contract is outstanding, changes in the fair value of the derivative are recorded as an unrealized gain or loss. When a derivative contract matures or is settled, the previously recorded unrealized gain or loss is reversed and the realized gain or loss of the contract is recorded.

Inventories

Inventories	3 Months Ended
	Mar. 31, 2019
Inventory Disclosure [Abstract]
Inventories	Inventories Inventories consist of the following: (In millions) March 31, 2019 December 31, 2018 NGLs $ 3 $ 9 Line fill 10 9 Spare parts, materials and supplies 61 59 Total inventories $ 74 $ 77

Property, Plant and Equipment

Property, Plant and Equipment	3 Months Ended
	Mar. 31, 2019
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment	Property, Plant and Equipment Property, plant and equipment with associated accumulated depreciation is shown below: (In millions) March 31, 2019 December 31, 2018 Natural gas gathering and NGL transportation pipelines and facilities $ 6,128 $ 5,926 Processing, fractionation and storage facilities 5,343 5,336 Pipelines and related assets 2,620 2,560 Barges and towing vessels 635 620 Terminals and related assets 1,184 1,178 Refinery related assets 943 938 Land, building, office equipment and other 978 957 Construction-in-progress 858 801 Total 18,689 18,316 Less accumulated depreciation 3,873 3,677 Property, plant and equipment, net $ 14,816 $ 14,639

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2019
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements Fair Values – Recurring Fair value measurements and disclosures relate primarily to MPLX’s derivative positions as discussed in Note 13 . The following table presents the financial instruments carried at fair value on a recurring basis as of March 31, 2019 and December 31, 2018 by fair value hierarchy level. MPLX has elected to offset the fair value amounts recognized for multiple derivative contracts executed with the same counterparty. March 31, 2019 December 31, 2018 (In millions) Assets Liabilities Assets Liabilities Significant unobservable inputs (Level 3) Embedded derivatives in commodity contracts $ — $ (65 ) $ — $ (61 ) Total carrying value on Consolidated Balance Sheets $ — $ (65 ) $ — $ (61 ) Level 3 instruments include all NGL transactions and embedded derivatives in commodity contracts. The embedded derivative liability relates to a natural gas purchase agreement embedded in a keep-whole processing agreement. The fair value calculation for these Level 3 instruments used significant unobservable inputs including: (1) NGL prices interpolated and extrapolated due to inactive markets ranging from $0.60 to $1.25 and (2) the probability of renewal of 91 percent for the first five -year term and 82 percent for the second five -year term of the gas purchase agreement and related keep-whole processing agreement. Increases or decreases in the fractionation spread result in an increase or decrease in the fair value of the embedded derivative liability, respectively. An increase in the probability of renewal would result in an increase in the fair value of the related embedded derivative liability. Beyond the embedded derivative discussed above, we had no outstanding commodity contracts as of March 31, 2019 or December 31, 2018 . Changes in Level 3 Fair Value Measurements The following table is a reconciliation of the net beginning and ending balances recorded for net assets and liabilities classified as Level 3 in the fair value hierarchy. Three Months Ended March 31, 2019 Three Months Ended March 31, 2018 (In millions) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Fair value at beginning of period $ — $ (61 ) $ (2 ) $ (64 ) Total (losses)/gains (realized and unrealized) included in earnings (1) — (6 ) — 3 Settlements — 2 — 3 Fair value at end of period — (65 ) (2 ) (58 ) The amount of total losses for the period included in earnings attributable to the change in unrealized losses relating to liabilities still held at end of period $ — $ (5 ) $ — $ 3 (1) Gains and losses on commodity derivative contracts classified as Level 3 are recorded in “Product sales” on the Consolidated Statements of Income. Gains and losses on derivatives embedded in commodity contracts are recorded in “Purchased product costs” and “Cost of revenues” on the Consolidated Statements of Income. Fair Values – Reported MPLX’s primary financial instruments are cash and cash equivalents, receivables, receivables from related parties, accounts payable, payables to related parties and long-term debt. MPLX’s fair value assessment incorporates a variety of considerations, including (1) the duration of the instruments, (2) MPC’s investment-grade credit rating and (3) the historical incurrence of and expected future insignificance of bad debt expense, which includes an evaluation of counterparty credit risk. MPLX believes the carrying values of its current assets and liabilities approximate fair value. The recorded value of the amounts outstanding under the bank revolving credit facility, if any, approximates fair value due to the variable interest rate that approximates current market rates. Derivative instruments are recorded at fair value, based on available market information (see Note 13 ). The fair value of MPLX’s long-term debt is estimated based on recent market non-binding indicative quotes. The fair value of the SMR liability is estimated using a discounted cash flow approach based on the contractual cash flows and MPLX’s unsecured borrowing rate. The long-term debt and SMR liability fair values are considered Level 3 measurements. The following table summarizes the fair value and carrying value of the long-term debt, excluding finance leases, and SMR liability: March 31, 2019 December 31, 2018 (In millions) Fair Value Carrying Value Fair Value Carrying Value Long-term debt $ 14,430 $ 13,921 $ 13,169 $ 13,484 SMR liability $ 94 $ 85 $ 92 $ 86

Derivative Financial Instrument

Derivative Financial Instruments	3 Months Ended
	Mar. 31, 2019
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Instruments and Hedging Activities Disclosure [Text Block]	Derivative Financial Instruments As of March 31, 2019 , MPLX had no outstanding commodity contracts beyond the embedded derivative discussed below. Embedded Derivative - MPLX has a natural gas purchase commitment embedded in a keep-whole processing agreement with a producer customer in the Southern Appalachian region expiring in December 2022. The customer has the unilateral option to extend the agreement for two consecutive five -year terms through December 2032. For accounting purposes, the natural gas purchase commitment and the term extending options have been aggregated into a single compound embedded derivative. The probability of the customer exercising its options is determined based on assumptions about the customer’s potential business strategy decision points that may exist at the time they would elect whether to renew the contract. The changes in fair value of this compound embedded derivative are based on the difference between the contractual and index pricing, the probability of the producer customer exercising its option to extend and the estimated favorability of these contracts compared to current market conditions. The changes in fair value are recorded in earnings through “Purchased product costs” on the Consolidated Statements of Income. As of March 31, 2019 and December 31, 2018 , the estimated fair value of this contract was a liability of $ 65 million and $ 61 million , respectively. Certain derivative positions are subject to master netting agreements, therefore, MPLX has elected to offset derivative assets and liabilities that are legally permissible to be offset. As of March 31, 2019 and December 31, 2018 , there were no derivative assets or liabilities that were offset on the Consolidated Balance Sheets. The impact of MPLX’s derivative instruments on its Consolidated Balance Sheets is summarized below: (In millions) March 31, 2019 December 31, 2018 Derivative contracts not designated as hedging instruments and their balance sheet location Asset Liability Asset Liability Commodity contracts (1) Other current assets / Other current liabilities $ — $ (8 ) $ — $ (7 ) Other noncurrent assets / Deferred credits and other liabilities — (57 ) — (54 ) Total $ — $ (65 ) $ — $ (61 ) (1) Includes embedded derivatives in commodity contracts as discussed above. For further information regarding the fair value measurement of derivative instruments, including the effect of master netting arrangements or collateral, see Note 12 . There were no material changes to MPLX’s policy regarding the accounting for Level 2 and Level 3 instruments as previously disclosed in MPLX’s Annual Report on Form 10-K for the year ended December 31, 2018 . MPLX does not designate any of its commodity derivative positions as hedges for accounting purposes. The impact of MPLX’s derivative contracts not designated as hedging instruments and the location of gains and losses recognized on the Consolidated Statements of Income is summarized below: Three Months Ended March 31, (In millions) 2019 2018 Product sales Realized (loss)/gain $ — $ — Unrealized (loss)/gain — 1 Total derivative (loss)/gain related to product sales — 1 Purchased product costs Realized (loss)/gain (2 ) (3 ) Unrealized (loss)/gain (4 ) 6 Total derivative (loss)/gain related to purchased product costs (6 ) 3 Cost of revenues Realized (loss)/gain — — Unrealized (loss)/gain — — Total derivative (loss)/gain related to cost of revenues — — Total derivative (loss)/gain $ (6 ) $ 4

Debt

Debt	3 Months Ended
	Mar. 31, 2019
Debt Disclosure [Abstract]
Debt	Debt MPLX’s outstanding borrowings consist of the following: (In millions) March 31, 2019 December 31, 2018 MPLX LP: Bank revolving credit facility due 2022 $ 425 $ — 3.375% senior notes due March 2023 500 500 4.500% senior notes due July 2023 989 989 4.875% senior notes due December 2024 1,149 1,149 4.000% senior notes due February 2025 500 500 4.875% senior notes due June 2025 1,189 1,189 4.125% senior notes due March 2027 1,250 1,250 4.000% senior notes due March 2028 1,250 1,250 4.800% senior notes due February 2029 750 750 4.500% senior notes due April 2038 1,750 1,750 5.200% senior notes due March 2047 1,000 1,000 4.700% senior notes due April 2048 1,500 1,500 5.500% senior notes due February 2049 1,500 1,500 4.900% senior notes due April 2058 500 500 Consolidated subsidiaries: MarkWest - 4.500% - 4.875% senior notes, due 2023-2025 23 23 Financing lease obligations (1) 8 6 Total 14,283 13,856 Unamortized debt issuance costs (96 ) (97 ) Unamortized discount (354 ) (366 ) Amounts due within one year (1 ) (1 ) Total long-term debt due after one year $ 13,832 $ 13,392 (1) See Note 19 for lease information. Credit Agreements MPLX has a $2.25 billion five -year bank revolving credit facility that expires in July 2022 (the “MPLX Credit Agreement”). During the three months ended March 31, 2019 , MPLX borrowed $825 million under the MPLX Credit Agreement, at an average interest rate of 3.916 percent , and repaid $400 million . At March 31, 2019 , MPLX had $425 million outstanding borrowings and $3 million letters of credit outstanding under the facility, resulting in total availability of $1.822 billion , or 81.0 percent of the borrowing capacity . Senior Notes On December 10, 2018, MPLX redeemed all of the $750 million 5.5 percent senior notes due February 15, 2023, $40 million of which was issued by the MarkWest subsidiary. These notes were redeemed at 101.833 percent of the principal amount, which resulted in a payment of $14 million related to the note premium and the immediate recognition of $46 million of unamortized debt issuance costs. On November 15, 2018, MPLX issued $2.25 billion aggregate principal amount of senior notes in a public offering, consisting of $750 million aggregate principal amount of 4.8 percent unsecured senior notes due February 2029 and $1.5 billion aggregate principal amount of 5.5 percent unsecured senior notes due February 2049 (collectively, the “November 2018 New Senior Notes”). The November 2018 New Senior Notes were offered at a price to the public of 99.432 percent and 98.031 percent of par, respectively. The proceeds were used to repay outstanding borrowings under the MPLX Credit Agreement and the MPC Loan Agreement and to redeem the $750 million 5.5 percent senior notes due February 2023, as well as for general business purposes. Interest on each series of the November 2018 New Senior Notes is payable semi-annually in arrears, commencing on February 15, 2019. On February 8, 2018 , MPLX issued $5.5 billion aggregate principal amount of senior notes in a public offering, consisting of $500 million aggregate principal amount of 3.375 percent unsecured senior notes due March 2023 , $1.25 billion aggregate principal amount of 4.0 percent unsecured senior notes due March 2028 , $1.75 billion aggregate principal amount of 4.5 percent unsecured senior notes due April 2038 , $ 1.5 billion aggregate principal amount of 4.7 percent unsecured senior notes due April 2048 , and $500 million aggregate principal amount of 4.9 percent unsecured senior notes due April 2058 (collectively, the “February 2018 New Senior Notes”). The February 2018 New Senior Notes were offered at a price to the public of 99.931 percent , 99.551 percent , 98.811 percent , 99.348 percent , and 99.289 percent of par, respectively. Also on February 8, 2018 , $4.1 billion of the net proceeds were used to repay a 364-day term loan facility, which was drawn on February 1, 2018 to fund the cash portion of the dropdown consideration for Refining Logistics and Fuels Distribution. The remaining proceeds were used to repay outstanding borrowings under the MPLX Credit Agreement and the MPC Loan Agreement, as well as for general business purposes. Interest on each series of notes due in 2023 and 2028 is payable semi-annually in arrears, commencing on September 15, 2018. Interest on each series of notes due in 2038, 2048 and 2058 is payable semi-annually in arrears, commencing on October 15, 2018.

Revenue

Revenue	3 Months Ended
	Mar. 31, 2019
Revenue from Contract with Customer [Abstract]
Revenue from Contract with Customer [Text Block]	Revenue Disaggregation of Revenue The following table represents a disaggregation of revenue for each reportable segment for the three months ended March 31, 2019 and 2018 : Three Months Ended March 31, 2019 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 34 $ 404 $ 438 Service revenue - related parties 578 — 578 Service revenue - product related — 34 34 Product sales 1 201 202 Product sales - related parties 2 9 11 Total revenues from contracts with customers $ 615 $ 648 1,263 Non-ASC 606 revenue (1) 383 Total revenues and other income $ 1,646 (1) Non-ASC 606 Revenue includes rental income, income from equity method investments, derivative gains and losses, mark-to-market adjustments, and other income. Three Months Ended March 31, 2018 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 28 $ 354 $ 382 Service revenue - related parties 471 — 471 Service revenue - product related — 44 44 Product sales (1) 1 205 206 Product sales - related parties 1 3 4 Total revenues from contracts with customers $ 501 $ 606 1,107 Non-ASC 606 revenue (2) 313 Total revenues and other income $ 1,420 (1) G&P “Product sales” for the three months ended March 31, 2018 includes approximately $1 million of revenue related to derivative gains and losses and mark-to-market adjustments. (2) Non-ASC 606 Revenue includes rental income, income from equity method investments, derivative gains and losses, mark-to-market adjustments, and other income. Contract Balances Contract assets typically relate to aid in construction agreements where the revenue recognized and MPLX’s rights to consideration for work completed exceeds the amount billed to the customer. Contract assets are generally classified as current and included in “Other current assets” on the Consolidated Balance Sheets. Contract liabilities, which we refer to as “Deferred revenue” and “Long-term deferred revenue,” typically relate to advance payments for aid in construction agreements and deferred customer credits associated with makeup rights and minimum volume commitments. Related to minimum volume commitments, breakage is estimated and recognized into service revenue in instances where it is probable the customer will not use the credit in future periods. We classify contract liabilities as current or long-term based on the timing of when we expect to recognize revenue. “Receivables, net” primarily relate to our commodity sales. Portions of the “Receivables, net” balance are attributed to the sale of commodity product controlled by MPLX prior to sale while a significant portion of the balance relates to the sale of commodity product on behalf of our producer customers. Both types of transactions are commingled and excluded from the table below. MPLX remits the net sales price back to our producer customers upon completion of the sale. Each period end, certain amounts within accounts payable relate to our payments to producer customers. Such amounts are not deemed material at period end as a result of when we settle with each producer. The table below reflects the changes in our contract balances for the period ended March 31, 2019 : (In millions) Balance at December 31, 2018 (1) Additions/ (Deletions) Revenue Recognized (2) Balance at March 31, 2019 Contract assets $ 4 $ 1 $ — $ 5 Deferred revenue 4 1 (1 ) 4 Deferred revenue - related parties 50 3 (9 ) 44 Long-term deferred revenue 10 3 — 13 Long-term deferred revenue - related parties $ 42 $ (1 ) $ — $ 41 (1) Balance represents ASC 606 portion of each respective line item. (2) No significant revenue was recognized related to past performance obligations in the current period. Remaining Performance Obligations The table below includes estimated revenue expected to be recognized in the future related to performance obligations that are unsatisfied (or partially unsatisfied) at the end of the reporting period. As of March 31, 2019 , the amounts allocated to contract assets and contract liabilities on the Consolidated Balance Sheets are $100 million and are reflected in the amounts below. This will be recognized as revenue as the obligations are satisfied, which is expected to occur over the next 25 years . Further, MPLX does not disclose variable consideration due to volume variability in the table below. (In millions) 2019 $ 884 2020 1,178 2021 1,193 2022 1,179 2023 and thereafter 5,651 Total revenue on remaining performance obligations (1),(2),(3) $ 10,085 (1) All fixed consideration from contracts with customers is included in the amounts presented above. Variable consideration that is constrained or not required to be estimated as it reflects our efforts to perform is excluded. (2) Arrangements deemed implicit leases are included in “Rental income” and are excluded from this table. (3) Only minimum volume commitments that are deemed fixed are included in the table above. MPLX has various minimum volume commitments in processing arrangements that vary based on the actual Btu content of the gas received. These amounts are deemed variable consideration and are excluded from the table above. We do not disclose information on the future performance obligations for any contract with an original expected duration of one year or less.

