MPLX (MPLX) 10-Q 30 Apr 18 2018 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2018	Apr. 26, 2018
Document Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q1
Trading Symbol	MPLX
Entity Registrant Name	MPLX LP
Entity Central Index Key	1,552,000
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Common Units [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		793,973,724
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, 2018		Mar. 31, 2017
Revenues and other income:
Service revenue		$ 382		$ 260
Service revenue - related parties		471		255
Service revenue - product related		44		0
Rental income		79		69
Rental income - related parties		145		67
Product sales		207	[1]	203
Product sales - related parties		4		2
Income from equity method investments	[2]	61		5
Other income		4		3
Other income - related parties		23		22
Total revenues and other income		1,420		886
Costs and expenses:
Cost of revenues (excludes items below)		206	[3]	113
Purchased product costs		187		131
Rental cost of sales		29		12
Rental cost of sales - related parties		1		0
Purchases - related parties		177		107
Depreciation and amortization		176		187
General and administrative expenses		69		58
Other taxes		18		13
Total costs and expenses		863		621
Income from operations		557		265
Related party interest and other financial costs		1		0
Interest expense (net of amounts capitalized of $9 million, $7 million, respectively)		112		66
Other financial costs		17		12
Income before income taxes		427		187
Provision for income taxes		4		0
Net income		423		187
Less: Net income (loss) attributable to noncontrolling interest		2		1
Less: Net income attributable to Predecessor		0		36
Net income (loss) attributable to MPLX LP	[4]	421		150
Less: Preferred unit distributions		16		16
Less: General partner’s GP interest in net income attributable to MPLX LP		0		62
Limited partners' interest in net income (loss) attributable to MPLX LP		$ 405		$ 72
Weighted average limited partner units outstanding:
Common - basic (in shares)		692		400
Common - diluted (in shares)		692		405
Limited Partners Common Units
Costs and expenses:
Net income (loss) attributable to MPLX LP	[4]	$ 405		$ 72
Net income (loss) attributable to MPLX LP per limited partner unit:
Common - basic (in USD per unit)		$ 0.61		$ 0.20
Common - diluted (in USD per unit)		$ 0.61		$ 0.19
Weighted average limited partner units outstanding:
Common - basic (in shares)		661		362
Common - diluted (in shares)		661		367
Cash distributions declared per limited partner common unit		$ 0.6175		$ 0.5400
[1]	G&P “Product sales” exclude approximately $1 million of revenue related to derivative gains/losses and mark-to-market adjustments.
[2]	“Income (loss) from equity method investments” includes the impact of any basis differential amortization or accretion.
[3]	Excludes “Purchased product costs,” “Rental cost of sales,” “Purchases,” “Depreciation and amortization,” “General and administrative expenses,” and “Other taxes.”
[4]	Allocation of net income (loss) attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.

Consolidated Statements of Inc3

Consolidated Statements of Income (Parenthetical) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Statement [Abstract]
Interest costs capitalized	$ 9	$ 7

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 423	$ 187
Other comprehensive income (loss), net of tax:
Remeasurements of pension and other postretirement benefits related to equity method investments, net of tax	(2)	0
Comprehensive income	421	187
Less comprehensive income (loss) attributable to:
Noncontrolling interests	2	1
Income attributable to Predecessor	0	36
Comprehensive income attributable to MPLX LP	$ 419	$ 150

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents		$ 2	$ 5
Receivables, net		300	292
Receivables - related parties		330	160
Inventories		64	65
Other current assets		33	37
Total current assets		729	559
Equity method investments		4,033	4,010
Property, plant and equipment, net		13,291	12,187
Intangibles, net		444	453
Goodwill		2,460	2,245
Long-term receivables - related parties		21	20
Other noncurrent assets		28	26
Total assets		21,006	19,500
Current liabilities:
Accounts payable		143	151
Payables - related parties		146	516
Deferred revenue - related parties		43	43
Accrued interest payable		99	88
Other current liabilities		445	506
Total current liabilities		876	1,304
Long-term deferred revenue		49	42
Long-term deferred revenue - related parties		49	43
Long-term debt		11,861	6,945
Deferred income taxes		10	5
Deferred credits and other liabilities		183	188
Total liabilities		13,028	8,527
Commitments and contingencies (see Note 20)
Redeemable preferred units		1,000	1,000
Equity
Total MPLX LP partners’ capital		6,832	9,827
Accumulated other comprehensive loss	[1]	(16)	(14)
Noncontrolling interests		146	146
Total equity		6,978	9,973
Total liabilities, preferred units and equity		21,006	19,500
MPC
Current assets:
Receivables - related parties		308	153
Long-term receivables - related parties		21	20
Current liabilities:
Payables - related parties		115	470
Limited Partners Common Units | Public
Equity
Total MPLX LP partners’ capital		8,385	8,379
Total equity		8,385	8,379
Limited Partners Common Units | MPC
Equity
Total MPLX LP partners’ capital		(1,537)	2,099
Total equity		(1,537)	2,099
General Partner | MPC
Equity
Total MPLX LP partners’ capital		0	(637)
Total equity		0	(637)
LOOP and Explorer
Equity
Accumulated other comprehensive loss		$ (16)	$ (14)
[1]	These accumulated other comprehensive loss components are included in the computation of net periodic benefit cost by LOOP and Explorer and are therefore included in 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, 2018	Dec. 31, 2017
Limited Partners Common Units | Public
Units issued	289	289
Units outstanding	289	289
Limited Partners Common Units | MPC
Units issued	505	118
Units outstanding	505	118
General Partner | MPC
GP units issued	0	8
GP units outstanding	0	8

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018		Mar. 31, 2017
Operating activities:
Net income		$ 423		$ 187
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization of deferred financing costs		16		12
Depreciation and amortization		176		187
Deferred income taxes		4		0
Asset retirement expenditures		(1)		(1)
Gain on disposal of assets		0		(1)
Income from equity method investments	[1]	(61)		(5)
Distributions from unconsolidated affiliates		68		33
Changes in:
Current receivables		(8)		44
Inventories		2		0
Fair value of derivatives		(9)		(18)
Current accounts payable and accrued liabilities		(44)		(59)
Receivables from / liabilities to related parties		(127)		(18)
Prepaid other current assets from related parties		1		0
Deferred revenue		7		7
All other, net		3		9
Net cash provided by operating activities		450		377
Investing activities:
Additions to property, plant and equipment		(455)		(280)
Acquisitions, net of cash acquired		0		(220)
Disposal of assets		2
Disposal of Assets				(1)
Investments - net related party loans		0		80
Investments in unconsolidated affiliates		(38)		(554)
Distributions from unconsolidated affiliates - return of capital		0		20
All other, net		1		0
Net cash used in investing activities		(490)		(955)
Financing activities:
Long-term debt - borrowings		9,610		2,241
Long-term debt - repayments		(4,655)		(1)
Related party debt - borrowings		452		12
Related party debt - repayments		(838)		(12)
Debt issuance costs		(53)		(21)
Net proceeds from equity offerings		0		151
Distributions of cash received from joint-interest acquisition entities to MPC		11		0
Distributions to MPC for acquisition		(4,100)		(1,511)
Distributions to MPC from Predecessor		0		(113)
Distributions to noncontrolling interests		(3)		(2)
Distributions to preferred unitholders		(16)		(16)
Distributions to unitholders and general partner		(347)		(242)
Contributions from noncontrolling interests		1		126
All other, net		(3)		(5)
Net cash provided by financing activities		37		607
Net (decrease) increase in cash, cash equivalents and restricted cash		(3)		29
Cash, cash equivalents and restricted cash at beginning of period		9	[2]	239
Cash, cash equivalents and restricted cash at end of period		$ 6	[2]	$ 268
[1]	“Income (loss) from equity method investments” includes the impact of any basis differential amortization or accretion.
[2]	As a result of the adoption of ASU 2016-18, 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	PublicClass B Unitholders	MPCLimited Partners Common Units	MPCGeneral Partner	Accumulated Other Comprehensive Loss	Noncontrolling Interests	Equity of PredecessorMPC
Beginning Balance at Dec. 31, 2016	$ 11,110	$ 8,086	$ 133	$ 1,069	$ 1,013		$ 18	$ 791
Distributions to MPC from Predecessor	(113)							(113)
Issuance of units under ATM Program	151	148			3
Net income	171	55		17	62		1	36
Contribution from MPC	12							12
Allocation of MPC's net investment at acquisition	0			923	(197)			(726)
Distribution to MPC for acquisitions	(1,511)			(430)	(1,081)
Distributions to unitholders and general partner	(242)	(140)		(45)	(57)
Distributions to noncontrolling interests	(2)						(2)
Contributions from noncontrolling interests	126						126
Other	(2)	(2)
Ending Balance at Mar. 31, 2017	9,700	8,147	133	1,534	(257)		143	0
Beginning Balance at Dec. 31, 2017	9,973	8,379	0	2,099	(637)	$ (14)	146	0
Distributions to MPC from Predecessor	0
Net income	407	180		225			2
Contribution from MPC	1,046							1,046
Allocation of MPC's net investment at acquisition	0			5,172	(4,126)			(1,046)
Distribution to MPC for acquisitions	(4,100)			(936)	(3,164)
Distributions to unitholders and general partner	(347)	(176)		(171)
Distributions to noncontrolling interests	(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	$ 0	$ (1,537)	$ 0	$ (16)	$ 146	$ 0

Description of the Business and

Description of the Business and Basis of Presentation	3 Months Ended
	Mar. 31, 2018
Accounting Policies [Abstract]
Description of the Business and Basis of Presentation	Description of the Business and Basis of Presentation Description of the Business – MPLX LP is a diversified, growth-oriented master limited partnership formed by Marathon Petroleum Corporation. References in this report to “MPLX LP,” “the Partnership,” “we,” “ours,” “us,” or like terms refer to MPLX LP and its subsidiaries (collectively, the “Partnership”). References to “MPC” refer collectively to Marathon Petroleum Corporation as our sponsor and its subsidiaries, other than the Partnership. The Partnership is engaged in the gathering, processing and transportation of natural gas; the gathering, transportation, fractionation, storage and marketing of NGLs; the transportation, storage and distribution of crude oil and refined petroleum products; as well as refining logistics and fuels distributions services. The Partnership’s principal executive office is located in Findlay, Ohio. Effective March 1, 2017 , the Partnership acquired pipeline, storage and terminal businesses that are operated through Hardin Street Transportation LLC (“HST”), Woodhaven Cavern LLC (“WHC”) and MPLX Terminals LLC (“MPLXT”) from MPC. Effective September 1, 2017 , the Partnership acquired certain ownership percentages in joint venture entities from MPC including: all of the membership interests of Lincoln Pipeline LLC, which holds a 35 percent interest in Illinois Extension Pipeline Company, L.L.C. (“Illinois Extension”); all of the membership interests of MPL Louisiana Holdings LLC, which holds a 41 percent interest in LOOP LLC (“LOOP”); a 59 percent interest in LOCAP LLC (“LOCAP”); and a 25 percent interest in Explorer Pipeline Company (“Explorer”). Effective February 1, 2018 , the Partnership acquired MPC’s refining logistics assets and fuels distribution services. These acquisitions are described further in Note 4 . The Partnership’s business consists of two segments based on the nature of services it offers: Logistics and Storage (“L&S”), which is focused on crude oil and refined petroleum products; and Gathering and Processing (“G&P”), which is focused on natural gas and NGLs. See Note 10 for additional information regarding operations. Basis of Presentation – The accompanying interim consolidated financial statements are unaudited; however, in the opinion of the Partnership’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, 2017 . The results of operations for the three months ended March 31, 2018 are not necessarily indicative of the results to be expected for the full year. The Partnership’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” in the accompanying Consolidated Balance Sheets. Intercompany investments, accounts and transactions have been eliminated. The Partnership’s investments in which the Partnership exercises significant influence but does not control and does not have a controlling financial interest are accounted for using the equity method. The Partnership’s investments in a VIE in which the Partnership 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, as discussed in Note 9 . Prior to 2018 , when distributions related to the IDRs were made, earnings equal to the amount of those distributions were first allocated to the general partner before the remaining earnings were allocated to the limited partner unitholders based on their respective ownership percentages. Subsequent to the conversion of the general partner to a non-economic interest as described in Note 8, no earnings will be allocated to the general partner. 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 7 .

Summary of Principal Accounting

Summary of Principal Accounting Policies	3 Months Ended
	Mar. 31, 2018
Accounting Policies [Abstract]
Significant Accounting Policies	Summary of Principal Accounting Policies Revenue Recognition – As a result of the adoption of the new revenue recognition standard, as described further in Note 3 , the Partnership has updated its policies as it relates to revenue recognition. Revenue is measured based on consideration specified in a contract with a customer. The Partnership recognizes revenue when it satisfies a performance obligation by transferring control over a product or providing services to a customer. The Partnership enters into a variety of contract types in order to generate “Product sales” and “Service revenue.” The Partnership provides services under the following different types of arrangements: • Fee-based arrangements – Under fee-based arrangements, the Partnership receives a fee or fees for one or more of the following services: gathering, processing and transportation of natural gas; gathering, transportation, fractionation, exchange and storage of NGLs; and transportation, storage and distribution of crude oil, refined products and other hydrocarbon-based products. The revenue the Partnership earns from these arrangements is generally directly related to the volume of natural gas, NGL refined products or crude oil that is handled by or flows through the Partnership’s systems and facilities and is not normally directly dependent on commodity prices. In certain cases, the Partnership’s arrangements provide for minimum annual payments or fixed demand charges. Fee-based arrangements are reported as “Service revenue” on the Consolidated Statements of Income. Revenue is recognized over time as services are performed in a series. In certain instances when specifically stated in the contract terms, the Partnership purchases product after fee-based services have been provided. Revenue from the sale of products purchased after services are provided is reported as “Product sales” on the Consolidated Statements of Income and recognized on a gross basis as the Partnership takes control of the product and is the principal in the transaction. • Percent-of-proceeds arrangements – Under percent-of-proceeds arrangements, the Partnership: gathers and processes natural gas on behalf of producers; sells the resulting residue gas, condensate and NGLs at market prices; and remits to producers an agreed-upon percentage of the proceeds. In other cases, instead of remitting cash payments to the producer, the Partnership delivers an agreed-upon percentage of the residue gas and NGLs to the producer (take-in-kind arrangements) and sells the volumes the Partnership retains to third parties. Revenue is recognized on a net basis when the Partnership acts as an agent and does not have control of the gross amount of gas and/or NGLs prior to it being sold. Percent-of-proceeds revenue is reported as “Service revenue - product related” on the Consolidated Statements of Income. • Keep-whole arrangements – Under keep-whole arrangements, the Partnership gathers natural gas from the producer, processes the natural gas and sells the resulting condensate and NGLs to third parties at market prices. Because the extraction of the condensate and NGLs from the natural gas during processing reduces the Btu content of the natural gas, the Partnership must either purchase natural gas at market prices for return to producers or make cash payment to the producers equal to the value of the energy content of this natural gas. Certain keep-whole arrangements also have provisions that require the Partnership to share a percentage of the keep-whole profits with the producers based on the oil to gas ratio or the NGL to gas ratio. “Service revenue - product related” is recorded based on the value of the NGLs received on the date the services are performed. Natural gas purchased to return to the producer and shared NGL profits are recorded as a reduction of “Service revenue - product related” in the Consolidated Statements of Income on the date the services are performed. Sales of NGLs under these arrangements are reported as “Product sales” on the Consolidated Statements of Income and are reported on a gross basis as the Partnership is the principal in the arrangement and controls the product prior to sale. The sale of the NGLs may occur shortly after services are performed at the tailgate of the plant, or after a period of time as determined by the Partnership. • Purchase arrangements – Under purchase arrangements, the Partnership purchases natural gas at either the wellhead or the tailgate of a plant. The Partnership then gathers and delivers the natural gas to pipelines where the Partnership may resell the natural gas. Wellhead purchase arrangements represent an arrangement with a supplier and are recorded in “Purchased product costs”. Often, the Partnership earns fees for services performed prior to taking control of the product in these arrangements and “Service revenue” is recorded for these fees. Revenue generated from the sale of product obtained in tailgate purchase arrangements are reported as “Product sales” on the Consolidated Statements of Income and are recognized on a gross basis as the Partnership purchases and takes control of the product prior to sale and is the principal in the transaction. In many cases, the Partnership provides services under contracts that contain a combination of more than one of the arrangements described above. When fees are charged (in addition to product received) under percent-of-proceeds arrangements, keep-whole arrangements or purchase arrangements, the Partnership records such fees as “Service revenue” on the Consolidated Statements of Income. The terms of the Partnership’s contracts vary based on gas quality conditions, the competitive environment when the contracts are signed and customer requirements. Performance obligations are determined based on the specific terms of the arrangements, economics of the geographical regions and based on the services offered and whether they are deemed distinct. The Partnership allocates the consideration earned between the performance obligations based on the stand-alone selling price when multiple performance obligations are identified. The Partnership’s service arrangements will generally be recognized over time when the performance obligation is satisfied as services are provided in a series. The Partnership has elected to use the output measure of progress to recognize revenue based on the units delivered, processed or transported. The transaction price has fixed components related to minimum volume commitments and variable components which are primarily dependent on volumes. Variable consideration will generally not be estimated at contract inception as the transaction price is specifically allocable to the services provided each period end. In instances in which tiered pricing structures do not reflect our efforts to perform, the Partnership will estimate variable consideration at contract inception. “Product sales” will be recognized at a point in time when control of the product transfers to the customer. Minimum volume commitments may create contract liabilities or deferred credits if current period payments can be used for future services. Breakage is estimated and recognized into service revenue in instances where it is probable the customer will not use the credit in future periods. No breakage was recognized in the current period. Amounts billed to customers for shipping and handling, electricity, and other costs to perform services are included in “Service revenue” on the Consolidated Statements of Income. Shipping and handling costs associated with product sales are included in “Purchased product costs” on the Consolidated Statements of Income. Facility expenses, costs of revenues and depreciation represent those expenses related to operating our various facilities and are necessary to provide both “Product sales” and “Service revenue.” Customers usually pay monthly based on the products purchased or services performed that month. Taxes collected from customers and remitted to the appropriate taxing authority are excluded from revenue. Based on the terms of certain natural gas gathering, transportation and processing agreements, the Partnership is considered to be the lessor under several implicit operating lease arrangements in accordance with GAAP. Revenue and costs related to the portion of the revenue earned under these contracts considered to be implicit leases are recorded as “Rental income” and “Rental cost of sales,” respectively, on the Consolidated Statements of Income. The allocation method used to allocate income between lease and non-lease components was updated as a result of ASC 606. Similarly, the Partnership is considered to be the lessor under implicit operating lease arrangements with MPC in accordance with GAAP. Revenue related to these agreements are recorded as “Rental income - related parties” on the Consolidated Statements of Income. “Rental income” and “Rental income - related parties” is not deemed to be revenue from contracts with customers. The Partnership routinely makes accruals based on estimates for both revenue and expenses due to the timing of compiling billing information, receiving certain third-party information and reconciling the Partnership’s records with those of third parties. The delayed information from third parties includes among other things; actual volumes purchased, transported or sold; adjustments to inventory and invoices for purchases; actual natural gas and NGL deliveries and other operating expenses. The Partnership makes accruals to reflect estimates for these items based on its internal records and information from third parties. Estimated accruals are adjusted when actual information is received from third parties and the Partnership’s internal records have been reconciled.

Accounting Standards

Accounting Standards	3 Months Ended
	Mar. 31, 2018
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Accounting Standards	Accounting Standards Recently Adopted ASU 2014-09, Revenue - Revenue from Contracts with Customers. In May 2014, the FASB issued ASU 2014-09 which created ASC Topic 606, “Revenue from Contracts with Customers” (“ASC 606”). The guidance in ASC 606 states that revenue is recognized when a customer obtains control of a good or service. Recognition of the revenue involves a multiple step approach including identifying the contract, identifying the separate performance obligations, determining the transaction price, allocating the price to the performance obligations and recognizing the revenue as the obligations are satisfied. Additional disclosures are required to provide adequate information to understand the nature, amount, timing and uncertainty of reported revenues and revenues expected to be recognized. The Partnership adopted the standard as of January 1, 2018 using the modified retrospective method by recognizing the cumulative effect of initially applying the new revenue standard as an adjustment to the opening balance of retained earnings. The comparative information has not been restated and continues to be reported under the accounting standards in effect for those periods. See Note 16 for further details. We also adopted the following standards during the first quarter of 2018 , none of which had a material impact to our financial statements or financial statement disclosures: ASU Effective Date 2017-09 Stock Compensation - Scope of Modification Accounting January 1, 2018 2017-05 Gains and Losses from the Derecognition of Nonfinancial Assets - Clarifying the Scope of Asset Derecognition Guidance January 1, 2018 2017-01 Business Combinations - Clarifying the Definition of a Business January 1, 2018 2016-18 Statement of Cash Flows - Restricted Cash January 1, 2018 2016-15 Statement of Cash Flows - Classification of Certain Cash Receipts and Cash Payments January 1, 2018 2016-01 Financial Instruments - Recognition and Measurement of Financial Assets and Liabilities January 1, 2018 Not Yet Adopted ASU 2017-12, Derivatives and Hedging - Targeted Improvements to Accounting for Hedging Activities. In August 2017, the FASB issued an accounting standards update to amend the hedge accounting rules to simplify the application of hedge accounting guidance and better portray the economic results of risk management activities in the financial statements. The guidance expands the ability to hedge nonfinancial and financial risk components, reduces complexity in fair value hedges of interest rate risk, eliminates the requirement to separately measure and report hedge ineffectiveness, as well as eases certain hedge effectiveness assessment requirements. The guidance is effective beginning in 2019 with early adoption permitted. The Partnership is currently evaluating the impact of this guidance, including transition elections and required disclosures, on our financial statements and the timing of adoption. However, since we have not historically designated our commodity derivatives as hedges, we do not expect the adoption of this accounting standards update to have a material impact on our consolidated financial statements. ASU 2017-04, Intangibles - Goodwill and Other - Simplifying the Test for Goodwill Impairment. In January 2017, the FASB issued an accounting standards update 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. The Partnership is currently evaluating the impact of this guidance on our financial statements and the timing of adoption. ASU 2016-13, Credit Losses - Measurement of Credit Losses on Financial Instruments. In June 2016, the FASB issued an accounting standards update 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. The Partnership does not expect application of this accounting standards update to have a material impact on our consolidated financial statements. ASU 2016-02, Leases. In February 2016, the FASB issued an accounting standards update requiring lessees to record virtually all leases on their balance sheets. The accounting standards update also requires expanded disclosures to help financial statement users better understand the amount, timing and uncertainty of cash flows arising from leases. For lessors, this amended guidance modifies the classification criteria and the accounting for sales-type and direct financing leases. The change will be effective on a modified retrospective basis for fiscal years beginning after December 15, 2018, and interim periods within those years, with early adoption permitted. The Partnership continues to evaluate the impact of this standard on our financial statements and disclosures, internal controls and accounting policies. This evaluation process includes reviewing all forms of leases, performing a completeness assessment over the lease population and analyzing the practical expedients in order to determine the best path of implementing changes to existing processes and controls. We are implementing a third-party supported lease accounting information system to account for our lease population in accordance with this new standard and establishing internal controls over the new system. We believe the adoption of the standard will have a material impact on our consolidated financial statements as virtually all leases will be recognized as a right of use asset and lease obligation.

