MPLX (MPLX) 10-Q 1 May 17 2017 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2017	Apr. 27, 2017
Document Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2017
Document Fiscal Year Focus	2,017
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		374,754,198
Class B Units [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		3,990,878
General Partner Units [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		7,722,352

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) shares in Millions, $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Revenues and other income:
Service revenue		$ 260	$ 229
Service revenue - related parties		255	177	[1]
Rental income		69	70
Rental income - related parties		67	38	[1]
Product sales		203	100
Product sales - related parties		2	3
Gain (Loss) on Disposition of Assets for Financial Service Operations		1	0
Income from equity method investments	[2]	5	5
Other income		2	2
Other income - related parties		22	21	[1]
Total revenues and other income		886	645	[1]
Costs and expenses:
Cost of revenues (excludes items below)		113	94	[1]
Purchased product costs		131	79
Rental cost of sales		12	14
Purchases - related parties		107	78	[1]
Depreciation and amortization		187	136	[1],[3]
Impairment expense		0	129
General and administrative expenses		58	53	[1]
Other taxes		13	12	[1]
Total costs and expenses		621	595	[1]
Income from operations		265	50	[1]
Related party interest and other financial costs		0	1
Interest expense (net of amounts capitalized of $7 million and $7 million, respectively)		66	55
Other financial costs		12	12
Income (loss) before income taxes		187	(18)	[1]
Benefit for income taxes		0	4
Net income (loss)		187	(14)	[1],[3]
Net income attributable to noncontrolling interest		1	0
Less: Net income attributable to Predecessor		36	46	[1]
Net income (loss) attributable to MPLX LP	[4]	150	(60)
Less: Preferred unit distributions		16	0
Less: General partner’s interest in net income attributable to MPLX LP		62	39
Net Income (Loss) Allocated to Limited Partners		$ 72	$ (99)
Weighted average limited partner units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic		400	307
Weighted Average Limited Partnership Units Outstanding, Diluted		405	307
MPC [Member]
Revenues and other income:
Service revenue - related parties		$ 255	$ 177
Rental income - related parties		67	38
Product sales - related parties	[5]	2	3
Other income - related parties		$ 11	$ 14
[1]	(1)Financial information has been retrospectively adjusted for the acquisition of HST and WHC from MPC. See Notes 1 and 3.
[2]	(1)Income (loss) from equity method investments includes the impact of any basis differential amortization or accretion.
[3]	(1)Financial information has been retrospectively adjusted for the acquisition of HST and WHC from MPC. See Notes 1 and 3.
[4]	llocation of net income (loss) attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.
[5]	(1)For the three months ended March 31, 2017 and 2016, there were $57 million and $5 million, respectively, of 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.

Consolidated Statements of Inc3

Consolidated Statements of Income Consolidated Statements of Income (Parenthetical) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Income Statement [Abstract]
Interest costs capitalized	$ 7	$ 7

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions		Mar. 31, 2017	Dec. 31, 2016		Mar. 31, 2016
Current assets:
Cash and cash equivalents		$ 265	$ 234		$ 4
Receivables, net		255	299	[1]
Receivables - related parties		198	247	[1]
Inventories		62	55	[1]
Other current assets		31	33
Total current assets		811	868	[1]
Equity method investments		3,306	2,471	[1]
Property, plant and equipment, net		11,411	11,408	[1]
Intangibles, net		482	492
Goodwill		2,245	2,245	[1]
Long-term receivables - related parties		13	11	[1]
Other noncurrent assets		17	14
Total assets		18,285	17,509	[1]
Current liabilities:
Accounts payable		121	140	[1]
Accrued liabilities		194	232	[1]
Payables - related parties		91	87	[1]
Deferred Revenue, Current		3	2
Deferred revenue - related parties		38	38	[1]
Accrued property, plant and equipment		138	146	[1]
Accrued taxes		31	38	[1]
Accrued interest payable		70	53
Other current liabilities		26	27	[1]
Total current liabilities		712	763	[1]
Long-term deferred revenue		19	12
Long-term deferred revenue - related parties		26	19	[1]
Long-term debt		6,654	4,422
Deferred income taxes		6	6	[1]
Deferred credits and other liabilities		168	177	[1]
Total liabilities		7,585	5,399	[1]
Commitments and contingencies (see Note 17)
Redeemable preferred units		1,000	1,000
Equity
Total MPLX LP partners’ capital		9,557	11,092	[1]
Noncontrolling interest		143	18
Total equity		9,700	11,110	[1],[2]	9,957	[2]
Total liabilities, preferred units and equity		18,285	17,509	[1]
MPC [Member]
Current assets:
Receivables - related parties		177	242
Long-term receivables - related parties		13	11
Current liabilities:
Payables - related parties		59	63
Limited Partners Common Units [Member] | MPC [Member]
Equity
Total MPLX LP partners’ capital		1,184	1,069
GP Held Limited Partners Common Units [Member] | MPC [Member]
Equity
Total MPLX LP partners’ capital		350			0
Limited Partners Class B Units [Member]
Equity
Total MPLX LP partners’ capital		133	133
General Partner [Member] | MPC [Member]
Equity
Total MPLX LP partners’ capital		(257)	1,013
Public [Member] | Limited Partners Common Units [Member]
Equity
Total MPLX LP partners’ capital		8,147	8,086
Predecessor [Member] | MPC [Member]
Equity
Total MPLX LP partners’ capital		$ 0	791	[1]
Total equity	[2]		$ 791		$ 302
[1]	Financial information has been retrospectively adjusted for the acquisition of HST, WHC and MPLXT from MPC. See Notes 1 and 3.
[2]	(1)Financial information has been retrospectively adjusted for the acquisition of HST, WHC and MPLXT from MPC. See Notes 1 and 3.