Supplemental Cash Flow Informat

Supplemental Cash Flow Information	3 Months Ended
	Mar. 31, 2019
Supplemental Cash Flow Elements [Abstract]
Supplemental Cash Flow Information [Text Block]	Supplemental Cash Flow Information (In millions) March 31, 2019 December 31, 2018 Cash and cash equivalents $ 93 $ 68 Restricted cash (1) — 8 Cash, cash equivalents and restricted cash (2) $ 93 $ 76 (1) The restricted cash balance is included within “Other current assets” on the Consolidated Balance Sheets. (2) As a result of the adoption of ASU 2016-18, Statement of Cash Flows - Restricted Cash, the Consolidated Statements of Cash Flows now explain the change during the period of both “Cash and cash equivalents” and “Restricted cash.” Three Months Ended March 31, (In millions) 2019 2018 Net cash provided by operating activities included: Interest paid (net of amounts capitalized) $ 143 $ 103 Cash paid for amounts included in the measurement of lease liabilities Payments on operating leases 19 — Non-cash investing and financing activities: Net transfers of property, plant and equipment from materials and supplies inventories 1 1 ROU assets obtained in exchange for new operating lease obligations 1 — ROU assets obtained in exchange for new finance lease obligations $ 2 $ — The Consolidated Statements of Cash Flows exclude changes to the Consolidated Balance Sheets that did not affect cash. The following is the change of additions to property, plant and equipment related to capital accruals: Three Months Ended March 31, (In millions) 2019 2018 Increase/(decrease) in capital accruals $ (74 ) $ (6 )

Accumulated Other Comprehensive

Accumulated Other Comprehensive Income	3 Months Ended
	Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block]	Accumulated Other Comprehensive Loss MPLX LP records an accumulated other comprehensive loss on the Consolidated Balance Sheets relating to pension and other post-retirement benefits provided by LOOP LLC (“LOOP”) and Explorer Pipeline Company (“Explorer”) to their employees. MPLX LP is not a sponsor of these benefit plans. The following table shows the changes in “Accumulated other comprehensive loss” by component during the period December 31, 2018 through March 31, 2019 . (In millions) Pension Other Total Balance at December 31, 2018 (1) $ (14 ) $ (2 ) $ (16 ) Other comprehensive income - remeasurements (2) — 1 1 Balance at March 31, 2019 (1) $ (14 ) $ (1 ) $ (15 ) The following table shows the changes in “Accumulated other comprehensive loss” by component during the period December 31, 2017 through March 31, 2018 . (In millions) Pension Benefits Other Post-Retirement Benefits Total Balance at December 31, 2017 (1) $ (13 ) $ (1 ) $ (14 ) Other comprehensive loss - remeasurements (2) (1 ) (1 ) (2 ) Balance at March 31, 2018 (1) $ (14 ) $ (2 ) $ (16 ) (1) These components of “Accumulated other comprehensive loss” are included in the computation of net periodic benefit cost by LOOP and Explorer and are therefore included on the Consolidated Statements of Income under the caption “Income/(loss) from equity method investments.” (2) Components of other comprehensive income/loss - remeasurements relate to actuarial gains and losses as well as amortization of prior service costs. MPLX records an adjustment to “Comprehensive income” in accordance with its ownership interest in LOOP and Explorer.

Equity-Based Compensation Plan

Equity-Based Compensation Plan	3 Months Ended
	Mar. 31, 2019
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Equity-Based Compensation Plan	Equity-Based Compensation Phantom Units – The following is a summary of phantom unit award activity of MPLX LP common units for the three months ended March 31, 2019 : Number Weighted Outstanding at December 31, 2018 1,154,335 $ 34.34 Granted 188,069 33.09 Settled (210,846 ) 32.20 Forfeited (9,375 ) 34.86 Outstanding at March 31, 2019 1,122,183 $ 34.53 Performance Units – MPLX grants performance units to certain officers of the general partner and certain eligible MPC officers who make significant contributions to its business. These performance units pay out 75 percent in cash and 25 percent in MPLX LP common units and often contain both market and performance conditions based on various metrics. Market conditions are valued using a Monte Carlo valuation while performance conditions are reevaluated periodically and valued at the compensation cost associated with the performance outcome deemed most probable. The performance units granted in 2019 are hybrid awards having a three-year performance period of January 1, 2019 through December 31, 2021. The payout of the award is dependent on two independent conditions, each constituting 50 percent of the overall target units granted. The awards have a performance condition based on MPLX LP’s distributable cash flow, and a market condition based on MPLX LP’s total unitholder return. The market condition was valued using a Monte Carlo valuation, resulting in a grant date fair value of $.68 per unit for the 2019 equity-classified performance units. Grant date fair value of the performance condition is based on potential payouts per unit of up to $2.00 per unit. Compensation cost associated with the performance condition is based on the grant date fair value of the payout deemed most probable to occur and is adjusted as the expectation for payout changes. During the first quarter of 2018, a performance award was granted; however, a grant date could not be established based on the nature of the award terms. Given that a grant date cannot be established, no expense or units have been recorded. When a grant date is established, the fair value of the award will be recognized over the remaining performance period. The following is a summary of the activity for performance unit awards to be settled in MPLX LP common units for the three months ended March 31, 2019 : Number of Outstanding at December 31, 2018 1,941,750 Granted 987,994 Settled (772,397 ) Forfeited — Outstanding at March 31, 2019 2,157,347

Leases Leases (Notes)

Leases Leases (Notes)	3 Months Ended
	Mar. 31, 2019
Disclosure Text Block [Abstract]
Lessee, Finance Leases [Text Block]	Lessee We lease a wide variety of facilities and equipment under leases from third parties, including land and building space, office and field equipment, storage facilities and transportation equipment, while our related party leases primarily relate to ground leases associated with our refining logistics assets. Our remaining lease terms range from less than one year to 60 years. Some long-term leases include renewal options ranging from one to 50 years and, in certain leases, also include purchase options. Renewal options and termination options were not included in the measurement of ROU assets and lease liabilities since it was determined they were not reasonably certain to be exercised. Under ASC 842, the components of lease cost were as follows: Three Months Ended March 31, 2019 (In millions) Related Party Third Party Components of lease costs: Operating lease costs $ 4 $ 14 Variable lease cost — 2 Short-term lease cost — 10 Total lease cost $ 4 $ 26 Supplemental balance sheet data related to leases were as follows: March 31, 2019 (In millions) Related Party Third Party Operating leases Assets Right of use assets $ 232 $ 262 Liabilities Operating lease liabilities 1 46 Long-term operating lease liabilities 231 216 Total operating lease liabilities $ 232 $ 262 Weighted average remaining lease term 47.92 years 6.97 years Weighted average discount rate 5.80 % 4.33 % Finance leases Assets Property, plant and equipment, gross $ 28 Accumulated depreciation 10 Property, plant and equipment, net 18 Liabilities Other current liabilities 1 Long-term debt 7 Total finance lease liabilities $ 8 Weighted average remaining lease term 31.05 years Weighted average discount rate 5.76 % As of March 31, 2019, maturities of lease liabilities for operating lease obligations and finance lease obligations having initial or remaining non-cancellable lease terms in excess of one year are as follows: (In millions) Related Party Operating Third Party Operating Finance 2019 $ 11 $ 42 $ 1 2020 14 53 6 2021 14 51 — 2022 14 46 — 2023 14 43 — 2024 and thereafter 619 69 7 Gross lease payments 686 304 14 Less: Imputed interest 454 42 6 Total lease liabilities $ 232 $ 262 $ 8 Future minimum commitments as of December 31, 2018, for capital lease obligations and for operating lease obligations having initial or remaining non-cancellable lease terms in excess of one year are as follows: (In millions) Operating Capital Lease Obligations 2019 $ 73 $ 2 2020 70 5 2021 67 — 2022 64 — 2023 58 — 2024 and thereafter 719 — Total minimum lease payments $ 1,051 7 Less: imputed interest costs 1 Present value of net minimum lease payments $ 6
Lessee, Operating Leases [Text Block]	Lessee We lease a wide variety of facilities and equipment under leases from third parties, including land and building space, office and field equipment, storage facilities and transportation equipment, while our related party leases primarily relate to ground leases associated with our refining logistics assets. Our remaining lease terms range from less than one year to 60 years. Some long-term leases include renewal options ranging from one to 50 years and, in certain leases, also include purchase options. Renewal options and termination options were not included in the measurement of ROU assets and lease liabilities since it was determined they were not reasonably certain to be exercised. Under ASC 842, the components of lease cost were as follows: Three Months Ended March 31, 2019 (In millions) Related Party Third Party Components of lease costs: Operating lease costs $ 4 $ 14 Variable lease cost — 2 Short-term lease cost — 10 Total lease cost $ 4 $ 26 Supplemental balance sheet data related to leases were as follows: March 31, 2019 (In millions) Related Party Third Party Operating leases Assets Right of use assets $ 232 $ 262 Liabilities Operating lease liabilities 1 46 Long-term operating lease liabilities 231 216 Total operating lease liabilities $ 232 $ 262 Weighted average remaining lease term 47.92 years 6.97 years Weighted average discount rate 5.80 % 4.33 % Finance leases Assets Property, plant and equipment, gross $ 28 Accumulated depreciation 10 Property, plant and equipment, net 18 Liabilities Other current liabilities 1 Long-term debt 7 Total finance lease liabilities $ 8 Weighted average remaining lease term 31.05 years Weighted average discount rate 5.76 % As of March 31, 2019, maturities of lease liabilities for operating lease obligations and finance lease obligations having initial or remaining non-cancellable lease terms in excess of one year are as follows: (In millions) Related Party Operating Third Party Operating Finance 2019 $ 11 $ 42 $ 1 2020 14 53 6 2021 14 51 — 2022 14 46 — 2023 14 43 — 2024 and thereafter 619 69 7 Gross lease payments 686 304 14 Less: Imputed interest 454 42 6 Total lease liabilities $ 232 $ 262 $ 8 Future minimum commitments as of December 31, 2018, for capital lease obligations and for operating lease obligations having initial or remaining non-cancellable lease terms in excess of one year are as follows: (In millions) Operating Capital Lease Obligations 2019 $ 73 $ 2 2020 70 5 2021 67 — 2022 64 — 2023 58 — 2024 and thereafter 719 — Total minimum lease payments $ 1,051 7 Less: imputed interest costs 1 Present value of net minimum lease payments $ 6
Lessor, Operating Leases [Text Block]	Lessor Based on the terms of fee-based transportation and storage services agreements with MPC as well as certain natural gas gathering, transportation and processing agreements, MPLX is considered to be the lessor under several operating lease arrangements in accordance with GAAP. The agreements with MPC have remaining terms ranging from less than one year to 12 years with renewal options ranging from zero to 10 years . MPLX’s primary natural gas lease operations relate to a natural gas gathering agreement in the Marcellus Shale for which it earns a fixed-fee for providing gathering services to a single producer using a dedicated gathering system. As the gathering system is expanded, the fixed-fee charged to the producer is adjusted to include the additional gathering assets in the lease. The primary term of the natural gas gathering arrangement expires in 2038 and will continue thereafter on a year-to-year basis until terminated by either party. Other significant natural gas implicit leases relate to a natural gas processing agreement in the Marcellus Shale and a natural gas processing agreement in the Southern Appalachia region for which MPLX earns minimum monthly fees for providing processing services to a single producer using a dedicated processing plant. The primary term of these natural gas processing agreements expires during 2023 and 2033. MPLX’s revenue from its lease arrangements, excluding executory costs, totaled approximately $248 million for the three months ended March 31, 2019 . MPLX did not elect to use the practical expedient to combine lease and non-lease components for lessor arrangements. The tables below represent the portion of the contract allocated to the lease component based on relative standalone selling price. Lessor agreements are currently deemed operating, as we elected the practical expedient to grandfather in old conclusions. We are still determining the impact of the new standard on these arrangements if and when a modification occurs and they are required to be assessed under ASC 842. MPLX may be required to re-classify existing operating leases to sales-type leases upon modification and related reassessment of the leases. MPLX’s lease arrangements related to the processing facilities contain contingent rental provisions whereby MPLX receives additional fees if the producer customer exceeds the monthly minimum processed volumes. During the three months ended March 31, 2019 , MPLX did not receive any material contingent lease payments. The following is a schedule of minimum future rental revenue on the non-cancellable operating leases as of March 31, 2019 : (In millions) Related Party Third Party Total 2019 $ 562 $ 144 $ 706 2020 752 168 920 2021 632 162 794 2022 633 160 793 2023 621 154 775 2024 and thereafter 2,401 1,205 3,606 Total minimum future rentals $ 5,601 $ 1,993 $ 7,594 The following is a schedule of minimum future rental revenue on the non-cancellable operating leases as of December 31, 2018: (In millions) Related Party Third Party Total 2019 $ 748 $ 160 $ 908 2020 750 159 909 2021 627 150 777 2022 627 148 775 2023 616 142 758 2024 and thereafter 2,321 1,111 3,432 Total minimum future rentals $ 5,689 $ 1,870 $ 7,559 The following schedule summarizes MPLX’s investment in assets held for operating lease by major classes as of March 31, 2019 and December 31, 2018: (In millions) March 31, 2019 December 31, 2018 Natural gas gathering and NGL transportation pipelines and facilities $ 1,036 $ 964 Processing, fractionation and storage facilities 1,548 1,398 Pipelines and related assets 274 266 Barges and towing vessels 636 619 Terminals and related assets 1,184 1,178 Refinery related assets 943 938 Land, building, office equipment and other 204 162 Total 5,825 5,525 Less accumulated depreciation 2,155 2,038 Property, plant and equipment, net $ 3,670 $ 3,487