Acquisitions

Acquisitions	3 Months Ended
	Mar. 31, 2018
Business Combinations [Abstract]
Acquisitions	Acquisitions 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 (“MPLX Logistics”) and (iii) 7,824,167 common units to MPLX Holdings Inc. (“MPLX Holdings”). The Partnership also issued 2,277,778 general partner units to MPLX GP in order to maintain its two percent general partner interest (“GP Interest”) in the Partnership. 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 will not receive $23.7 million of the distributions that would have otherwise accrued on such common units with respect to the first quarter 2018. Immediately following this transaction, the GP Interest was converted into a non-economic general partner interest as discussed in Note 8 . The Partnership 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 Refining Logistics and Fuels Distribution respectively. Both the refining logistics assets and the fuels distribution services are accounted for within the L&S segment. The refining logistics assets are owned by MPLX Refining Logistics LLC (“Refining Logistics”) and include 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 is the sole and exclusive provider of 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. MPLX Fuels Distribution LLC (“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 sole and exclusive agent. The amounts of revenue and income from operations associated with these investments included in the Consolidated Statements of Income, since the February 1, 2018 acquisition date, were as follows: (In millions) Two Months Ended March 31, 2018 Revenues and other income $ 265 Income from operations 181 Joint-Interest Acquisition On September 1, 2017 , the Partnership entered into a Membership Interests and Shares Contributions Agreement (the “September 2017 Contributions Agreement”) with MPLX GP, MPLX Logistics, MPLX Holdings and MPC Investment LLC (“MPC Investment”), each a wholly-owned subsidiary of MPC, whereby the Partnership agreed to acquire certain ownership interests in joint venture entities indirectly held by MPC. Pursuant to the September 2017 Contributions Agreement, MPC Investment agreed to contribute: all of the membership interests of Lincoln Pipeline LLC, which holds a 35 percent interest in Illinois Extension; all of the membership interests of MPL Louisiana Holdings LLC, which holds a 41 percent interest in LOOP; a 59 percent interest in LOCAP; and a 25 percent interest in Explorer, through a series of intercompany contributions to the Partnership for an agreed upon purchase price of approximately $420 million in cash and equity consideration valued at approximately $630 million for total consideration of $1.05 billion (collectively, the “Joint-Interest Acquisition”). The number of common units representing the equity consideration was then determined by dividing the contribution amount by the simple average of the ten day trading volume weighted average NYSE price of a common unit for the ten trading days ending at market close on August 31, 2017 . The fair value of the common and general partner units issued was approximately $653 million based on the closing common unit price as of September 1, 2017 , as recorded on the Consolidated Statements of Equity, for a total purchase price of $1.07 billion . The equity issued consisted of: (i) 13,719,017 common units to MPLX GP, (ii) 3,350,893 common units to MPLX Logistics and (iii) 1,441,224 common units to MPLX Holdings. The Partnership also issued 377,778 general partner units to MPLX GP in order to maintain its two percent GP Interest in the Partnership. Illinois Extension operates the 168 -mile, 24 -inch diameter Southern Access Extension (“SAX”) crude oil pipeline from Flanagan, Illinois to Patoka, Illinois, as well as additional tankage and two pump stations. LOOP owns and operates midstream crude oil infrastructure, including a deep water oil port offshore of Louisiana, pipelines and onshore storage facilities. LOOP also manages the operations of LOCAP, an affiliate pipeline system. LOCAP owns and operates a crude oil pipeline and tank facility in St. James, Louisiana, that distributes oil received from LOOP’s storage facilities and other connecting pipelines to nearby refineries and into the mid-continent region of the United States. Explorer owns and operates an approximate 1,830 -mile common carrier pipeline that primarily transports gasoline, diesel, diluent and jet fuel from the Gulf Coast refining complex to the Midwest United States. The Partnership accounts for the Joint-Interest Acquisition entities as equity method investments within its L&S segment. As a transfer between entities under common control, the Partnership recorded the Joint-Interest Acquisition on its Consolidated Balance Sheets at MPC’s historical basis, which included accumulated other comprehensive loss. The Partnership recognizes an accumulated other comprehensive loss on its Consolidated Balance Sheets relating to pension and other post-retirement benefits provided by the LOOP and Explorer joint-interests to their employees. MPLX LP is not a sponsor of these benefit plans. Distributions of cash received from the entities and interests acquired in the Joint-Interest Acquisition related to periods prior to the acquisition will be prorated on a daily basis with MPLX LP retaining the portion of distributions beginning on the closing date. All amounts distributed to MPLX LP related to periods before the acquisition will be paid to MPC. Additionally, MPLX LP has agreed to pay MPC for any distributions of cash from LOOP related to the sale of LOOP’s excess crude oil inventory. Because the future distributions or payments cannot be reasonably quantified, a liability was not recorded in connection with the acquisition. MPLX LP subsequently received distributions related to the time period prior to the acquisition and recorded a liability to MPC and a corresponding decrease to the general partner’s equity for $32 million . The Partnership accounts for the interests acquired in the Joint-Interest Acquisition in arrears using the most recently available information. The amount of income (loss) associated with these investments included in the Consolidated Statements of Income under the caption “ Income from equity method investments ” for the three months ended March 31, 2018 totaled $37 million . The Partnership’s investment balance at March 31, 2018 related to the acquired interests is approximately $635 million and reported under the caption “Equity method investments” on the Consolidated Balance Sheets. MPC agreed to waive approximately two-thirds of the third quarter 2017 distributions on the common units issued in connection with the Joint-Interest Acquisition. As a result of this waiver, MPC did not receive approximately two-thirds of the distributions or IDRs that would have otherwise accrued on such common units with respect to the third quarter 2017 distributions. The value of these waived distributions was $10 million . Acquisition of Hardin Street Transportation LLC, Woodhaven Cavern LLC and MPLX Terminals LLC MPC contributed the assets of HST, WHC and MPLXT to newly created and wholly-owned subsidiaries and entered into commercial agreements related to services provided by these new entities to MPC on January 1, 2015 for HST and WHC and April 1, 2016 for MPLXT. Pursuant to a Membership Interests Contributions Agreement entered into on March 1, 2017 by the Partnership with MPLX GP, MPLX Logistics, MPLX Holdings and MPC Investment, each a wholly-owned subsidiary of MPC, MPC Investment agreed to contribute the outstanding membership interests in HST, WHC and MPLXT through a series of intercompany contributions to the Partnership for approximately $1.5 billion in cash and equity consideration valued at approximately $504 million (the “Transaction”). The number of common units representing the equity consideration was determined by dividing the contribution amount by the simple average of the ten day trailing volume weighted average NYSE price of a common unit for the ten trading days ending at market close on February 28, 2017. The fair value of the common and general partner units issued was approximately $503 million , and consisted of (i) 9,197,900 common units to MPLX GP, (ii) 2,630,427 common units to MPLX Logistics and (iii) 1,132,049 common units to MPLX Holdings. The Partnership also issued 264,497 general partner units to MPLX GP in order to maintain its two percent GP Interest in the Partnership. MPC agreed to waive two-thirds of the first quarter 2017 distributions on the common units issued in connection with the Transaction. As a result of this waiver, MPC did not receive two-thirds of the general partner distributions or IDRs that would have otherwise accrued on such common units with respect to the first quarter 2017 distributions. The value of these waived distributions was $6 million . HST owns and operates various private crude oil and refined product pipeline systems and associated storage tanks. As of the acquisition date, these pipeline systems consisted of 174 miles of crude oil pipelines and 430 miles of refined products pipelines. WHC owns and operates eight butane and propane storage caverns located in Michigan with approximately 1.8 million barrels of NGL storage capacity. As of the acquisition date, MPLXT owned and operated 59 terminals for the receipt, storage, blending, additization, handling and redelivery of refined petroleum products. Additionally, MPLXT operated one leased terminal and had partial ownership interest in two terminals. Collectively, these 62 terminals have a combined shell capacity of approximately 23.6 million barrels. The terminal facilities are located primarily in the Midwest, Gulf Coast and Southeast regions of the United States. The Partnership accounts for these businesses within its L&S segment. Acquisition of Ozark Pipeline On March 1, 2017, the Partnership acquired the Ozark pipeline from Enbridge Pipelines (Ozark) LLC for approximately $219 million , including purchase price adjustments made in the second quarter of 2017. Based on the final fair value estimates of assets acquired and liabilities assumed at the acquisition date, the purchase price was primarily allocated to property, plant and equipment. The Ozark pipeline is a 433 -mile, 22 -inch crude oil pipeline originating in Cushing, Oklahoma, and terminating in Wood River, Illinois, capable of transporting approximately 230 mbpd. The Partnership accounts for the Ozark pipeline within its L&S segment. MarEn Bakken On February 15, 2017, the Partnership closed on a joint venture, MarEn Bakken Company LLC (“MarEn Bakken”), with Enbridge Energy Partners LP in which MPLX LP acquired a partial, indirect interest in the Dakota Access Pipeline and Energy Transfer Crude Oil Company Pipeline projects, collectively referred to as the Bakken Pipeline system, from Energy Transfer Partners, L.P. and Sunoco Logistics Partners, LP. The Partnership contributed $500 million of the $2.0 billion purchase price paid by MarEn Bakken to acquire a 37 percent indirect interest in the Bakken Pipeline system. The Partnership holds, through a subsidiary, a 25 percent interest in MarEn Bakken, which equates to a 9 percent indirect interest in the Bakken Pipeline system. The Partnership accounts for its investment in MarEn Bakken as an equity method investment and bases the equity method accounting for this joint venture in arrears using the most recently available information. The amount of income (loss) associated with these investments included in the Consolidated Statements of Income under the caption “ Income from equity method investments ” for the three months ended March 31, 2018 totaled $7 million . The Partnership’s investment balance at March 31, 2018 is approximately $529 million and reported under the caption “Equity method investments” on the Consolidated Balance Sheets. In connection with the Partnership’s acquisition of a partial, indirect equity interest in the Bakken Pipeline system, MPC agreed to waive its right to receive incentive distributions of $1.6 million per quarter for twelve consecutive quarters, beginning with distributions declared in the first quarter of 2017 and paid to MPC in the second quarter of 2017, which was prorated to $0.8 million from the acquisition date. This waiver is no longer applicable as a result of the conversion of the GP Interest to a non-economic general partner interest as discussed in Note 8 .

Equity Method Investments

Equity Method Investments	3 Months Ended
	Mar. 31, 2018
Equity Method Investments and Joint Ventures [Abstract]
Investments and Noncontrolling Interests [Text Block]	Investments and Noncontrolling Interests The following table presents the Partnership’s equity method investments at the dates indicated: Ownership as of Carrying value at March 31, March 31, December 31, (In millions) 2018 2018 2017 Centrahoma Processing LLC 40% $ 121 $ 121 Explorer 25% 97 89 Illinois Extension 35% 288 284 LOCAP 59% 26 24 LOOP 41% 225 225 MarEn Bakken 25% 529 520 MarkWest EMG Jefferson Dry Gas Gathering Company, L.L.C. 67% 178 164 MarkWest Utica EMG, L.L.C. 56% 2,116 2,139 Ohio Condensate Company, L.L.C. 60% 11 11 Panola Pipeline Company, LLC 15% 23 24 Sherwood Midstream LLC 50% 260 236 Sherwood Midstream Holdings LLC 62% 151 165 Other 8 8 Total $ 4,033 $ 4,010 Summarized financial information for the Partnership’s equity method investments for the three months ended March 31, 2018 and 2017 is as follows: Three Months Ended March 31, 2018 (In millions) MarkWest Utica EMG, L.L.C. Other VIEs Non-VIEs Total Revenues and other income $ 63 $ 43 $ 297 $ 403 Costs and expenses 44 18 155 217 Income from operations 19 25 142 186 Net income 19 25 129 173 Income from equity method investments (1) 1 14 46 61 Three Months Ended March 31, 2017 (In millions) MarkWest Utica EMG, L.L.C. Other VIEs Non-VIEs Total Revenues and other income $ 50 $ 8 $ 45 $ 103 Costs and expenses 25 8 33 66 Income from operations 25 — 12 37 Net income 25 — 8 33 Income (loss) from equity method investments (1) 4 (1 ) 2 5 (1) “Income (loss) from equity method investments” includes the impact of any basis differential amortization or accretion. Summarized balance sheet information for the Partnership’s equity method investments as of March 31, 2018 and December 31, 2017 is as follows: March 31, 2018 (In millions) MarkWest Utica EMG, L.L.C. (1) Other VIEs Non-VIEs Total Current assets $ 71 $ 71 $ 363 $ 505 Noncurrent assets 2,035 1,002 4,696 7,733 Current liabilities 24 87 183 294 Noncurrent liabilities 4 11 870 885 December 31, 2017 (In millions) MarkWest Utica EMG, L.L.C. (1) Other VIEs Non-VIEs Total Current assets $ 65 $ 46 $ 399 $ 510 Noncurrent assets 2,077 930 4,624 7,631 Current liabilities 39 44 220 303 Noncurrent liabilities 3 11 904 918 (1) MarkWest Utica EMG, L.L.C.’s (“MarkWest Utica EMG”) noncurrent assets include its investment in its subsidiary, Ohio Gathering Company, L.L.C. (“Ohio Gathering”), which does not appear elsewhere in this table. The investment was $784 million and $790 million as of March 31, 2018 and December 31, 2017 , respectively. As of March 31, 2018 and December 31, 2017 , the carrying value of the Partnership’s equity method investments exceeded the underlying net assets of its investees by $1.0 billion . This basis difference is being amortized or accreted into net income over the remaining estimated useful lives of the underlying net assets, except for $459 million of excess related to goodwill. MarkWest Utica EMG Effective January 1, 2012, MarkWest Utica Operating Company, L.L.C. (“Utica Operating”), a wholly-owned and consolidated subsidiary of MarkWest, and EMG Utica, LLC (“EMG Utica” and together with Utica Operating, the “Members”) executed agreements to form a joint venture, MarkWest Utica EMG, to develop significant natural gas gathering, processing and NGL fractionation, transportation and marketing infrastructure in eastern Ohio. MarkWest Utica EMG is deemed to be a VIE. Utica Operating is not deemed to be the primary beneficiary, due to EMG Utica’s voting rights on significant matters. The Partnership’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. The Partnership 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, 2018 . 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, 2018 , the Partnership 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, the Partnership reports its portion of Ohio Gathering’s net assets as a component of its investment in MarkWest Utica EMG. Sherwood Midstream Effective January 1, 2017, MarkWest Liberty Midstream & Resources, L.L.C. (“MarkWest Liberty Midstream”), a wholly-owned and consolidated subsidiary of MarkWest, and Antero Midstream Partners, LP (“Antero Midstream”) formed a joint venture, Sherwood Midstream LLC (“Sherwood Midstream”), to support Antero Resources Corporation’s development in the Marcellus Shale. Pursuant to the terms of the related limited liability company agreement (the “LLC Agreement”), MarkWest Liberty Midstream contributed assets then under construction with a fair value of approximately $134 million and cash of approximately $20 million . Antero Midstream made an initial capital contribution of approximately $154 million . Also effective January 1, 2017, MarkWest Liberty Midstream converted all of its ownership interests in MarkWest Ohio Fractionation Company, L.L.C. (“Ohio Fractionation”), a previously wholly-owned subsidiary, to Class A Interests and amended its LLC Agreement to create Class B-3 Interests, which were sold to Sherwood Midstream for $126 million in cash. The Class B-3 Interests provide Sherwood Midstream with the right to fractionation revenue and the obligation to pay expenses related to 20 mbpd of capacity in the Hopedale 3 fractionator. Sherwood Midstream accounts for its investment in Ohio Fractionation, which is a VIE, as an equity method investment as Sherwood Midstream does not control Ohio Fractionation. MarkWest Liberty Midstream 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” in the Consolidated Statements of Income and “Noncontrolling interests” in the Consolidated Balance Sheets. Sherwood Midstream is deemed to be a VIE. MarkWest Liberty Midstream is not deemed to be the primary beneficiary, due to Antero Midstream’s voting rights on significant matters. The Partnership’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. The Partnership did not provide any financial support to Sherwood Midstream that it was not contractually obligated to provide during the three months ended March 31, 2018 . Sherwood Midstream Holdings Effective January 1, 2017, MarkWest Liberty Midstream and Sherwood Midstream formed a joint venture, Sherwood Midstream Holdings LLC (“Sherwood Midstream Holdings”), for the purpose of owning, operating and maintaining all of the shared assets that support the operations of the gas plants and other assets owned by Sherwood Midstream and the gas plants and deethanization facilities owned by MarkWest Liberty Midstream. MarkWest Liberty Midstream initially contributed certain real property, equipment and facilities with a fair value of approximately $209 million to Sherwood Midstream Holdings in exchange for a 79 percent initial ownership interest. Sherwood Midstream contributed cash of approximately $44 million to Sherwood Midstream Holdings in exchange for a 21 percent ownership interest. During the second quarter ended June 30, 2017, true-ups to the initial contributions were finalized. MarkWest Liberty Midstream contributed certain additional real property, equipment and facilities with a fair value of approximately $10 million to Sherwood Midstream Holdings and Sherwood Midstream contributed cash of approximately $4 million to Sherwood Midstream Holdings. The contribution was determined to be an in-substance sale of real estate. During the three months ended March 31, 2018 , MarkWest Liberty Midstream sold to Sherwood Midstream six percent of their equity ownership in Sherwood Midstream Holdings for $15 million . The Partnership accounts for 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. The Partnership’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. The Partnership 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, 2018 . 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, the Partnership also reports its portion of Sherwood Midstream Holdings’ net assets as a component of its investment in Sherwood Midstream. As of March 31, 2018 , the Partnership has an 18.9 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, 2018
Related Party Transactions [Abstract]
Related Party Agreements and Transactions	Related Party Agreements and Transactions The Partnership’s material related parties include: • MPC, which refines, markets and transports crude oil and petroleum products, primarily in the Midwest, Gulf Coast, East Coast and Southeast regions of the United States. • MarkWest Utica EMG, in which MPLX LP has a 56 percent interest as of March 31, 2018 . 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, 2018 . 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, 2018 . 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 an 81 percent total direct and indirect interest as of March 31, 2018 . 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 deethanization facilities. • MarkWest EMG Jefferson Dry Gas Gathering Company, LLC (“Jefferson Dry Gas”), in which MPLX LP has a 67 percent interest as of March 31, 2018 . Jefferson Dry Gas provides natural dry gas gathering and related services in the Utica Shale region of Ohio. Related Party Agreements The Partnership has various long-term, fee-based commercial agreements with MPC. Under these agreements, the Partnership provides transportation, terminal, fuels distribution, marketing and storage services to MPC. MPC has committed to provide the Partnership with minimum quarterly throughput volumes on crude oil and refined products systems and minimum storage volumes of crude oil, refined products and butane. In addition, the Partnership is party to a loan agreement with MPC Investment. Under the terms of the agreement, MPC Investment will make a loan or loans to the Partnership on a revolving basis as requested by the Partnership and as agreed to by MPC Investment, in an amount or amounts that do not result in the aggregate principal amount of all loans outstanding exceeding $500 million 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 . During the three months ended March 31, 2018 , the Partnership borrowed $452 million and repaid $838 million . At March 31, 2018 there was no outstanding balance for this loan on the Consolidated Balance Sheets. Borrowings were at an average interest rate of 3.073 percent , per annum, for the three months ended March 31, 2018 . During the year ended December 31, 2017 , the Partnership borrowed $2.4 billion and repaid $2.0 billion , resulting in $386 million outstanding balance at December 31, 2017 . Borrowings were at an average interest rate of 2.777 percent , per annum, for the year ended December 31, 2017 . For additional information regarding the Partnership’s commercial and other agreements with MPC, see Item 1. Business in the Annual Report on Form 10-K for the year ended December 31, 2017 . Refining Logistics and Fuels Distribution Agreements As discussed in Note 4 , the Partnership acquired Refining Logistics and Fuels Distribution on February 1, 2018 . Refining Logistics and Fuels Distribution, along with their subsidiaries, have various storage services agreements and a fuels distribution services agreement with MPC which were assumed by the Partnership with the closing of the transaction. The commercial agreements with MPC include: Fuels distribution services agreement – Fuels Distribution is a party to a services agreement with MPC in connection with the dropdown of the fuels distribution services. Under this agreement, Fuels Distribution provides services related to the scheduling and marketing of certain petroleum products to MPC. Fuels Distribution does not provide the same services to third parties without the prior written consent of MPC, and Fuels Distribution is MPC’s sole provider of these services. This agreement has an initial term of 10 years, subject to a five -year renewal period under terms to be renegotiated at that time. Under the Fuels Distribution Services Agreement, MPC pays Fuels Distribution a tiered monthly fee based on the volume of MPC’s products sold by Fuels Distribution each month, subject to a maximum annual volume. Fuels Distribution has agreed to use commercially reasonable efforts to sell not less than a minimum quarterly volume of MPC’s products during each calendar quarter. If Fuels Distribution sells less than the minimum quarterly volume of MPC’s products during any calendar quarter despite its commercially reasonable efforts, MPC will pay Fuels Distribution a deficiency payment equal to the volume deficiency multiplied by the applicable tiered fee. The dollar amount of actual sales volume of MPC’s products that exceeds the minimum quarterly volume (an “Excess Sale”) for a particular quarter will be applied as a credit, on a first-in-first-out basis, against any future deficiency payment owed by MPC to Fuels Distribution during the four calendar quarters immediately following the calendar quarter in which the Excess Sale occurs. Storage services agreements – Refining Logistics is party to multiple storage services agreements with each of MPC’s refineries in connection with the dropdown of the refining logistics assets. Under these agreements, the subsidiaries of Refining Logistics provide certain services exclusively 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. These agreements have initial terms of 10 years, subject to five -year renewal periods under terms to be renegotiated at that time. MPC pays Refining Logistics monthly fees for such storage and logistical services calculated as set forth in the agreements. The storage and logistical facilities subject to the agreements are to be allocated exclusively to MPC for the term of the agreement. Co-location services agreements – Refining Logistics is party to multiple co-location services agreements with each of MPC’s refineries in connection with the dropdown of the refining logistics assets. Under these agreements, MPC provides management, operational and other services to the subsidiaries of Refining Logistics. Refining Logistics pays MPC monthly fixed fees and direct reimbursements for such services calculated as set forth in the agreements. These agreements have initial terms of 50 years. Related Party Transactions Related party sales to MPC consisted of crude oil and refined products pipeline transportation services based on regulated tariff rates; storage, terminal, throughput 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. Revenue received from related parties related to service and product sales were as follows: Three Months Ended March 31, (In millions) 2018 2017 Service revenues MPC $ 471 $ 255 Rental income MPC $ 145 $ 67 Product sales (1) MPC $ 4 $ 2 (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, 2018 , these sales totaled $79 million . For the three months ended March 31, 2017 , these sales totaled $57 million . The Partnership has operating agreements with MPC under which it receives a fee for operating MPC’s retained pipeline assets. The Partnership receives management fee revenue for engineering and construction and administrative services for operating certain of its equity method investments, and is also reimbursed for personnel services. The Partnership has an agreement with MPC under which it receives a fixed annual fee for providing oversight and management services required to run the marine business. The revenue received from these related parties, included in “Other income - related parties” on the Consolidated Statements of Income, was as follows: Three Months Ended March 31, (In millions) 2018 2017 MPC $ 10 $ 11 MarkWest Utica EMG 4 4 Ohio Gathering 4 4 Jefferson Dry Gas 1 1 Sherwood Midstream 3 1 Other 1 1 Total $ 23 $ 22 MPC provides executive management services and certain general and administrative services to the Partnership under the terms of an omnibus agreement. Expenses incurred under this agreement are shown in the table below by the income statement line where they were recorded. Charges for services included in “Purchases - related parties” primarily relate to services that support the Partnership’s operations and maintenance activities, as well as compensation expenses. Charges for services included in “General and administrative expenses” primarily relate to services that support the Partnership’s executive management, accounting and human resources activities. These charges were as follows: Three Months Ended March 31, (In millions) 2018 2017 Purchases - related parties $ 36 $ 15 General and administrative expenses 16 8 Total $ 52 $ 23 Also under terms of the omnibus agreement, some service costs related to engineering services are associated with assets under construction. These costs added to “Property, plant and equipment, net” were as follows: Three Months Ended March 31, (In millions) 2018 2017 MPC $ 22 $ 10 MPLX LP obtains employee services from MPC under employee services agreements. Expenses incurred under these agreements are shown in the table below by the income statement line where they were recorded. The costs of personnel directly involved in or supporting operations and maintenance activities related to rental services are classified as “Rental cost of sales - related parties”. The costs of personnel directly involved in or supporting operations and maintenance activities related to other services are classified as “Purchases - related parties.” The costs of personnel involved in executive management, accounting and human resources activities are classified as “General and administrative expenses” in the Consolidated Statements of Income. These charges were as follows: Three Months Ended March 31, (In millions) 2018 2017 Rental cost of sales - related parties $ 1 $ — Purchases - related parties 114 92 General and administrative expenses 23 25 Total $ 138 $ 117 The following table shows other purchases from MPC classified as “Purchases - related parties”. These purchases include product purchases, payments made to MPC in their capacity as general contractor to MPLX LP, and certain rent and lease agreements. Three Months Ended March 31, (In millions) 2018 2017 MPC $ 27 $ — Receivables from related parties were as follows: (In millions) March 31, 2018 December 31, 2017 MPC $ 308 $ 153 MarkWest Utica EMG 1 1 Ohio Gathering 3 2 Jefferson Dry Gas 1 2 Sherwood Midstream Holdings 15 — Other 2 2 Total $ 330 $ 160 Long-term receivables with related parties, which includes straight-line rental income, were as follows: (In millions) March 31, 2018 December 31, 2017 MPC $ 21 $ 20 Payables to related parties were as follows: (In millions) March 31, 2018 December 31, 2017 MPC $ 115 $ 470 MarkWest Utica EMG 20 29 Ohio Gathering — 8 Sherwood Midstream 11 8 Other — 1 Total $ 146 $ 516 During the three months ended March 31, 2018 and the year ended December 31, 2017 , MPC did not ship its minimum committed volumes on certain pipeline systems. Under the Partnership’s pipeline transportation services agreements, if MPC fails to transport its minimum throughput volumes during any quarter, then MPC will pay the Partnership a deficiency payment equal to the volume of the deficiency multiplied by the tariff rate then in effect. The deficiency amounts are recorded as “Deferred revenue - 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 during the following four or eight quarters under the terms of the applicable transportation services agreement. The Partnership 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 “Deferred revenue - related parties.” In addition, capital projects the Partnership is undertaking at the request of MPC are reimbursed in cash and recognized in income over the remaining term of the applicable agreements. The “Deferred revenue - related parties” balance associated with the minimum volume deficiencies and project reimbursements were as follows: (In millions) March 31, 2018 December 31, 2017 Minimum volume deficiencies - MPC $ 46 $ 53 Project reimbursements - MPC 46 33 Total $ 92 $ 86