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - shares shares in Millions	Mar. 31, 2017	Dec. 31, 2016
Limited Partners Common Units [Member] | Public [Member]
Units issued	275	271
Units outstanding	275	271
Limited Partners Class B Units [Member]
Units issued	4	4
Units outstanding	4	4
MPC [Member] | Limited Partners Common Units [Member]
Units issued	90	86
Units outstanding	90	86
MPC [Member] | GP Held Limited Partners Common Units [Member]
Units issued	9	0
Units outstanding	9	0
MPC [Member] | General Partner [Member]
GP units issued	8	7
GP units outstanding	8	7

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Operating activities:
Net income (loss)		$ 187	$ (14)	[1],[2]
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Amortization of deferred financing costs		12	11
Depreciation and amortization		187	136	[1],[2]
Impairment expense		0	129
Deferred income taxes		0	(4)
Asset retirement expenditures		1	0
Gain (loss) on disposal of assets		1	0
Income from equity method investments	[3]	5	5
Distributions from unconsolidated affiliates		33	38
Changes in:
Current receivables		44	(5)
Inventories		0	(1)
Change in fair value of derivatives		18	(12)
Current accounts payable and accrued liabilities		(59)	(18)	[2]
Receivables from / liabilities to related parties		(18)	23
All other, net		16	17	[2]
Net cash provided by operating activities		377	321	[2]
Investing activities:
Additions to property, plant and equipment		(280)	(304)
Acquisitions, net of cash acquired		(220)
Disposal of assets		(1)	0
Investments - loans from (to) related parties		80	64	[2]
Investments in unconsolidated affiliates		554	29
Distributions from unconsolidated affiliates - return of capital		20
All other, net		2	3
Net cash used in investing activities		(953)	(266)	[2]
Financing activities:
Long-term debt - borrowings		2,241	306
Long-term debt - repayments		(1)	(857)
Related party debt - borrowings		12	1,437
Related party debt - repayments		12	1,007
Debt issuance costs		21	0
Net proceeds from equity offerings		151	321
Payments of Distributions on Preferred Units		(16)	0
Distributions to unitholders and general partner		(242)	(190)
Distributions to noncontrolling interests		(2)	(1)
Contributions from noncontrolling interests		126	2
All other, net		(5)	(1)
Net cash provided by (used in) financing activities		607	(94)
Net increase (decrease) in cash and cash equivalents		31	(39)
Cash and cash equivalents at beginning of period		234	43
Cash and cash equivalents at end of period		265	4
MPC [Member]
Financing activities:
Distributions to MPC from Predecessor		(1,511)
Predecessor [Member] | MPC [Member]
Financing activities:
Distributions to MPC from Predecessor		$ (113)	$ (104)
[1]	(1)Financial information has been retrospectively adjusted for the acquisition of HST and WHC from MPC. See Notes 1 and 3.
[2]	(1)Financial information has been retrospectively adjusted for the acquisition of HST and WHC from MPC. See Notes 1 and 3.
[3]	(1)Income (loss) from equity method investments includes the impact of any basis differential amortization or accretion.

Consolidated Statements of Equi

Consolidated Statements of Equity - USD ($) $ in Millions		Total		MPC [Member]	Limited Partners Common Units [Member]Public [Member]	Limited Partners Common Units [Member]MPC [Member]	GP Held Limited Partners Common Units [Member]MPC [Member]	Limited Partners Class B Units [Member]Public [Member]	General Partner [Member]MPC [Member]	Noncontrolling Interest [Member]	Noncontrolling Interest [Member]MPC [Member]
Beginning Balance (Predecessor [Member]) at Dec. 31, 2015	[1]			$ 692
Beginning Balance at Dec. 31, 2015		$ 9,946	[1]		$ 7,691	$ 465		$ 266	$ 819	$ 13
Distributions to MPC from Predecessor | Predecessor [Member]				(104)
Contributions from noncontrolling interests		2
Ending Balance (Predecessor [Member]) at Mar. 31, 2016	[1]			302
Ending Balance at Mar. 31, 2016		9,957	[1]		7,805	1,086		266	484		$ 14
Beginning Balance (Predecessor [Member]) at Dec. 31, 2015	[1]			692
Beginning Balance at Dec. 31, 2015		9,946	[1]		7,691	465		266	819	13
Distributions to MPC from Predecessor | Predecessor [Member]				(104)
Distributions to MPC from Predecessor				(104)
Issuance of units under ATM Program		321			315				6
Net income | Predecessor [Member]	[1]			46
Net income		(14)	[1]		(80)	(19)			39	0
Allocation of MPC's net investment at acquisition | Predecessor [Member]				(332)
Allocation of MPC's net investment at acquisition						669			(337)
Distributions to unitholders and general partner		(190)			(120)	(29)			(41)
Distributions to noncontrolling interest		(1)								(1)
Contributions from noncontrolling interests		2								2
Equity-based compensation		(2)			(2)
Deferred income tax impact from changes in equity		(5)			(3)	0			(2)
Ending Balance (Predecessor [Member]) at Dec. 31, 2016	[1]			791
Ending Balance at Dec. 31, 2016		11,110	[1],[2]		8,086	1,069		133	1,013	18
Distributions to MPC from Predecessor | Predecessor [Member]				(113)
Distributions to MPC from Predecessor				(1,511)		(430)			(1,081)
Issuance of units under ATM Program		151			148				3
Net income | Predecessor [Member]				36
Net income		171			55	17			62	1
Contribution from MPC | Predecessor [Member]				12
Contribution from MPC		12				0			0
Allocation of MPC's net investment at acquisition | Predecessor [Member]				$ (726)
Allocation of MPC's net investment at acquisition						573	$ 350		(197)
Distributions to unitholders and general partner		(242)			(140)	(45)			(57)
Distributions to noncontrolling interest		(2)								(2)
Contributions from noncontrolling interests		126								126
Equity-based compensation		(1)			(1)
Stockholders' Equity, Other		(1)			(1)
Ending Balance at Mar. 31, 2017		$ 9,700			$ 8,147	$ 1,184	$ 350	$ 133	$ (257)	$ 143
[1]	(1)Financial information has been retrospectively adjusted for the acquisition of HST, WHC and MPLXT from MPC. See Notes 1 and 3.
[2]	Financial information has been retrospectively adjusted for the acquisition of HST, WHC and MPLXT from MPC. See Notes 1 and 3.