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies Disclosure [Text Block]	Commitments and Contingencies MPLX is the subject of, or a party to, a number of pending or threatened legal actions, contingencies and commitments involving a variety of matters, including laws and regulations relating to the environment. Some of these matters are discussed below. For matters for which MPLX has not recorded an accrued liability, MPLX is unable to estimate a range of possible losses for the reasons discussed in more detail below. However, the ultimate resolution of some of these contingencies could, individually or in the aggregate, be material. Environmental Matters – MPLX is subject to federal, state and local laws and regulations relating to the environment. These laws generally provide for control of pollutants released into the environment and require responsible parties to undertake remediation of hazardous waste disposal sites. Penalties may be imposed for non-compliance. At March 31, 2019 and December 31, 2018 , accrued liabilities for remediation totaled $17 million and $14 million , respectively. However, it is not presently possible to estimate the ultimate amount of all remediation costs that might be incurred or the penalties, if any, which may be imposed. At March 31, 2019 there were $2 million in receivables from MPC for indemnification of environmental costs. At December 31, 2018 there were no balances with MPC for these costs. MPLX is involved in environmental enforcement matters arising in the ordinary course of business. While the outcome and impact on MPLX LP cannot be predicted with certainty, management believes the resolution of these environmental matters will not, individually or collectively, have a material adverse effect on its consolidated results of operations, financial position or cash flows. Other Lawsuits – The Partnership, MarkWest, MarkWest Liberty Midstream, MarkWest Liberty Bluestone, L.L.C., Ohio Fractionation and MarkWest Utica EMG (collectively, the “MPLX Parties”) are parties to various lawsuits with Bilfinger Westcon, Inc. (“Westcon”) that were instituted in 2016 and 2017 in the Court of Common Pleas in Butler County, Pennsylvania, the Circuit Court in Wetzel County, West Virginia, and the Court of Common Pleas in Harrison County, Ohio. The lawsuits relate to disputes regarding construction work performed by Westcon at the Bluestone, Mobley and Cadiz processing complexes in Pennsylvania, West Virginia and Ohio, respectively, and the Hopedale fractionation complex in Ohio. With respect to work performed by Westcon at the Mobley and Bluestone processing complexes, one or more of the MPLX Parties have asserted breach of contract, fraud, and with respect to work performed at the Mobley processing complex, MarkWest Liberty Midstream has also asserted negligent misrepresentation claims against Westcon. Westcon has also asserted claims against one or more of the MPLX Parties regarding these construction projects for breach of contract, unjust enrichment, promissory estoppel, fraud and constructive fraud, tortious interference with contractual relations, and civil conspiracy. Collectively, in the several cases, the MPLX Parties seek in excess of $10 million , plus an unspecified amount of punitive damages. Collectively, in the several cases, Westcon seeks in excess of $40 million , plus an unspecified amount of punitive damages. It is possible that, in connection with these lawsuits, the MPLX Parties will incur material amounts of damages. While the ultimate outcome and impact to MPLX cannot be predicted with certainty, and MPLX is not able to estimate a reasonably possible loss (or range of loss), if any, for these matters, MPLX believes the resolution of these claims will not have a material adverse effect on its consolidated financial position, results of operations, or cash flows. In 2003, the State of Illinois brought an action against the Premcor Refining Group, Inc. (“Premcor”) and Apex Refining Company (“Apex”) asserting claims for environmental cleanup related to the refinery owned by these entities in the Hartford/Wood River, Illinois area. In 2006, Premcor and Apex filed third-party complaints against numerous owners and operators of petroleum products facilities in the Hartford/Wood River, Illinois area, including Marathon Pipe Line LLC (“MPL”). These complaints, which have been amended since filing, assert claims of common law nuisance and contribution under the Illinois Contribution Act and other laws for environmental cleanup costs that may be imposed on Premcor and Apex by the State of Illinois. On September 6, 2016, the trial court approved a settlement between Apex and the State of Illinois whereby Apex agreed to settle all claims against it for a $10 million payment. Premcor filed a motion for permissive appeal and requested a stay to the proceeding until the motion is ruled upon. Premcor reached a settlement with the State of Illinois in the second quarter of 2018, which has been objected to by certain third-party defendants, including MPL, and is subject to court approval. Several third-party defendants in the litigation including MPL have asserted cross-claims in contribution against the various third-party defendants. This litigation is currently pending in the Third Judicial Circuit Court, Madison County, Illinois. The trial concerning Premcor’s claims against third-party defendants, including MPL, previously scheduled to commence September 10, 2018, has been postponed and a new trial date has not been set. While the ultimate outcome of these litigated matters remains uncertain, neither the likelihood of an unfavorable outcome nor the ultimate liability, if any, with respect to this matter can be determined at this time and MPLX is unable to estimate a reasonably possible loss (or range of loss) for this litigation. Under the omnibus agreement, MPC will indemnify MPLX for the full cost of any losses should MPL be deemed responsible for any damages in this lawsuit. MPLX is also a party to a number of other lawsuits and other proceedings arising in the ordinary course of business. While the ultimate outcome and impact to MPLX cannot be predicted with certainty, MPLX believes the resolution of these other lawsuits and proceedings will not have a material adverse effect on its consolidated financial position, results of operations or cash flows. Guarantees – Over the years, MPLX has sold various assets in the normal course of its business. Certain of the related agreements contain performance and general guarantees, including guarantees regarding inaccuracies in representations, warranties, covenants and agreements, and environmental and general indemnifications that require MPLX to perform upon the occurrence of a triggering event or condition. These guarantees and indemnifications are part of the normal course of selling assets. MPLX is typically not able to calculate the maximum potential amount of future payments that could be made under such contractual provisions because of the variability inherent in the guarantees and indemnities. Most often, the nature of the guarantees and indemnities is such that there is no appropriate method for quantifying the exposure because the underlying triggering event has little or no past experience upon which a reasonable prediction of the outcome can be based. In connection with our approximate 9 percent indirect interest in a joint venture that owns and operates the Dakota Access Pipeline and Energy Transfer Crude Oil Pipeline projects, collectively referred to as the Bakken Pipeline system, we have entered into a Contingent Equity Contribution Agreement whereby MPLX LP, along with the other joint venture owners in the Bakken Pipeline system, have agreed to make equity contributions to the joint venture upon certain events occurring to allow the entities that own and operate the Bakken Pipeline system to satisfy their senior note payment obligations. The senior notes were issued to repay amounts owed by the pipeline companies to fund the cost of construction of Bakken Pipeline system. As of March 31, 2019, our maximum potential undiscounted payments under the Contingent Equity Contribution Agreement was approximately $230 million . Contractual Commitments and Contingencies – At March 31, 2019 , MPLX’s contractual commitments to acquire property, plant and equipment totaled $726 million . These commitments were primarily related to plant expansion projects for the Marcellus and Southwest Operations. In addition, from time to time and in the ordinary course of business, MPLX and its affiliates provide guarantees of MPLX’s subsidiaries payment and performance obligations in the G&P segment. Certain natural gas processing and gathering arrangements require MPLX to construct new natural gas processing plants, natural gas gathering pipelines and NGL pipelines and contain certain fees and charges if specified construction milestones are not achieved for reasons other than force majeure. In certain cases, certain producers may have the right to cancel the processing arrangements if there are significant delays that are not due to force majeure. As of March 31, 2019 , management does not believe there are any indications that MPLX will not be able to meet the construction milestones, that force majeure does not apply or that such fees and charges will otherwise be triggered.

Subsequent Events Subsequent Ev

Subsequent Events Subsequent Events (Notes)	3 Months Ended
	Mar. 31, 2019
Subsequent Event [Line Items]
Subsequent Events [Text Block]	Subsequent Events On May 8, 2019 , MPC, MPLX and Andeavor Logistics LP (“ANDX”) announced that MPLX and ANDX have entered into a definitive merger agreement whereby MPLX will acquire ANDX in a unit-for-unit transaction. Under the terms of the merger agreement, ANDX public unitholders will receive 1.135 MPLX common units for each ANDX common unit held and MPC will receive 1.0328 MPLX common units for each ANDX common unit held. The MPLX common units to be issued in addition to the assumption of ANDX debt and ANDX preferred units represents an equity value of approximately $9 billion and an enterprise value of approximately $14 billion for the acquired entity. MPLX’s acquisition of ANDX will be treated as a common control transaction, which requires the recognition of assets and liabilities acquired using MPC’s historical basis as of October 1, 2018. The transaction has been approved by MPLX’s and ANDX’s respective Conflicts Committees and both Boards of Directors. Subject to the satisfaction of customary closing conditions and receipt of regulatory approvals, the transaction is expected to close in the second half of 2019.

Description of the Business a_2

Description of the Business and Basis of Presentation Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
Business Description and Basis of Presentation [Text Block]	Description of the Business and Basis of Presentation Description of the Business – MPLX LP is a diversified, large-cap master limited partnership formed by Marathon Petroleum Corporation that owns and operates midstream energy infrastructure and logistics assets, and provides fuels distribution services. References in this report to “MPLX LP,” “MPLX,” “the Partnership,” “we,” “ours,” “us,” or like terms refer to MPLX LP and its subsidiaries. References to “MPC” refer collectively to Marathon Petroleum Corporation as our sponsor and its subsidiaries, other than the Partnership. We are engaged in the transportation, storage and distribution of crude oil and refined petroleum products; the gathering, processing and transportation of natural gas; and the gathering, transportation, fractionation, storage and marketing of NGLs. MPLX’s principal executive office is located in Findlay, Ohio. MPLX’s business consists of two segments based on the nature of services it offers: Logistics and Storage (“L&S”), which relates primarily to crude oil and refined petroleum products; and Gathering and Processing (“G&P”), which relates primarily to natural gas and NGLs. See Note 9 for additional information regarding the operations and results of these segments. Basis of Presentation – The accompanying interim consolidated financial statements are unaudited; however, in the opinion of MPLX’s management, these statements reflect all adjustments necessary for a fair statement of the results for the periods reported. All such adjustments are of a normal, recurring nature unless otherwise disclosed. These interim consolidated financial statements, including the notes, have been prepared in accordance with the rules and regulations of the SEC applicable to interim period financial statements and do not include all of the information and disclosures required by GAAP for complete financial statements. Certain amounts in prior years have been reclassified to conform to current year presentation. These interim consolidated financial statements should be read in conjunction with the audited consolidated financial statements and notes thereto included in the Annual Report on Form 10-K for the year ended December 31, 2018 . The results of operations for the three months ended March 31, 2019 are not necessarily indicative of the results to be expected for the full year. MPLX’s consolidated financial statements include all majority-owned and controlled subsidiaries. For non-wholly-owned consolidated subsidiaries, the interests owned by third parties have been recorded as “Noncontrolling interests” on the accompanying Consolidated Balance Sheets. Intercompany investments, accounts and transactions have been eliminated. MPLX’s investments in which MPLX exercises significant influence but does not control and does not have a controlling financial interest are accounted for using the equity method. MPLX’s investments in a VIE in which MPLX exercises significant influence but does not control and is not the primary beneficiary are also accounted for using the equity method. In preparing the Consolidated Statements of Equity, net income attributable to MPLX LP is allocated to preferred unitholders based on a fixed distribution schedule. Distributions, although earned, are not accrued until declared. The allocation of net income attributable to MPLX LP for purposes of calculating net income per limited partner unit is described in Note 6 .
Use of Estimates, Policy [Policy Text Block]	The accompanying interim consolidated financial statements are unaudited; however, in the opinion of MPLX’s management, these statements reflect all adjustments necessary for a fair statement of the results for the periods reported. All such adjustments are of a normal, recurring nature unless otherwise disclosed. These interim consolidated financial statements, including the notes, have been prepared in accordance with the rules and regulations of the SEC applicable to interim period financial statements and do not include all of the information and disclosures required by GAAP for complete financial statements. Certain amounts in prior years have been reclassified to conform to current year presentation.
Earnings Per Share, Policy [Policy Text Block]	In preparing the Consolidated Statements of Equity, net income attributable to MPLX LP is allocated to preferred unitholders based on a fixed distribution schedule. Distributions, although earned, are not accrued until declared. The allocation of net income attributable to MPLX LP for purposes of calculating net income per limited partner unit is described in Note 6 . Net income/(loss) per unit applicable to common limited partner units is computed by dividing net income/(loss) attributable to MPLX LP less income/(loss) allocated to participating securities by the weighted average number of common units outstanding. The classes of participating securities include common units, certain equity-based compensation awards and Series A Convertible preferred units.

Equity Method Investments (Tabl

Equity Method Investments (Tables)	3 Months Ended
	Mar. 31, 2019
Schedule of Equity Method Investments [Line Items]
Equity Method Investments [Table Text Block]	The following table presents MPLX’s equity method investments at the dates indicated: Ownership as of Carrying value at March 31, March 31, December 31, (In millions, except ownership percentages) 2019 2019 2018 Explorer Pipeline Company 25% $ 85 $ 90 Illinois Extension Pipeline Company, L.L.C. 35% 280 275 LOCAP LLC 59% 27 27 LOOP LLC 41% 232 226 MarEn Bakken Bakken Company LLC 25% 492 498 Centrahoma Processing LLC 40% 158 160 MarkWest EMG Jefferson Dry Gas Gathering Company, L.L.C. 67% 247 236 MarkWest Utica EMG, L.L.C. 56% 2,017 2,039 Sherwood Midstream LLC 50% 455 366 Sherwood Midstream Holdings LLC 56% 163 157 Other 114 100 Total $ 4,270 $ 4,174
Summarized Financial Information For Equity Method Investees Table [Table Text Block]	Summarized financial information for MPLX’s equity method investments for the three months ended March 31, 2019 and 2018 is as follows: Three Months Ended March 31, 2019 (In millions) VIEs Non-VIEs Total Revenues and other income $ 143 $ 340 $ 483 Costs and expenses 70 172 242 Income from operations 73 168 241 Net income 66 156 222 Income from equity method investments (1) $ 24 $ 46 $ 70 Three Months Ended March 31, 2018 (In millions) VIEs Non-VIEs Total Revenues and other income $ 106 $ 297 $ 403 Costs and expenses 62 155 217 Income from operations 44 142 186 Net income 44 129 173 Income from equity method investments (1) $ 15 $ 46 $ 61 (1) “ Income from equity method investments ” includes the impact of any basis differential amortization or accretion. Summarized balance sheet information for MPLX’s equity method investments as of March 31, 2019 and December 31, 2018 is as follows: March 31, 2019 (In millions) VIEs Non-VIEs Total Current assets $ 162 $ 327 $ 489 Noncurrent assets 4,438 4,693 9,131 Current liabilities 129 221 350 Noncurrent liabilities $ 193 $ 843 $ 1,036 December 31, 2018 (In millions) VIEs Non-VIEs Total Current assets $ 235 $ 379 $ 614 Noncurrent assets 3,535 4,715 8,250 Current liabilities 155 246 401 Noncurrent liabilities $ 189 $ 841 $ 1,030

Related Party Agreements and _2

Related Party Agreements and Transactions (Tables)	3 Months Ended
	Mar. 31, 2019
Related Party Transactions [Abstract]
Other Long-term Debt, Noncurrent	Activity on the MPC Loan Agreement was as follows: (In millions) Three Months Ended March 31, 2019 Year Ended December 31, 2018 Borrowings $ 851 $ 3,962 Average interest rate of borrowings 3.988 % 3.473 % Repayments $ 851 $ 4,347 Outstanding balance at end of period (1) $ — $ — (1) Included in “Current liabilities - related parties” on the Consolidated Balance Sheets.
Schedule of Sales to Related Parties	Revenue received from related parties included on the Consolidated Statements of Income was as follows: Three Months Ended March 31, (In millions) 2019 2018 Service revenues - related parties MPC $ 578 $ 471 Rental income - related parties MPC 193 145 Product sales - related parties (1) MPC 11 4 Other income - related parties MPC 10 10 MarkWest Utica EMG 4 4 Ohio Gathering 4 4 Sherwood Midstream 4 3 Jefferson Dry Gas 2 1 Other 2 1 Total Other income - related parties $ 26 $ 23 (1) There were additional product sales to MPC that net to zero within the consolidated financial statements as the transactions are recorded net due to the terms of the agreements under which such product was sold. For the three months ended March 31, 2019 and March 31, 2018 , these sales totaled $86 million and $79 million , respectively.
Schedule of Employee Services Expenses from Related Parties	Expenses incurred from MPC under the omnibus and employee services agreements as well as other purchases from MPC included on the Consolidated Statements of Income are as follows: Three Months Ended March 31, (In millions) 2019 2018 Rental cost of sales - related parties $ 3 $ 1 Purchases - related parties 212 177 General and administrative expenses 50 39 Total $ 265 $ 217
Schedule of Related Party Transactions [Table Text Block]	Assets and liabilities with related parties appearing on the Consolidated Balance Sheets are detailed in the table below. This table identifies the various components of related party assets and liabilities, including those associated with leases (see Note 19 for additional information) and deferred revenue on minimum volume commitments. During the three months ended March 31, 2019 and the year ended December 31, 2018 , MPC did not ship its minimum committed volumes on certain pipelines. Under MPLX’s pipeline transportation services agreements, if MPC fails to transport its minimum throughput volumes during any quarter, then MPC will pay MPLX a deficiency payment equal to the volume of the deficiency multiplied by the tariff rate then in effect. The deficiency amounts are recorded as “Current liabilities - related parties.” MPC may then apply the amount of any such deficiency payments as a credit for volumes transported on the applicable pipeline in excess of its minimum volume commitment in future periods under the terms of the applicable transportation services agreement. MPLX recognizes related party revenues for the deficiency payments when credits are used for volumes transported in excess of minimum quarterly volume commitments, when it becomes impossible to physically transport volumes necessary to utilize the credits or upon the expiration of the credits. The use or expiration of the credits is a decrease in “Current liabilities - related parties.” In addition, capital projects MPLX is undertaking at the request of MPC are reimbursed in cash and recognized in income over the remaining term of the applicable agreements. (In millions) March 31, 2019 December 31, 2018 Current assets - related parties Receivables - MPC $ 372 $ 281 Receivables - Other 7 8 Prepaid - MPC 7 1 Total 386 290 Noncurrent assets - related parties Long-term receivables - MPC 24 24 Right of use assets - MPC 232 — Total 256 24 Current liabilities - related parties Payables - MPC 119 131 Payables - MarkWest Utica EMG 16 51 Payables - Sherwood Midstream 18 16 Payables - Other 5 5 Operating lease liabilities - MPC 1 — Deferred revenue - Minimum volume deficiencies - MPC 38 44 Deferred revenue - Project reimbursements - MPC 7 7 Total 204 254 Long-term liabilities - related parties Long-term operating lease liabilities - MPC 231 — Long-term deferred revenue - Project reimbursements - MPC 42 43 Total $ 273 $ 43