Net Income Per Limited Partner

Net Income Per Limited Partner Unit	3 Months Ended
	Mar. 31, 2018
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 the respective limited partners’ interest in net income (loss) attributable to MPLX LP by the weighted average number of common units outstanding. Because the Partnership has more than one class of participating securities, it uses the two-class method when calculating the net income (loss) per unit applicable to limited partners. The classes of participating securities include common units, Series A Convertible Preferred units (the "Preferred units") and certain equity-based compensation awards; and in prior periods, general partner units and IDRs. For the three months ended March 31, 2018 , the Partnership had dilutive potential common units consisting of certain equity-based compensation awards. For the three months ended March 31, 2017 , the Partnership had dilutive potential common units consisting of certain equity-based compensation awards and Class B units. Potential common units omitted from the diluted earnings per unit calculation for the three months ended March 31, 2018 and March 31, 2017 were less than 1 million . Three Months Ended March 31, (In millions) 2018 2017 Net income attributable to MPLX LP $ 421 $ 150 Less: Limited partners’ distributions declared on Preferred units (1) 16 16 General partner’s distributions declared (including IDRs) (1) — 65 Limited partners’ distributions declared on common units (including common units of general partner) (1) 467 198 Undistributed net loss attributable to MPLX LP $ (62 ) $ (129 ) (1) See Note 8 for distribution information. 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 $ 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 Three Months Ended March 31, 2017 (In millions, except per unit data) General Partner 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) $ 65 $ 198 $ 16 $ 279 Undistributed net loss attributable to MPLX LP (3 ) (126 ) — (129 ) Net income attributable to MPLX LP (1) $ 62 $ 72 $ 16 $ 150 Weighted average units outstanding: Basic 7 362 31 400 Diluted 7 367 31 405 Net income attributable to MPLX LP per limited partner unit: Basic $ 0.20 Diluted $ 0.19 (1) Allocation of net income (loss) 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, 2018
Equity [Abstract]
Equity	Equity The changes in the number of units outstanding during the three months ended March 31, 2018 are summarized below: (In units) Common General Partner Total Balance at December 31, 2017 407,130,020 8,308,773 415,438,793 Unit-based compensation awards (1) 150,257 140 150,397 Contribution of refining logistics and fuels distribution assets (2) 111,611,111 2,277,778 113,888,889 Conversion of GP economic interests 275,000,000 (10,586,691 ) 264,413,309 Balance at March 31, 2018 793,891,388 — 793,891,388 (1) As a result of the unit-based compensation awards issued during the period, MPLX GP contributed less than $1 million in exchange for 140 general partner units to maintain its two percent GP Interest. (2) 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 will not receive $23.7 million of the distributions that would have otherwise accrued on such common units with respect to the first quarter 2018 . See Note 4 for information regarding this acquisition. GP/IDR Exchange – On February 1, 2018 , MPC cancelled its IDRs and converted its economic GP Interest in MPLX LP to a non-economic general partner interest, in exchange for 275 million newly issued MPLX LP common units. These units had a fair value of $10.4 billion as of the transaction date as recorded on the Consolidated Statements of Equity. As a result of this transaction, the general partner units and IDRs were eliminated, are no longer outstanding and no longer participate in distributions of cash from the Partnership. MPC continues to own the non-economic GP Interest in MPLX LP. See Note 7 for more information on the net income per unit calculation. Net Income Allocation – In preparing the Consolidated Statements of Equity, net income (loss) attributable to MPLX LP is allocated to preferred unitholders based on a fixed distribution schedule, as discussed in Note 9 , and subsequently allocated to the limited partner unitholders in accordance with their respective ownership percentages. Prior to 2018, when distributions related to the IDRs were made, earnings equal to the amount of those distributions were first allocated to the general partner before the remaining earnings were allocated to the unitholders, based on their respective ownership percentages. The following table presents the allocation of the general partner’s GP Interest in net income attributable to MPLX LP, for income statement periods occurring prior to the exchange of the GP economic interests: Three Months Ended March 31, (In millions) 2017 Net income attributable to MPLX LP $ 150 Less: Preferred unit distributions 16 General partner's IDRs and other 61 Net income attributable to MPLX LP available to general and limited partners $ 73 General partner's two percent GP Interest in net income attributable to MPLX LP $ 1 General partner's IDRs and other 61 General partner's GP Interest in net income attributable to MPLX LP $ 62 Cash distributions – The Partnership Agreement sets forth the calculation to be used to determine the amount and priority of cash distributions that the common unitholders and preferred unitholders will receive. In accordance with the Partnership Agreement, on April 25, 2018 , the Partnership declared a quarterly cash distribution, based on the results of the first quarter of 2018 , totaling $467 million , or $0.6175 per common unit. These distributions will be paid on May 15, 2018 to common unitholders of record on May 7, 2018 . The allocation of total quarterly cash distributions to general, limited and preferred unitholders is as follows for the three months ended March 31, 2018 and 2017 . The Partnership’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) 2018 2017 General partner's distributions: General partner's distributions on general partner units $ — $ 5 General partner's distributions on IDRs — 60 Total distribution on general partner units and IDRs $ — $ 65 Common and preferred unit distributions: Common unitholders, includes common units of general partner $ 467 $ 198 Preferred unit distributions 16 16 Total cash distributions declared $ 483 $ 279

Redeemable Preferred Units

Redeemable Preferred Units	3 Months Ended
	Mar. 31, 2018
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 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 partnership purposes. The Preferred units rank senior to all common units with respect to distributions and rights upon liquidation. The holders of the Preferred units are entitled to receive cumulative quarterly distributions equal to $0.528125 per unit. Following the second anniversary of the issuance of the Preferred units, the holders of the Preferred units will receive as a distribution the greater of $0.528125 per unit or the amount of per unit distributions paid to holders of MPLX LP common units. The changes in the redeemable preferred balance from December 31, 2017 through March 31, 2018 are summarized below: (In millions) Redeemable Preferred Units Balance at December 31, 2017 $ 1,000 Net income 16 Distributions received by preferred unitholders (16 ) Balance at March 31, 2018 $ 1,000 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, the Partnership may convert the Preferred units into common units at any time, in whole or in part, subject to certain minimum conversion amounts and conditions, if the closing price of MPLX LP common units is greater than $48.75 for the 20 day trading period immediately preceding the conversion notice date. The conversion rate for the Preferred units shall be the quotient of (a) the sum of (i) $32.50, plus (ii) any unpaid cash distributions on the applicable Preferred unit, divided by (b) $32.50. The holders of the Preferred units are entitled to vote on an as-converted basis with the common unitholders and will have certain other class voting rights with respect to any amendment to the Partnership Agreement that would adversely affect any rights, preferences or privileges of the Preferred units. In addition, upon certain events involving a change 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 the Partnership’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 decreased 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, 2018
Segment Reporting [Abstract]
Segment Reporting Disclosure [Text Block]	Segment Information The Partnership’s chief operating decision maker is the chief executive officer (“CEO”) of its general partner. The CEO reviews the Partnership’s discrete financial information, makes operating decisions, assesses financial performance and allocates resources on a type of service basis. The Partnership has two reportable segments: L&S and G&P. Each of these segments are 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. Segment information is not included for periods prior to the Joint-Interest Acquisition and the Ozark pipeline acquisitions. See Note 4 for more detail of these acquisitions. • G&P – gathers, processes and transports natural gas; gathers, transports, fractionates, stores and markets NGLs. The Partnership has investments in entities that are accounted for using the equity method of accounting (see Note 5 ). However, the CEO only views the Partnership-operated equity method investments’ financial information as if those investments were consolidated, in contrast to the non-operated equity method investments. Segment operating income represents income from operations attributable to the reportable segments. Corporate general and administrative expenses, unrealized derivative gains (losses), certain management fees and depreciation and amortization are not allocated to the reportable segments. Management does not consider these items allocable to or controllable by any individual segment and, therefore, excludes these items when evaluating segment performance. Segment results are also adjusted to exclude the portion of income from operations attributable to the noncontrolling interests related to partially-owned entities that are either consolidated or accounted for as equity method investments. Segment operating income attributable to MPLX LP excludes the operating income related to Predecessors of the HST, WHC and MPLXT businesses prior to the dates they were acquired by MPLX LP. The tables below present information about income from operations and capital expenditures for the reported segments: Three Months Ended March 31, 2018 (In millions) L&S G&P Total Revenues and other income: Segment revenues $ 646 $ 569 $ 1,215 Product related revenues — 249 249 Segment other income 12 6 18 Total segment revenues and other income 658 824 1,482 Costs and expenses: Segment cost of revenues 234 235 469 Purchased product costs — 194 194 Segment operating income before portion attributable to noncontrolling interests and Predecessor 424 395 819 Segment portion attributable to noncontrolling interests and Predecessor — 45 45 Segment operating income attributable to MPLX LP $ 424 $ 350 $ 774 Three Months Ended March 31, 2017 (In millions) L&S G&P Total Revenues and other income: Segment revenues $ 345 $ 401 $ 746 Product related revenues — 196 196 Segment other income 12 1 13 Total segment revenues and other income 357 598 955 Costs and expenses: Segment cost of revenues 148 113 261 Purchased product costs — 140 140 Segment operating income before portion attributable to noncontrolling interests and Predecessor 209 345 554 Segment portion attributable to noncontrolling interests and Predecessor 53 36 89 Segment operating income attributable to MPLX LP $ 156 $ 309 $ 465 Three Months Ended March 31, (In millions) 2018 2017 Reconciliation to Income from operations: Segment operating income attributable to MPLX LP $ 774 $ 465 Segment portion attributable to unconsolidated affiliates (53 ) (40 ) Segment portion attributable to Predecessor — 53 Income from equity method investments 61 5 Other income - related parties 13 11 Unrealized derivative gains (1) 7 16 Depreciation and amortization (176 ) (187 ) General and administrative expenses (69 ) (58 ) Income from operations $ 557 $ 265 Three Months Ended March 31, (In millions) 2018 2017 Reconciliation to Total revenues and other income: Total segment revenues and other income $ 1,482 $ 955 Revenue adjustment from unconsolidated affiliates (137 ) (92 ) Income from equity method investments 61 5 Other income - related parties 13 11 Unrealized derivative gains related to product sales (1) 1 7 Total revenues and other income $ 1,420 $ 886 (1) The Partnership makes a distinction between realized or 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. Three Months Ended March 31, (In millions) 2018 2017 Reconciliation to Net income attributable to noncontrolling interests and Predecessor: Segment portion attributable to noncontrolling interests and Predecessor $ 45 $ 89 Portion of noncontrolling interests and Predecessor related to items below segment income from operations (19 ) (36 ) Portion of operating income attributable to noncontrolling interests of unconsolidated affiliates (24 ) (16 ) Net income attributable to noncontrolling interests and Predecessor $ 2 $ 37 The following table reconciles segment capital expenditures to total capital expenditures: Three Months Ended March 31, (In millions) 2018 2017 L&S segment capital expenditures $ 190 $ 97 G&P segment capital expenditures 319 307 Total segment capital expenditures 509 404 Less: Capital expenditures for Partnership-operated, non-wholly-owned subsidiaries in G&P segment 54 124 Total capital expenditures $ 455 $ 280 Total assets by reportable segment were: (In millions) March 31, 2018 December 31, 2017 Cash and cash equivalents $ 2 $ 5 L&S 5,958 4,611 G&P 15,046 14,884 Total assets $ 21,006 $ 19,500 Equity method investments included in L&S assets were $1.17 billion at March 31, 2018 and $1.15 billion at December 31, 2017 . Equity method investments included in G&P assets were $2.86 billion at March 31, 2018 and December 31, 2017 , respectively.

Inventories

Inventories	3 Months Ended
	Mar. 31, 2018
Inventory Disclosure [Abstract]
Inventories	Inventories Inventories consist of the following: (In millions) March 31, 2018 December 31, 2017 NGLs $ 2 $ 4 Line fill 7 8 Spare parts, materials and supplies 55 53 Total inventories $ 64 $ 65

Property, Plant and Equipment

Property, Plant and Equipment	3 Months Ended
	Mar. 31, 2018
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, 2018 December 31, 2017 Natural gas gathering and NGL transportation pipelines and facilities $ 5,253 $ 5,178 Processing, fractionation and storage facilities 4,729 3,893 Pipelines and related assets 2,388 2,253 Barges and towing vessels 553 490 Terminals and related assets 827 821 Refinery and related assets 839 — Land, building, office equipment and other 873 770 Construction-in-progress 992 1,057 Total 16,454 14,462 Less accumulated depreciation 3,163 2,275 Property, plant and equipment, net $ 13,291 $ 12,187

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2018
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements Fair Values – Recurring Fair value measurements and disclosures relate primarily to the Partnership’s derivative positions as discussed in Note 14 . The following table presents the financial instruments carried at fair value on a recurring basis as of March 31, 2018 and December 31, 2017 by fair value hierarchy level. The Partnership has elected to offset the fair value amounts recognized for multiple derivative contracts executed with the same counterparty. March 31, 2018 December 31, 2017 (In millions) Assets Liabilities Assets Liabilities Significant unobservable inputs (Level 3) Commodity contracts $ — $ (1 ) $ — $ (2 ) Embedded derivatives in commodity contracts — (59 ) — (64 ) Total carrying value in Consolidated Balance Sheets $ — $ (60 ) $ — $ (66 ) 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 $0.26 to $1.47 and (2) the probability of renewal of 62.50 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. For these contracts, increases in forward NGL prices result in a decrease in the fair value of the derivative assets and an increase in the fair value of derivative liabilities. The forward prices for NGL products generally increase or decrease in positive correlation with one another. Increases or decreases in forward NGL prices result in an increase or decrease in the fair value of the embedded derivative. Increases or decreases in the frac spread result in an increase or decrease in the fair value of the embedded derivative liability. An increase in the probability of renewal would result in an increase in the fair value of the related embedded derivative liability. 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, 2018 Three Months Ended March 31, 2017 (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 $ (2 ) $ (64 ) $ (6 ) $ (54 ) Total gains (losses) (realized and unrealized) included in earnings (1) — 3 5 8 Settlements — 3 1 2 Fair value at end of period $ (2 ) $ (58 ) $ — $ (44 ) 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 $ — $ 3 $ 5 $ 8 (1) Gains and losses on Commodity Derivative Contracts classified as Level 3 are recorded in “Product sales” in the accompanying Consolidated Statements of Income. Gains and losses derivatives embedded in commodity contracts are recorded in “Purchased product costs” and “Cost of revenues.” Fair Values – Reported The Partnership’s primary financial instruments are cash and cash equivalents, receivables, receivables from related parties, accounts payable, payables to related parties and long-term debt. The Partnership’s fair value assessment incorporates a variety of considerations, including (1) the short-term 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. The Partnership 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 14 ). The fair value of the Partnership’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 the Partnership’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 capital leases, and SMR liability: March 31, 2018 December 31, 2017 (In millions) Fair Value Carrying Value Fair Value Carrying Value Long-term debt $ 12,442 $ 11,934 $ 7,718 $ 6,966 SMR liability 100 90 104 91

Derivative Financial Instrument

Derivative Financial Instruments	3 Months Ended
	Mar. 31, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Instruments and Hedging Activities Disclosure [Text Block]	Derivative Financial Instruments As of March 31, 2018 , the Partnership had the following outstanding commodity contracts that were executed to manage the cash flow risk associated with future sales of NGLs and purchases of natural gas: Derivative contracts not designated as hedging instruments Financial Position Notional Quantity (net) Natural Gas (MMBtu) Long 745,045 NGLs (Gal) Short 7,696,503 Embedded Derivative - The Partnership 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, these natural gas purchase commitment and 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 in the Consolidated Statements of Income. As of March 31, 2018 and December 31, 2017 , the estimated fair value of this contract was a liability of $ 59 million and $ 64 million , respectively. Certain derivative positions are subject to master netting agreements; therefore, the Partnership has elected to offset derivative assets and liabilities that are legally permissible to be offset. As of March 31, 2018 and December 31, 2017 , there were no derivative assets or liabilities that were offset in the Consolidated Balance Sheets. The impact of the Partnership’s derivative instruments on its Consolidated Balance Sheets is summarized below: (In millions) March 31, 2018 December 31, 2017 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 $ — $ (12 ) $ — $ (14 ) Other noncurrent assets / deferred credits and other liabilities — (48 ) — (52 ) Total $ — $ (60 ) $ — $ (66 ) (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 13 . There were no material changes to the Partnership’s policy regarding the accounting for Level 2 and Level 3 instruments as previously disclosed in the Partnership’s Annual Report on Form 10-K for the year ended December 31, 2017 . The Partnership does not designate any of its commodity derivative positions as hedges for accounting purposes. The impact of the Partnership’s derivative contracts not designated as hedging instruments and the location of (loss) or gain recognized in the Consolidated Statements of Income is summarized below: Three Months Ended March 31, (In millions) 2018 2017 Product sales Realized loss $ — $ (1 ) Unrealized gain 1 7 Total derivative gain related to product sales 1 6 Purchased product costs Realized loss (3 ) (2 ) Unrealized gain 6 9 Total derivative gain related to purchased product costs 3 7 Cost of revenues Realized (loss) gain — — Unrealized (loss) gain — — Total derivative (loss) gain related to cost of revenues — — Total derivative gains $ 4 $ 13