Description of the Business and

Description of the Business and Basis of Presentation	3 Months Ended
	Mar. 31, 2017
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. MPLX LP and its subsidiaries (collectively, the “Partnership”) are engaged in the gathering, processing and transportation of natural gas; the gathering, transportation, fractionation, storage and marketing of NGLs; and the transportation, storage and distribution of crude oil and refined petroleum products principally for our sponsor. The Partnership’s business consists of two segments based on the nature of services it offers: Logistics and Storage (“L&S”) focused on crude oil and refined petroleum products and Gathering and Processing (“G&P”) focused on natural gas and NGLs. See Note 9 for additional information regarding operations. Basis of Presentation – 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 interest 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. Effective March 1, 2017 , the Partnership acquired pipeline, storage and terminal businesses which are operated through HST, WHC and MPLXT (collectively with HSM, the “Predecessor”) from MPC. The acquisition from MPC was considered a transfer between entities under common control. Accordingly, the Partnership recorded the acquisition from MPC on its Consolidated Balance Sheets at MPC’s historical basis instead of fair value. Transfers of businesses between entities under common control require prior periods to be retrospectively adjusted to furnish comparative information since inception of common control. Therefore, the accompanying consolidated financial statements and related notes of MPLX LP have been retrospectively adjusted to include the historical results of the assets acquired from MPC prior to the effective dates of the acquisition. See Note 3 for additional information regarding the HST, WHC and MPLXT acquisition. The accompanying financial statements and related notes present the combined financial position, results of operations, cash flows and equity of the Predecessor at historical cost. The financial statements of the Predecessor have been prepared from the separate records maintained by MPC and may not necessarily be indicative of the conditions or the results of operations that would have existed if the Predecessor had been operated as an unaffiliated entity. 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 8 , and subsequently allocated to the general partner and limited partner unitholders. Distributions, although earned, are not accrued for until declared. However, when distributions related to the incentive distribution rights are made, earnings equal to the amount of those distributions are first allocated to the general partner before the remaining earnings are allocated to the limited partner unitholders based on their respective ownership percentages. The allocation of net income attributable to MPLX LP for purposes of calculating net income per limited partner unit is described in Note 6 . 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. 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, 2016. The results of operations for the three months ended March 31, 2017 are not necessarily indicative of the results to be expected for the full year.