Net Income Per Limited Partne_2

Net Income Per Limited Partner Unit (Tables)	3 Months Ended
	Mar. 31, 2019
Earnings Per Share [Abstract]
Schedule of Distributions By Partner By Class	For the three months ended March 31, 2019 and 2018 , MPLX had dilutive potential common units consisting of certain equity-based compensation awards. Potential common units omitted from the diluted earnings per unit calculation for the three months ended March 31, 2019 and 2018 were less than 1 million . Three Months Ended March 31, (In millions) 2019 2018 Net income attributable to MPLX LP $ 503 $ 421 Less: Limited partners’ distributions declared on preferred units (1) 20 16 Limited partners’ distributions declared on common units (including common units of general partner) (1) 523 467 Undistributed net loss attributable to MPLX LP $ (40 ) $ (62 ) (1) See Note 7 for distribution information.
Schedule of Basic and Diluted Earnings Per Unit	Three Months Ended March 31, 2019 (In millions, except per unit data) Limited Partners’ Common Units Redeemable Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit Net income attributable to MPLX LP: Distributions declared $ 523 $ 20 $ 543 Undistributed net loss attributable to MPLX LP (40 ) — (40 ) Net income attributable to MPLX LP (1) $ 483 $ 20 $ 503 Weighted average units outstanding: Basic 794 31 825 Diluted 795 31 826 Net income attributable to MPLX LP per limited partner unit: Basic $ 0.61 Diluted $ 0.61 Three Months Ended March 31, 2018 (In millions, except per unit data) Limited Partners’ Common Units Redeemable Preferred Units Total Basic and diluted net income attributable to MPLX LP per unit Net income attributable to MPLX LP: Distributions declared (including IDRs) $ 467 $ 16 $ 483 Undistributed net loss attributable to MPLX LP (62 ) — (62 ) Net income attributable to MPLX LP (1) $ 405 $ 16 $ 421 Weighted average units outstanding: Basic 661 31 692 Diluted 661 31 692 Net income attributable to MPLX LP per limited partner unit: Basic $ 0.61 Diluted $ 0.61 (1) Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.

Equity (Tables)

Equity (Tables)	3 Months Ended
	Mar. 31, 2019
Equity [Abstract]
Schedule of Stockholders Equity [Table Text Block]	The changes in the number of common units outstanding during the three months ended March 31, 2019 are summarized below: (In units) Common Balance at December 31, 2018 794,089,518 Unit-based compensation awards 148,379 Balance at March 31, 2019 794,237,897
Distributions Made to Limited Partner, by Distribution [Table Text Block]	The allocation of total quarterly cash distributions to limited and preferred unitholders is as follows for the three months ended March 31, 2019 and 2018 . MPLX’s distributions are declared subsequent to quarter end; therefore, the following table represents total cash distributions applicable to the period in which the distributions were earned. Three Months Ended March 31, (In millions) 2019 2018 Common and preferred unit distributions: Common unitholders, includes common units of general partner $ 523 $ 467 Preferred unit distributions 20 16 Total cash distributions declared $ 543 $ 483

Redeemable Preferred Units (Tab

Redeemable Preferred Units (Tables)	3 Months Ended
	Mar. 31, 2019
Temporary Equity Disclosure [Abstract]
Rollforward of Redeemable Preferred Units	The changes in the redeemable preferred balance from December 31, 2018 through March 31, 2019 are summarized below: (In millions) Redeemable Preferred Units Balance at December 31, 2018 $ 1,004 Net income allocated 20 Distributions received by preferred unitholders (20 ) Balance at March 31, 2019 $ 1,004

Segment Information (Tables)

Segment Information (Tables)	3 Months Ended
	Mar. 31, 2019
Segment Reporting [Abstract]
Schedule of Segment Reporting Information, by Segment [Table Text Block]	The tables below present information about revenues and other income, capital expenditures and total assets for our reportable segments: Three Months Ended March 31, (In millions) 2019 2018 L&S Service revenue $ 612 $ 499 Rental income 199 145 Product related revenue 3 2 Income from equity method investments 41 44 Other income 11 12 Total segment revenues and other income (1) 866 702 Segment Adjusted EBITDA (2) 559 437 Maintenance capital expenditures 13 22 Growth capital expenditures 103 154 G&P Service revenue 404 354 Rental income 88 79 Product related revenue 244 253 Income from equity method investments 29 17 Other income 15 15 Total segment revenues and other income (1) 780 718 Segment Adjusted EBITDA (2) 371 323 Maintenance capital expenditures 6 3 Growth capital expenditures $ 261 $ 271
Reconciliation of Assets from Segment to Consolidated [Table Text Block]	(In millions) March 31, 2019 December 31, 2018 Segment assets Cash and cash equivalents $ 93 $ 68 L&S (1) 7,040 6,566 G&P (1) 16,451 16,145 Total assets $ 23,584 $ 22,779 (1) Equity method investments included in L&S assets were $1.12 billion at March 31, 2019 and December 31, 2018 , respectively. Equity method investments included in G&P assets were $3.15 billion at March 31, 2019 and $3.05 billion at December 31, 2018 .
Reconciliation of Other Significant Reconciling Items from Segments to Consolidated [Table Text Block]	Three Months Ended March 31, (In millions) 2019 2018 Reconciliation to Net income: L&S Segment Adjusted EBITDA $ 559 $ 437 G&P Segment Adjusted EBITDA 371 323 Total reportable segments 930 760 Depreciation and amortization (1) (211 ) (176 ) Benefit/(provision) for income taxes 2 (4 ) Amortization of deferred financing costs (13 ) (16 ) Non-cash equity-based compensation (6 ) (4 ) Net interest and other financial costs (158 ) (114 ) Income from equity method investments 70 61 Distributions/adjustments related to equity method investments (108 ) (90 ) Unrealized derivative (losses)/gains (2) (4 ) 7 Acquisition costs — (3 ) Adjusted EBITDA attributable to noncontrolling interests 7 2 Net income $ 509 $ 423 (1) Depreciation and amortization attributable to L&S was $70 million and $48 million for the three months ended March 31, 2019 and 2018 , respectively. Depreciation and amortization attributable to G&P was $141 million and $128 million for the three months ended March 31, 2019 and 2018 , respectively. (2) MPLX makes a distinction between realized and unrealized gains and losses on derivatives. During the period when a derivative contract is outstanding, changes in the fair value of the derivative are recorded as an unrealized gain or loss. When a derivative contract matures or is settled, the previously recorded unrealized gain or loss is reversed and the realized gain or loss of the contract is recorded.

Inventories (Tables)

Inventories (Tables)	3 Months Ended
	Mar. 31, 2019
Inventory Disclosure [Abstract]
Summary of Inventories	Inventories consist of the following: (In millions) March 31, 2019 December 31, 2018 NGLs $ 3 $ 9 Line fill 10 9 Spare parts, materials and supplies 61 59 Total inventories $ 74 $ 77

Property, Plant and Equipment (

Property, Plant and Equipment (Tables)	3 Months Ended
	Mar. 31, 2019
Property, Plant and Equipment [Abstract]
Summary of Property, Plant and Equipment	Property, plant and equipment with associated accumulated depreciation is shown below: (In millions) March 31, 2019 December 31, 2018 Natural gas gathering and NGL transportation pipelines and facilities $ 6,128 $ 5,926 Processing, fractionation and storage facilities 5,343 5,336 Pipelines and related assets 2,620 2,560 Barges and towing vessels 635 620 Terminals and related assets 1,184 1,178 Refinery related assets 943 938 Land, building, office equipment and other 978 957 Construction-in-progress 858 801 Total 18,689 18,316 Less accumulated depreciation 3,873 3,677 Property, plant and equipment, net $ 14,816 $ 14,639

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2019
Fair Value Disclosures [Abstract]
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block]	The following table presents the financial instruments carried at fair value on a recurring basis as of March 31, 2019 and December 31, 2018 by fair value hierarchy level. MPLX has elected to offset the fair value amounts recognized for multiple derivative contracts executed with the same counterparty. March 31, 2019 December 31, 2018 (In millions) Assets Liabilities Assets Liabilities Significant unobservable inputs (Level 3) Embedded derivatives in commodity contracts $ — $ (65 ) $ — $ (61 ) Total carrying value on Consolidated Balance Sheets $ — $ (65 ) $ — $ (61 )
Fair Value Inputs Assets and Liabilities Quantitative Information [Table Text Block]	Level 3 instruments include all NGL transactions and embedded derivatives in commodity contracts. The embedded derivative liability relates to a natural gas purchase agreement embedded in a keep-whole processing agreement. The fair value calculation for these Level 3 instruments used significant unobservable inputs including: (1) NGL prices interpolated and extrapolated due to inactive markets ranging from $0.60 to $1.25 and (2) the probability of renewal of 91 percent for the first five -year term and 82 percent for the second five -year term of the gas purchase agreement and related keep-whole processing agreement. Increases or decreases in the fractionation spread result in an increase or decrease in the fair value of the embedded derivative liability, respectively. An increase in the probability of renewal would result in an increase in the fair value of the related embedded derivative liability. Beyond the embedded derivative discussed above, we had no outstanding commodity contracts as of March 31, 2019 or December 31, 2018 .
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block]	The following table is a reconciliation of the net beginning and ending balances recorded for net assets and liabilities classified as Level 3 in the fair value hierarchy. Three Months Ended March 31, 2019 Three Months Ended March 31, 2018 (In millions) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Fair value at beginning of period $ — $ (61 ) $ (2 ) $ (64 ) Total (losses)/gains (realized and unrealized) included in earnings (1) — (6 ) — 3 Settlements — 2 — 3 Fair value at end of period — (65 ) (2 ) (58 ) The amount of total losses for the period included in earnings attributable to the change in unrealized losses relating to liabilities still held at end of period $ — $ (5 ) $ — $ 3 (1) Gains and losses on commodity derivative contracts classified as Level 3 are recorded in “Product sales” on the Consolidated Statements of Income. Gains and losses on derivatives embedded in commodity contracts are recorded in “Purchased product costs” and “Cost of revenues”
Fair Value Carrying Value by Balance Sheet Grouping [Table Text Block]	The following table summarizes the fair value and carrying value of the long-term debt, excluding finance leases, and SMR liability: March 31, 2019 December 31, 2018 (In millions) Fair Value Carrying Value Fair Value Carrying Value Long-term debt $ 14,430 $ 13,921 $ 13,169 $ 13,484 SMR liability $ 94 $ 85 $ 92 $ 86

Derivative Financial Instrume_2

Derivative Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 2019
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Schedule of Notional Amounts of Outstanding Derivative Positions [Table Text Block]	As of March 31, 2019 , MPLX had no outstanding commodity contracts beyond the embedded derivative discussed below.
Schedule of Derivative Instruments in Statement of Financial Position, Fair Value [Table Text Block]	The impact of MPLX’s derivative instruments on its Consolidated Balance Sheets is summarized below: (In millions) March 31, 2019 December 31, 2018 Derivative contracts not designated as hedging instruments and their balance sheet location Asset Liability Asset Liability Commodity contracts (1) Other current assets / Other current liabilities $ — $ (8 ) $ — $ (7 ) Other noncurrent assets / Deferred credits and other liabilities — (57 ) — (54 ) Total $ — $ (65 ) $ — $ (61 ) (1) Includes embedded derivatives in commodity contracts as discussed above.
Derivative Instruments, Gain (Loss) [Table Text Block]	The impact of MPLX’s derivative contracts not designated as hedging instruments and the location of gains and losses recognized on the Consolidated Statements of Income is summarized below: Three Months Ended March 31, (In millions) 2019 2018 Product sales Realized (loss)/gain $ — $ — Unrealized (loss)/gain — 1 Total derivative (loss)/gain related to product sales — 1 Purchased product costs Realized (loss)/gain (2 ) (3 ) Unrealized (loss)/gain (4 ) 6 Total derivative (loss)/gain related to purchased product costs (6 ) 3 Cost of revenues Realized (loss)/gain — — Unrealized (loss)/gain — — Total derivative (loss)/gain related to cost of revenues — — Total derivative (loss)/gain $ (6 ) $ 4

Debt (Tables)

Debt (Tables)	3 Months Ended
	Mar. 31, 2019
Debt Disclosure [Abstract]
Summary of Outstanding Borrowings	MPLX’s outstanding borrowings consist of the following: (In millions) March 31, 2019 December 31, 2018 MPLX LP: Bank revolving credit facility due 2022 $ 425 $ — 3.375% senior notes due March 2023 500 500 4.500% senior notes due July 2023 989 989 4.875% senior notes due December 2024 1,149 1,149 4.000% senior notes due February 2025 500 500 4.875% senior notes due June 2025 1,189 1,189 4.125% senior notes due March 2027 1,250 1,250 4.000% senior notes due March 2028 1,250 1,250 4.800% senior notes due February 2029 750 750 4.500% senior notes due April 2038 1,750 1,750 5.200% senior notes due March 2047 1,000 1,000 4.700% senior notes due April 2048 1,500 1,500 5.500% senior notes due February 2049 1,500 1,500 4.900% senior notes due April 2058 500 500 Consolidated subsidiaries: MarkWest - 4.500% - 4.875% senior notes, due 2023-2025 23 23 Financing lease obligations (1) 8 6 Total 14,283 13,856 Unamortized debt issuance costs (96 ) (97 ) Unamortized discount (354 ) (366 ) Amounts due within one year (1 ) (1 ) Total long-term debt due after one year $ 13,832 $ 13,392

Revenue (Tables)

Revenue (Tables)	3 Months Ended
	Mar. 31, 2019
Revenue from Contract with Customer [Abstract]
Disaggregation of Revenue [Table Text Block]	The following table represents a disaggregation of revenue for each reportable segment for the three months ended March 31, 2019 and 2018 : Three Months Ended March 31, 2019 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 34 $ 404 $ 438 Service revenue - related parties 578 — 578 Service revenue - product related — 34 34 Product sales 1 201 202 Product sales - related parties 2 9 11 Total revenues from contracts with customers $ 615 $ 648 1,263 Non-ASC 606 revenue (1) 383 Total revenues and other income $ 1,646 (1) Non-ASC 606 Revenue includes rental income, income from equity method investments, derivative gains and losses, mark-to-market adjustments, and other income. Three Months Ended March 31, 2018 (In millions) L&S G&P Total Revenues and other income: Service revenue $ 28 $ 354 $ 382 Service revenue - related parties 471 — 471 Service revenue - product related — 44 44 Product sales (1) 1 205 206 Product sales - related parties 1 3 4 Total revenues from contracts with customers $ 501 $ 606 1,107 Non-ASC 606 revenue (2) 313 Total revenues and other income $ 1,420 (1) G&P “Product sales” for the three months ended March 31, 2018 includes approximately $1 million of revenue related to derivative gains and losses and mark-to-market adjustments. (2) Non-ASC 606 Revenue includes rental income, income from equity method investments, derivative gains and losses, mark-to-market adjustments, and other income.
Contract with Customer, Asset and Liability [Table Text Block]	The table below reflects the changes in our contract balances for the period ended March 31, 2019 : (In millions) Balance at December 31, 2018 (1) Additions/ (Deletions) Revenue Recognized (2) Balance at March 31, 2019 Contract assets $ 4 $ 1 $ — $ 5 Deferred revenue 4 1 (1 ) 4 Deferred revenue - related parties 50 3 (9 ) 44 Long-term deferred revenue 10 3 — 13 Long-term deferred revenue - related parties $ 42 $ (1 ) $ — $ 41 (1) Balance represents ASC 606 portion of each respective line item. (2) No significant revenue was recognized related to past performance obligations in the current period.
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Table Text Block]	As of March 31, 2019 , the amounts allocated to contract assets and contract liabilities on the Consolidated Balance Sheets are $100 million and are reflected in the amounts below. This will be recognized as revenue as the obligations are satisfied, which is expected to occur over the next 25 years . Further, MPLX does not disclose variable consideration due to volume variability in the table below. (In millions) 2019 $ 884 2020 1,178 2021 1,193 2022 1,179 2023 and thereafter 5,651 Total revenue on remaining performance obligations (1),(2),(3) $ 10,085 (1) All fixed consideration from contracts with customers is included in the amounts presented above. Variable consideration that is constrained or not required to be estimated as it reflects our efforts to perform is excluded. (2) Arrangements deemed implicit leases are included in “Rental income” and are excluded from this table. (3) Only minimum volume commitments that are deemed fixed are included in the table above. MPLX has various minimum volume commitments in processing arrangements that vary based on the actual Btu content of the gas received. These amounts are deemed variable consideration and are excluded from the table above.