Debt

Debt	3 Months Ended
	Mar. 31, 2018
Debt Disclosure [Abstract]
Debt	Debt The Partnership’s outstanding borrowings consisted of the following: (In millions) March 31, 2018 December 31, 2017 MPLX LP: Bank revolving credit facility due 2022 $ — $ 505 5.500% senior notes due February 2023 710 710 3.375% senior notes due March 2023 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 — 4.500% senior notes due April 2038 1,750 — 5.200% senior notes due March 2047 1,000 1,000 4.700% senior notes due April 2048 1,500 — 4.900% senior notes due April 2058 500 — Consolidated subsidiaries: MarkWest - 4.500% - 5.500% senior notes, due 2023-2025 63 63 MPL - capital lease obligations due 2020 7 7 Total 12,357 7,362 Unamortized debt issuance costs (79 ) (27 ) Unamortized discount (416 ) (389 ) Amounts due within one year (1 ) (1 ) Total long-term debt due after one year $ 11,861 $ 6,945 Credit Agreements On July 21, 2017, the Partnership entered into a syndicated credit agreement to replace its previously outstanding $2.0 billion five -year bank revolving credit facility with a $2.25 billion five -year bank revolving credit facility that expires in July 2022 (the “MPLX Credit Agreement”). The financial covenants and the interest rate terms contained in the new credit agreement are substantially the same as those contained in the previous bank revolving credit facility. During the three months ended March 31, 2018 , the Partnership borrowed $50 million under the MPLX Credit Agreement, at an average interest rate of 2.975 percent and repaid $555 million . At March 31, 2018 , the Partnership had no outstanding borrowings and $3 million letters of credit outstanding under the new facility, resulting in total availability of $2.247 billion , or 99.9 percent of the borrowing capacity . On January 2, 2018 , the Partnership entered into a term loan agreement with a syndicate of lenders providing for a $ 4.1 billion , 364-day term loan facility. The Partnership drew the entire amount of the term loan facility in a single borrowing on February 1, 2018 with the entire amount then being repaid on February 8, 2018 as described below. The proceeds from the term loan facility were used to fund the cash portion of the dropdown consideration. Senior Notes On February 8, 2018 , the Partnership issued $5.5 billion 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 “2018 New Senior Notes”). The 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 the 364-day term loan facility, which was drawn on February 1, 2018 to fund the cash portion of the dropdown consideration. The remaining proceeds were used to repay outstanding borrowings under the MPLX Credit Agreement and the intercompany loan agreement with MPC Investment, as well as for general partnership purposes. On February 10, 2017, the Partnership completed a public offering of $2.25 billion aggregate principal amount of unsecured senior notes, consisting of (i) $1.25 billion aggregate principal amount of 4.125 percent senior notes due in March 2027 and (ii) $1.0 billion aggregate principal amount of 5.200 percent senior notes due in March 2047 (collectively, the “2017 New Senior Notes”). The net proceeds from the New Senior Notes totaled approximately $2.22 billion , after deducting underwriting discounts, and were used for general partnership purposes and capital expenditures. Interest on each series of the notes is payable semi-annually in arrears on March 1 and September 1, commencing on September 1, 2017.

Revenue

Revenue	3 Months Ended
	Mar. 31, 2018
Revenue from Contract with Customer [Abstract]
Revenue from Contract with Customer [Text Block]	Revenue Effect of ASC 606 Adoption The Partnership adopted ASC 606 on January 1, 2018 for all contracts that were not yet completed as of the date of adoption. The details of significant changes and quantitative impact of the new revenue standard are disclosed below. • Third-party reimbursements – Third-party reimbursements, such as electricity costs, are presented gross on the income statement rather than net within cost of revenues. The gross-up for third-party reimbursements (e.g., increase in revenue; increase in cost of revenues) was $78 million for the period ending March 31, 2018 . The Partnership updated the allocation between lease and non-lease components for implicit leases as a result of this ASC 606 gross up. As a result, “Rental income” and “Rental cost of sales” increased by $16 million for the period ending March 31, 2018 . • Noncash consideration – Under certain processing agreements, the Partnership is entitled to retain NGLs or other liquids from the customer. We obtain control of these NGLs and are able to direct the use of the goods. Service revenues are recorded based on the value of the NGLs received on the date the services are performed. Historically, revenue was not recorded on these arrangements until the product was sold. The impact to this change was an increase of $11 million to “Service revenue - product related” for the period ending March 31, 2018 . NGL inventory related to keep-whole volumes was also revalued as a result of this change, with a cumulative adjustment of $1 million . Historically, revenue was not recorded on product received until it was sold and further, the increase in the inventory basis increased “Purchased product costs” by $12 million for the period ending March 31, 2018 . • Percent-of-proceeds revenues – The Partnership’s percentage of proceeds revenue received was historically recorded in product revenues. Upon adoption of ASC 606, these revenues have been classified in service revenue, as the performance obligation related to these contracts is to provide gathering and processing services. Revenues will continue to be recorded net under these arrangements as the Partnership does not control the product prior to sale. For the period ending March 31, 2018 , $33 million has now been recorded in “Service revenue - product related” as opposed to “Product sales.” • Imbalances – Historically, all imbalances were recorded net. In certain instances, the Partnership’s arrangements are structured such that imbalances are cashed-out each period end which results in the transfer of control of a commodity and creates a purchase and/or sale of a commodity under ASC 606. Thus, certain imbalances will be grossed up as a result of adoption. The impact of this change was an increase of $12 million to “Product sales” and “Purchased product costs” for the period ending March 31, 2018 . • Aid in construction – Historically, all aid in construction amounts received were deferred and recognized into revenue. Payments received from non-customers will no longer be deferred as the accounting will not be subject to ASC 606. Such payments will be recorded as a reduction to plant, property and equipment. The cumulative adjustment wrote down $3 million of net plant, property and equipment. • Oil Allowances – Historically, oil allowances were recorded when received as consideration for services performed. Under ASC 606, the Partnership does not believe such amounts represent consideration from a customer. Any excess product obtained and sold as a result of these allowances is recorded as product sale. This change decreased “Service revenues” and “Service revenues - related party” by $1 million for the period ending March 31, 2018 and increased Product sales and Product sales related party by $1 million . The cumulative effect of the changes made to our consolidated January 1, 2018 balance sheet for the adoption of ASC 606 was as follows: (In millions) Balance at December 31, 2017 ASC 606 Adjustment Total Assets Inventories $ 65 $ 1 $ 66 Property, plant and equipment, net 12,187 (3 ) 12,184 Liabilities Long-term deferred revenue 42 (3 ) 39 Equity Common unitholders - public $ 8,379 $ 1 $ 8,380 Aside from the adjustments to the opening balances noted above, the impact of adoption on our Consolidated Balance Sheets for the period ended March 31, 2018 was immaterial. The disclosure of the impact of adoption on our Consolidated Statements of Income for the period ended March 31, 2018 was as follows: Three Months Ended March 31, 2018 (In millions) ASC 606 Balance ASC 605 Balance Effect of Change Higher/ (Lower) Revenues and other income: Service revenue $ 382 $ 305 $ 77 Service revenue - related parties 471 472 (1 ) Service revenue - product related 44 — 44 Rental income 79 63 16 Product sales (1) 207 228 (21 ) Product sales - related parties 4 3 1 Costs and expenses: Cost of revenues (2) 206 128 78 Rental cost of sales 29 13 16 Purchased product costs 187 163 24 Depreciation and amortization 176 176 — Net income $ 423 $ 425 $ (2 ) (1) G&P “Product sales” exclude approximately $1 million of revenue related to derivative gains/losses and mark-to-market adjustments. (2) Excludes “Purchased product costs,” “Rental cost of sales,” “Purchases,” “Depreciation and amortization,” “General and administrative expenses,” and “Other taxes.” Disaggregation of Revenue The following table represents a disaggregation of revenue for each reportable segment for the period ended March 31, 2018 : 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 Revenue adjustment for unconsolidated affiliates 137 (Income) from equity method investments (61 ) Other income - related parties (13 ) Unrealized derivative (gain) related to product sales (1 ) Total segment revenues and other income $ 1,482 (1) G&P “Product sales” exclude approximately $1 million of revenue related to derivative gains/losses and mark-to-market adjustments. (2) Non-ASC 606 Revenue includes rental income, income from equity method investments, derivative gains/losses, mark-to-market adjustments, and other income. Contract Balances Contract assets typically relate to aid in construction agreements where the revenue recognized and the Partnership’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” in our 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. 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 the Partnership prior to sale while a significant portion of the balance relates to the sale of commodity product on behalf of our producer customer. The sales and related “Receivables, net” are commingled and excluded from the table below. The Partnership 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, 2018 : (In millions) Balance at January 1, 2018 (1) Additions/ (Deletions) Revenue Recognized (2) Balance at March 31, 2018 Contract assets $ 4 $ — $ — $ 4 Deferred revenue 5 2 (2 ) 5 Deferred revenue - related parties 42 9 (9 ) 42 Long-term deferred revenue 7 1 — 8 Long-term deferred revenue - related parties 39 6 — 45 (1) Balance represents ASC 606 portion of each respective line items. (2) No revenue was recognized related to past performance obligations, in the current period. Changes in long-term amounts represent reclassifications to current balances. 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, 2018 , the amounts allocated to contract assets and contract liabilities on the Consolidated Balance Sheets are $96 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 19 years . The Company does not disclose information about remaining performance obligations that have original expected durations of one year or less. Further, the Company does not disclose variable consideration due to volume variability in the table below. (In millions) Logistics & Storage Services Gathering & Processing Services (3) 2018 $ 704 $ 91 2019 929 118 2020 929 114 2021 929 113 2022 and thereafter 5,577 404 Total revenue on remaining performance obligations (1),(2) $ 9,068 $ 840 (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. The Partnership 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. Practical Expedients 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, 2018
Supplemental Cash Flow Elements [Abstract]
Supplemental Cash Flow Information [Text Block]	Supplemental Cash Flow Information (In millions) March 31, 2018 December 31, 2017 Cash and cash equivalents $ 2 $ 5 Restricted cash (1) 4 4 Cash, cash equivalents and restricted cash (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, 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) 2018 2017 Net cash provided by operating activities included: Interest paid (net of amounts capitalized) $ 103 $ 49 Non-cash investing and financing activities: Net transfers of property, plant and equipment from materials and supplies inventories 1 6 Contribution of fixed assets to joint venture (1) $ — $ 328 (1) Contribution of assets to Sherwood Midstream and Sherwood Midstream Holdings. See Note 5 . 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) 2018 2017 (Decrease) increase in capital accruals $ (6 ) $ 2

Accumulated Other Comprehensive

Accumulated Other Comprehensive Income	3 Months Ended
	Mar. 31, 2018
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 its Consolidated Balance Sheets relating to pension and other post-retirement benefits provided by the LOOP and Explorer joint-interests to their employees. MPLX LP is not a sponsor of these benefit plans. As a transfer between entities under common control, the Partnership recorded the Joint-Interest Acquisition from MPC on its Consolidated Balance Sheets at MPC’s historical basis, which included accumulated other comprehensive loss. MPLX LP’s assumption of the accumulated other comprehensive loss balance had no effect on MPLX LP’s comprehensive income during the period as the balance was accumulated while under the ownership of MPC. 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 as of December 1, 2017 (1) $ (13 ) $ (1 ) $ (14 ) Other comprehensive loss - remeasurements (2) (1 ) (1 ) (2 ) Balance as of March 31, 2018 (1) $ (14 ) $ (2 ) $ (16 ) (1) These accumulated other comprehensive loss components are included in the computation of net periodic benefit cost by LOOP and Explorer and are therefore included in the Consolidated Statements of Income under the caption “Income (loss) from equity method investments.” (2) Components of “Other comprehensive loss - remeasurements” relate to actuarial gains and losses as well as amortization of prior service costs. The Partnership 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, 2018
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, 2018 : Number Weighted Outstanding at December 31, 2017 1,351,523 $ 34.53 Granted 169,310 35.00 Settled (217,093 ) 33.83 Forfeited (46,330 ) 34.95 Outstanding at March 31, 2018 1,257,410 34.70 Performance Units – The Partnership grants performance units under the MPLX LP 2012 Incentive Compensation Plan 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. During the three months ended March 31, 2018 , an 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 awards fair value will be recognized over the remaining service 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, 2018 : Number of Outstanding at December 31, 2017 2,536,594 Granted — Settled (538,594 ) Forfeited (50,000 ) Outstanding at March 31, 2018 1,948,000

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies Disclosure [Text Block]	Commitments and Contingencies The Partnership 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 the Partnership has not recorded an accrued liability, the Partnership 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 – The Partnership 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, 2018 and December 31, 2017 , accrued liabilities for remediation totaled $15 million and $13 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 December 31, 2017 , there was less than $1 million in payables to MPC for indemnification of environmental costs related to incidents occurring prior to the Initial Offering. At March 31, 2018 there was $3 million in receivables from MPC for these costs. MarkWest Liberty Midstream and its affiliates agreed in principle to pay a cash penalty of approximately $0.6 million and to undertake certain supplemental environmental projects with an estimated cost of approximately $2.4 million , related to civil enforcement allegations associated with permitting and other regulatory obligations for launcher/receiver and compressor station facilities in southeastern Ohio and western Pennsylvania. On April 24, 2018, MarkWest Liberty Midstream and its affiliates entered into a Consent Decree with the EPA and the Pennsylvania Department of Environmental Protection resolving these issues, subject to a 30-day public comment period, pursuant to which MarkWest Liberty Midstream will pay a penalty of $0.6 million and undertake certain supplemental environmental projects with an estimated cost of approximately $2.4 million , in addition to other related projects that are substantially complete. The Partnership is involved in a number of other 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 the Partnership cannot be predicted with certainty, and the Partnership is not able to estimate a reasonably possible loss (or range of loss), if any, for these matters, the Partnership 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 has objected to this ruling and is seeking an appeal. There are several third-party defendants in the litigation and MPL has 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 State’s case against Premcor is currently scheduled to commence trial on June 25, 2018, and Premcor’s claims against third-party defendants, including MPL, is currently scheduled to commence August 13, 2018. 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 the Partnership is unable to estimate a reasonably possible loss (or range of loss) for this litigation. Under the omnibus agreement, MPC will indemnify the Partnership for the full cost of any losses should MPL be deemed responsible for any damages in this lawsuit. The Partnership 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 the Partnership cannot be predicted with certainty, the Partnership 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, the Partnership 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 the Partnership to perform upon the occurrence of a triggering event or condition. These guarantees and indemnifications are part of the normal course of selling assets. The Partnership 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. Contractual Commitments and Contingencies – At March 31, 2018 , the Partnership’s contractual commitments to acquire property, plant and equipment totaled $718 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, the Partnership and its affiliates provide guarantees of the Partnership’s subsidiaries payment and performance obligations in the G&P segment. Certain natural gas processing and gathering arrangements require the Partnership 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, 2018 , management does not believe there are any indications that the Partnership 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 Events	3 Months Ended
	Mar. 31, 2018
Subsequent Event [Line Items]
Subsequent Events [Text Block]	Subsequent Events On December 4, 2015 , the Partnership entered into a loan agreement with MPC Investment (the “MPC Loan Agreement”). Under the terms of the MPC Loan Agreement, MPC Investment may make loans to the Partnership on a revolving basis, as requested by the Partnership and agreed to by MPC Investment, up to $500 million at any time outstanding. On April 27, 2018, the Partnership and MPC Investment entered into a First Amendment to the MPC Loan Agreement (the “First Amendment”) to increase the borrowing capacity under the MPC Loan Agreement from $500 million to $1 billion at any time outstanding.

Summary of Principal Accounti30

Summary of Principal Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2018
Accounting Policies [Abstract]
Use of Estimates	The accompanying interim consolidated financial statements are unaudited; however, in the opinion of the Partnership’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.
Earnings Per Share	In preparing the Consolidated Statements of Equity, net income attributable to MPLX LP is allocated to preferred unitholders based on a fixed distribution schedule, as discussed in Note 9 . Prior to 2018 , when distributions related to the IDRs were made, earnings equal to the amount of those distributions were first allocated to the general partner before the remaining earnings were allocated to the limited partner unitholders based on their respective ownership percentages. Subsequent to the conversion of the general partner to a non-economic interest as described in Note 8, no earnings will be allocated to the general partner. 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 7 . Net income (loss) per unit applicable to common limited partner units is computed by dividing the respective limited partners’ interest in net income (loss) attributable to MPLX LP by the weighted average number of common units outstanding. Because the Partnership has more than one class of participating securities, it uses the two-class method when calculating the net income (loss) per unit applicable to limited partners. The classes of participating securities include common units, Series A Convertible Preferred units (the "Preferred units") and certain equity-based compensation awards; and in prior periods, general partner units and IDRs.
Revenue Recognition	Revenue Recognition – As a result of the adoption of the new revenue recognition standard, as described further in Note 3 , the Partnership has updated its policies as it relates to revenue recognition. Revenue is measured based on consideration specified in a contract with a customer. The Partnership recognizes revenue when it satisfies a performance obligation by transferring control over a product or providing services to a customer. The Partnership enters into a variety of contract types in order to generate “Product sales” and “Service revenue.” The Partnership provides services under the following different types of arrangements: • Fee-based arrangements – Under fee-based arrangements, the Partnership receives a fee or fees for one or more of the following services: gathering, processing and transportation of natural gas; gathering, transportation, fractionation, exchange and storage of NGLs; and transportation, storage and distribution of crude oil, refined products and other hydrocarbon-based products. The revenue the Partnership earns from these arrangements is generally directly related to the volume of natural gas, NGL refined products or crude oil that is handled by or flows through the Partnership’s systems and facilities and is not normally directly dependent on commodity prices. In certain cases, the Partnership’s arrangements provide for minimum annual payments or fixed demand charges. Fee-based arrangements are reported as “Service revenue” on the Consolidated Statements of Income. Revenue is recognized over time as services are performed in a series. In certain instances when specifically stated in the contract terms, the Partnership purchases product after fee-based services have been provided. Revenue from the sale of products purchased after services are provided is reported as “Product sales” on the Consolidated Statements of Income and recognized on a gross basis as the Partnership takes control of the product and is the principal in the transaction. • Percent-of-proceeds arrangements – Under percent-of-proceeds arrangements, the Partnership: gathers and processes natural gas on behalf of producers; sells the resulting residue gas, condensate and NGLs at market prices; and remits to producers an agreed-upon percentage of the proceeds. In other cases, instead of remitting cash payments to the producer, the Partnership delivers an agreed-upon percentage of the residue gas and NGLs to the producer (take-in-kind arrangements) and sells the volumes the Partnership retains to third parties. Revenue is recognized on a net basis when the Partnership acts as an agent and does not have control of the gross amount of gas and/or NGLs prior to it being sold. Percent-of-proceeds revenue is reported as “Service revenue - product related” on the Consolidated Statements of Income. • Keep-whole arrangements – Under keep-whole arrangements, the Partnership gathers natural gas from the producer, processes the natural gas and sells the resulting condensate and NGLs to third parties at market prices. Because the extraction of the condensate and NGLs from the natural gas during processing reduces the Btu content of the natural gas, the Partnership must either purchase natural gas at market prices for return to producers or make cash payment to the producers equal to the value of the energy content of this natural gas. Certain keep-whole arrangements also have provisions that require the Partnership to share a percentage of the keep-whole profits with the producers based on the oil to gas ratio or the NGL to gas ratio. “Service revenue - product related” is recorded based on the value of the NGLs received on the date the services are performed. Natural gas purchased to return to the producer and shared NGL profits are recorded as a reduction of “Service revenue - product related” in the Consolidated Statements of Income on the date the services are performed. Sales of NGLs under these arrangements are reported as “Product sales” on the Consolidated Statements of Income and are reported on a gross basis as the Partnership is the principal in the arrangement and controls the product prior to sale. The sale of the NGLs may occur shortly after services are performed at the tailgate of the plant, or after a period of time as determined by the Partnership. • Purchase arrangements – Under purchase arrangements, the Partnership purchases natural gas at either the wellhead or the tailgate of a plant. The Partnership then gathers and delivers the natural gas to pipelines where the Partnership may resell the natural gas. Wellhead purchase arrangements represent an arrangement with a supplier and are recorded in “Purchased product costs”. Often, the Partnership earns fees for services performed prior to taking control of the product in these arrangements and “Service revenue” is recorded for these fees. Revenue generated from the sale of product obtained in tailgate purchase arrangements are reported as “Product sales” on the Consolidated Statements of Income and are recognized on a gross basis as the Partnership purchases and takes control of the product prior to sale and is the principal in the transaction. In many cases, the Partnership provides services under contracts that contain a combination of more than one of the arrangements described above. When fees are charged (in addition to product received) under percent-of-proceeds arrangements, keep-whole arrangements or purchase arrangements, the Partnership records such fees as “Service revenue” on the Consolidated Statements of Income. The terms of the Partnership’s contracts vary based on gas quality conditions, the competitive environment when the contracts are signed and customer requirements. Performance obligations are determined based on the specific terms of the arrangements, economics of the geographical regions and based on the services offered and whether they are deemed distinct. The Partnership allocates the consideration earned between the performance obligations based on the stand-alone selling price when multiple performance obligations are identified. The Partnership’s service arrangements will generally be recognized over time when the performance obligation is satisfied as services are provided in a series. The Partnership has elected to use the output measure of progress to recognize revenue based on the units delivered, processed or transported. The transaction price has fixed components related to minimum volume commitments and variable components which are primarily dependent on volumes. Variable consideration will generally not be estimated at contract inception as the transaction price is specifically allocable to the services provided each period end. In instances in which tiered pricing structures do not reflect our efforts to perform, the Partnership will estimate variable consideration at contract inception. “Product sales” will be recognized at a point in time when control of the product transfers to the customer. Minimum volume commitments may create contract liabilities or deferred credits if current period payments can be used for future services. Breakage is estimated and recognized into service revenue in instances where it is probable the customer will not use the credit in future periods. No breakage was recognized in the current period. Amounts billed to customers for shipping and handling, electricity, and other costs to perform services are included in “Service revenue” on the Consolidated Statements of Income. Shipping and handling costs associated with product sales are included in “Purchased product costs” on the Consolidated Statements of Income. Facility expenses, costs of revenues and depreciation represent those expenses related to operating our various facilities and are necessary to provide both “Product sales” and “Service revenue.” Customers usually pay monthly based on the products purchased or services performed that month. Taxes collected from customers and remitted to the appropriate taxing authority are excluded from revenue. Based on the terms of certain natural gas gathering, transportation and processing agreements, the Partnership is considered to be the lessor under several implicit operating lease arrangements in accordance with GAAP. Revenue and costs related to the portion of the revenue earned under these contracts considered to be implicit leases are recorded as “Rental income” and “Rental cost of sales,” respectively, on the Consolidated Statements of Income. The allocation method used to allocate income between lease and non-lease components was updated as a result of ASC 606. Similarly, the Partnership is considered to be the lessor under implicit operating lease arrangements with MPC in accordance with GAAP. Revenue related to these agreements are recorded as “Rental income - related parties” on the Consolidated Statements of Income. “Rental income” and “Rental income - related parties” is not deemed to be revenue from contracts with customers. The Partnership routinely makes accruals based on estimates for both revenue and expenses due to the timing of compiling billing information, receiving certain third-party information and reconciling the Partnership’s records with those of third parties. The delayed information from third parties includes among other things; actual volumes purchased, transported or sold; adjustments to inventory and invoices for purchases; actual natural gas and NGL deliveries and other operating expenses. The Partnership makes accruals to reflect estimates for these items based on its internal records and information from third parties. Estimated accruals are adjusted when actual information is received from third parties and the Partnership’s internal records have been reconciled.