Accounting Standards

Accounting Standards	3 Months Ended
	Mar. 31, 2017
Accounting Standards [Abstract]
Accounting Standards	Accounting Standards Recently Adopted In October 2016, the FASB issued an accounting standard update to amend the consolidation guidance issued in February 2015 to require that a decision maker consider, in the determination of the primary beneficiary, its indirect interest in a VIE held by a related party that is under common control on a proportionate basis only. The change was effective for the financial statements for fiscal years beginning after December 15, 2016, and interim periods within those fiscal years. The Partnership was required to apply the standard retrospectively to January 1, 2016, the date on which the Partnership adopted the consolidation guidance issued in February 2015. The Partnership adopted this accounting standard update in the first quarter of 2017 and it did not have an impact on the consolidated financial statements. In March 2016, the FASB issued an accounting standard update on the accounting for employee share-based payments. This accounting standard update requires the recognition of income tax effects of awards through the income statement when awards vest or are settled. It will also increase the amount an employer can withhold for tax purposes without triggering liability accounting. Lastly, it allows employers to make a policy election to account for forfeitures as they occur. The changes were effective for fiscal years beginning after December 15, 2016, and interim periods within those fiscal years. Under the new guidance, the Partnership will continue estimating forfeiture rates to calculate compensation cost. The Partnership adopted this accounting standard update in the first quarter of 2017 and it did not have a material impact on the consolidated financial statements. Not Yet Adopted In February 2017, the FASB issued an accounting standard update addressing the derecognition of nonfinancial assets. The guidance defines in substance nonfinancial assets, and states that the derecognition of business activities should be evaluated under the consolidation guidance, with limited exceptions related to conveyances of oil and gas mineral rights or contracts with customers. The standard eliminates the previous exclusion for businesses that are in-substance real estate, and eliminates some differences based on whether a transferred set is that of assets or a business and whether the transfer is to a joint venture. The standard must be implemented in conjunction with the implementation date of the revenue recognition accounting standard update, which the Partnership will implement January 1, 2018. The Partnership plans to adopt the new standard using the modified retrospective method and is in the process of determining the impact of the accounting standard update on the consolidated financial statements together with its evaluation of the new revenue recognition standard, as described further below. In January 2017, the FASB issued an accounting standard 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; 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 in the process of determining the impact of the accounting standard update on the consolidated financial statements. In January 2017, the FASB issued an accounting standard update to clarify the definition of a business with the objective of adding guidance to assist entities with evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. The standard is intended to narrow the definition of a business by specifying the minimum inputs and processes and by narrowing the definition of outputs. The change is effective for fiscal years beginning after December 15, 2017, and interim periods within those fiscal years. The guidance will be applied prospectively and early adoption is permitted for certain transactions. The Partnership is in the process of determining the impact of the accounting standard update on the consolidated financial statements. In November 2016, the FASB issued an accounting standard update requiring that the statement of cash flows explain the change during the period in the total of cash, cash equivalents, and amounts generally described as restricted cash or restricted cash equivalents. The change is effective for fiscal years beginning after December 15, 2017, and interim periods within those fiscal years, with early adoption permitted. Retrospective application is required. The application of this accounting standard update will not have a material impact on the Consolidated Statements of Cash Flows. In August 2016, the FASB issued an accounting standard update related to the classification of certain cash flows. The accounting standard update provides specific guidance on eight cash flow classification issues, including debt prepayment or debt extinguishment costs and distributions received from equity method investees. The change is effective for fiscal years beginning after December 15, 2017, and interim periods within those fiscal years, with early adoption permitted. The Partnership does not expect application of this accounting standard update to have a material impact on the Consolidated Statements of Cash Flows. In June 2016, the FASB issued an accounting standard update related to the accounting for credit losses on certain financial instruments. The guidance requires that for most financial assets, losses are 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 standard update to have a material impact on the consolidated financial statements. In February 2016, the FASB issued an accounting standard update requiring lessees to record virtually all leases on their balance sheets. The accounting standard 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 fiscal years, with early adoption permitted. The Partnership is currently evaluating the impact of this standard on the Partnership’s 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 to implementation. The Partnership does not plan to early adopt the standard. In January 2016, the FASB issued an accounting standard update requiring unconsolidated equity investments, not accounted for under the equity method, to be measured at fair value with changes in fair value recognized in net income. The accounting standard update also requires the use of the exit price notion when measuring the fair value of financial instruments for disclosure purposes and the separate presentation of financial assets and liabilities by measurement category and form on the balance sheet and accompanying notes. The accounting standard update eliminates the requirement to disclose the methods and assumptions used in estimating the fair value of financial instruments measured at amortized cost. Lastly, the accounting standard update requires separate presentation in other comprehensive income of the portion of the total change in the fair value of a liability resulting from a change in the instrument-specific credit risk when electing to measure the liability at fair value in accordance with the fair value option for financial instruments. The changes are effective for fiscal years and interim periods within those fiscal years beginning after December 15, 2017. Early adoption is permitted only for guidance regarding presentation of the liability’s credit risk. The application of this accounting standard update will not have a material impact on the Partnership’s consolidated financial statements. In May 2014, the FASB issued an initial accounting standard update for revenue recognition for contracts with customers. The guidance in the accounting standard update states that revenue is recognized when a customer obtains control of a good or service. Recognition of the revenue will involve 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 then recognizing the revenue as the obligations are satisfied. Additional disclosures will be required to provide adequate information to understand the nature, amount, timing and uncertainty of reported revenues and revenues expected to be recognized. The change will be effective on a retrospective or modified retrospective basis for fiscal years beginning after December 15, 2017, and interim periods within those years, with early adoption permitted no earlier than January 1, 2017. The Partnership is currently evaluating the impact of the revenue recognition standard on the Partnership’s financial statements and disclosures, internal controls, and accounting policies. This evaluation process includes a phased approach, the first phase of which includes reviewing a sample of contracts and transaction types across segments. The Partnership is currently in the process of completing this first phase. Based on the results of the first phase assessment to date, the Partnership has reached tentative conclusions for most contract types and does not believe revenue recognition patterns for fee-based or percent-of-proceeds contracts will change materially. The Partnership is currently finalizing the accounting impact on keep-whole and percent-of-liquids agreements under the new standard, specifically related to the accounting for noncash consideration received in the form of a commodity product. The Partnership does expect certain amounts to be grossed up in revenue as a result of implementation. In the first quarter of 2017, the Partnership started to calculate the impact of the new standard on 2016 operating results and write new policies and procedures related to the application upon adoption. The Partnership will provide updates as qualitative and quantitative conclusions are reached throughout 2017. The Partnership will adopt the revenue recognition standard during the first quarter of 2018. The Partnership plans to adopt the new standard using the modified retrospective method which will result in a cumulative effect adjustment as of the date of adoption. By selecting this adoption method, the Partnership will disclose the amount by which each financial statement line item is affected by the standard in the current reporting period as compared with the guidance that was in effect before adoption.