Supplemental Cash Flow Inform_2

Supplemental Cash Flow Information (Tables)	3 Months Ended
	Mar. 31, 2019
Supplemental Cash Flow Elements [Abstract]
Schedule of Cash and Cash Equivalents [Table Text Block]	(In millions) March 31, 2019 December 31, 2018 Cash and cash equivalents $ 93 $ 68 Restricted cash (1) — 8 Cash, cash equivalents and restricted cash (2) $ 93 $ 76 (1) The restricted cash balance is included within “Other current assets” on the Consolidated Balance Sheets. (2) As a result of the adoption of ASU 2016-18, Statement of Cash Flows - Restricted Cash, the Consolidated Statements of Cash Flows now explain the change during the period of both “Cash and cash equivalents” and “Restricted cash.”
Summary of Supplemental Cash Flow Information [Table Text Block]	Three Months Ended March 31, (In millions) 2019 2018 Net cash provided by operating activities included: Interest paid (net of amounts capitalized) $ 143 $ 103 Cash paid for amounts included in the measurement of lease liabilities Payments on operating leases 19 — Non-cash investing and financing activities: Net transfers of property, plant and equipment from materials and supplies inventories 1 1 ROU assets obtained in exchange for new operating lease obligations 1 — ROU assets obtained in exchange for new finance lease obligations $ 2 $ —
Summary of Reconciliation of Additions to Property, Plant and Equipment to Total Capital Expenditures [Table Text Block]	The following is the change of additions to property, plant and equipment related to capital accruals: Three Months Ended March 31, (In millions) 2019 2018 Increase/(decrease) in capital accruals $ (74 ) $ (6 )

Accumulated Other Comprehensi_2

Accumulated Other Comprehensive Income (Tables)	3 Months Ended
	Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block]	The following table shows the changes in “Accumulated other comprehensive loss” by component during the period December 31, 2018 through March 31, 2019 . (In millions) Pension Other Total Balance at December 31, 2018 (1) $ (14 ) $ (2 ) $ (16 ) Other comprehensive income - remeasurements (2) — 1 1 Balance at March 31, 2019 (1) $ (14 ) $ (1 ) $ (15 ) The following table shows the changes in “Accumulated other comprehensive loss” by component during the period December 31, 2017 through March 31, 2018 . (In millions) Pension Benefits Other Post-Retirement Benefits Total Balance at December 31, 2017 (1) $ (13 ) $ (1 ) $ (14 ) Other comprehensive loss - remeasurements (2) (1 ) (1 ) (2 ) Balance at March 31, 2018 (1) $ (14 ) $ (2 ) $ (16 ) (1) These components of “Accumulated other comprehensive loss” are included in the computation of net periodic benefit cost by LOOP and Explorer and are therefore included on the Consolidated Statements of Income under the caption “Income/(loss) from equity method investments.” (2) Components of other comprehensive income/loss - remeasurements relate to actuarial gains and losses as well as amortization of prior service costs. MPLX records an adjustment to “Comprehensive income” in accordance with its ownership interest in LOOP and Explorer.

Equity-Based Compensation Plan

Equity-Based Compensation Plan (Tables)	3 Months Ended
	Mar. 31, 2019
Phantom Units
Equity Transactions And Share Based Compensation [Line Items]
Summary of Share-based Compensation, Restricted Stock Units Award Activity	The following is a summary of phantom unit award activity of MPLX LP common units for the three months ended March 31, 2019 : Number Weighted Outstanding at December 31, 2018 1,154,335 $ 34.34 Granted 188,069 33.09 Settled (210,846 ) 32.20 Forfeited (9,375 ) 34.86 Outstanding at March 31, 2019 1,122,183 $ 34.53
Performance Shares [Member]
Equity Transactions And Share Based Compensation [Line Items]
Summary of Share-based Compensation, Restricted Stock Units Award Activity	The following is a summary of the activity for performance unit awards to be settled in MPLX LP common units for the three months ended March 31, 2019 : Number of Outstanding at December 31, 2018 1,941,750 Granted 987,994 Settled (772,397 ) Forfeited — Outstanding at March 31, 2019 2,157,347

Leases Lessee Disclosure (Table

Leases Lessee Disclosure (Tables)	3 Months Ended
	Mar. 31, 2019
Lessee Disclosure [Abstract]
Lease, Cost [Table Text Block]	Under ASC 842, the components of lease cost were as follows: Three Months Ended March 31, 2019 (In millions) Related Party Third Party Components of lease costs: Operating lease costs $ 4 $ 14 Variable lease cost — 2 Short-term lease cost — 10 Total lease cost $ 4 $ 26
Supplemental Balance Sheet Disclosures [Text Block]	Supplemental balance sheet data related to leases were as follows: March 31, 2019 (In millions) Related Party Third Party Operating leases Assets Right of use assets $ 232 $ 262 Liabilities Operating lease liabilities 1 46 Long-term operating lease liabilities 231 216 Total operating lease liabilities $ 232 $ 262 Weighted average remaining lease term 47.92 years 6.97 years Weighted average discount rate 5.80 % 4.33 % Finance leases Assets Property, plant and equipment, gross $ 28 Accumulated depreciation 10 Property, plant and equipment, net 18 Liabilities Other current liabilities 1 Long-term debt 7 Total finance lease liabilities $ 8 Weighted average remaining lease term 31.05 years Weighted average discount rate 5.76 %
Schedule of Future Minimum Lease Payments for Operating and Finance Leases [Table Text Block]	As of March 31, 2019, maturities of lease liabilities for operating lease obligations and finance lease obligations having initial or remaining non-cancellable lease terms in excess of one year are as follows: (In millions) Related Party Operating Third Party Operating Finance 2019 $ 11 $ 42 $ 1 2020 14 53 6 2021 14 51 — 2022 14 46 — 2023 14 43 — 2024 and thereafter 619 69 7 Gross lease payments 686 304 14 Less: Imputed interest 454 42 6 Total lease liabilities $ 232 $ 262 $ 8 Future minimum commitments as of December 31, 2018, for capital lease obligations and for operating lease obligations having initial or remaining non-cancellable lease terms in excess of one year are as follows: (In millions) Operating Capital Lease Obligations 2019 $ 73 $ 2 2020 70 5 2021 67 — 2022 64 — 2023 58 — 2024 and thereafter 719 — Total minimum lease payments $ 1,051 7 Less: imputed interest costs 1 Present value of net minimum lease payments $ 6

Leases Lessor Disclosure (Table

Leases Lessor Disclosure (Tables)	3 Months Ended
	Mar. 31, 2019
Lessor Disclosure [Abstract]
Lessor, Operating Lease, Payments to be Received, Maturity [Table Text Block]	The following is a schedule of minimum future rental revenue on the non-cancellable operating leases as of March 31, 2019 : (In millions) Related Party Third Party Total 2019 $ 562 $ 144 $ 706 2020 752 168 920 2021 632 162 794 2022 633 160 793 2023 621 154 775 2024 and thereafter 2,401 1,205 3,606 Total minimum future rentals $ 5,601 $ 1,993 $ 7,594
Schedule of Property Subject to or Available for Operating Lease [Table Text Block]	The following schedule summarizes MPLX’s investment in assets held for operating lease by major classes as of March 31, 2019 and December 31, 2018: (In millions) March 31, 2019 December 31, 2018 Natural gas gathering and NGL transportation pipelines and facilities $ 1,036 $ 964 Processing, fractionation and storage facilities 1,548 1,398 Pipelines and related assets 274 266 Barges and towing vessels 636 619 Terminals and related assets 1,184 1,178 Refinery related assets 943 938 Land, building, office equipment and other 204 162 Total 5,825 5,525 Less accumulated depreciation 2,155 2,038 Property, plant and equipment, net $ 3,670 $ 3,487
Schedule of Future Minimum Rental Payments Receivable for Operating Leases [Table Text Block]	The following is a schedule of minimum future rental revenue on the non-cancellable operating leases as of December 31, 2018: (In millions) Related Party Third Party Total 2019 $ 748 $ 160 $ 908 2020 750 159 909 2021 627 150 777 2022 627 148 775 2023 616 142 758 2024 and thereafter 2,321 1,111 3,432 Total minimum future rentals $ 5,689 $ 1,870 $ 7,559

Description of Business and Bas

Description of Business and Basis of Presentation - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2019
Description Of Business And Basis Of Presentation [Line Items]
Number of reportable segments	2

Accounting Standards Impact of

Accounting Standards Impact of Adoption of ASC 842 (Details) - Accounting Standards Update 2016-02 [Member] $ in Millions	Jan. 01, 2019USD ($)
Operating Lease Right Of Use Asset [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	$ 505
Operating Lease Liability [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	$ 502

Acquisitions Mt. Airy Terminal

Acquisitions Mt. Airy Terminal (Details) bbl / d in Thousands, bbl in Millions, $ in Millions	Sep. 26, 2018USD ($)bbl / dbbl	Mar. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Goodwill, Purchase Accounting Adjustments		$ (5)
Goodwill		2,581	$ 2,586
Mt. Airy Terminal [Member]
Payments to Acquire Businesses, Gross	$ 451	446
Storage Capacity | bbl	4
Barrels Handled | bbl / d	120
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment		336
Goodwill		$ 121

Acquisitions Refining Logistics

Acquisitions Refining Logistics & Fuels Distribution (Details) bbl in Millions, $ in Millions	Feb. 01, 2018USD ($)Tanksharesbbl	Mar. 31, 2018USD ($)	Mar. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Business Acquisition [Line Items]
Unitholders		$ 1,046
Property, plant and equipment, net			$ 14,816	$ 14,639
Goodwill			$ 2,581	$ 2,586
Number of storage tanks | Tank	619
Number of rail and truck racks	32
Number of docks and gasoline blenders	18
Refining Logistics & Fuels Distribution
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross	$ 4,100
Equity interest issued or issuable, fair value assigned	4,300
Total consideration, fair value assigned	$ 8,400
Limited Liability Company (LLC) or Limited Partnership (LP), Managing Member or General Partner, Ownership Interest	2.00%
Property, plant and equipment, net	$ 830
Refining Logistics [Member]
Business Acquisition [Line Items]
Goodwill	85
Fuels Distribution [Member]
Business Acquisition [Line Items]
Goodwill	$ 130
General Partner Common Units | Refining Logistics & Fuels Distribution
Business Acquisition [Line Items]
Units acquired | shares	85,610,278
MPLX LP | Limited Partners Common Units | Refining Logistics & Fuels Distribution
Business Acquisition [Line Items]
Units acquired | shares	111,611,111
MPLX LP | General Partner Units | Refining Logistics & Fuels Distribution
Business Acquisition [Line Items]
Units acquired | shares	2,277,778
MPLX Logistics LLC | Limited Partners Common Units | Refining Logistics & Fuels Distribution
Business Acquisition [Line Items]
Units acquired | shares	18,176,666
MPLX Holdings Inc | Limited Partners Common Units | Refining Logistics & Fuels Distribution
Business Acquisition [Line Items]
Units acquired | shares	7,824,167
MPC | Refining Logistics & Fuels Distribution
Business Acquisition [Line Items]
Unitholders		$ 23.7
Crude Oil [Member]
Business Acquisition [Line Items]
Storage Capacity | bbl	56

Equity Method Investments - Sum

Equity Method Investments - Summary of Equity Method Investment Financial Information (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Equity method investments		$ 4,270		$ 4,174
Revenues and other income		483	$ 403
Costs and expenses		242	217
Income (loss) from operations		241	186
Net income (loss)		222	173
Income from equity method investments	[1]	70	61
Current assets		489		614
Noncurrent assets		9,131		8,250
Current liabilities		350		401
Noncurrent liabilities		$ 1,036		1,030
Centrahoma Processing LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		40.00%
Equity method investments		$ 158		160
Explorer
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		25.00%
Equity method investments		$ 85		90
Illinois Extension
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		35.00%
Equity method investments		$ 280		275
LOCAP
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		59.00%
Equity method investments		$ 27		27
LOOP
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		41.00%
Equity method investments		$ 232		226
MarEn Bakken Company LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		25.00%
Equity method investments		$ 492		498
Jefferson Dry Gas
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		67.00%
Equity method investments		$ 247		236
MarkWest Utica EMG
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		56.00%
Equity method investments		$ 2,017		2,039
Sherwood Midstream
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		50.00%
Equity method investments		$ 455		366
Sherwood Midstream Holdings
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		78.00%
Equity method investments		$ 163		157
Other VIEs and Non-VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments		114		100
Other VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Revenues and other income		143	106
Costs and expenses		70	62
Income (loss) from operations		73	44
Net income (loss)		66	44
Income from equity method investments	[1]	24	15
Current assets		162		235
Noncurrent assets		4,438		3,535
Current liabilities		129		155
Noncurrent liabilities		193		189
Non-VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Revenues and other income		340	297
Costs and expenses		172	155
Income (loss) from operations		168	142
Net income (loss)		156	129
Income from equity method investments	[1]	46	$ 46
Current assets		327		379
Noncurrent assets		4,693		4,715
Current liabilities		221		246
Noncurrent liabilities		$ 843		841
Ohio Gathering
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		34.00%
Investment in subsidiary		$ 766		750
Direct Ownership Interest [Member] | Sherwood Midstream Holdings
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		56.00%
G&P
Schedule of Equity Method Investments [Line Items]
Difference between carrying amount and underlying equity		$ 1,000		1,000
Difference between carrying amount and underlying equity portion related to goodwill		459		459
L&S
Schedule of Equity Method Investments [Line Items]
Difference between carrying amount and underlying equity		112		114
Difference between carrying amount and underlying equity portion related to goodwill		$ 39		$ 39
[1]	“Income from equity method investments” includes the impact of any basis differential amortization or accretion.