Acquisitions (Tables)

Acquisitions (Tables)	3 Months Ended
	Mar. 31, 2018
Business Combinations [Abstract]
Business Acquisition, Pro Forma Information [Table Text Block]	The amounts of revenue and income from operations associated with these investments included in the Consolidated Statements of Income, since the February 1, 2018 acquisition date, were as follows: (In millions) Two Months Ended March 31, 2018 Revenues and other income $ 265 Income from operations 181

Equity Method Investments (Tabl

Equity Method Investments (Tables)	3 Months Ended
	Mar. 31, 2018
Schedule of Equity Method Investments [Line Items]
Equity Method Investments [Table Text Block]	The following table presents the Partnership’s equity method investments at the dates indicated: Ownership as of Carrying value at March 31, March 31, December 31, (In millions) 2018 2018 2017 Centrahoma Processing LLC 40% $ 121 $ 121 Explorer 25% 97 89 Illinois Extension 35% 288 284 LOCAP 59% 26 24 LOOP 41% 225 225 MarEn Bakken 25% 529 520 MarkWest EMG Jefferson Dry Gas Gathering Company, L.L.C. 67% 178 164 MarkWest Utica EMG, L.L.C. 56% 2,116 2,139 Ohio Condensate Company, L.L.C. 60% 11 11 Panola Pipeline Company, LLC 15% 23 24 Sherwood Midstream LLC 50% 260 236 Sherwood Midstream Holdings LLC 62% 151 165 Other 8 8 Total $ 4,033 $ 4,010
Summarized Financial Information For Equity Method Investees Table [Table Text Block]	Summarized financial information for the Partnership’s equity method investments for the three months ended March 31, 2018 and 2017 is as follows: Three Months Ended March 31, 2018 (In millions) MarkWest Utica EMG, L.L.C. Other VIEs Non-VIEs Total Revenues and other income $ 63 $ 43 $ 297 $ 403 Costs and expenses 44 18 155 217 Income from operations 19 25 142 186 Net income 19 25 129 173 Income from equity method investments (1) 1 14 46 61 Three Months Ended March 31, 2017 (In millions) MarkWest Utica EMG, L.L.C. Other VIEs Non-VIEs Total Revenues and other income $ 50 $ 8 $ 45 $ 103 Costs and expenses 25 8 33 66 Income from operations 25 — 12 37 Net income 25 — 8 33 Income (loss) from equity method investments (1) 4 (1 ) 2 5 (1) “Income (loss) from equity method investments” includes the impact of any basis differential amortization or accretion. Summarized balance sheet information for the Partnership’s equity method investments as of March 31, 2018 and December 31, 2017 is as follows: March 31, 2018 (In millions) MarkWest Utica EMG, L.L.C. (1) Other VIEs Non-VIEs Total Current assets $ 71 $ 71 $ 363 $ 505 Noncurrent assets 2,035 1,002 4,696 7,733 Current liabilities 24 87 183 294 Noncurrent liabilities 4 11 870 885 December 31, 2017 (In millions) MarkWest Utica EMG, L.L.C. (1) Other VIEs Non-VIEs Total Current assets $ 65 $ 46 $ 399 $ 510 Noncurrent assets 2,077 930 4,624 7,631 Current liabilities 39 44 220 303 Noncurrent liabilities 3 11 904 918 (1) MarkWest Utica EMG, L.L.C.’s (“MarkWest Utica EMG”) noncurrent assets include its investment in its subsidiary, Ohio Gathering Company, L.L.C. (“Ohio Gathering”), which does not appear elsewhere in this table. The investment was $784 million and $790 million as of March 31, 2018 and December 31, 2017 , respectively.

Related Party Agreements and 33

Related Party Agreements and Transactions (Tables)	3 Months Ended
	Mar. 31, 2018
Related Party Transactions [Abstract]
Schedule of Sales to Related Parties	Revenue received from related parties related to service and product sales were as follows: Three Months Ended March 31, (In millions) 2018 2017 Service revenues MPC $ 471 $ 255 Rental income MPC $ 145 $ 67 Product sales (1) MPC $ 4 $ 2 (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, 2018 , these sales totaled $79 million . For the three months ended March 31, 2017 , these sales totaled $57 million .
Summary of Fees Received from Related Parties Included in Other Income - Related Parties	The revenue received from these related parties, included in “Other income - related parties” on the Consolidated Statements of Income, was as follows: Three Months Ended March 31, (In millions) 2018 2017 MPC $ 10 $ 11 MarkWest Utica EMG 4 4 Ohio Gathering 4 4 Jefferson Dry Gas 1 1 Sherwood Midstream 3 1 Other 1 1 Total $ 23 $ 22
Schedule of Entity Wide Information Allocated Related Party Omnibus Agreement Costs by Income Stmt Line [Table Text Block]	Charges for services included in “General and administrative expenses” primarily relate to services that support the Partnership’s executive management, accounting and human resources activities. These charges were as follows: Three Months Ended March 31, (In millions) 2018 2017 Purchases - related parties $ 36 $ 15 General and administrative expenses 16 8 Total $ 52 $ 23
Summary of Related Party Costs Added to Property, Plant and Equipment	These costs added to “Property, plant and equipment, net” were as follows: Three Months Ended March 31, (In millions) 2018 2017 MPC $ 22 $ 10
Schedule of Employee Services Expenses from Related Parties	These charges were as follows: Three Months Ended March 31, (In millions) 2018 2017 Rental cost of sales - related parties $ 1 $ — Purchases - related parties 114 92 General and administrative expenses 23 25 Total $ 138 $ 117
Schedule of Entity Wide Information Purchases From Related Parties	The following table shows other purchases from MPC classified as “Purchases - related parties”. These purchases include product purchases, payments made to MPC in their capacity as general contractor to MPLX LP, and certain rent and lease agreements. Three Months Ended March 31, (In millions) 2018 2017 MPC $ 27 $ —
Schedule of Receivables from Related Parties	Receivables from related parties were as follows: (In millions) March 31, 2018 December 31, 2017 MPC $ 308 $ 153 MarkWest Utica EMG 1 1 Ohio Gathering 3 2 Jefferson Dry Gas 1 2 Sherwood Midstream Holdings 15 — Other 2 2 Total $ 330 $ 160
Schedule of Long Term Receivables with Related Parties	Long-term receivables with related parties, which includes straight-line rental income, were as follows: (In millions) March 31, 2018 December 31, 2017 MPC $ 21 $ 20
Schedule of Payables to Related Parties	Payables to related parties were as follows: (In millions) March 31, 2018 December 31, 2017 MPC $ 115 $ 470 MarkWest Utica EMG 20 29 Ohio Gathering — 8 Sherwood Midstream 11 8 Other — 1 Total $ 146 $ 516
Summary of Deferred Revenue - Related Parties	The “Deferred revenue - related parties” balance associated with the minimum volume deficiencies and project reimbursements were as follows: (In millions) March 31, 2018 December 31, 2017 Minimum volume deficiencies - MPC $ 46 $ 53 Project reimbursements - MPC 46 33 Total $ 92 $ 86

Net Income Per Limited Partne34

Net Income Per Limited Partner Unit (Tables)	3 Months Ended
	Mar. 31, 2018
Earnings Per Share [Abstract]
Schedule of Distributions By Partner By Class	For the three months ended March 31, 2018 , the Partnership had dilutive potential common units consisting of certain equity-based compensation awards. For the three months ended March 31, 2017 , the Partnership had dilutive potential common units consisting of certain equity-based compensation awards and Class B units. Potential common units omitted from the diluted earnings per unit calculation for the three months ended March 31, 2018 and March 31, 2017 were less than 1 million . Three Months Ended March 31, (In millions) 2018 2017 Net income attributable to MPLX LP $ 421 $ 150 Less: Limited partners’ distributions declared on Preferred units (1) 16 16 General partner’s distributions declared (including IDRs) (1) — 65 Limited partners’ distributions declared on common units (including common units of general partner) (1) 467 198 Undistributed net loss attributable to MPLX LP $ (62 ) $ (129 ) (1) See Note 8 for distribution information.
Schedule of Basic and Diluted Earnings Per Unit	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 $ 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 Three Months Ended March 31, 2017 (In millions, except per unit data) General Partner 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) $ 65 $ 198 $ 16 $ 279 Undistributed net loss attributable to MPLX LP (3 ) (126 ) — (129 ) Net income attributable to MPLX LP (1) $ 62 $ 72 $ 16 $ 150 Weighted average units outstanding: Basic 7 362 31 400 Diluted 7 367 31 405 Net income attributable to MPLX LP per limited partner unit: Basic $ 0.20 Diluted $ 0.19 (1) Allocation of net income (loss) 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, 2018
Equity [Abstract]
Schedule of Stockholders Equity [Table Text Block]	The changes in the number of units outstanding during the three months ended March 31, 2018 are summarized below: (In units) Common General Partner Total Balance at December 31, 2017 407,130,020 8,308,773 415,438,793 Unit-based compensation awards (1) 150,257 140 150,397 Contribution of refining logistics and fuels distribution assets (2) 111,611,111 2,277,778 113,888,889 Conversion of GP economic interests 275,000,000 (10,586,691 ) 264,413,309 Balance at March 31, 2018 793,891,388 — 793,891,388 (1) As a result of the unit-based compensation awards issued during the period, MPLX GP contributed less than $1 million in exchange for 140 general partner units to maintain its two percent GP Interest. (2) 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 will not receive $23.7 million of the distributions that would have otherwise accrued on such common units with respect to the first quarter 2018 . See Note 4 for information regarding this acquisition.
Schedule Of Calculation Of Net Income Applicable to Partners [Table Text Block]	The following table presents the allocation of the general partner’s GP Interest in net income attributable to MPLX LP, for income statement periods occurring prior to the exchange of the GP economic interests: Three Months Ended March 31, (In millions) 2017 Net income attributable to MPLX LP $ 150 Less: Preferred unit distributions 16 General partner's IDRs and other 61 Net income attributable to MPLX LP available to general and limited partners $ 73 General partner's two percent GP Interest in net income attributable to MPLX LP $ 1 General partner's IDRs and other 61 General partner's GP Interest in net income attributable to MPLX LP $ 62
Distributions Made to Limited Partner, by Distribution [Table Text Block]	The allocation of total quarterly cash distributions to general, limited and preferred unitholders is as follows for the three months ended March 31, 2018 and 2017 . The Partnership’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) 2018 2017 General partner's distributions: General partner's distributions on general partner units $ — $ 5 General partner's distributions on IDRs — 60 Total distribution on general partner units and IDRs $ — $ 65 Common and preferred unit distributions: Common unitholders, includes common units of general partner $ 467 $ 198 Preferred unit distributions 16 16 Total cash distributions declared $ 483 $ 279

Redeemable Preferred Units (Tab

Redeemable Preferred Units (Tables)	3 Months Ended
	Mar. 31, 2018
Temporary Equity Disclosure [Abstract]
Rollforward of Redeemable Preferred Units	The changes in the redeemable preferred balance from December 31, 2017 through March 31, 2018 are summarized below: (In millions) Redeemable Preferred Units Balance at December 31, 2017 $ 1,000 Net income 16 Distributions received by preferred unitholders (16 ) Balance at March 31, 2018 $ 1,000

Segment Information (Tables)

Segment Information (Tables)	3 Months Ended
	Mar. 31, 2018
Segment Reporting [Abstract]
Schedule of Segment Reporting Information, by Segment [Table Text Block]	The tables below present information about income from operations and capital expenditures for the reported segments: Three Months Ended March 31, 2018 (In millions) L&S G&P Total Revenues and other income: Segment revenues $ 646 $ 569 $ 1,215 Product related revenues — 249 249 Segment other income 12 6 18 Total segment revenues and other income 658 824 1,482 Costs and expenses: Segment cost of revenues 234 235 469 Purchased product costs — 194 194 Segment operating income before portion attributable to noncontrolling interests and Predecessor 424 395 819 Segment portion attributable to noncontrolling interests and Predecessor — 45 45 Segment operating income attributable to MPLX LP $ 424 $ 350 $ 774 Three Months Ended March 31, 2017 (In millions) L&S G&P Total Revenues and other income: Segment revenues $ 345 $ 401 $ 746 Product related revenues — 196 196 Segment other income 12 1 13 Total segment revenues and other income 357 598 955 Costs and expenses: Segment cost of revenues 148 113 261 Purchased product costs — 140 140 Segment operating income before portion attributable to noncontrolling interests and Predecessor 209 345 554 Segment portion attributable to noncontrolling interests and Predecessor 53 36 89 Segment operating income attributable to MPLX LP $ 156 $ 309 $ 465
Reconciliation of Operating Profit (Loss) from Segments to Consolidated [Table Text Block]	Three Months Ended March 31, (In millions) 2018 2017 Reconciliation to Income from operations: Segment operating income attributable to MPLX LP $ 774 $ 465 Segment portion attributable to unconsolidated affiliates (53 ) (40 ) Segment portion attributable to Predecessor — 53 Income from equity method investments 61 5 Other income - related parties 13 11 Unrealized derivative gains (1) 7 16 Depreciation and amortization (176 ) (187 ) General and administrative expenses (69 ) (58 ) Income from operations $ 557 $ 265
Reconciliation of Revenue from Segments to Consolidated [Table Text Block]	Three Months Ended March 31, (In millions) 2018 2017 Reconciliation to Total revenues and other income: Total segment revenues and other income $ 1,482 $ 955 Revenue adjustment from unconsolidated affiliates (137 ) (92 ) Income from equity method investments 61 5 Other income - related parties 13 11 Unrealized derivative gains related to product sales (1) 1 7 Total revenues and other income $ 1,420 $ 886 (1) The Partnership makes a distinction between realized or 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.
Reconciliation of Net Income Attributable to Noncontrolling Interests [Table Text Block]	Three Months Ended March 31, (In millions) 2018 2017 Reconciliation to Net income attributable to noncontrolling interests and Predecessor: Segment portion attributable to noncontrolling interests and Predecessor $ 45 $ 89 Portion of noncontrolling interests and Predecessor related to items below segment income from operations (19 ) (36 ) Portion of operating income attributable to noncontrolling interests of unconsolidated affiliates (24 ) (16 ) Net income attributable to noncontrolling interests and Predecessor $ 2 $ 37
Reconciliation of Other Significant Reconciling Items from Segments to Consolidated [Table Text Block]	The following table reconciles segment capital expenditures to total capital expenditures: Three Months Ended March 31, (In millions) 2018 2017 L&S segment capital expenditures $ 190 $ 97 G&P segment capital expenditures 319 307 Total segment capital expenditures 509 404 Less: Capital expenditures for Partnership-operated, non-wholly-owned subsidiaries in G&P segment 54 124 Total capital expenditures $ 455 $ 280
Reconciliation of Assets from Segment to Consolidated [Table Text Block]	Total assets by reportable segment were: (In millions) March 31, 2018 December 31, 2017 Cash and cash equivalents $ 2 $ 5 L&S 5,958 4,611 G&P 15,046 14,884 Total assets $ 21,006 $ 19,500

Inventories (Tables)

Inventories (Tables)	3 Months Ended
	Mar. 31, 2018
Inventory Disclosure [Abstract]
Summary of Inventories	Inventories consist of the following: (In millions) March 31, 2018 December 31, 2017 NGLs $ 2 $ 4 Line fill 7 8 Spare parts, materials and supplies 55 53 Total inventories $ 64 $ 65

Property, Plant and Equipment (

Property, Plant and Equipment (Tables)	3 Months Ended
	Mar. 31, 2018
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, 2018 December 31, 2017 Natural gas gathering and NGL transportation pipelines and facilities $ 5,253 $ 5,178 Processing, fractionation and storage facilities 4,729 3,893 Pipelines and related assets 2,388 2,253 Barges and towing vessels 553 490 Terminals and related assets 827 821 Refinery and related assets 839 — Land, building, office equipment and other 873 770 Construction-in-progress 992 1,057 Total 16,454 14,462 Less accumulated depreciation 3,163 2,275 Property, plant and equipment, net $ 13,291 $ 12,187

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2018
Fair Value Disclosures [Abstract]
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block]	March 31, 2018 December 31, 2017 (In millions) Assets Liabilities Assets Liabilities Significant unobservable inputs (Level 3) Commodity contracts $ — $ (1 ) $ — $ (2 ) Embedded derivatives in commodity contracts — (59 ) — (64 ) Total carrying value in Consolidated Balance Sheets $ — $ (60 ) $ — $ (66 )
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 $0.26 to $1.47 and (2) the probability of renewal of 62.50 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. For these contracts, increases in forward NGL prices result in a decrease in the fair value of the derivative assets and an increase in the fair value of derivative liabilities. The forward prices for NGL products generally increase or decrease in positive correlation with one another. Increases or decreases in forward NGL prices result in an increase or decrease in the fair value of the embedded derivative. Increases or decreases in the frac spread result in an increase or decrease in the fair value of the embedded derivative liability. An increase in the probability of renewal would result in an increase in the fair value of the related embedded derivative liability.
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, 2018 Three Months Ended March 31, 2017 (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 $ (2 ) $ (64 ) $ (6 ) $ (54 ) Total gains (losses) (realized and unrealized) included in earnings (1) — 3 5 8 Settlements — 3 1 2 Fair value at end of period $ (2 ) $ (58 ) $ — $ (44 ) 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 $ — $ 3 $ 5 $ 8 (1) Gains and losses on Commodity Derivative Contracts classified as Level 3 are recorded in “Product sales” in the accompanying Consolidated Statements of Income. Gains and losses 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 capital leases, and SMR liability: March 31, 2018 December 31, 2017 (In millions) Fair Value Carrying Value Fair Value Carrying Value Long-term debt $ 12,442 $ 11,934 $ 7,718 $ 6,966 SMR liability 100 90 104 91

Derivative Financial Instrume41

Derivative Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Schedule of Notional Amounts of Outstanding Derivative Positions [Table Text Block]	As of March 31, 2018 , the Partnership had the following outstanding commodity contracts that were executed to manage the cash flow risk associated with future sales of NGLs and purchases of natural gas: Derivative contracts not designated as hedging instruments Financial Position Notional Quantity (net) Natural Gas (MMBtu) Long 745,045 NGLs (Gal) Short 7,696,503
Schedule of Derivative Instruments in Statement of Financial Position, Fair Value [Table Text Block]	(In millions) March 31, 2018 December 31, 2017 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 $ — $ (12 ) $ — $ (14 ) Other noncurrent assets / deferred credits and other liabilities — (48 ) — (52 ) Total $ — $ (60 ) $ — $ (66 ) (1) Includes embedded derivatives in commodity contracts as discussed above.
Derivative Instruments, Gain (Loss) [Table Text Block]	Three Months Ended March 31, (In millions) 2018 2017 Product sales Realized loss $ — $ (1 ) Unrealized gain 1 7 Total derivative gain related to product sales 1 6 Purchased product costs Realized loss (3 ) (2 ) Unrealized gain 6 9 Total derivative gain related to purchased product costs 3 7 Cost of revenues Realized (loss) gain — — Unrealized (loss) gain — — Total derivative (loss) gain related to cost of revenues — — Total derivative gains $ 4 $ 13

Debt (Tables)

Debt (Tables)	3 Months Ended
	Mar. 31, 2018
Debt Disclosure [Abstract]
Summary of Outstanding Borrowings	The Partnership’s outstanding borrowings consisted of the following: (In millions) March 31, 2018 December 31, 2017 MPLX LP: Bank revolving credit facility due 2022 $ — $ 505 5.500% senior notes due February 2023 710 710 3.375% senior notes due March 2023 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 — 4.500% senior notes due April 2038 1,750 — 5.200% senior notes due March 2047 1,000 1,000 4.700% senior notes due April 2048 1,500 — 4.900% senior notes due April 2058 500 — Consolidated subsidiaries: MarkWest - 4.500% - 5.500% senior notes, due 2023-2025 63 63 MPL - capital lease obligations due 2020 7 7 Total 12,357 7,362 Unamortized debt issuance costs (79 ) (27 ) Unamortized discount (416 ) (389 ) Amounts due within one year (1 ) (1 ) Total long-term debt due after one year $ 11,861 $ 6,945