Acquisitions

Acquisitions	3 Months Ended
	Mar. 31, 2017
Equity [Abstract]
Acquisitions	Acquisitions 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. Prior to these dates, these entities were not considered businesses and therefore there are no financial results from which to recast. Pursuant to a Membership Interests Contributions Agreement (the “Contributions Agreement”) entered into on March 1, 2017 by the Partnership with MPLX GP LLC (“MPLX GP”), MPLX Logistics Holdings LLC (“MPLX Logistics”), MPLX Holdings Inc. (“MPLX Holdings”) and MPC Investment LLC (“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 New York Stock Exchange 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 , as recorded on the Consolidated Statements of Equity, and consisted of (i) 9,197,900 common units representing limited partner interests in the Partnership 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 general partner interest (GP Interest) in the Partnership. MPC agreed to waive two-thirds of the first quarter 2017 distributions on the MPLX LP common units issued in connection with the Transaction. MPC did not receive two-thirds of the general partner distributions or incentive distribution rights that would have otherwise accrued on such MPLX LP common units with respect to the first quarter 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. These pipeline systems consist of 174 miles of crude oil pipelines and 430 miles of refined products pipelines. WHC owns and operates nine butane and propane storage caverns located in Michigan with approximately 1.8 million barrels of natural gas liquids storage capacity. MPLXT owns and operates 59 terminals for the receipt, storage, blending, additization, handling and redelivery of refined petroleum products. Additionally, MPLXT operates one leased terminal and has partial ownership interest in two terminals. Collectively, these 62 terminals have a combined total 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 the L&S segment. The Partnership retrospectively adjusted the historical financial results for all periods to give effect to the acquisition of HST and WHC effective January 1, 2015 and the acquisition of MPLXT effective April 1, 2016, as required for transactions between entities under common control. The following table presents the Partnership’s previously reported unaudited Consolidated Statements of Income, retrospectively adjusted for the acquisition of HST and WHC: Three Months Ended March 31, 2016 (In millions, except per unit data) MPLX LP (Previously Reported) HST/WHC Eliminations (1) MPLX LP (Currently Reported) Revenues and other income: Service revenue $ 229 $ — $ — $ 229 Service revenue to related parties 150 27 — 177 Rental income 70 — — 70 Rental income to related parties 26 12 — 38 Product sales 100 — — 100 Product sales to related parties 3 — — 3 Income from equity method investments 5 — — 5 Other income 2 — — 2 Other income - related parties 24 — (3 ) 21 Total revenues and other income 609 39 (3 ) 645 Costs and expenses: Cost of revenues (excludes items below) 89 5 — 94 Purchased product costs 79 — — 79 Rental cost of sales 14 — — 14 Purchases from related parties 76 5 (3 ) 78 Depreciation and amortization 132 4 — 136 Impairment expense 129 — — 129 General and administrative expenses 52 1 — 53 Other taxes 11 1 — 12 Total costs and expenses 582 16 (3 ) 595 Income from operations 27 23 — 50 Related party interest and other financial income 1 — — 1 Interest expense (net of amounts capitalized) 55 — — 55 Other financial costs 12 — — 12 (Loss) income before income taxes (41 ) 23 — (18 ) Benefit for income taxes (4 ) — — (4 ) Net (loss) income (37 ) 23 — (14 ) Net income attributable to Predecessor 23 23 — 46 Net loss attributable to MPLX LP (60 ) — — (60 ) Less: General partner’s interest in net income attributable to MPLX LP 39 — — 39 Limited partners’ interest in net loss attributable to MPLX LP $ (99 ) $ — $ — $ (99 ) (1) Represents intercompany transactions eliminated during the consolidation process, in accordance with GAAP. The following table presents the Partnership’s previously reported Consolidated Balance Sheet, retrospectively adjusted for the acquisition of HST, WHC and MPLXT: December 31, 2016 (In millions) MPLX LP (Previously Reported) HST/WHC MPLXT Eliminations (1) MPLX LP (Currently Reported) Assets Current assets: Cash and cash equivalents $ 234 $ — $ — $ — $ 234 Receivables, net 297 1 1 — 299 Receivables from related parties 122 91 38 (4 ) 247 Inventories 54 1 — — 55 Other current assets 33 — — — 33 Total current assets 740 93 39 (4 ) 868 Equity method investments 2,467 — 4 — 2,471 Property, plant and equipment, net 10,730 265 413 — 11,408 Intangibles, net 492 — — — 492 Goodwill 2,199 25 21 — 2,245 Long-term receivables from related parties 4 — 7 — 11 Other noncurrent assets 14 — — — 14 Total assets $ 16,646 $ 383 $ 484 $ (4 ) $ 17,509 Liabilities Current liabilities: Accounts payable $ 123 $ 5 $ 12 $ — $ 140 Accrued liabilities 228 4 — — 232 Payables to related parties 75 4 12 (4 ) 87 Deferred revenue 2 — — — 2 Deferred revenue - related parties 34 4 — — 38 Accrued property, plant and equipment 132 9 5 — 146 Accrued taxes 33 2 3 — 38 Accrued interest payable 53 — — — 53 Other current liabilities 24 1 2 — 27 Total current liabilities 704 29 34 (4 ) 763 Long-term deferred revenue 12 — — — 12 Long-term deferred revenue - related parties 15 — 4 — 19 