Investments & NCI Ohio Gatherin

Investments & NCI Ohio Gathering (Details)	Mar. 31, 2019
Ohio Gathering
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	34.00%

Investments & NCI Sherwood Mids

Investments & NCI Sherwood Midstream (Details) bbl / d in Thousands	Mar. 31, 2019bbl / d
MarkWest Ohio Fractionation Company, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Capacity	20

Investments & NCI Sherwood Mi_2

Investments & NCI Sherwood Midstream Holdings (Details) - Sherwood Midstream Holdings - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2019
Schedule of Equity Method Investments [Line Items]
Ownership Interest In Assets Sold By Company In Affiliate	6.00%
Proceeds From Sale Of Ownership Interest In Assets Sold By Company In Affiliate	$ 15
Equity method investment, ownership percentage		78.00%
Indirect Ownership Interest [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		21.80%

Related Party Agreements and _3

Related Party Agreements and Transactions - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Related Party Transaction [Line Items]
Related Party Transaction, Purchases from Related Party	$ 1	$ 1
Revenue from Related Parties	$ 1	$ 1
MarkWest Utica EMG
Related Party Transaction [Line Items]
Equity method investment, ownership percentage	56.00%
Ohio Gathering
Related Party Transaction [Line Items]
Equity method investment, ownership percentage	34.00%
Sherwood Midstream
Related Party Transaction [Line Items]
Equity method investment, ownership percentage	50.00%
Sherwood Midstream Holdings
Related Party Transaction [Line Items]
Equity method investment, ownership percentage	78.00%
Jefferson Dry Gas
Related Party Transaction [Line Items]
Equity method investment, ownership percentage	67.00%

Related Party Agreements and _4

Related Party Agreements and Transactions MPC Loan Agreement (Details) - Related Party Revolving Credit Agreement [Member] - MPC Investment [Member] - USD ($) $ in Millions	Apr. 27, 2018	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Apr. 26, 2018
Related Party Transaction [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 1,000				$ 500
Line of Credit Facility, Expiration Date	Dec. 4, 2020
Debt Instrument, Description of Variable Rate Basis	LIBOR plus 1.50 percent
Proceeds from Lines of Credit		$ 851	$ 3,962
Line of Credit Facility, Interest Rate During Period		3.988%	3.473%
Repayments of Lines of Credit		$ 851	$ 4,347
Long-term Line of Credit		$ 0		$ 0

Sales to Related Parties (Detai

Sales to Related Parties (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Related Party Transaction [Line Items]
Rental income - related parties	$ 193	$ 145
Other income - related parties	26	23
MPC
Related Party Transaction [Line Items]
Service revenue - related parties	578	471
Rental income - related parties	193	145
Product sales - related parties	11	4
Other income - related parties	10	10
Product sales to MPC that net to zero	86	79
MarkWest Utica EMG
Related Party Transaction [Line Items]
Other income - related parties	4	4
Ohio Gathering
Related Party Transaction [Line Items]
Other income - related parties	4	4
Sherwood Midstream [Member]
Related Party Transaction [Line Items]
Other income - related parties	4	3
Jefferson Dry Gas
Related Party Transaction [Line Items]
Other income - related parties	2	1
Other Affiliates [Member]
Related Party Transaction [Line Items]
Other income - related parties	$ 2	$ 1

Summary of Charges for Employee

Summary of Charges for Employee Services and Omnibus Agreements (Detail) - MPC - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Related Party Transaction [Line Items]
Labor and Related Expense	$ 265	$ 217
Rental cost of sales - related parties
Related Party Transaction [Line Items]
Labor and Related Expense	3	1
Purchases - related parties
Related Party Transaction [Line Items]
Labor and Related Expense	212	177
General and administrative expenses
Related Party Transaction [Line Items]
Labor and Related Expense	50	39
Construction-in-progress
Related Party Transaction [Line Items]
Property, Plant and Equipment, Additions	$ 41	$ 22

Other Assets and Liabilities fr

Other Assets and Liabilities from Related Parties (Detail) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Related Party Transaction [Line Items]
Current Assets, Related Parties	$ 386	$ 290
Operating Lease, Right-of-Use Asset	262	0
Other Assets, Related Parties, Noncurrent	256	24
Operating Lease, Liability, Current	46	0
Current Liabilities, Related Parties	204	254
Operating Lease, Liability, Noncurrent	216	0
Liabilities, Related Parties, Noncurrent	273	43
MPC
Related Party Transaction [Line Items]
Receivables - related parties	372	281
Prepaid Insurance	7	1
Due from Related Parties, Noncurrent	24	24
Operating Lease, Right-of-Use Asset	232	0
Due to Related Parties, Current	119	131
Operating Lease, Liability, Current	1	0
Operating Lease, Liability, Noncurrent	231	0
Sherwood Midstream [Member]
Related Party Transaction [Line Items]
Due to Related Parties, Current	18	16
MarkWest Utica EMG
Related Party Transaction [Line Items]
Due to Related Parties, Current	16	51
Other Affiliates [Member]
Related Party Transaction [Line Items]
Receivables - related parties	7	8
Due to Related Parties, Current	5	5
Minimum Committed Volume Contracts [Member] | MPC
Related Party Transaction [Line Items]
Deferred Revenue Related Parties	38	44
Reimbursable Projects [Member] | MPC
Related Party Transaction [Line Items]
Deferred Revenue Related Parties	7	7
Deferred Revenue, Noncurrent, Related Parties	$ 42	$ 43

Net Income Per Limited Partne_3

Net Income Per Limited Partner Unit - Schedule of Distributions by Partner by Class (Detail) - USD ($) shares in Millions, $ in Millions		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018
Net Income Per Share [Line Items]
Potentially dilutive units		1	1
Net income attributable to MPLX LP	[1]	$ 503	$ 421
Less: Distribution declared		543	483
Undistributed net loss attributable to MPLX LP		(40)	(62)
Preferred Units
Net Income Per Share [Line Items]
Net income attributable to MPLX LP	[1]	20	16
Less: Distribution declared	[2]	20	16
Limited Partners Common Units
Net Income Per Share [Line Items]
Net income attributable to MPLX LP	[1]	483	405
Less: Distribution declared	[2]	$ 523	$ 467
[1]	Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.
[2]	See Note 7 for distribution information.

Net Income Per Limited Partne_4

Net Income Per Limited Partner Unit - Basic and Diluted Earnings Per Unit (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018
Net income (loss) attributable to MPLX LP:
Distributions declared (including IDRs)		$ 543	$ 483
Undistributed net income (loss) attributable to MPLX LP		(40)	(62)
Net income attributable to MPLX LP	[1]	$ 503	$ 421
Weighted average units outstanding:
Common - basic (in shares)		825	692
Common - diluted (in shares)		826	692
Limited Partners Common Units
Net income (loss) attributable to MPLX LP:
Distributions declared (including IDRs)	[2]	$ 523	$ 467
Undistributed net income (loss) attributable to MPLX LP		(40)	(62)
Net income attributable to MPLX LP	[1]	$ 483	$ 405
Weighted average units outstanding:
Common - basic (in shares)		794	661
Common - diluted (in shares)		795	661
Net income attributable to MPLX LP per limited partner unit:
Basic (in USD per unit)		$ 0.61	$ 0.61
Diluted (in USD per unit)		$ 0.61	$ 0.61
Preferred Units
Net income (loss) attributable to MPLX LP:
Distributions declared (including IDRs)	[2]	$ 20	$ 16
Undistributed net income (loss) attributable to MPLX LP		0	0
Net income attributable to MPLX LP	[1]	$ 20	$ 16
Weighted average units outstanding:
Common - basic (in shares)		31	31
Common - diluted (in shares)		31	31
[1]	Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.
[2]	See Note 7 for distribution information.

Equity - Changes in Partners Ca

Equity - Changes in Partners Capital, Unit Rollforward (Details) - Limited Partners Common Units	3 Months Ended
	Mar. 31, 2019shares
Stockholders Equity [Line Items]
Balance at December 31, 2018	794,089,518
Unit-based compensation awards	148,379
Balance at March 31, 2019	794,237,897

Equity - Cash Distributions (De

Equity - Cash Distributions (Details) - USD ($) $ / shares in Units, $ in Millions		Apr. 29, 2019	Mar. 31, 2019	Mar. 31, 2018
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distributions declared (including IDRs)			$ 543	$ 483
Partners' distributions			543	483
Limited Partners Common Units
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Partners' distributions			523	467
Preferred Units
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Partners' distributions			20	16
Subsequent Event
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Declaration date		Apr. 29, 2019
Cash distributions declared per limited partner common unit		$ 0.6575
Distribution date		May 15, 2019
Date of record		May 9, 2019
Limited Partners Common Units
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distributions declared (including IDRs)	[1]		$ 523	$ 467
Cash distributions declared per limited partner common unit				$ 0.6175
Limited Partners Common Units | Subsequent Event
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distributions declared (including IDRs)		$ 523
[1]	See Note 7 for distribution information.

Redeemable Preferred Units (Nar

Redeemable Preferred Units (Narrative) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	Apr. 29, 2019	May 13, 2016
Series A Convertible Preferred Units
Redeemable Noncontrolling Interest [Line Items]
Issuance of preferred units		30.8
Dividend rate, percentage		6.50%
Price per share		$ 32.50
Issuance of redeemable preferred units		$ 984
Dividend rate, per-dollar-amount		$ 0.528125
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 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 change of control conversion rate.
Subsequent Event
Redeemable Noncontrolling Interest [Line Items]
Distribution Made to Limited Partner, Declaration Date	Apr. 29, 2019
Distribution Made to Limited Partner, Distributions Declared, Per Unit	$ 0.6575

Redeemable Preferred Units (Rol

Redeemable Preferred Units (Rollforward of Redeemable Preferred Units) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Redeemable Noncontrolling Interest [Line Items]
Balance at December 31, 2018	$ 1,004
Distributions to unitholders and general partner	(515)	$ (347)
Balance at March 31, 2019	1,004
Series A Convertible Preferred Units
Redeemable Noncontrolling Interest [Line Items]
Balance at December 31, 2018	1,004
Net income allocated	20
Distributions to unitholders and general partner	(20)
Balance at March 31, 2019	$ 1,004

Segment Information (Details)

Segment Information (Details)	3 Months Ended
	Mar. 31, 2019
Segment Reporting [Abstract]
Number of reportable segments	2

Segment Information - Segment A

Segment Information - Segment Adjusted EBITDA (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		$ 1,263	$ 1,107
Rental income		94	79
Income from equity method investments	[1]	70	61
Other income		0	4
Total segment revenues and other income		1,646	1,420
L&S
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		615	501
G&P
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		648	606
Operating Segments
Segment Reporting Information [Line Items]
Adjusted EBITDA		930	760
Operating Segments | L&S
Segment Reporting Information [Line Items]
Rental income		199	145
Income from equity method investments		41	44
Other income		11	12
Total segment revenues and other income	[2]	866	702
Adjusted EBITDA	[3]	559	437
Capital Expenditure		13	22
Growth Capital Expenditures		103	154
Operating Segments | G&P
Segment Reporting Information [Line Items]
Rental income		88	79
Income from equity method investments		29	17
Other income		15	15
Total segment revenues and other income	[2]	780	718
Adjusted EBITDA	[3]	371	323
Capital Expenditure		6	3
Growth Capital Expenditures		261	271
Service [Member]
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		438	382
Service [Member] | L&S
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		34	28
Service [Member] | G&P
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		404	354
Service [Member] | Operating Segments | L&S
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		612	499
Service [Member] | Operating Segments | G&P
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		404	354
Product [Member]
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		202	207
Product [Member] | Operating Segments | L&S
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		3	2
Product [Member] | Operating Segments | G&P
Segment Reporting Information [Line Items]
Total revenues from contracts with customers		244	253
Third Party [Member] | L&S
Segment Reporting Information [Line Items]
Total segment revenues and other income		82	74
Third Party [Member] | G&P
Segment Reporting Information [Line Items]
Total segment revenues and other income		$ 757	$ 703
[1]	“Income from equity method investments” includes the impact of any basis differential amortization or accretion.
[2]	Within the total segment revenues and other income amounts presented above, third party revenues for the L&S segment were $82 million and $74 million for the three months ended March 31, 2019 and 2018, respectively. Third party revenues for the G&P segment were $757 million and $703 million for the three months ended March 31, 2019 and 2018, respectively.
[3]	See below for the reconciliation from Segment Adjusted EBITDA to “Net income.”

Segment Information - Assets by

Segment Information - Assets by Segment (Details) - USD ($) $ in Millions		Mar. 31, 2019	Dec. 31, 2018
Segment Reporting, Other Significant Reconciling Item [Line Items]
Cash and cash equivalents		$ 93	$ 68
Assets		23,584	22,779
Equity method investments		4,270	4,174
L&S | Operating Segments
Segment Reporting, Other Significant Reconciling Item [Line Items]
Assets	[1]	7,040	6,566
Equity method investments		1,120	1,120
G&P | Operating Segments
Segment Reporting, Other Significant Reconciling Item [Line Items]
Assets	[1]	16,451	16,145
Equity method investments		$ 3,150	$ 3,050
[1]	Equity method investments included in L&S assets were $1.12 billion at March 31, 2019 and December 31, 2018, respectively. Equity method investments included in G&P assets were $3.15 billion at March 31, 2019 and $3.05 billion at December 31, 2018.

Segment Information - Reconcili

Segment Information - Reconciliation to Net Income (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018
Segment Reporting, Revenue Reconciling Item [Line Items]
Depreciation and amortization	[1]	$ (211)	$ (176)
Income Tax Expense (Benefit)		2	(4)
Amortization of Debt Issuance Costs		(13)	(16)
Allocated Share-based Compensation Expense		(6)	(4)
Interest and Other Financial Costs		(158)	(114)
Income from equity method investments	[2]	70	61
Proceeds from Equity Method Investment, Distribution, Return of Capital		(101)	(68)
Acquisition Costs, Period Cost		0	(3)
Net income		509	423
Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Adjusted EBITDA		930	760
Amortization of Debt Issuance Costs		(13)	(16)
Segment Reconciling Items [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Proceeds from Equity Method Investment, Distribution, Return of Capital		(108)	(90)
Adjusted EBITDA attributable to noncontrolling interests		7	2
L&S
Segment Reporting, Revenue Reconciling Item [Line Items]
Depreciation and amortization		70	48
L&S | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Adjusted EBITDA	[3]	559	437
Income from equity method investments		41	44
G&P
Segment Reporting, Revenue Reconciling Item [Line Items]
Depreciation and amortization		141	128
G&P | Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Adjusted EBITDA	[3]	371	323
Income from equity method investments		29	17
Not Designated as Hedging Instrument [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Unrealized Gain (Loss) on Derivatives and Commodity Contracts		$ (4)	$ 7
[1]	Depreciation and amortization attributable to L&S was $70 million and $48 million for the three months ended March 31, 2019 and 2018, respectively. Depreciation and amortization attributable to G&P was $141 million and $128 million for the three months ended March 31, 2019 and 2018, respectively.
[2]	“Income from equity method investments” includes the impact of any basis differential amortization or accretion.
[3]	See below for the reconciliation from Segment Adjusted EBITDA to “Net income.”