Revenue (Tables)

Revenue (Tables)	3 Months Ended
	Mar. 31, 2018
Revenue from Contract with Customer [Abstract]
Revenue, Initial Application Period Cumulative Effect Transition [Table Text Block]	The cumulative effect of the changes made to our consolidated January 1, 2018 balance sheet for the adoption of ASC 606 was as follows: (In millions) Balance at December 31, 2017 ASC 606 Adjustment Total Assets Inventories $ 65 $ 1 $ 66 Property, plant and equipment, net 12,187 (3 ) 12,184 Liabilities Long-term deferred revenue 42 (3 ) 39 Equity Common unitholders - public $ 8,379 $ 1 $ 8,380 Aside from the adjustments to the opening balances noted above, the impact of adoption on our Consolidated Balance Sheets for the period ended March 31, 2018 was immaterial. The disclosure of the impact of adoption on our Consolidated Statements of Income for the period ended March 31, 2018 was as follows: Three Months Ended March 31, 2018 (In millions) ASC 606 Balance ASC 605 Balance Effect of Change Higher/ (Lower) Revenues and other income: Service revenue $ 382 $ 305 $ 77 Service revenue - related parties 471 472 (1 ) Service revenue - product related 44 — 44 Rental income 79 63 16 Product sales (1) 207 228 (21 ) Product sales - related parties 4 3 1 Costs and expenses: Cost of revenues (2) 206 128 78 Rental cost of sales 29 13 16 Purchased product costs 187 163 24 Depreciation and amortization 176 176 — Net income $ 423 $ 425 $ (2 ) (1) G&P “Product sales” exclude approximately $1 million of revenue related to derivative gains/losses and mark-to-market adjustments. (2) Excludes “Purchased product costs,” “Rental cost of sales,” “Purchases,” “Depreciation and amortization,” “General and administrative expenses,” and “Other taxes.”
Disaggregation of Revenue [Table Text Block]	The following table represents a disaggregation of revenue for each reportable segment for the period ended March 31, 2018 : 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 Revenue adjustment for unconsolidated affiliates 137 (Income) from equity method investments (61 ) Other income - related parties (13 ) Unrealized derivative (gain) related to product sales (1 ) Total segment revenues and other income $ 1,482 (1) G&P “Product sales” exclude approximately $1 million of revenue related to derivative gains/losses and mark-to-market adjustments. (2) Non-ASC 606 Revenue includes rental income, income from equity method investments, derivative gains/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, 2018 : (In millions) Balance at January 1, 2018 (1) Additions/ (Deletions) Revenue Recognized (2) Balance at March 31, 2018 Contract assets $ 4 $ — $ — $ 4 Deferred revenue 5 2 (2 ) 5 Deferred revenue - related parties 42 9 (9 ) 42 Long-term deferred revenue 7 1 — 8 Long-term deferred revenue - related parties 39 6 — 45 (1) Balance represents ASC 606 portion of each respective line items. (2) No revenue was recognized related to past performance obligations, in the current period. Changes in long-term amounts represent reclassifications to current balances.
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Table Text Block]	The Company does not disclose information about remaining performance obligations that have original expected durations of one year or less. Further, the Company does not disclose variable consideration due to volume variability in the table below. (In millions) Logistics & Storage Services Gathering & Processing Services (3) 2018 $ 704 $ 91 2019 929 118 2020 929 114 2021 929 113 2022 and thereafter 5,577 404 Total revenue on remaining performance obligations (1),(2) $ 9,068 $ 840 (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. The Partnership 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 Inform44

Supplemental Cash Flow Information (Tables)	3 Months Ended
	Mar. 31, 2018
Supplemental Cash Flow Elements [Abstract]
Schedule of Cash and Cash Equivalents [Table Text Block]	(In millions) March 31, 2018 December 31, 2017 Cash and cash equivalents $ 2 $ 5 Restricted cash (1) 4 4 Cash, cash equivalents and restricted cash (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, 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) 2018 2017 Net cash provided by operating activities included: Interest paid (net of amounts capitalized) $ 103 $ 49 Non-cash investing and financing activities: Net transfers of property, plant and equipment from materials and supplies inventories 1 6 Contribution of fixed assets to joint venture (1) $ — $ 328 (1) Contribution of assets to Sherwood Midstream and Sherwood Midstream Holdings. See Note 5 .
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) 2018 2017 (Decrease) increase in capital accruals $ (6 ) $ 2

Accumulated Other Comprehensi45

Accumulated Other Comprehensive Income (Tables)	3 Months Ended
	Mar. 31, 2018
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, 2017 through March 31, 2018 . (In millions) Pension Benefits Other Post-Retirement Benefits Total Balance as of December 1, 2017 (1) $ (13 ) $ (1 ) $ (14 ) Other comprehensive loss - remeasurements (2) (1 ) (1 ) (2 ) Balance as of March 31, 2018 (1) $ (14 ) $ (2 ) $ (16 ) (1) These accumulated other comprehensive loss components are included in the computation of net periodic benefit cost by LOOP and Explorer and are therefore included in the Consolidated Statements of Income under the caption “Income (loss) from equity method investments.” (2) Components of “Other comprehensive loss - remeasurements” relate to actuarial gains and losses as well as amortization of prior service costs. The Partnership 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, 2018
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, 2018 : Number Weighted Outstanding at December 31, 2017 1,351,523 $ 34.53 Granted 169,310 35.00 Settled (217,093 ) 33.83 Forfeited (46,330 ) 34.95 Outstanding at March 31, 2018 1,257,410 34.70
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, 2018 : Number of Outstanding at December 31, 2017 2,536,594 Granted — Settled (538,594 ) Forfeited (50,000 ) Outstanding at March 31, 2018 1,948,000

Description of Business and Bas

Description of Business and Basis of Presentation - Additional Information (Detail)	Feb. 01, 2018	Sep. 01, 2017	Mar. 01, 2017	Mar. 31, 2018
Description Of Business And Basis Of Presentation [Line Items]
Number of reportable segments				2
Illinois Extension
Description Of Business And Basis Of Presentation [Line Items]
Equity method investment, ownership percentage		35.00%		35.00%
LOOP
Description Of Business And Basis Of Presentation [Line Items]
Equity method investment, ownership percentage		41.00%		41.00%
LOCAP
Description Of Business And Basis Of Presentation [Line Items]
Equity method investment, ownership percentage		59.00%		59.00%
Explorer
Description Of Business And Basis Of Presentation [Line Items]
Equity method investment, ownership percentage		25.00%		25.00%
LOOP LOCAP SAX and Explorer Llc
Description Of Business And Basis Of Presentation [Line Items]
Effective date of acquisition		Sep. 1, 2017
HST & WHC & MPLXT
Description Of Business And Basis Of Presentation [Line Items]
Effective date of acquisition			Mar. 1, 2017
Refining Logistics & Fuels Distribution
Description Of Business And Basis Of Presentation [Line Items]
Effective date of acquisition	Feb. 1, 2018

Acquisitions Refining Logistics

Acquisitions Refining Logistics & Fuels Distribution (Details) bbl in Millions, $ in Millions		Feb. 01, 2018USD ($)Tanksharesbbl	Mar. 01, 2017USD ($)	Mar. 31, 2018USD ($)	Mar. 31, 2018USD ($)shares	Sep. 30, 2017USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2017USD ($)
Business Acquisition [Line Items]
Contribution from MPC					$ 1,046		$ 12
Units acquired | shares	[1]				113,888,889
Number of storage tanks | Tank		619
Property, plant and equipment, net				$ 13,291	$ 13,291			$ 12,187
Goodwill				2,460	$ 2,460			$ 2,245
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
Revenues and other income				265
Income from operations				$ 181
Equity interest issued or issuable, fair value assigned		4,300
Total consideration, fair value assigned		8,400
Property, plant and equipment, net		830
Refining Logistics [Member]
Business Acquisition [Line Items]
Goodwill		85
Fuels Distribution [Member]
Business Acquisition [Line Items]
Goodwill		$ 130
Limited Partners Common Units
Business Acquisition [Line Items]
Units acquired | shares	[1]				111,611,111
General Partner Units
Business Acquisition [Line Items]
Units acquired | shares	[1]				2,277,778
General Partner Common Units | Refining Logistics & Fuels Distribution
Business Acquisition [Line Items]
Units acquired | shares	[1]	85,610,278
MPLX LP | Limited Partners Common Units | Refining Logistics & Fuels Distribution
Business Acquisition [Line Items]
Units acquired | shares	[1]	111,611,111
MPLX LP | General Partner Units | Refining Logistics & Fuels Distribution
Business Acquisition [Line Items]
Units acquired | shares	[1]	2,277,778
MPLX Logistics LLC | Limited Partners Common Units | Refining Logistics & Fuels Distribution
Business Acquisition [Line Items]
Units acquired | shares	[1]	18,176,666
MPLX Holdings Inc | Limited Partners Common Units | Refining Logistics & Fuels Distribution
Business Acquisition [Line Items]
Units acquired | shares	[1]	7,824,167
MPC
Business Acquisition [Line Items]
Contribution from MPC			$ 6			$ 10
MPC | Refining Logistics & Fuels Distribution
Business Acquisition [Line Items]
Contribution from MPC					$ 23.7
Crude Oil [Member]
Business Acquisition [Line Items]
Storage Capacity | bbl		56
[1]	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 will not receive $23.7 million of the distributions that would have otherwise accrued on such common units with respect to the first quarter 2018. See Note 4 for information regarding this acquisition.

Acquisitions Joint-Interest Acq

Acquisitions Joint-Interest Acquisition (Details) $ in Millions		Sep. 01, 2017USD ($)inmishares	Mar. 01, 2017USD ($)	Mar. 31, 2018USD ($)shares	Sep. 30, 2017USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2017USD ($)
Schedule of Equity Method Investments [Line Items]
Units acquired | shares	[1]			113,888,889
General partner ownership interest		2.00%	2.00%	2.00%
Distributions of cash received from joint-interest acquisition entities to MPC				$ 11		$ 0
Income from equity method investments	[2]			61		5
Equity method investments				4,033			$ 4,010
Contribution from MPC				1,046		$ 12
LOOP LOCAP SAX and Explorer Llc
Schedule of Equity Method Investments [Line Items]
Payments to acquire interest in joint venture		$ 420
Equity interest issued or issuable, value assigned		630
Total consideration, value assigned		1,050
Equity interest issued or issuable, fair value assigned		653
Total consideration, fair value assigned		$ 1,070
Income from equity method investments				37
Equity method investments				$ 635
Illinois Extension
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		35.00%		35.00%
Pipeline length | mi		168
Pipeline diameter | in		24
Number of pump stations		2
Equity method investments				$ 288			284
LOOP
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		41.00%		41.00%
Equity method investments				$ 225			225
LOCAP
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		59.00%		59.00%
Equity method investments				$ 26			24
Explorer
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		25.00%		25.00%
Pipeline length | mi		1,830
Equity method investments				$ 97			$ 89
Limited Partners Common Units
Schedule of Equity Method Investments [Line Items]
Units acquired | shares	[1]			111,611,111
General Partner Units
Schedule of Equity Method Investments [Line Items]
Units acquired | shares	[1]			2,277,778
General Partner Units | LOOP LOCAP SAX and Explorer Llc
Schedule of Equity Method Investments [Line Items]
Units acquired | shares	[1]	377,778
General Partner Common Units | LOOP LOCAP SAX and Explorer Llc
Schedule of Equity Method Investments [Line Items]
Units acquired | shares	[1]	13,719,017
MPLX Logistics LLC | Limited Partners Common Units | LOOP LOCAP SAX and Explorer Llc
Schedule of Equity Method Investments [Line Items]
Units acquired | shares	[1]	3,350,893
MPLX Holdings Inc | Limited Partners Common Units | LOOP LOCAP SAX and Explorer Llc
Schedule of Equity Method Investments [Line Items]
Units acquired | shares	[1]	1,441,224
MPC
Schedule of Equity Method Investments [Line Items]
Contribution from MPC			$ 6		$ 10
General Partner | MPC | LOOP LOCAP SAX and Explorer Llc
Schedule of Equity Method Investments [Line Items]
Distributions of cash received from joint-interest acquisition entities to MPC				$ 32
[1]	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 will not receive $23.7 million of the distributions that would have otherwise accrued on such common units with respect to the first quarter 2018. See Note 4 for information regarding this acquisition.
[2]	“Income (loss) from equity method investments” includes the impact of any basis differential amortization or accretion.

Acquisition of HST, WHC & MPLXT

Acquisition of HST, WHC & MPLXT (Details) bbl in Millions, $ in Millions		Sep. 01, 2017	Mar. 01, 2017USD ($)misharesbbl	Mar. 31, 2018USD ($)shares	Sep. 30, 2017USD ($)	Mar. 31, 2017USD ($)
Business Acquisition [Line Items]
Units acquired	[1]			113,888,889
General partner ownership interest		2.00%	2.00%	2.00%
Contribution from MPC | $				$ 1,046		$ 12
Number of natural gas liquids storage caverns			8
Number of light product terminals			62
General Partner Units
Business Acquisition [Line Items]
Units acquired	[1]			2,277,778
Limited Partners Common Units
Business Acquisition [Line Items]
Units acquired	[1]			111,611,111
HST & WHC & MPLXT
Business Acquisition [Line Items]
Equity interest issued or issuable, value assigned | $			$ 504
Equity interest issued or issuable, fair value assigned | $			503
HST & WHC & MPLXT | Cash and Cash Equivalents [Member]
Business Acquisition [Line Items]
Payments to acquire businesses, gross | $			$ 1,500
HST & WHC & MPLXT | General Partner Units
Business Acquisition [Line Items]
Units acquired			264,497
HST & WHC & MPLXT | General Partner Common Units
Business Acquisition [Line Items]
Units acquired			9,197,900
MPLX Logistics LLC | HST & WHC & MPLXT | Limited Partners Common Units
Business Acquisition [Line Items]
Units acquired			2,630,427
MPLX Holdings Inc | HST & WHC & MPLXT | Limited Partners Common Units
Business Acquisition [Line Items]
Units acquired			1,132,049
Natural gas liquids storage caverns [Member]
Business Acquisition [Line Items]
Storage Capacity | bbl			1.8
Light-product terminal [Member]
Business Acquisition [Line Items]
Storage Capacity | bbl			23.6
Refined Products [Member]
Business Acquisition [Line Items]
Miles of pipeline | mi			430
Crude Oil [Member]
Business Acquisition [Line Items]
Miles of pipeline | mi			174
Wholly Owned Properties [Member]
Business Acquisition [Line Items]
Number of light product terminals			59
Property Subject to Operating Lease [Member]
Business Acquisition [Line Items]
Number of light product terminals			1
Partially Owned Properties [Member]
Business Acquisition [Line Items]
Number of light product terminals			2
MPC
Business Acquisition [Line Items]
Contribution from MPC | $			$ 6		$ 10
[1]	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 will not receive $23.7 million of the distributions that would have otherwise accrued on such common units with respect to the first quarter 2018. See Note 4 for information regarding this acquisition.

Acquisition of Ozark Pipeline (

Acquisition of Ozark Pipeline (Details) bbl / d in Thousands, $ in Millions	Mar. 01, 2017USD ($)bbl / dinmi	Mar. 31, 2018USD ($)	Mar. 31, 2017USD ($)
Business Acquisition [Line Items]
Additions to property, plant and equipment		$ (455)	$ (280)
Ozark Pipeline [Member]
Business Acquisition [Line Items]
Additions to property, plant and equipment	$ (219)
Pipeline length | mi	433
Pipeline diameter | in	22
Pipeline barrels capable of transportation | bbl / d	230

Acquisitions MarEn Bakken (Deta

Acquisitions MarEn Bakken (Details) $ in Millions		Feb. 15, 2017USD ($)Quarterly_reporting_period	Mar. 31, 2018USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2017USD ($)
Schedule of Equity Method Investments [Line Items]
Investments in unconsolidated affiliates			$ 38	$ 554
Income from equity method investments	[1]		61	$ 5
Equity method investments			4,033		$ 4,010
MarEn Bakken [Member]
Schedule of Equity Method Investments [Line Items]
Investments in unconsolidated affiliates		$ 500
Percentage of Ownership Interest in Joint Venture Acquired		25.00%
Income from equity method investments			$ 7
Bakken Pipeline System [Member]
Schedule of Equity Method Investments [Line Items]
Percentage of Ownership Interest in Joint Venture Acquired		37.00%
Equity method investment, ownership percentage		9.00%
MarEn Bakken Company LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage			25.00%
Equity method investments			$ 529		$ 520
MarEn Bakken Company LLC [Member]
Schedule of Equity Method Investments [Line Items]
Investments in unconsolidated affiliates		$ 2,000
MPC
Schedule of Equity Method Investments [Line Items]
Incentive Distribution Rights Forfeited per Quarter		$ 1.6
Number of Quarters IDRs Forfeited | Quarterly_reporting_period		12
Prorated Incentive Distribution Rights Forfeited		$ 0.8
[1]	“Income (loss) from equity method investments” includes the impact of any basis differential amortization or accretion.

Equity Method Investments - Sum

Equity Method Investments - Summary of Equity Method Investment Financial Information (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Sep. 01, 2017	Jan. 01, 2017
Schedule of Equity Method Investments [Line Items]
Revenues and other income		$ 403	$ 103
Costs and expenses		217	66
Income (loss) from operations		186	37
Net income (loss)		173	33
Income from equity method investments	[1]	61	5
Current assets		505		$ 510
Noncurrent assets		7,733		7,631
Current liabilities		294		303
Noncurrent liabilities		885		918
Difference between carrying amount and underlying equity		1,000		1,000
Difference between carrying amount and underlying equity portion related to goodwill		459		459
Equity method investments		$ 4,033		4,010
Centrahoma Processing LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		40.00%
Equity method investments		$ 121		121
MarkWest Utica EMG
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		56.00%
Revenues and other income		$ 63	50
Costs and expenses		44	25
Income (loss) from operations		19	25
Net income (loss)		19	25
Income from equity method investments	[1]	1	4
Current assets		71		65
Noncurrent assets	[2]	2,035		2,077
Current liabilities		24		39
Noncurrent liabilities		4		3
Equity method investments		2,116		2,139
Other VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Revenues and other income		43	8
Costs and expenses		18	8
Income (loss) from operations		25	0
Net income (loss)		25	0
Income from equity method investments	[1]	14	(1)
Current assets		71		46
Noncurrent assets		1,002		930
Current liabilities		87		44
Noncurrent liabilities		11		11
Equity method investments		8		8
Non-VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Revenues and other income		297	45
Costs and expenses		155	33
Income (loss) from operations		142	12
Net income (loss)		129	8
Income from equity method investments	[1]	46	$ 2
Current assets		363		399
Noncurrent assets		4,696		4,624
Current liabilities		183		220
Noncurrent liabilities		$ 870		904
Ohio Condensate Company, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		60.00%
Equity method investments		$ 11		11
Ohio Gathering
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		34.00%
Investment in subsidiary		$ 784		790
Explorer
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		25.00%			25.00%
Equity method investments		$ 97		89
Illinois Extension
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		35.00%			35.00%
Equity method investments		$ 288		284
LOCAP
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		59.00%			59.00%
Equity method investments		$ 26		24
LOOP
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		41.00%			41.00%
Equity method investments		$ 225		225
MarEn Bakken Company LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		25.00%
Equity method investments		$ 529		520
Jefferson Dry Gas
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		67.00%
Equity method investments		$ 178		164
Panola Pipeline Company LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		15.00%
Equity method investments		$ 23		24
Sherwood Midstream
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		50.00%
Equity method investments		$ 260		236
Sherwood Midstream Holdings
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		81.00%
Equity method investments		$ 151		$ 165
Direct Ownership Interest [Member] | Sherwood Midstream Holdings
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		62.00%				79.00%
[1]	“Income (loss) from equity method investments” includes the impact of any basis differential amortization or accretion.
[2]	MarkWest Utica EMG, L.L.C.’s (“MarkWest Utica EMG”) noncurrent assets include its investment in its subsidiary, Ohio Gathering Company, L.L.C. (“Ohio Gathering”), which does not appear elsewhere in this table. The investment was $784 million and $790 million as of March 31, 2018 and December 31, 2017, respectively.

Equity Method Investments Utica

Equity Method Investments Utica EMG (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 4,033	$ 4,010
MarkWest Utica EMG
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 2,116	$ 2,139

Investments & NCI Ohio Gatherin

Investments & NCI Ohio Gathering (Details)	Mar. 31, 2018
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, $ in Millions	Jan. 01, 2017USD ($)bbl / d	Mar. 31, 2018USD ($)	Mar. 31, 2017USD ($)	[1]	Dec. 31, 2017USD ($)
Schedule of Equity Method Investments [Line Items]
Contribution of fixed assets to joint venture		$ 0	$ 328
Equity method investments		$ 4,033			$ 4,010
Sherwood Midstream
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		50.00%
Contribution of fixed assets to joint venture	$ 134
Payments to acquire interest in joint venture	$ 20
Equity method investments		$ 260			$ 236
MarkWest Ohio Fractionation Company, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Capacity | bbl / d	20
Sherwood Midstream | MarkWest Ohio Fractionation Company, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Payments to acquire interest in joint venture	$ 126
Antero Midstream Partners L.P. [Member] | Sherwood Midstream
Schedule of Equity Method Investments [Line Items]
Payments to acquire interest in joint venture	$ 154
[1]	Contribution of assets to Sherwood Midstream and Sherwood Midstream Holdings. See Note 5.