Long-term debt 4,422 — — — 4,422 Deferred income taxes 5 — 1 — 6 Deferred credits and other liabilities 169 2 6 — 177 Total liabilities 5,327 31 45 (4 ) 5,399 Redeemable preferred units 1,000 — — — 1,000 Equity Common unitholders - public 8,086 — — — 8,086 Class B unitholders 133 — — — 133 Common unitholder - MPC 1,069 — — — 1,069 General partner - MPC 1,013 — — — 1,013 Equity of Predecessor — 352 439 — 791 Total MPLX LP partners’ capital 10,301 352 439 — 11,092 Noncontrolling interest 18 — — — 18 Total equity 10,319 352 439 — 11,110 Total liabilities and equity $ 16,646 $ 383 $ 484 $ (4 ) $ 17,509 (1) Represents intercompany transactions eliminated during the consolidation process, in accordance with GAAP. The following table presents the Partnership’s previously reported unaudited Consolidated Statements of Cash Flows, retrospectively adjusted for the acquisition of HST and WHC: Three Months Ended March 31, 2016 (In millions) MPLX LP (Previously Reported) HST/WHC MPLX LP (Currently Reported) Increase (decrease) in cash and cash equivalents Operating activities: Net (loss) income $ (37 ) $ 23 $ (14 ) Adjustments to reconcile net income to net cash provided by operating activities: Amortization of deferred financing costs 11 — 11 Depreciation and amortization 132 4 136 Impairment expense 129 — 129 Deferred income taxes (4 ) — (4 ) Income from equity method investments (5 ) — (5 ) Distributions from unconsolidated affiliates 38 — 38 Changes in: Current receivables (5 ) — (5 ) Inventories 1 — 1 Change in fair value of derivatives 12 — 12 Current accounts payable and accrued liabilities (15 ) (3 ) (18 ) Receivables from / liabilities to related parties 23 — 23 All other, net 15 2 17 Net cash provided by operating activities 295 26 321 Investing activities: Additions to property, plant and equipment (291 ) (13 ) (304 ) Investments - loans from (to) related parties 77 (13 ) 64 Investments in unconsolidated affiliates (29 ) — (29 ) All other, net 3 — 3 Net cash used in investing activities (240 ) (26 ) (266 ) Financing activities: Long-term debt - borrowings 306 — 306 - repayments (857 ) — (857 ) Related party debt - borrowings 1,437 — 1,437 - repayments (1,007 ) — (1,007 ) Net proceeds from equity offerings 321 — 321 Distributions to unitholders and general partner (190 ) — (190 ) Distributions to noncontrolling interests (1 ) — (1 ) Contributions from noncontrolling interests 2 — 2 All other, net (1 ) — (1 ) Distributions to MPC from Predecessor (104 ) — (104 ) Net cash used in financing activities (94 ) — (94 ) Net decrease in cash and cash equivalents (39 ) — (39 ) Cash and cash equivalents at beginning of period 43 — 43 Cash and cash equivalents at end of period $ 4 $ — $ 4 Acquisition of Ozark Pipeline On March 1, 2017, the Partnership acquired the Ozark pipeline from Enbridge Pipelines (Ozark) LLC for approximately $220 million . 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. The components of the fair value of consideration transferred was $220 million of cash. The Partnership is still completing its analysis of the final purchase price allocation for property, plant and equipment, intangibles and resulting goodwill. The estimated fair value of assets acquired and liabilities assumed at the acquisition date has been allocated primarily to property, plant and equipment. The amounts of revenue and income from operations associated with the acquisition included in the Consolidated Statements of Income from the acquisition date, March 1, 2017, to March 31, 2017 are as follows: (In millions) One Month Ended March 31, 2017 Revenues and other income $ 7 Income from operations 2 Assuming the acquisition of the Ozark pipeline had occurred on January 1, 2016, the consolidated pro forma results would not have been materially different from reported results. Acquisition of Hardin Street Marine LLC On March 14, 2016, the Partnership entered into a Membership Interests Contribution Agreement (the “Contribution Agreement”) with MPLX GP, MPLX Logistics and MPC Investment, each a wholly-owned subsidiary of MPC, related to the acquisition of HSM, MPC’s inland marine business, from MPC. Pursuant to the Contribution Agreement, the transaction was valued at $600 million consisting of a fixed number of common units and general partner units of 22,534,002 and 459,878 , respectively. The general partner units maintain MPC’s two percent GP Interest in the Partnership. The acquisition closed on March 31, 2016 and the fair value of the common units and general partner units issued was $669 million and $14 million , respectively, as recorded on the Consolidated Statements of Equity. MPC agreed to waive distributions in the first quarter of 2016 on MPLX LP common units issued in connection with this transaction. MPC did not receive general partner distributions or incentive distribution rights that would have otherwise accrued on such MPLX LP common units with respect to the first quarter 2016 distributions. The value of these waived distributions was $15 million . The inland marine business, comprised of 18 tow boats and 205 barges which transport light products, heavy oils, crude oil, renewable fuels, chemicals and feedstocks in the Midwest and U.S. Gulf Coast regions, accounted for nearly 60 percent of the total volumes MPC shipped by inland marine vessels as of March 31, 2016. The Partnership accounts for HSM as a reporting unit of the L&S segment.