Inventories (Summary of Invento

Inventories (Summary of Inventories) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Inventory Disclosure [Abstract]
NGLs	$ 3	$ 9
Line fill	10	9
Spare parts, materials and supplies	61	59
Total inventories	$ 74	$ 77

Property, Plant and Equipment_2

Property, Plant and Equipment (Summary of Property, Plant and Equipment) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 18,689	$ 18,316
Less accumulated depreciation	3,873	3,677
Property, plant and equipment, net	14,816	14,639
Natural gas gathering and NGL transportation pipelines and facilities
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	6,128	5,926
Processing, fractionation and storage facilities
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	5,343	5,336
Pipelines and related assets
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	2,620	2,560
Barges and towing vessels
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	635	620
Terminals and related assets
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	1,184	1,178
Refineries and related assets
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	943	938
Land, building, office equipment and other
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	978	957
Construction-in-progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 858	$ 801

Fair Value Measurements - Recur

Fair Value Measurements - Recurring - Financial Instruments by Valuation Hierarchy (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	$ 0	$ 0
Derivative liability	65	61
Embedded derivatives in commodity contracts | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	0	0
Derivative liability	$ 65	$ 61

Fair Value Measurments - Recurr

Fair Value Measurments - Recurring - Significant Unobservable Inputs in Level 3 Valuation (Details) - Fair Value, Inputs, Level 3 [Member]	3 Months Ended
	Mar. 31, 2019
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Embedded Derivative Renewal Term	5 years
Embedded derivatives in commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Probability of Renewal	91.00%
Fair Value Inputs Probability of Renewal Second Term	82.00%

Fair Value Measurements - Rec_2

Fair Value Measurements - Recurring - Changes in Level 3 Measurements (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018
Commodity Derivative Contracts (net)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value at beginning of period		$ 0	$ (2)
Total gains (losses) (realized and unrealized) included in earnings	[1]	0	0
Settlements		0	0
Fair value at end of period		0	(2)
The amount of total gains (losses) for the period included in earnings attributable to the change in unrealized losses relating to liabilities still held at end of period		0	0
Embedded Derivatives in Commodity Contracts (net)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value at beginning of period		(61)	(64)
Total gains (losses) (realized and unrealized) included in earnings	[1]	(6)	3
Settlements		2	3
Fair value at end of period		(65)	(58)
The amount of total gains (losses) for the period included in earnings attributable to the change in unrealized losses relating to liabilities still held at end of period		$ (5)	$ 3
[1]	Gains and losses on commodity derivative contracts classified as Level 3 are recorded in “Product sales” on the Consolidated Statements of Income. Gains and losses on derivatives embedded in commodity contracts are recorded in “Purchased product costs” and “Cost of revenues” on the Consolidated Statements of Income.

Fair Value Measurements - Repor

Fair Value Measurements - Reported (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	$ 13,921	$ 13,484
SMR liability	85	86
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	14,430	13,169
SMR liability	$ 94	$ 92

Derivative Financial Instrume_3

Derivative Financial Instruments - Embedded Derivatives in Commodity Contracts (Details) - Natural Gas [Member] - Embedded derivatives in commodity contracts $ in Millions	3 Months Ended
	Mar. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Derivative [Line Items]
Number of Renewals	2
Embedded Derivative Renewal Term	5 years
Embedded Derivative Fair Value of Embedded Derivative Liability Including Inception Value Allocable to Host Contract	$ 65	$ 61

Derivative Financial Instrume_4

Derivative Financial Instruments - Derivatives Balance Sheet Location (Details) - Not Designated as Hedging Instrument [Member] - Commodity contracts - USD ($) $ in Millions		Mar. 31, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	$ 0	$ 0
Derivative Liability, Fair Value, Gross Liability	[1]	65	61
Other current assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	0	0
Other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	[1]	8	7
Other noncurrent assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	0	0
Other Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	[1]	$ 57	$ 54
[1]	Includes embedded derivatives in commodity contracts as discussed above.

Derivatives Financial Instrumen

Derivatives Financial Instruments - Derivative Income Statement Location (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Derivative [Line Items]
Total gain (loss)	$ (6)	$ 4
Product sales
Derivative [Line Items]
Realized gain (loss)	0	0
Total gain (loss)	0	1
Purchased product costs
Derivative [Line Items]
Realized gain (loss)	(2)	(3)
Total gain (loss)	(6)	3
Cost of revenues
Derivative [Line Items]
Realized gain (loss)	0	0
Total gain (loss)	0	0
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Unrealized Gain (Loss) on Derivatives and Commodity Contracts	(4)	7
Not Designated as Hedging Instrument [Member] | Product sales
Derivative [Line Items]
Unrealized Gain (Loss) on Derivatives and Commodity Contracts	0	1
Not Designated as Hedging Instrument [Member] | Purchased product costs
Derivative [Line Items]
Unrealized Gain (Loss) on Derivatives and Commodity Contracts	(4)	6
Not Designated as Hedging Instrument [Member] | Cost of revenues
Derivative [Line Items]
Unrealized Gain (Loss) on Derivatives and Commodity Contracts	$ 0	$ 0

Debt - Summary of Outstanding B

Debt - Summary of Outstanding Borrowings (Detail) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Nov. 15, 2018	Feb. 08, 2018
Debt Instrument [Line Items]
Financing lease obligations(1)	$ 8
Total	14,283	$ 13,856
Unamortized debt issuance costs	(96)	(97)
Unamortized discount	(354)	(366)
Amounts due within one year	(1)	(1)
Total long-term debt due after one year	13,832	13,392
Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross			$ 2,250	$ 5,500
Finance Lease [Member] | Marathon Pipe Line LLC [Member]
Debt Instrument [Line Items]
Financing lease obligations(1)	8	6
Bank revolving credit facility due 2022 | Line of Credit [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	425	0
3.375% senior notes due March 2023 | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	500	500		500
4.500% senior notes due July 2023 | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	989	989
4.875% senior notes due December 2024 | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	1,149	1,149
4.000% senior notes due February 2025 | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	500	500
4.875% senior notes due June 2025 | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	1,189	1,189
4.125% senior notes due March 2027 | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	1,250	1,250
4.000% senior notes due March 2028 | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	1,250	1,250		1,250
Senior Notes Due February 2029 [Member] | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	750	750	750
4.500% senior notes due April 2038 | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	1,750	1,750		1,750
5.200% senior notes due March 2047 | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	1,000	1,000
4.700% senior notes due April 2048 | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	1,500	1,500		1,500
Senior Notes Due February 2049 [Member] | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	1,500	1,500	$ 1,500
4.900% senior notes due April 2058 | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	500	500		$ 500
MarkWest - 4.500% - 4.875% senior notes, due 2023-2025 | Senior Notes [Member] | MarkWest [Member]
Debt Instrument [Line Items]
Long-term debt, gross	$ 23	$ 23

Debt Debt - Summary of Outstand

Debt Debt - Summary of Outstanding Borrowings - Interest Rates and Table Due Dates (Details)	3 Months Ended
	Mar. 31, 2019	Nov. 15, 2018	Feb. 08, 2018
MarkWest [Member] | Senior Notes [Member] | 4.500% senior notes due July 2023
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.50%
Debt Instrument, Maturity Date	Jul. 15, 2023
MarkWest [Member] | Senior Notes [Member] | 4.875% senior notes due December 2024
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.875%
Debt Instrument, Maturity Date	Dec. 1, 2024
MarkWest [Member] | Senior Notes [Member] | 4.875% senior notes due June 2025
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.875%
Debt Instrument, Maturity Date	Jun. 1, 2025
MPLX LP | Line of Credit [Member] | Bank revolving credit facility due 2022
Debt Instrument [Line Items]
Line of Credit Facility, Expiration Date	Jul. 21, 2022
MPLX LP | Senior Notes [Member] | Senior Notes Due March 2023 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	3.375%		3.375%
Debt Instrument, Maturity Date	Mar. 15, 2023
MPLX LP | Senior Notes [Member] | 4.500% senior notes due July 2023
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.50%
Debt Instrument, Maturity Date	Jul. 15, 2023
MPLX LP | Senior Notes [Member] | 4.875% senior notes due December 2024
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.875%
Debt Instrument, Maturity Date	Dec. 1, 2024
MPLX LP | Senior Notes [Member] | 4.000% senior notes due February 2025
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.00%
Debt Instrument, Maturity Date	Feb. 15, 2025
MPLX LP | Senior Notes [Member] | 4.875% senior notes due June 2025
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.875%
Debt Instrument, Maturity Date	Jun. 1, 2025
MPLX LP | Senior Notes [Member] | 4.125% senior notes due March 2027
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.125%
Debt Instrument, Maturity Date	Mar. 1, 2027
MPLX LP | Senior Notes [Member] | Senior Notes Due March 2028 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.00%		4.00%
Debt Instrument, Maturity Date	Mar. 15, 2028
MPLX LP | Senior Notes [Member] | Senior Notes Due April 2038 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.50%		4.50%
Debt Instrument, Maturity Date	Apr. 15, 2038
MPLX LP | Senior Notes [Member] | 5.200% senior notes due March 2047
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.20%
Debt Instrument, Maturity Date	Mar. 1, 2047
MPLX LP | Senior Notes [Member] | Senior Notes Due April 2048 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.70%		4.70%
Debt Instrument, Maturity Date	Apr. 15, 2048
MPLX LP | Senior Notes [Member] | Senior Notes Due February 2049 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.50%	5.50%
Debt Instrument, Maturity Date	Feb. 15, 2049
MPLX LP | Senior Notes [Member] | Senior Notes Due April 2058 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.90%		4.90%
Debt Instrument, Maturity Date	Apr. 15, 2058
Marathon Pipe Line LLC [Member] | Finance Lease [Member]
Debt Instrument [Line Items]
Capital Lease Due Date Year	2020

Debt - Additional Information (

Debt - Additional Information (Detail) - USD ($) $ in Millions	Feb. 08, 2018	Jul. 21, 2017	Mar. 31, 2019	Dec. 31, 2018	Dec. 10, 2018	Nov. 15, 2018
Debt Instrument [Line Items]
Extinguishment of Debt, Gain (Loss), Net of Tax				$ (46)
Bank revolving credit facility due 2022 | MPLX LP
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 2,250
Debt Instrument, Term		5 years
Proceeds from Lines of Credit			$ 825
Debt Instrument, Interest Rate, Effective Percentage			3.916%
Repayments of Long-term Lines of Credit			$ 400
Letters of Credit Outstanding			3
Line of Credit Facility, Remaining Borrowing Capacity			$ 1,822
Line of Credit Facility, Remaining Borrowing Capacity, Percentage			81.00%
MPLX 364-Day Term Loan [Member] | MPLX LP
Debt Instrument [Line Items]
Repayments of Short-term Debt	$ 4,100
Line of Credit [Member] | Bank revolving credit facility due 2022 | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross			$ 425	0
Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	5,500					$ 2,250
Senior Notes [Member] | Senior Notes Due February 2023 [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross					$ 750
Debt Instrument, Interest Rate, Stated Percentage					5.50%
Percent of Par					101.833%
Senior Notes [Member] | Senior Notes Due February 2023 [Member] | MarkWest [Member]
Debt Instrument [Line Items]
Long-term debt, gross					$ 40
Senior Notes [Member] | Senior Notes Due February 2029 [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross			750	750		$ 750
Debt Instrument, Interest Rate, Stated Percentage						4.80%
Percent of Par						99.432%
Senior Notes [Member] | Senior Notes Due February 2049 [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross			$ 1,500	1,500		$ 1,500
Debt Instrument, Interest Rate, Stated Percentage			5.50%			5.50%
Percent of Par						98.031%
Senior Notes [Member] | Senior Notes Due March 2023 [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	$ 500		$ 500	500
Debt Instrument, Interest Rate, Stated Percentage	3.375%		3.375%
Percent of Par	99.931%
Senior Notes [Member] | Senior Notes Due March 2028 [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	$ 1,250		$ 1,250	1,250
Debt Instrument, Interest Rate, Stated Percentage	4.00%		4.00%
Percent of Par	99.551%
Senior Notes [Member] | Senior Notes Due April 2038 [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	$ 1,750		$ 1,750	1,750
Debt Instrument, Interest Rate, Stated Percentage	4.50%		4.50%
Percent of Par	98.811%
Senior Notes [Member] | Senior Notes Due April 2048 [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	$ 1,500		$ 1,500	1,500
Debt Instrument, Interest Rate, Stated Percentage	4.70%		4.70%
Percent of Par	99.348%
Senior Notes [Member] | Senior Notes Due April 2058 [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	$ 500		$ 500	500
Debt Instrument, Interest Rate, Stated Percentage	4.90%		4.90%
Percent of Par	99.289%
Debt Premium [Member]
Debt Instrument [Line Items]
Extinguishment of Debt, Gain (Loss), Net of Tax				$ (14)

Revenue Disaggregation of Reven

Revenue Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019		Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	$ 1,263		$ 1,107
Other income	0		4
Total segment revenues and other income	1,646		1,420
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(6)		4
L&S
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	615		501
G&P
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	648		606
Other Income [Member]
Disaggregation of Revenue [Line Items]
Other income	383	[1]	313	[2]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	0		1
Service [Member]
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	438		382
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties	578		471
Service [Member] | L&S
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	34		28
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties	578		471
Service [Member] | G&P
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	404		354
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties	0		0
Service, Other [Member]
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	34		44
Service, Other [Member] | L&S
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	0		0
Service, Other [Member] | G&P
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	34		44
Product [Member]
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	202		207
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties	11		4
Revenue from Contract with Customer, excluding Assessed Tax and Non-ASC 606 Revenue	202		206	[3]
Product [Member] | L&S
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties	2		1
Revenue from Contract with Customer, excluding Assessed Tax and Non-ASC 606 Revenue	1		1
Product [Member] | G&P
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax, Related Parties	9		3
Revenue from Contract with Customer, excluding Assessed Tax and Non-ASC 606 Revenue	$ 201		$ 205	[3]
[1]	Non-ASC 606 Revenue includes rental income, income from equity method investments, derivative gains and losses, mark-to-market adjustments, and other income.
[2]	Non-ASC 606 Revenue includes rental income, income from equity method investments, derivative gains and losses, mark-to-market adjustments, and other income.
[3]	G&P “Product sales” for the three months ended March 31, 2018 includes approximately $1 million of revenue related to derivative gains and losses and mark-to-market adjustments.

Revenue Contract Balances (Deta

Revenue Contract Balances (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019USD ($)
Long-term deferred revenue, beginning balance	$ 80
Long-term deferred revenue, ending balance	94
Liability, change in timeframe, performance obligation satisfied, revenue recognized	0
ASC 606
Contract assets, beginning balance	4	[1]
Contract assets, additions/(deletions)	1
Contract assets, revenue recognized	0	[2]
Contract assets, ending balance	5
Deferred revenue, beginning balance	4	[1]
Deferred revenue, additions/(deletions)	1
Deferred revenue, revenue recognized	(1)	[2]
Deferred revenue, ending balance	4
Deferred revenue - related parties, beginning balance	50	[1]
Deferred revenue - related party, additions/(deletions)	3
Deferred revenue - related parties, revenue recognized	(9)	[2]
Deferred revenue - related parties, ending balance	44
Long-term deferred revenue, beginning balance	10	[1]
Long-term deferred revenue, additions/(deletions)	3
Long-term deferred revenue, revenue recognized	0	[2]
Long-term deferred revenue, ending balance	13
Long-term deferred revenue - related parties, beginning balance	42	[1]
Long-term deferred revenue - related party, additions/(deletions)	(1)
Long-term deferred revenue - related parties, revenue recognized	0	[2]
Long-term deferred revenue - related parties, ending balance	$ 41
[1]	Balance represents ASC 606 portion of each respective line item.
[2]	No significant revenue was recognized related to past performance obligations in the current period.

Revenue Remaining Performance O

Revenue Remaining Performance Obligations (Details) $ in Millions	Mar. 31, 2019USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Contract with customer, liability	$ 100
Remaining performance obligation, expected timing of satisfaction, years	25 years
2018	$ 884
2019	1,178
2020	1,193
2022	1,179
2023 and thereafter	5,651
Total revenue on remaining performance obligations	$ 10,085	[1],[2],[3]
[1]	All fixed consideration from contracts with customers is included in the amounts presented above. Variable consideration that is constrained or not required to be estimated as it reflects our efforts to perform is excluded.
[2]	Arrangements deemed implicit leases are included in “Rental income” and are excluded from this table.
[3]	Only minimum volume commitments that are deemed fixed are included in the table above. MPLX has various minimum volume commitments in processing arrangements that vary based on the actual Btu content of the gas received. These amounts are deemed variable consideration and are excluded from the table above.