Investments & NCI Sherwood Mi57

Investments & NCI Sherwood Midstream Holdings (Details) - USD ($) $ in Millions	Jan. 01, 2017	Mar. 31, 2018	Jun. 30, 2017	Mar. 31, 2017	[1]	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Contribution of fixed assets to joint venture		$ 0		$ 328
Difference between carrying amount and underlying equity		1,000				$ 1,000
Equity method investments		$ 4,033				4,010
Sherwood Midstream Holdings
Schedule of Equity Method Investments [Line Items]
Fair Value Of Assets Contributed	$ 209		$ 10
Equity method investment, ownership percentage		81.00%
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 investments		$ 151				$ 165
Direct Ownership Interest [Member] | Sherwood Midstream Holdings
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	79.00%	62.00%
Indirect Ownership Interest [Member] | Sherwood Midstream Holdings
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		18.90%
Sherwood Midstream | Sherwood Midstream Holdings
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	21.00%
Payments to acquire interest in joint venture	$ 44		$ 4
[1]	Contribution of assets to Sherwood Midstream and Sherwood Midstream Holdings. See Note 5.

Related Party Agreements and 58

Related Party Agreements and Transactions - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Mar. 31, 2018	Sep. 30, 2017	Dec. 31, 2017
Related Party Revolving Credit Agreement | MPC Investment
Related Party Transaction [Line Items]
Current borrowing capacity	$ 500
Expiration date	Dec. 4, 2020
Description of variable rate basis	LIBOR plus 1.50 percent
Proceeds from short-term debt	$ 452		$ 2,400
Repayments of short-term debt	838		2,000
Intercompany loan outstanding	$ 0		$ 386
Interest rate during period	3.073%		2.777%
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	81.00%
Jefferson Dry Gas
Related Party Transaction [Line Items]
Equity method investment, ownership percentage	67.00%
Gas Distribution [Member] | Ten Year Storage Services Agreement [Member] | Commercial Agreements [Member] | MPC
Related Party Transaction [Line Items]
Term Of Agreements	10 years
Storage Services Butane and Propane Caverns [Member] | Ten Year Storage Services Agreement [Member] | Commercial Agreements [Member] | MPC
Related Party Transaction [Line Items]
Term Of Agreements	10 years
Storage Services Tank Farm [Member] | Three Year Storage Services Agreement [Member] | Commercial Agreements [Member] | MPC
Related Party Transaction [Line Items]
Term Of Agreements	5 years	5 years
Management Services Agreements [Member] | Ten Year Storage Services Agreement [Member] | Commercial Agreements [Member] | MPC
Related Party Transaction [Line Items]
Term Of Agreements	50 years

Related Party Agreements and 59

Related Party Agreements and Transactions - Sales to Related Parties (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Related Party Transaction [Line Items]
Service revenue - related parties		$ 471	$ 255
Rental income - related parties		145	67
Product sales - related parties		4	2
MPC
Related Party Transaction [Line Items]
Service revenue - related parties		471	255
Rental income - related parties		145	67
Product sales - related parties	[1]	4	2
Product sales to MPC that net to zero		$ 79	$ 57
[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, 2018, these sales totaled $79 million. For the three months ended March 31, 2017, these sales totaled $57 million.

Related Party Agreements and 60

Related Party Agreements and Transactions - Other Income - Related Parties (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Related Party Transaction [Line Items]
Other income - related parties	$ 23	$ 22
MPC
Related Party Transaction [Line Items]
Other income - related parties	10	11
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
Jefferson Dry Gas
Related Party Transaction [Line Items]
Other income - related parties	1	1
Sherwood Midstream
Related Party Transaction [Line Items]
Other income - related parties	3	1
Other
Related Party Transaction [Line Items]
Other income - related parties	$ 1	$ 1

Related Party Agreements and 61

Related Party Agreements and Transaction - Summary of Charges for Omnibus Agreement (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
MPC
Related Party Transaction [Line Items]
Charges for services under an omnibus agreement	$ 52	$ 23
MPC | Purchases - related parties
Related Party Transaction [Line Items]
Charges for services under an omnibus agreement	36	15
MPC | General and administrative expenses
Related Party Transaction [Line Items]
Charges for services under an omnibus agreement	16	$ 8
Related Party Revolving Credit Agreement | MPC Investment
Related Party Transaction [Line Items]
Current borrowing capacity	$ 500

Related Party Agreements and 62

Related Party Agreements and Transactions - Summary of Related Party Costs Added to Property, Plant and Equipment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
MPC | Construction-in-progress
Property, Plant and Equipment [Line Items]
Costs added to property, plant and equipment	$ 22	$ 10

Related Party Agreements and 63

Related Party Agreements and Transactions - Employee Services Agreements (Detail) - MPC - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	$ 138	$ 117
Rental cost of sales - related parties
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	1	0
Purchases - related parties
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	114	92
General and administrative expenses
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	$ 23	$ 25

Related Party Agreements and 64

Related Party Agreements and Transactions - Purchases from Related Party (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
MPC
Related Party Transaction [Line Items]
Related party purchases	$ 27	$ 0

Related Party Agreements and 65

Related Party Agreements and Transactions - Receivables from Related Parties (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Receivables - related parties	$ 330	$ 160
MPC
Related Party Transaction [Line Items]
Receivables - related parties	308	153
MarkWest Utica EMG
Related Party Transaction [Line Items]
Receivables - related parties	1	1
Ohio Gathering
Related Party Transaction [Line Items]
Receivables - related parties	3	2
Jefferson Dry Gas
Related Party Transaction [Line Items]
Receivables - related parties	1	2
Sherwood Midstream Holdings
Related Party Transaction [Line Items]
Receivables - related parties	15	0
Other
Related Party Transaction [Line Items]
Receivables - related parties	$ 2	$ 2

Related Party Agreements and 66

Related Party Agreements and Transactions Related Party Agreements and Transactions - Long-term Receivables from Related Parties (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Long-term receivables - related parties	$ 21	$ 20
MPC
Related Party Transaction [Line Items]
Long-term receivables - related parties	$ 21	$ 20

Related Party Agreements and 67

Related Party Agreements and Transactions - Payables to Related Parties (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Payables - related parties	$ 146	$ 516
MPC
Related Party Transaction [Line Items]
Payables - related parties	115	470
MarkWest Utica EMG
Related Party Transaction [Line Items]
Payables - related parties	20	29
Ohio Gathering
Related Party Transaction [Line Items]
Payables - related parties	0	8
Sherwood Midstream
Related Party Transaction [Line Items]
Payables - related parties	11	8
Other
Related Party Transaction [Line Items]
Payables - related parties	0	1
Related Party Revolving Credit Agreement | MPC Investment
Related Party Transaction [Line Items]
Intercompany loan outstanding	$ 0	$ 386

Related Party Agreements and 68

Related Party Agreements and Transactions - Summary of Deferred Revenue - Related Parties (Detail) - MPC - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Deferred revenue related parties	$ 92	$ 86
Minimum volume deficiencies [Member]
Related Party Transaction [Line Items]
Deferred revenue related parties	46	53
Project reimbursements [Member]
Related Party Transaction [Line Items]
Deferred revenue related parties	$ 46	$ 33

Net Income Per Limited Partne69

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, 2018	Mar. 31, 2017
Net Income Per Share [Line Items]
Potentially dilutive units		1	1
Net income attributable to MPLX LP	[1]	$ 421	$ 150
Less: Distribution declared		483	279
Undistributed net loss attributable to MPLX LP		(62)	(129)
Preferred Units
Net Income Per Share [Line Items]
Net income attributable to MPLX LP	[1]	16	16
Less: Distribution declared	[2]	16	16
General Partner | MPC
Net Income Per Share [Line Items]
Net income attributable to MPLX LP	[1]		62
Less: Distribution declared	[2]	0	65
Limited Partners Common Units
Net Income Per Share [Line Items]
Net income attributable to MPLX LP	[1]	405	72
Less: Distribution declared	[2]	$ 467	$ 198
[1]	Allocation of net income (loss) attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.
[2]	See Note 8 for distribution information.

Net Income Per Limited Partne70

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, 2018	Mar. 31, 2017
Net income (loss) attributable to MPLX LP:
Distributions declared (including IDRs)		$ 483	$ 279
Undistributed net income (loss) attributable to MPLX LP		(62)	(129)
Net income attributable to MPLX LP	[1]	$ 421	$ 150
Weighted average units outstanding:
Common - basic (in shares)		692	400
Common - diluted (in shares)		692	405
Limited Partners Common Units
Net income (loss) attributable to MPLX LP:
Distributions declared (including IDRs)	[2]	$ 467	$ 198
Undistributed net income (loss) attributable to MPLX LP		(62)	(126)
Net income attributable to MPLX LP	[1]	$ 405	$ 72
Weighted average units outstanding:
Common - basic (in shares)		661	362
Common - diluted (in shares)		661	367
Net income attributable to MPLX LP per limited partner unit:
Basic (in USD per unit)		$ 0.61	$ 0.20
Diluted (in USD per unit)		$ 0.61	$ 0.19
Preferred Units
Net income (loss) attributable to MPLX LP:
Distributions declared (including IDRs)	[2]	$ 16	$ 16
Undistributed net income (loss) attributable to MPLX LP		0	0
Net income attributable to MPLX LP	[1]	$ 16	$ 16
Weighted average units outstanding:
Common - basic (in shares)		31	31
Common - diluted (in shares)		31	31
MPC | General Partner
Net income (loss) attributable to MPLX LP:
Distributions declared (including IDRs)	[2]	$ 0	$ 65
Undistributed net income (loss) attributable to MPLX LP			(3)
Net income attributable to MPLX LP	[1]		$ 62
Weighted average units outstanding:
Common - basic (in shares)			7
Common - diluted (in shares)			7
[1]	Allocation of net income (loss) attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.
[2]	See Note 8 for distribution information.

Equity - Changes in Partners Ca

Equity - Changes in Partners Capital, Unit Rollforward (Details) - USD ($) $ in Millions		Feb. 01, 2018	[2]	Sep. 01, 2017	Mar. 01, 2017	Mar. 31, 2018	Sep. 30, 2017	Mar. 31, 2017
Stockholders Equity [Line Items]
Balance at December 31, 2017						415,438,793
Unit-based compensation awards	[1]					150,397
Contribution of refining logistics and fuels distribution assets	[2]					113,888,889
Conversion of GP economic interests						264,413,309
Balance at March 31, 2018						793,891,388
Sale of units for GP Interest								$ 151
General partner ownership interest				2.00%	2.00%	2.00%
Contribution from MPC						$ 1,046		$ 12
Limited Partners Common Units
Stockholders Equity [Line Items]
Balance at December 31, 2017						407,130,020
Unit-based compensation awards	[1]					150,257
Contribution of refining logistics and fuels distribution assets	[2]					111,611,111
Conversion of GP economic interests		275,000,000				275,000,000
Balance at March 31, 2018						793,891,388
General Partner Units
Stockholders Equity [Line Items]
Balance at December 31, 2017						8,308,773
Unit-based compensation awards	[1]					140
Contribution of refining logistics and fuels distribution assets	[2]					2,277,778
Conversion of GP economic interests						(10,586,691)
Balance at March 31, 2018						0
Sale of units for GP Interest						$ 1
MPC
Stockholders Equity [Line Items]
Contribution from MPC					$ 6		$ 10
MPC | Refining Logistics & Fuels Distribution
Stockholders Equity [Line Items]
Contribution from MPC						$ 23.7
[1]	As a result of the unit-based compensation awards issued during the period, MPLX GP contributed less than $1 million in exchange for 140 general partner units to maintain its two percent GP Interest.
[2]	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 will not receive $23.7 million of the distributions that would have otherwise accrued on such common units with respect to the first quarter 2018. See Note 4 for information regarding this acquisition.

Equity - GP_IDR Exchange (Detai

Equity - GP/IDR Exchange (Details) - USD ($) $ in Millions	Feb. 01, 2018		Mar. 31, 2018
Stockholders Equity Note [Line Items]
Conversion of GP economic interests			264,413,309
Partners' Capital Account, Exchanges and Conversions			$ 0
Limited Partners Common Units
Stockholders Equity Note [Line Items]
Conversion of GP economic interests	275,000,000	[1]	275,000,000
Partners' Capital Account, Exchanges and Conversions	$ 10,400
[1]	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 will not receive $23.7 million of the distributions that would have otherwise accrued on such common units with respect to the first quarter 2018. See Note 4 for information regarding this acquisition.

Equity - Net Income Allocation

Equity - Net Income Allocation (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Equity [Abstract]
Net income attributable to MPLX LP	[1]	$ 421	$ 150
Less: Preferred unit distributions		16	16
General partner's IDRs and other			61
Net income attributable to MPLX LP available to general and limited partners			73
General partner's two percent GP Interest in net income attributable to MPLX LP			1
Less: General partner’s GP interest in net income attributable to MPLX LP		$ 0	$ 62
[1]	Allocation of net income (loss) attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.

Equity - Cash Distributions (De

Equity - Cash Distributions (Details) - USD ($) $ / shares in Units, $ in Millions	Apr. 25, 2018	Mar. 31, 2018	Mar. 31, 2017
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distributions declared (including IDRs)		$ 483	$ 279
General partner's distributions		0	5
General partner's incentive distribution rights		0	60
Total general partner's distributions		0	65
Partners' distributions		483	279
Limited Partners Common Units
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Partners' distributions		467	198
Preferred Units
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Partners' distributions		$ 16	$ 16
Subsequent Event
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Declaration date	Apr. 25, 2018
Distributions declared (including IDRs)	$ 467
Cash distributions declared per limited partner common unit	$ 0.6175
Distribution date	May 15, 2018
Date of record	May 7, 2018

Redeemable Preferred Units (Nar

Redeemable Preferred Units (Narrative) (Details) - Series A Convertible Preferred Units $ / shares in Units, shares in Millions, $ in Millions	May 13, 2016USD ($)$ / sharesshares
Redeemable Noncontrolling Interest [Line Items]
Issuance of preferred units | shares	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, the Partnership may convert the Preferred units into common units at any time, in whole or in part, subject to certain minimum conversion amounts and conditions, if the closing price of MPLX LP common units is greater than $48.75 for the 20 day trading period immediately preceding the conversion notice date. The conversion rate for the Preferred units shall be the quotient of (a) the sum of (i) $32.50, plus (ii) any unpaid cash distributions on the applicable Preferred unit, divided by (b) $32.50. The holders of the Preferred units are entitled to vote on an as-converted basis with the common unitholders and will have certain other class voting rights with respect to any amendment to the Partnership Agreement that would adversely affect any rights, preferences or privileges of the Preferred units. In addition, upon certain events involving a change 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.

Redeemable Preferred Units (Rol

Redeemable Preferred Units (Rollforward of Redeemable Preferred Units) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Redeemable Noncontrolling Interest [Line Items]
Balance at December 31, 2017	$ 1,000
Distributions to unitholders and general partner	(347)	$ (242)
Balance at March 31, 2018	1,000
Series A Convertible Preferred Units
Redeemable Noncontrolling Interest [Line Items]
Balance at December 31, 2017	1,000
Net income	16
Distributions to unitholders and general partner	(16)
Balance at March 31, 2018	$ 1,000

Segment Information (Details)

Segment Information (Details)	3 Months Ended
	Mar. 31, 2018
Segment Reporting [Abstract]
Number of reportable segments	2

Segment Information - Segment O

Segment Information - Segment Operating Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018		Mar. 31, 2017
Segment Reporting Information [Line Items]
Service revenue	$ 382		$ 260
Segment revenues	207	[1]	203
Segment other income	4		3
Total segment revenues and other income	1,420		886
Purchased product costs	187		131
L&S
Segment Reporting Information [Line Items]
Service revenue	28
G&P
Segment Reporting Information [Line Items]
Service revenue	354
Operating Segments
Segment Reporting Information [Line Items]
Service revenue	1,215		746
Segment revenues	249		196
Segment other income	18		13
Total segment revenues and other income	1,482		955
Segment cost of revenues	469		261
Purchased product costs	194		140
Segment operating income before portion attributable to noncontrolling interests and Predecessor	819		554
Segment portion attributable to noncontrolling interests and Predecessor	45		89
Segment operating income attributable to MPLX LP	774		465
Operating Segments | L&S
Segment Reporting Information [Line Items]
Service revenue	646		345
Segment revenues	0		0
Segment other income	12		12
Total segment revenues and other income	658		357
Segment cost of revenues	234		148
Purchased product costs	0		0
Segment operating income before portion attributable to noncontrolling interests and Predecessor	424		209
Segment portion attributable to noncontrolling interests and Predecessor	0		53
Segment operating income attributable to MPLX LP	424		156
Operating Segments | G&P
Segment Reporting Information [Line Items]
Service revenue	569		401
Segment revenues	249		196
Segment other income	6		1
Total segment revenues and other income	824		598
Segment cost of revenues	235		113
Purchased product costs	194		140
Segment operating income before portion attributable to noncontrolling interests and Predecessor	395		345
Segment portion attributable to noncontrolling interests and Predecessor	45		36
Segment operating income attributable to MPLX LP	$ 350		$ 309
[1]	G&P “Product sales” exclude approximately $1 million of revenue related to derivative gains/losses and mark-to-market adjustments.

Segment Information - Reconcili

Segment Information - Reconciliation to Income from Operations (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Segment Reporting, Revenue Reconciling Item [Line Items]
Segment portion attributable to Predecessor		$ 0	$ 36
Income from equity method investments	[1]	61	5
Other income - related parties		23	22
Depreciation and amortization		(176)	(187)
General and administrative expenses		(69)	(58)
Income from operations		557	265
Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Segment operating income attributable to MPLX LP		774	465
Segment Reconciling Items
Segment Reporting, Revenue Reconciling Item [Line Items]
Segment portion attributable to unconsolidated affiliates		(53)	(40)
Segment portion attributable to Predecessor		0	53
Income from equity method investments		61	5
Other income - related parties		13	11
Unrealized derivative gains	[2]	$ 7	$ 16
[1]	“Income (loss) from equity method investments” includes the impact of any basis differential amortization or accretion.
[2]	The Partnership makes a distinction between realized or 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

Segment Information - Reconci80

Segment Information - Reconciliation to Total Revenues and Other Income (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues and other income		$ 1,420	$ 886
Income from equity method investments	[1]	61	5
Other income - related parties		23	22
Operating Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues and other income		1,482	955
Segment Reconciling Items
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue adjustment from unconsolidated affiliates		(137)	(92)
Income from equity method investments		61	5
Other income - related parties		13	11
Unrealized derivative gains related to product sales(1)	[2]	$ 1	$ 7
[1]	“Income (loss) from equity method investments” includes the impact of any basis differential amortization or accretion.
[2]	The Partnership makes a distinction between realized or 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

Segment Information - Reconci81

Segment Information - Reconciliation to Net Income Attributable to Noncontrolling Interests (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Reconciliation to Net Income Attributable to Noncontrolling Interests [Line Items]
Net income attributable to noncontrolling interests and Predecessor	$ 2	$ 37
Operating Segments
Reconciliation to Net Income Attributable to Noncontrolling Interests [Line Items]
Segment portion attributable to noncontrolling interests and Predecessor	45	89
Segment Reconciling Items
Reconciliation to Net Income Attributable to Noncontrolling Interests [Line Items]
Portion of noncontrolling interests and Predecessor related to items below segment income from operations	(19)	(36)
Portion of operating income attributable to noncontrolling interests of unconsolidated affiliates	$ (24)	$ (16)

Segment Information - Reconci82

Segment Information - Reconciliation of Capital Expenditures (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting, Other Significant Reconciling Item [Line Items]
Additions to property, plant and equipment	$ (455)	$ (280)
Operating Segments
Segment Reporting, Other Significant Reconciling Item [Line Items]
Additions to property, plant and equipment	(509)	(404)
Operating Segments | L&S
Segment Reporting, Other Significant Reconciling Item [Line Items]
Additions to property, plant and equipment	(190)	(97)
Operating Segments | G&P
Segment Reporting, Other Significant Reconciling Item [Line Items]
Additions to property, plant and equipment	(319)	(307)
Segment Reconciling Items
Segment Reporting, Other Significant Reconciling Item [Line Items]
Additions to property, plant and equipment	$ 54	$ 124

Segment Information - Assets by

Segment Information - Assets by Segment (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Segment Reporting, Other Significant Reconciling Item [Line Items]
Equity method investments	$ 4,033	$ 4,010
Cash and cash equivalents	2	5
Assets	21,006	19,500
L&S | Operating Segments
Segment Reporting, Other Significant Reconciling Item [Line Items]
Equity method investments	1,170	1,150
Assets	5,958	4,611
G&P | Operating Segments
Segment Reporting, Other Significant Reconciling Item [Line Items]
Equity method investments	2,860	2,860
Assets	$ 15,046	$ 14,884

Inventories (Summary of Invento

Inventories (Summary of Inventories) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Inventory Disclosure [Abstract]
NGLs	$ 2	$ 4
Line fill	7	8
Spare parts, materials and supplies	55	53
Total inventories	$ 64	$ 65

Property, Plant and Equipment85

Property, Plant and Equipment (Summary of Property, Plant and Equipment) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 16,454	$ 14,462
Less accumulated depreciation	3,163	2,275
Property, plant and equipment, net	13,291	12,187
Natural gas gathering and NGL transportation pipelines and facilities
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	5,253	5,178
Processing, fractionation and storage facilities
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	4,729	3,893
Pipelines and related assets
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	2,388	2,253
Barges and towing vessels
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	553	490
Terminals and related assets
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	827	821
Refineries and related assets
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	839	0
Land, building, office equipment and other
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	873	770
Construction-in-progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 992	$ 1,057

Fair Value Measurements - Recur

Fair Value Measurements - Recurring - Financial Instruments by Valuation Hierarchy (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Embedded Derivative Renewal Term	5 years
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	$ 0	$ 0
Derivative liability	60,000,000	66,000,000
Commodity contracts | Fair Value, Measurements, Recurring [Member] | 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	$ 1,000,000	2,000,000
Embedded derivatives in commodity contracts | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs Probability of Renewal	62.50%
Fair Value Inputs Probability of Renewal Second Term	82.00%
Embedded derivatives in commodity contracts | Fair Value, Measurements, Recurring [Member] | 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	59,000,000	$ 64,000,000
Minimum [Member] | Commodity contracts | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs Forward Commodity Price	0.26
Maximum [Member] | Commodity contracts | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs Forward Commodity Price	$ 1.47

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, 2018
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	62.50%
Fair Value Inputs Probability of Renewal Second Term	82.00%

Fair Value Measurements - Rec88

Fair Value Measurements - Recurring - Changes in Level 3 Measurements (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
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		$ (2)	$ (6)
Total gains (losses) (realized and unrealized) included in earnings	[1]	0	5
Settlements		0	1
Fair value at end of period		(2)	0
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	5
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		(64)	(54)
Total gains (losses) (realized and unrealized) included in earnings	[1]	3	8
Settlements		3	2
Fair value at end of period		(58)	(44)
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		$ 3	$ 8
[1]	(1)Gains and losses on Commodity Derivative Contracts classified as Level 3 are recorded in “Product sales” in the accompanying Consolidated Statements of Income. Gains and losses derivatives embedded in commodity contracts are recorded in “Purchased product costs” and “Cost of revenues.”