Equity Method Investments (Note

Equity Method Investments (Notes)	3 Months Ended
	Mar. 31, 2017
Equity Method Investments and Joint Ventures [Abstract]
Equity Method Investments and Joint Ventures Disclosure [Text Block]	Investments and Non-Controlling Interests Summarized financial information for the Partnership’s equity method investments for the three months ended March 31, 2017 and 2016 is as follows: Three Months Ended March 31, 2017 (In millions) MarkWest Utica EMG Other VIEs Non-VIEs Total Revenue 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 Three Months Ended March 31, 2016 (In millions) MarkWest Utica EMG Other VIEs Non-VIEs Total Revenue and other income $ 61 $ 5 $ 29 $ 95 Costs and expenses 22 4 20 46 Income from operations 39 1 9 49 Net income 38 1 9 48 Income from equity method investments (1) 4 — 1 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, 2017 and December 31, 2016 is as follows: March 31, 2017 (In millions) MarkWest Utica EMG (1) Other VIEs Non-VIEs Total Current assets $ 63 $ 48 $ 34 $ 145 Noncurrent assets 2,145 832 379 3,356 Current liabilities 29 96 20 145 Noncurrent liabilities 2 10 — 12 December 31, 2016 (In millions) MarkWest Utica EMG (1) Other VIEs Non-VIEs Total Current assets $ 45 $ 2 $ 40 $ 87 Noncurrent assets 2,173 132 390 2,695 Current liabilities 30 4 26 60 Noncurrent liabilities 2 13 — 15 (1) MarkWest Utica EMG’s noncurrent assets includes its investment in its subsidiary Ohio Gathering, which does not appear elsewhere in this table. The investment was $792 million and $794 million as of March 31, 2017 and December 31, 2016 , respectively. As of March 31, 2017 and December 31, 2016 , the carrying value of the Partnership’s equity method investments exceeded the underlying net assets of its investees by $1.1 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, LLC (“Utica Operating”), a wholly-owned and consolidated subsidiary of MarkWest, and EMG Utica (together 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. The related limited liability company agreement has been amended from time to time (the limited liability company agreement currently in effect is referred to as the “Amended LLC Agreement”). The aggregate funding commitment of EMG Utica was $950 million (the “Minimum EMG Investment”). Thereafter, Utica Operating was required to fund, as needed, 100 percent of future capital for MarkWest Utica EMG until the aggregate capital that had been contributed by the Members reached $2.0 billion , which occurred prior to the MarkWest Merger. Until such time as the investment balances of Utica Operating and EMG Utica are in the ratio of 70 percent and 30 percent , respectively (such time being referred to as the “Second Equalization Date”), EMG Utica will have the right, but not the obligation, to fund up to 10 percent of each capital call for MarkWest Utica EMG, and Utica Operating will be required to fund all remaining capital not elected to be funded by EMG Utica. After the Second Equalization Date, Utica Operating and EMG Utica will have the right, but not the obligation, to fund their pro rata portion (based on their respective investment balances) of any additional required capital and may also fund additional capital that the other party elects not to fund. As of March 31, 2017 , EMG Utica has contributed approximately $1.2 billion and Utica Operating has contributed approximately $1.5 billion to MarkWest Utica EMG. Under the Amended LLC Agreement, prior to December 31, 2016, EMG Utica’s investment balance was increased by a quarterly special non-cash allocation of income (“Preference Amount”), calculated based upon the amount of capital contributed by EMG Utica in excess of $500 million . After December 31, 2016, no Preference Amount will accrue to EMG Utica’s investment balance. EMG Utica received a Preference Amount totaling approximately $4 million for the three months ended March 31, 2016. Under the Amended LLC Agreement, after December 31, 2016, cash generated by MarkWest Utica EMG that is available for distribution (the “Distribution”) will be allocated to the Members in proportion to their respective investment balances. As of March 31, 2017 , Utica Operating’s investment balance in MarkWest Utica EMG was approximately 56 percent. 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 portion of MarkWest Utica EMG’s net assets, which was $2.2 billion at March 31, 2017 and December 31, 2016 , is reported under the caption Equity method investments on the Consolidated Balance Sheets. 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, 2017 and 2016 , respectively. The Partnership receives management fee revenue for engineering and construction and administrative services for operating MarkWest Utica EMG, and is also reimbursed for personnel services (“Operational Service revenue”). The amount of Operational Service revenue related to MarkWest Utica EMG for the three months ended March 31, 2017 and 2016 , totaled $4 million and $2 million , respectively, and is reported as Other income-related parties in the Consolidated Statements of Income. 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 (“Summit”). As of March 31, 2017 , we have a 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. The Partnership receives Operational Service revenue for operating Ohio Gathering. The amount of Operational Service revenue related to Ohio Gathering for the three months ended March 31, 2017 and 2016 , was approximately $4 million , and is reported as Other income-related parties in the Consolidated Statements of Income. Sherwood Midstream Effective January 1, 2017, MarkWest Liberty Midstream & Resources, LLC (“MarkWest Liberty Midstream”), a wholly-owned and consolidated subsidiary of MarkWest, and Antero Midstream Partners, LP (“Antero Midstream”) formed a joint venture, Sherwood Midstream, to support Antero Resources Corporation’s development of 195,000 gross acres in the Marcellus Shale. MarkWest Liberty Midstream has a 50 percent ownership interest in Sherwood Midstream. Pursuant to the terms of the related limited liability company agreement (the “LLC Agreement”), MarkWest Liberty Midstream contributed assets currently 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, LLC (“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 carrying amounts of assets and liabilities included in the Partnership’s Consolidated Balance Sheets pertaining to Ohio Fractionation at March 31, 2017, were current assets of $12 million , non-current assets of $23 million , and current liabilities were $31 million . 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 interest in the Consolidated Statements of Income and Noncontrolling interests in the Consolidated Balance Sheets. Under the LLC Agreement, cash generated by Sherwood Midstream that is available for distribution (the “Distribution”) will be allocated to the members in proportion to their respective investment balances. For the three months ended March 31, 2017 , there was no cash available for the Distribution. 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 portion of Sherwood Midstream’s net assets, which was approximately $147 million at March 31, 2017 , is reported under the caption Equity method investments on the Consolidated Balance Sheets. 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, 2017 . The Partnership receives Operational Service revenue for operating Sherwood Midstream. The amount of Operational Service revenue related to Sherwood Midstream for the three months ended March 31, 2017 totaled $1 million , and is reported as Other income-related parties in the Consolidated Statements of Income. Sherwood Midstream Holdings Effective January 1, 2017, MarkWest Liberty Midstream and Sherwood Midstream formed a joint venture, 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 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. Collectively, the real property, equipment, facilities and cash initially contributed, or that may be subsequently constructed by or contributed, to Sherwood Midstream Holdings are referred to as the “Shared Assets.” The net book value of the contributed assets was approximately $194 million . The contribution was determined to be an in-substance sale of real estate. As such, the Partnership only recognized a gain for the portion attributable to Antero Midstream’s indirect interest of approximately $2 million , included in Gain on sale of assets in the Consolidated Statements of Income. MarkWest Liberty Midstream’s portion of the gain attributable to its direct and indirect interests of approximately $13 million is included in its investment in Sherwood Midstream Holdings and is reported under the caption Equity method investments on the Consolidated Balance Sheets. Contemporaneous with the closing, MarkWest Liberty Midstream received a special distribution of approximately $41 million . MarkWest Liberty Midstream’s and Sherwood Midstream’s ownership interests in Sherwood Midstream Holdings will fluctuate over time. As new Shared Assets are constructed, the members will make additional capital contributions to Sherwood Midstream Holdings. The amount that each member must contribute will be based on the expected utilization of the Shared Asset, as defined in Sherwood Midstream’s LLC Agreement. Pursuant to the terms of the LLC Agreement, MarkWest Liberty Midstream will serve as the Operator for Sherwood Midstream Holdings. 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 portion of Sherwood Midstream Holdings’ net assets, which was approximately $166 million at March 31, 2017, is reported under the caption Equity method investments on the Consolidated Balance Sheets. 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, 2017 . 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, 2017, we have a 10.5 percent indirect ownership interest in Sherwood Midstream Holdings through Sherwood Midstream. MarEn Bakken On February 15, 2017, MPLX LP closed on a joint venture, MarEn Bakken, with Enbridge Energy Partners L.P. (“Enbridge Energy Partners”) in which MPLX LP acquired a partial, indirect interest in the Dakota Access Pipeline (“DAPL”) and Energy Transfer Crude Oil Company Pipeline (“ETCOP”) projects, collectively referred to as the Bakken Pipeline system, from Energy Transfer Partners, L.P. (“ETP”) and Sunoco Logistics Partners, L.P. (“SXL”). MPLX LP contributed $500 million of the $2.0 billion purchase price paid by MarEn Bakken to acquire a 36.75 percent indirect interest in the Bakken Pipeline system. MPLX LP holds, through a subsidiary, a 25 percent interest in MarEn Bakken, which equates to an approximate 9.1875 percent indirect interest in the Bakken Pipeline system. The Bakken Pipeline system is currently expected to deliver in excess of 470 mbpd of crude oil from the Bakken/Three Forks production area in North Dakota to the Midwest through Patoka, Illinois and ultimately to the Gulf Coast. 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 based on the most recent available information. The Partnership’s investment balance at March 31, 2017 is comprised of the Partnership’s initial contribution plus capitalized interest for a balance of approximately $503 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, which has been prorated to $0.8 million from the acquisition date.