Supplemental Cash Flow Inform_3

Supplemental Cash Flow Information - Change in Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Millions		Mar. 31, 2019		Dec. 31, 2018		Mar. 31, 2018	Dec. 31, 2017
Supplemental Cash Flow Elements [Abstract]
Cash and cash equivalents		$ 93		$ 68
Restricted cash	[1]	0		8
Cash, cash equivalents and restricted cash		$ 93	[2]	$ 76	[2]	$ 6	$ 9
[1]	The restricted cash balance is included within “Other current assets” on the Consolidated Balance Sheets.
[2]	As a result of the adoption of ASU 2016-18, Statement of Cash Flows - Restricted Cash, the Consolidated Statements of Cash Flows now explain the change during the period of both “Cash and cash equivalents” and “Restricted cash.”

Supplemental Cash Flow Inform_4

Supplemental Cash Flow Information - Summary of Supplemental Cash Flow Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Net cash provided by operating activities included:
Interest paid (net of amounts capitalized)	$ 143	$ 103
Operating Lease, Payments	19	0
Non-cash investing and financing activities:
Net transfers of property, plant and equipment from materials and supplies inventories	1	1
Right-of-Use Asset Obtained in Exchange for Operating Lease Liability	1	0
Right-of-Use Asset Obtained in Exchange for Finance Lease Liability	$ 2	$ 0

Supplemental Cash Flow Inform_5

Supplemental Cash Flow Information - Summary of Reconciliation of Additions to Property, Plant and Equipment to Total Capital Expenditures (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Supplemental Cash Flow Elements [Abstract]
Increase/(decrease) in capital accruals	$ (74)	$ (6)

Accumulated Other Comprehensi_3

Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss	[1]	$ (15)	$ (16)	$ (16)	$ (14)
Other Comprehensive Income (Loss), Net of Tax	[2]	1	(2)
Pension Plan [Member] | Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss	[1]	(14)	(14)	(14)	(13)
Other Comprehensive Income (Loss), Net of Tax	[2]	0	(1)
Other Postretirement Benefits Plan [Member] | Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss	[1]	(1)	(2)	$ (2)	$ (1)
Other Comprehensive Income (Loss), Net of Tax	[2]	$ 1	$ (1)
[1]	These components of “Accumulated other comprehensive loss” are included in the computation of net periodic benefit cost by LOOP and Explorer and are therefore included on the Consolidated Statements of Income under the caption “Income/(loss) from equity method investments.”
[2]	Components of other comprehensive income/loss - remeasurements relate to actuarial gains and losses as well as amortization of prior service costs. MPLX records an adjustment to “Comprehensive income” in accordance with its ownership interest in LOOP and Explorer.

Equity-Based Compensation Pla_2

Equity-Based Compensation Plan - Summary of Phantom Unit Award Activity (Details) - Phantom Units	3 Months Ended
	Mar. 31, 2019$ / sharesshares
Number of Units
Outstanding at December 31, 2018 | shares	1,154,335
Granted | shares	188,069
Settled | shares	210,846
Forfeited | shares	9,375
Outstanding at March 31, 2019 | shares	1,122,183
Weighted Average Fair Value
Outstanding at December 31, 2018 | $ / shares	$ 34.34
Granted | $ / shares	33.09
Settled | $ / shares	32.20
Forfeited | $ / shares	34.86
Outstanding at March 31, 2019 | $ / shares	$ 34.53

Equity-Based Compensation Pla_3

Equity-Based Compensation Plan - Additional Information (Detail) - Officer - Performance Shares [Member] - MPLX LP 2012 Incentive Compensation Plan [Member]	3 Months Ended
	Mar. 31, 2019
Equity Transactions And Share Based Compensation [Line Items]
Percentage paid out in cash	75.00%
Share Based Compensation Arrangement By Share Based Payment Award Percentage Paid Out In Stock	25.00%

Equity-Based Compensation Pla_4

Equity-Based Compensation Plan - Summary of Performance Unit Award Activity (Detail)	3 Months Ended
	Mar. 31, 2019$ / sharesshares
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Outstanding at December 31, 2018	1,941,750
Granted	987,994
Settled	772,397
Forfeited	0
Outstanding at March 31, 2019	2,157,347
Performance Units Market Condition [Member]
Equity Transactions And Share Based Compensation [Line Items]
Performance units grant date fair value (in USD per unit) | $ / shares	$ 0.68
Maximum [Member] | Performance Unit Performance Condition [Member]
Equity Transactions And Share Based Compensation [Line Items]
Performance units grant date fair value (in USD per unit) | $ / shares	$ 2

Leases Leases Narrative (Detail

Leases Leases Narrative (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019USD ($)
Operating lease revenue, excluding executory costs	$ 248
Minimum [Member]
Renewal Term Agreement	1 year
Term Of Agreements	1 year
Lessor, Operating Lease, Term of Contract	1 year
Lessor, Operating Lease, Renewal Term	0 years
Maximum [Member]
Renewal Term Agreement	50 years
Term Of Agreements	60 years
Lessor, Operating Lease, Term of Contract	12 years
Lessor, Operating Lease, Renewal Term	10 years

Leases Lessee Lease Costs (Deta

Leases Lessee Lease Costs (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019USD ($)
Lessee, Lease, Description [Line Items]
Operating Lease, Cost	$ 14
Variable Lease, Cost	2
Short-term Lease, Cost	10
Lease, Cost	26
MPC
Lessee, Lease, Description [Line Items]
Operating Lease, Cost	4
Variable Lease, Cost	0
Short-term Lease, Cost	0
Lease, Cost	$ 4

Leases Lessee Balance Sheet Lea

Leases Lessee Balance Sheet Lease Detail (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Lessee, Lease, Description [Line Items]
Operating Lease, Right-of-Use Asset	$ 262	$ 0
Operating Lease, Liability, Current	46	0
Operating Lease, Liability, Noncurrent	216	0
Operating Lease, Liability	$ 262
Operating Lease, Weighted Average Remaining Lease Term	6 years 11 months 18 days
Operating Lease, Weighted Average Discount Rate, Percent	4.33%
Property, plant and equipment, gross	$ 18,689	18,316
Less accumulated depreciation	3,873	3,677
Property, plant and equipment, net	14,816	14,639
Other current liabilities	75	83
Long-term debt	13,832	13,392
Financing lease obligations(1)	$ 8
Finance Lease, Weighted Average Remaining Lease Term	31 years 20 days
Finance Lease, Weighted Average Discount Rate, Percent	5.76%
MPC
Lessee, Lease, Description [Line Items]
Operating Lease, Right-of-Use Asset	$ 232	0
Operating Lease, Liability, Current	1	0
Operating Lease, Liability, Noncurrent	231	$ 0
Operating Lease, Liability	$ 232
Operating Lease, Weighted Average Remaining Lease Term	47 years 11 months
Operating Lease, Weighted Average Discount Rate, Percent	5.80%
Finance Lease [Member]
Lessee, Lease, Description [Line Items]
Property, plant and equipment, gross	$ 28
Less accumulated depreciation	10
Property, plant and equipment, net	18
Other current liabilities	1
Long-term debt	$ 7

Leases Lessee Maturity Table (D

Leases Lessee Maturity Table (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Lessee, Lease, Description [Line Items]
Operating Leases, Future Minimum Payments Due, Next Twelve Months		$ 73
Lessee, Operating Lease, Liability, Payments, Remainder of Fiscal Year	$ 42
Finance Lease, Liability, Payments, Remainder of Fiscal Year	1
Lessee, Operating Lease, Liability, Payments, Due Next Twelve Months	53
Finance Lease, Liability, Payments, Due Next Twelve Months	6
Finance Lease, Liability, Payments, Due Next Twelve Months		2
Operating Leases, Future Minimum Payments, Due in Two Years		70
Lessee, Operating Lease, Liability, Payments, Due Year Two	51
Finance Lease, Liability, Payments, Due Year Two	0
Finance Lease, Liability, Payments, Due Year Two		5
Operating Leases, Future Minimum Payments, Due in Three Years		67
Lessee, Operating Lease, Liability, Payments, Due Year Three	46
Finance Lease, Liability, Payments, Due Year Three	0
Finance Lease, Liability, Payments, Due Year Three		0
Operating Leases, Future Minimum Payments, Due in Four Years		64
Lessee, Operating Lease, Liability, Payments, Due Year Four	43
Finance Lease, Liability, Payments, Due Year Four	0
Finance Lease, Liability, Payments, Due Year Four		0
Operating Leases, Future Minimum Payments, Due in Five Years		58
Lessee, Operating Lease, Liability, Payments, Due after Year Five	69
Finance Lease, Liability, Payments, Due after Year Five	7
Finance Lease, Liability, Payments, Due after Year Five		0
Operating Leases, Future Minimum Payments Due		1,051
Lessee, Operating Lease, Liability, Payments, Due	304
Finance Lease, Liability, Payments, Due	14
Finance Lease, Liability, Payments, Due		7
Lessee, Operating Lease, Liability, Undiscounted Excess Amount	42
Finance Lease, Interest Payment on Liability	6
Finance Lease, Interest Payment on Liability		1
Operating Lease, Liability	262
Finance Lease, Liability		6
Finance Lease, Liability, Payments, Due Year Five		0
Operating Leases, Future Minimum Payments, Due Thereafter		$ 719
Finance Lease, Liability	8
MPC
Lessee, Lease, Description [Line Items]
Lessee, Operating Lease, Liability, Payments, Remainder of Fiscal Year	11
Lessee, Operating Lease, Liability, Payments, Due Next Twelve Months	14
Lessee, Operating Lease, Liability, Payments, Due Year Two	14
Lessee, Operating Lease, Liability, Payments, Due Year Three	14
Lessee, Operating Lease, Liability, Payments, Due Year Four	14
Lessee, Operating Lease, Liability, Payments, Due after Year Five	619
Lessee, Operating Lease, Liability, Payments, Due	686
Lessee, Operating Lease, Liability, Undiscounted Excess Amount	454
Operating Lease, Liability	$ 232

Leases Lessor Maturity Table (D

Leases Lessor Maturity Table (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Lessor, Lease, Description [Line Items]
Operating Leases, Future Minimum Payments Receivable, Current		$ 908
Operating Leases, Future Minimum Payments Receivable, in Two Years		909
Operating Leases, Future Minimum Payments Receivable, in Three Years		777
Operating Leases, Future Minimum Payments Receivable, in Four Years		775
Operating Leases, Future Minimum Payments Receivable, in Five Years		758
Operating Leases, Future Minimum Payments Receivable, Thereafter		3,432
Operating Leases, Future Minimum Payments Receivable		7,559
Lessor, Operating Lease, Payments to be Received, Remainder of Fiscal Year	$ 706
Lessor, Operating Lease, Payments to be Received, Next Twelve Months	920
Lessor, Operating Lease, Payments to be Received, Two Years	794
Lessor, Operating Lease, Payments to be Received, Three Years	793
Lessor, Operating Lease, Payments to be Received, Four Years	775
Lessor, Operating Lease, Payments to be Received, Thereafter	3,606
Lessor, Operating Lease, Payments to be Received	7,594
Related Party [Member]
Lessor, Lease, Description [Line Items]
Operating Leases, Future Minimum Payments Receivable, Current		748
Operating Leases, Future Minimum Payments Receivable, in Two Years		750
Operating Leases, Future Minimum Payments Receivable, in Three Years		627
Operating Leases, Future Minimum Payments Receivable, in Four Years		627
Operating Leases, Future Minimum Payments Receivable, in Five Years		616
Operating Leases, Future Minimum Payments Receivable, Thereafter		2,321
Operating Leases, Future Minimum Payments Receivable		5,689
Lessor, Operating Lease, Payments to be Received, Remainder of Fiscal Year	562
Lessor, Operating Lease, Payments to be Received, Next Twelve Months	752
Lessor, Operating Lease, Payments to be Received, Two Years	632
Lessor, Operating Lease, Payments to be Received, Three Years	633
Lessor, Operating Lease, Payments to be Received, Four Years	621
Lessor, Operating Lease, Payments to be Received, Thereafter	2,401
Lessor, Operating Lease, Payments to be Received	5,601
Third Party [Member]
Lessor, Lease, Description [Line Items]
Operating Leases, Future Minimum Payments Receivable, Current		160
Operating Leases, Future Minimum Payments Receivable, in Two Years		159
Operating Leases, Future Minimum Payments Receivable, in Three Years		150
Operating Leases, Future Minimum Payments Receivable, in Four Years		148
Operating Leases, Future Minimum Payments Receivable, in Five Years		142
Operating Leases, Future Minimum Payments Receivable, Thereafter		1,111
Operating Leases, Future Minimum Payments Receivable		$ 1,870
Lessor, Operating Lease, Payments to be Received, Remainder of Fiscal Year	144
Lessor, Operating Lease, Payments to be Received, Next Twelve Months	168
Lessor, Operating Lease, Payments to be Received, Two Years	162
Lessor, Operating Lease, Payments to be Received, Three Years	160
Lessor, Operating Lease, Payments to be Received, Four Years	154
Lessor, Operating Lease, Payments to be Received, Thereafter	1,205
Lessor, Operating Lease, Payments to be Received	$ 1,993

Leases Lessor Operating Leased

Leases Lessor Operating Leased Assets (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2017
Lessor, Lease, Description [Line Items]
Property Subject to or Available for Operating Lease, Gross	$ 5,825	$ 5,525
Property Subject to or Available for Operating Lease, Accumulated Depreciation	2,155	2,038
Property Subject to or Available for Operating Lease, Net	3,670	3,487
Natural gas gathering and NGL transportation pipelines and facilities
Lessor, Lease, Description [Line Items]
Property Subject to or Available for Operating Lease, Gross	1,036	964
Processing, fractionation and storage facilities
Lessor, Lease, Description [Line Items]
Property Subject to or Available for Operating Lease, Gross	1,548	1,398
Pipelines and related assets
Lessor, Lease, Description [Line Items]
Property Subject to or Available for Operating Lease, Gross	274	266
Barges and towing vessels
Lessor, Lease, Description [Line Items]
Property Subject to or Available for Operating Lease, Gross	636	619
Terminals and related assets
Lessor, Lease, Description [Line Items]
Property Subject to or Available for Operating Lease, Gross	1,184	1,178
Refineries and related assets
Lessor, Lease, Description [Line Items]
Property Subject to or Available for Operating Lease, Gross	943	938
Land, building, office equipment and other
Lessor, Lease, Description [Line Items]
Property Subject to or Available for Operating Lease, Gross	$ 204	$ 162

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	Sep. 06, 2016	Mar. 31, 2019	Dec. 31, 2018
Commitments And Contingencies [Line Items]
Accrued liabilities for environmental remediation		$ 17	$ 14
Contractual commitments to acquire property, plant and equipment		726
MPC
Commitments And Contingencies [Line Items]
Due to Related Parties, Current		119	131
Environmental Loss Contingency [Member] | MPC
Commitments And Contingencies [Line Items]
Receivables from MPC for indemnification of environmental costs		2
Due to Related Parties, Current			$ 0
MarkWest, MarkWest Liberty Midstream, MarkWest Bluestone, Ohio Fractionation, MarkWest Utica EMG [Member] [Member]
Commitments And Contingencies [Line Items]
Loss Contingency, Damages Sought, Value		10
Bilfinger Westcon, Inc. [Member]
Commitments And Contingencies [Line Items]
Loss Contingency, Damages Sought, Value		$ 40
Apex [Member]
Commitments And Contingencies [Line Items]
Apex litigation settlement amount	$ 10
Indirect Ownership Interest [Member] | Bakken Pipeline System [Member]
Commitments And Contingencies [Line Items]
Equity method investment, ownership percentage		9.00%
Guarantee Type, Other [Member]
Commitments And Contingencies [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted		$ 230

Subsequent Events Subsequent _2

Subsequent Events Subsequent Events (Details) - ANDX LP [Member] - Subsequent Event $ in Billions	May 08, 2019USD ($)shares
Subsequent Event [Line Items]
Subsequent Event, Date	May 8, 2019
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned | $	$ 9
Enterprise Value | $	$ 14
Public
Subsequent Event [Line Items]
Common Units Conversion Ratio - ANDX to MPLX | shares	1.135
Nonpublic [Member]
Subsequent Event [Line Items]
Common Units Conversion Ratio - ANDX to MPLX | shares	1.0328