Fair Value Measurements - Repor

Fair Value Measurements - Reported (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	$ 11,934	$ 6,966
SMR liability	90	91
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	12,442	7,718
SMR liability	$ 100	$ 104

Derivative Financial Instrume90

Derivative Financial Instruments - Volume of Commodity Derivative Activity (Details) - Not Designated as Hedging Instrument [Member] - Commodity contracts	Mar. 31, 2018MMBTUgal
Short
Derivative [Line Items]
Nonmonetary Notional Amount of Price Risk Derivatives of Natural Gas Liquids | gal	7,696,503
Long
Derivative [Line Items]
Nonmonetary Notional Amount of Price Risk Derivatives Of Natural Gas | MMBTU	745,045

Derivative Financial Instrume91

Derivative Financial Instruments - Embedded Derivatives in Commodity Contracts (Details) $ in Millions		3 Months Ended
		Mar. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Natural Gas [Member] | Embedded derivatives in commodity contracts
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		$ 59	$ 64
Not Designated as Hedging Instrument [Member] | Commodity contracts
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability	[1]	60	66
Not Designated as Hedging Instrument [Member] | Other Noncurrent Liabilities [Member] | Commodity contracts
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability	[1]	$ 48	$ 52
[1]	Includes embedded derivatives in commodity contracts as discussed above.

Derivative Financial Instrume92

Derivative Financial Instruments - Derivatives Balance Sheet Location (Details) - Not Designated as Hedging Instrument [Member] - Commodity contracts - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	$ 0	$ 0
Derivative Liability, Fair Value, Gross Liability	[1]	60	66
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]	12	14
Other noncurrent assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	$ 0	$ 0
[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, 2018	Mar. 31, 2017
Derivative [Line Items]
Total gain (loss)	$ 4	$ 13
Product sales
Derivative [Line Items]
Realized gain (loss)	0	(1)
Unrealized gain (loss)	1	7
Total gain (loss)	1	6
Purchased product costs
Derivative [Line Items]
Realized gain (loss)	(3)	(2)
Unrealized gain (loss)	6	9
Total gain (loss)	3	7
Cost of revenues
Derivative [Line Items]
Realized gain (loss)	0	0
Unrealized gain (loss)	0	0
Total gain (loss)	$ 0	$ 0

Debt - Summary of Outstanding B

Debt - Summary of Outstanding Borrowings (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Feb. 08, 2018	Jan. 02, 2018	Dec. 31, 2017	Jul. 21, 2017	Feb. 10, 2017
Debt Instrument [Line Items]
Total	$ 12,357			$ 7,362
Unamortized debt issuance costs	(79)			(27)
Unamortized discount	(416)			(389)
Amounts due within one year	(1)			(1)
Total long-term debt due after one year	11,861			6,945
Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross		$ 5,500
Capital Lease Obligations [Member] | Marathon Pipe Line LLC [Member]
Debt Instrument [Line Items]
MPL - capital lease obligations due 2020	7			7
Bank revolving credit facility due 2022 | MPLX LP
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity					$ 2,250
Repayments of Long-term Lines of Credit	555
Bank revolving credit facility due 2022 | Line of Credit [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	0			505
5.500% senior notes due February 2023 | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	710			710
Senior Notes Due March 2023 [Member] | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	500			0
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		$ 1,250
Senior Notes Due March 2028 [Member] | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	1,250			0
Senior Notes Due April 2038 [Member] | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	1,750			0
5.200% senior notes due March 2047 | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	1,000			1,000		$ 1,000
Senior Notes Due April 2048 [Member] | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	1,500			0
Senior Notes Due April 2058 [Member] | Senior Notes [Member] | MPLX LP
Debt Instrument [Line Items]
Long-term debt, gross	500			0
MarkWest - 4.500% - 5.500% senior notes, due 2023-2025 | Senior Notes [Member] | MarkWest [Member]
Debt Instrument [Line Items]
Long-term debt, gross	$ 63			$ 63
MPLX 364-Day Term Loan [Member] | MPLX LP
Debt Instrument [Line Items]
Current borrowing capacity			$ 4,100

Debt Debt - Summary of Outstand

Debt Debt - Summary of Outstanding Borrowings - Interest Rates and Table Due Dates (Details)	3 Months Ended
	Mar. 31, 2018	Feb. 10, 2017
MarkWest [Member] | Senior Notes [Member] | 5.500% senior notes due February 2023
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.50%
Debt Instrument, Maturity Date	Feb. 15, 2023
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]
Expiration date	Jul. 21, 2022
MPLX LP | Senior Notes [Member] | 5.500% senior notes due February 2023
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.50%
Debt Instrument, Maturity Date	Feb. 15, 2023
MPLX LP | Senior Notes [Member] | Senior Notes Due March 2023 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	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%	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%
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%
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%	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%
Debt Instrument, Maturity Date	Apr. 15, 2048
MPLX LP | Senior Notes [Member] | Senior Notes Due April 2058 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.90%
Debt Instrument, Maturity Date	Apr. 15, 2058
Marathon Pipe Line LLC [Member] | Capital Lease Obligations [Member]
Debt Instrument [Line Items]
Capital Lease Due Date Year	2,020

Debt - Additional Information (

Debt - Additional Information (Detail) - MPLX LP - USD ($) $ in Millions	Feb. 08, 2018	Jul. 21, 2017	Oct. 27, 2015	Mar. 31, 2018	Jan. 02, 2018	Dec. 31, 2017	Feb. 10, 2017	Dec. 31, 2016
MPLX Revolver [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity								$ 2,000
Debt Instrument, Term			5 years
Bank revolving credit facility due 2022
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 2,250
Debt Instrument, Term		5 years
Proceeds from Lines of Credit				$ 50
Debt Instrument, Interest Rate, Effective Percentage				2.975%
Letters of Credit Outstanding				$ 3
Line of Credit Facility, Remaining Borrowing Capacity				$ 2,247
Line of Credit Facility, Remaining Borrowing Capacity, Percentage				99.90%
MPLX 364-Day Term Loan [Member]
Debt Instrument [Line Items]
Current borrowing capacity					$ 4,100
Repayments of Short-term Debt	$ 4,100
Line of Credit [Member] | Bank revolving credit facility due 2022
Debt Instrument [Line Items]
Long-term debt, gross				$ 0		$ 505
Senior Notes [Member]
Debt Instrument [Line Items]
Long-term debt, gross	$ 5,500
Debt Instrument, Face Amount							$ 2,250
Proceeds from new debt				2,220
Senior Notes [Member] | Senior Notes Due March 2023 [Member]
Debt Instrument [Line Items]
Long-term debt, gross				$ 500		0
Debt Instrument, Interest Rate, Stated Percentage				3.375%
Percent of Par	99.931%
Senior Notes [Member] | 4.125% senior notes due March 2027
Debt Instrument [Line Items]
Long-term debt, gross				$ 1,250		1,250	$ 1,250
Debt Instrument, Interest Rate, Stated Percentage				4.125%			4.125%
Senior Notes [Member] | 5.200% senior notes due March 2047
Debt Instrument [Line Items]
Long-term debt, gross				$ 1,000		1,000	$ 1,000
Debt Instrument, Interest Rate, Stated Percentage				5.20%			5.20%
Senior Notes [Member] | Senior Notes Due March 2028 [Member]
Debt Instrument [Line Items]
Long-term debt, gross				$ 1,250		0
Debt Instrument, Interest Rate, Stated Percentage				4.00%
Percent of Par	99.551%
Senior Notes [Member] | Senior Notes Due April 2038 [Member]
Debt Instrument [Line Items]
Long-term debt, gross				$ 1,750		0
Debt Instrument, Interest Rate, Stated Percentage				4.50%
Percent of Par	98.811%
Senior Notes [Member] | Senior Notes Due April 2048 [Member]
Debt Instrument [Line Items]
Long-term debt, gross				$ 1,500		0
Debt Instrument, Interest Rate, Stated Percentage				4.70%
Percent of Par	99.348%
Senior Notes [Member] | Senior Notes Due April 2058 [Member]
Debt Instrument [Line Items]
Long-term debt, gross				$ 500		$ 0
Debt Instrument, Interest Rate, Stated Percentage				4.90%
Percent of Par	99.289%

Revenue Effect of ASC 606 Adopt

Revenue Effect of ASC 606 Adoption (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018		Mar. 31, 2017	Dec. 31, 2017
Cost of revenues		$ 206	[1]	$ 113
Rental income		79		69
Rental cost of sales		29		12
Service revenue - product related		44		0
Purchased product costs		187		131
Product sales		207	[2]	203
Service revenue - related parties		471		255
Product sales - related parties		4		2
Service revenue		382		$ 260
Difference between Revenue Guidance in Effect before and after Topic 606
Cost of revenues	[1]	78
Rental income		16
Rental cost of sales		16
Service revenue - product related		44
Purchased product costs		24
Product sales	[2]	(21)
Service revenue - related parties		(1)
Product sales - related parties		1
Service revenue		77
Difference between Revenue Guidance in Effect before and after Topic 606 | Third Party Reimbursements
Cost of revenues	[1]	78
Rental income		16
Rental cost of sales		16
Service revenue		78
Difference between Revenue Guidance in Effect before and after Topic 606 | Non-cash Consideration
Service revenue - product related		11
Purchased product costs		12
Difference between Revenue Guidance in Effect before and after Topic 606 | Percent-of-Proceeds
Service revenue - product related		33
Product sales		(33)
Difference between Revenue Guidance in Effect before and after Topic 606 | Imbalances
Purchased product costs		12
Product sales	[2]	12
Difference between Revenue Guidance in Effect before and after Topic 606 | Oil Allowances
Product sales	[2]	1
Service revenue - related parties		(1)
Product sales - related parties		1
Service revenue		$ (1)
ASC 606 | Inventories
Cumulative effect of new accounting principle in period of adoption					$ 1
ASC 606 | Property, plant and equipment
Cumulative effect of new accounting principle in period of adoption					$ (3)
[1]	Excludes “Purchased product costs,” “Rental cost of sales,” “Purchases,” “Depreciation and amortization,” “General and administrative expenses,” and “Other taxes.”
[2]	G&P “Product sales” exclude approximately $1 million of revenue related to derivative gains/losses and mark-to-market adjustments.

Revenue ASC 606 Cumulative Effe

Revenue ASC 606 Cumulative Effect Adjustments (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Inventories	$ 64	$ 65
Property, plant and equipment, net	13,291	12,187
Long-term deferred revenue	49	42
Common unitholders - public	6,832	9,827
Cumulative Adjusted 606 Balance
Inventories		66
Property, plant and equipment, net		12,184
Long-term deferred revenue		39
ASC 606
Long-term deferred revenue	8	7	[1]
ASC 606 | Inventories
Cumulative effect of new accounting principle in period of adoption		1
ASC 606 | Property, plant and equipment
Cumulative effect of new accounting principle in period of adoption		(3)
ASC 606 | Long-term deferred revenue
Cumulative effect of new accounting principle in period of adoption		(3)
Limited Partners Common Units | Public
Common unitholders - public	$ 8,385	8,379
Limited Partners Common Units | Public | Cumulative Adjusted 606 Balance
Common unitholders - public		8,380
Limited Partners Common Units | Public | ASC 606 | Equity
Cumulative effect of new accounting principle in period of adoption		$ 1
[1]	Balance represents ASC 606 portion of each respective line items.

Revenue Impact of Adoption (Det

Revenue Impact of Adoption (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018		Mar. 31, 2017
Revenues and other income:
Service revenue		$ 382		$ 260
Service revenue - related parties		471		255
Service revenue - product related		44		0
Rental income		79		69
Product sales		207	[1]	203
Product sales - related parties		4		2
Costs and expenses:
Cost of revenues		206	[2]	113
Rental cost of sales		29		12
Purchased product costs		187		131
Depreciation and amortization		176		187
Net income		423		187
Product sales
Costs and expenses:
Unrealized gain (loss)		1		$ 7
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Revenues and other income:
Service revenue		305
Service revenue - related parties		472
Service revenue - product related		0
Rental income		63
Product sales	[1]	228
Product sales - related parties		3
Costs and expenses:
Cost of revenues	[2]	128
Rental cost of sales		13
Purchased product costs		163
Depreciation and amortization		176
Net income		425
Difference between Revenue Guidance in Effect before and after Topic 606
Revenues and other income:
Service revenue		77
Service revenue - related parties		(1)
Service revenue - product related		44
Rental income		16
Product sales	[1]	(21)
Product sales - related parties		1
Costs and expenses:
Cost of revenues	[2]	78
Rental cost of sales		16
Purchased product costs		24
Depreciation and amortization		0
Net income		$ (2)
[1]	G&P “Product sales” exclude approximately $1 million of revenue related to derivative gains/losses and mark-to-market adjustments.
[2]	Excludes “Purchased product costs,” “Rental cost of sales,” “Purchases,” “Depreciation and amortization,” “General and administrative expenses,” and “Other taxes.”

Revenue Disaggregation of Reven

Revenue Disaggregation of Revenue (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Service revenue		$ 382	$ 260
Service revenue - related parties		471	255
Service revenue - product related		44	0
Product sales	[1]	206
Product sales - related parties		4	2
Total revenues from contracts with customers		1,107
Other income		4	3
Total segment revenues and other income		1,420	886
Income from equity method investments	[2]	(61)	(5)
Other income - related parties		(23)	(22)
L&S
Disaggregation of Revenue [Line Items]
Service revenue		28
Service revenue - related parties		471
Service revenue - product related		0
Product sales	[1]	1
Product sales - related parties		1
Total revenues from contracts with customers		501
G&P
Disaggregation of Revenue [Line Items]
Service revenue		354
Service revenue - related parties		0
Service revenue - product related		44
Product sales	[1]	205
Product sales - related parties		3
Total revenues from contracts with customers		606
Segment Reconciling Items
Disaggregation of Revenue [Line Items]
Revenue adjustment from unconsolidated affiliates		137	92
Income from equity method investments		(61)	(5)
Other income - related parties		(13)	(11)
Unrealized derivative gains related to product sales	[3]	(1)	(7)
Operating Segments
Disaggregation of Revenue [Line Items]
Service revenue		1,215	746
Other income		18	13
Total segment revenues and other income		1,482	955
Operating Segments | L&S
Disaggregation of Revenue [Line Items]
Service revenue		646	345
Other income		12	12
Total segment revenues and other income		658	357
Operating Segments | G&P
Disaggregation of Revenue [Line Items]
Service revenue		569	401
Other income		6	1
Total segment revenues and other income		824	598
Product sales
Disaggregation of Revenue [Line Items]
Unrealized gain (loss)		1	$ 7
Other Income [Member]
Disaggregation of Revenue [Line Items]
Other income	[4]	$ 313
[1]	G&P “Product sales” exclude approximately $1 million of revenue related to derivative gains/losses and mark-to-market adjustments.
[2]	“Income (loss) from equity method investments” includes the impact of any basis differential amortization or accretion.
[3]	The Partnership makes a distinction between realized or 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
[4]	Non-ASC 606 Revenue includes rental income, income from equity method investments, derivative gains/losses, mark-to-market adjustments, and other income.

Revenue Contract Balances (Deta

Revenue Contract Balances (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018USD ($)
Deferred revenue - related parties, beginning balance	$ 43
Deferred revenue - related parties, ending balance	43
Long-term deferred revenue, beginning balance	42
Long-term deferred revenue, ending balance	49
Long-term deferred revenue - related parties, beginning balance	43
Long-term deferred revenue - related parties, ending balance	49
Liability, change in timeframe, performance obligation satisfied, revenue recognized	0
ASC 606
Contract assets, beginning balance	4	[1]
Contract assets, additions/(deletions)	0
Contract assets, revenue recognized	0	[2]
Contract assets, ending balance	4
Deferred revenue, beginning balance	5	[1]
Deferred revenue, additions/(deletions)	2
Deferred revenue, revenue recognized	(2)	[2]
Deferred revenue, ending balance	5
Deferred revenue - related parties, beginning balance	42	[1]
Deferred revenue - related party, additions/(deletions)	9
Deferred revenue - related parties, revenue recognized	(9)	[2]
Deferred revenue - related parties, ending balance	42
Long-term deferred revenue, beginning balance	7	[1]
Long-term deferred revenue, additions/(deletions)	1
Long-term deferred revenue, revenue recognized	0	[2]
Long-term deferred revenue, ending balance	8
Long-term deferred revenue - related parties, beginning balance	39	[1]
Long-term deferred revenue - related party, additions/(deletions)	6
Long-term deferred revenue - related parties, revenue recognized	0	[2]
Long-term deferred revenue - related parties, ending balance	$ 45
[1]	Balance represents ASC 606 portion of each respective line items.
[2]	No revenue was recognized related to past performance obligations, in the current period. Changes in long-term amounts represent reclassifications to current balances.

Revenue Remaining Performance O

Revenue Remaining Performance Obligations (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Contract with customer, liability	$ 96
Remaining performance obligation, expected timing of satisfaction, years	19 years
L&S
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
2,018	$ 704
2,019	929
2,020	929
2,021	929
2022 and thereafter	5,577
Total revenue on remaining performance obligations	9,068	[1],[2]
G&P
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
2,018	91	[3]
2,019	118	[3]
2,020	114	[3]
2,021	113	[3]
2022 and thereafter	404	[3]
Total revenue on remaining performance obligations	$ 840	[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. The Partnership 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 Infor103

Supplemental Cash Flow Information - Change in Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Millions		Mar. 31, 2018		Dec. 31, 2017		Mar. 31, 2017	Dec. 31, 2016
Supplemental Cash Flow Elements [Abstract]
Cash and cash equivalents		$ 2		$ 5
Restricted cash	[1]	4		4
Cash, cash equivalents and restricted cash		$ 6	[2]	$ 9	[2]	$ 268	$ 239
[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, 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 Infor104

Supplemental Cash Flow Information - Summary of Supplemental Cash Flow Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Net cash provided by operating activities included:
Interest paid (net of amounts capitalized)	$ 103	$ 49
Non-cash investing and financing activities:
Net transfers of property, plant and equipment from materials and supplies inventories	1	6
Contribution of fixed assets to joint venture	$ 0	$ 328	[1]
[1]	Contribution of assets to Sherwood Midstream and Sherwood Midstream Holdings. See Note 5.

Supplemental Cash Flow Infor105

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, 2018	Mar. 31, 2017
Supplemental Cash Flow Elements [Abstract]
(Decrease) increase in capital accruals	$ (6)	$ 2

Accumulated Other Comprehens106

Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Mar. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss	[1]	$ (14)	$ (16)
Other Comprehensive Income (Loss), Net of Tax	[2]	(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]	(13)	(14)
Other Comprehensive Income (Loss), Net of Tax	[2]	(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)
Other Comprehensive Income (Loss), Net of Tax	[2]	$ (1)
[1]	These accumulated other comprehensive loss components are included in the computation of net periodic benefit cost by LOOP and Explorer and are therefore included in the Consolidated Statements of Income under the caption “Income (loss) from equity method investments.”
[2]	Components of “Other comprehensive loss - remeasurements” relate to actuarial gains and losses as well as amortization of prior service costs. The Partnership records an adjustment to comprehensive income in accordance with its ownership interest in LOOP and Explorer.

Equity-Based Compensation Pl107

Equity-Based Compensation Plan - Summary of Phantom Unit Award Activity (Details) - Phantom Units	3 Months Ended
	Mar. 31, 2018$ / sharesshares
Number of Units
Outstanding at December 31, 2017 | shares	1,351,523
Granted | shares	169,310
Settled | shares	217,093
Forfeited | shares	46,330
Outstanding at March 31, 2018 | shares	1,257,410
Weighted Average Fair Value
Outstanding at December 31, 2017 | $ / shares	$ 34.53
Granted | $ / shares	35
Settled | $ / shares	33.83
Forfeited | $ / shares	34.95
Outstanding at March 31, 2018 | $ / shares	$ 34.70

Equity-Based Compensation Pl108

Equity-Based Compensation Plan - Additional Information (Detail) - Officer - Performance Shares [Member] - MPLX LP 2012 Incentive Compensation Plan [Member]	3 Months Ended
	Mar. 31, 2018
Equity Transactions And Share Based Compensation [Line Items]
Percentage paid out in cash	25.00%
Percentage paid out in stock	75.00%

Equity-Based Compensation Pl109

Equity-Based Compensation Plan - Summary of Performance Unit Award Activity (Detail) - Performance Shares [Member]	3 Months Ended
	Mar. 31, 2018shares
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Outstanding at December 31, 2017	2,536,594
Granted	0
Settled	538,594
Forfeited	(50,000)
Outstanding at March 31, 2018	1,948,000

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	Sep. 06, 2016	Mar. 31, 2018	Dec. 31, 2017
Commitments And Contingencies [Line Items]
Accrued liabilities for environmental remediation		$ 15	$ 13
Payables - related parties		146	516
Contractual commitments to acquire property, plant and equipment		718
MPC
Commitments And Contingencies [Line Items]
Payables - related parties		115	470
Environmental Loss Contingency [Member] | MPC
Commitments And Contingencies [Line Items]
Payables - related parties			$ 1
Receivables from MPC for indemnification of environmental costs		3
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
Markwest Liberty Midstream [Member]
Commitments And Contingencies [Line Items]
EPA proposed penalty		0.6
Estimated cost of proposed supplemental environmental projects		$ 2.4

Subsequent Events (Details)

Subsequent Events (Details) - MPC Investment - Related Party Revolving Credit Agreement - USD ($) $ in Millions	Apr. 26, 2018	Mar. 31, 2018
Subsequent Event [Line Items]
Current borrowing capacity		$ 500
Subsequent Event
Subsequent Event [Line Items]
Current borrowing capacity	$ 1,000