Related Party Agreements and Tr

Related Party Agreements and Transactions	3 Months Ended
	Mar. 31, 2017
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. • Centennial Pipeline LLC (“Centennial”), in which MPC has a 50 percent interest as of March 31, 2017 . Centennial owns a products pipeline and storage facility. • Muskegon Pipeline LLC (“Muskegon”), in which MPC has a 60 percent interest as of March 31, 2017 . Muskegon owns a common carrier products pipeline. • MarkWest Utica EMG, in which MPLX LP has a 56 percent interest as of March 31, 2017 . MarkWest Utica EMG is engaged in significant natural gas processing and NGL fractionation, transportation and marketing in the State of Ohio. • Ohio Gathering, in which MPLX LP has a 34 percent indirect interest as of March 31, 2017 . 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, 2017. Sherwood Midstream supports the development of Antero Resources Corporation’s extensive Marcellus Shale acreage in the prolific rich-gas corridor of West Virginia. • Sherwood Midstream Holdings, in which MPLX LP has a 90 percent direct and indirect interest at March 31, 2017. 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. • MarEn Bakken, in which MPLX LP has a 25 percent interest at March 31, 2017. MarEn Bakken owns a 36.75 percent interest in the Bakken Pipeline system. Related Party Agreements The Partnership has various long-term, fee-based commercial agreements with MPC. Under these agreements, the Partnership provides transportation, terminal and storage services to MPC, and 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, a wholly-owned subsidiary of MPC. 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, 2017 , the Partnership made no borrowings and no repayments on the loan, resulting in no outstanding balance at March 31, 2017 . During the year ended December 31, 2016 , the Partnership borrowed $2.5 billion and repaid $2.5 billion , resulting in no outstanding balance at December 31, 2016 . Borrowings were at an average interest rate of 1.939 percent , per annum, for the year ended December 31, 2016 . 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, 2016 . The Partnership believes the terms and conditions under its agreements with MPC are generally comparable to those with unrelated parties. HST, WHC and MPLXT Agreements As discussed in Note 3 , the Partnership acquired HST, WHC and MPLXT on March 1, 2017. HST, WHC and MPLXT have various operating, transportation services, terminal services, storage services and employee services agreements with MPC, which were assumed by the Partnership with the closing of the Transaction. HST is a party to a transportation services agreement with MPC dated January 1, 2015. Under this agreement, HST provides pipeline transportation of crude oil and refined products, as well as related services, for MPC. MPC pays HST for such services based on contractual rates related to MPC crude oil and refined product deliveries as well as any viscosity surcharges, loading, handling transfers or other related charges. This agreement is set to expire on December 31, 2026 and automatically renews for two additional renewal terms of four years each unless terminated by either party. On January 1, 2015, WHC entered into a long-term, fee-based storage and services agreement with MPC related to storage at its butane and propane caverns with an initial term of 10 years. Under this storage and services agreement, WHC receives a monthly fee from MPC based on a contractual rate per barrel multiplied by the total commitment volume respective to each storage cavern. The contractual rate per barrel includes utilization of the caverns and related services. The agreement is subject to an annual review and adjustment for inflation. On January 1, 2015, HST entered into various three -year term storage services agreements with MPC. Under the storage services agreements, HST receives a monthly fee from MPC based on a contractual rate per barrel multiplied by the total commitment volume respective to each storage tank. The contractual rate per barrel is subject to an annual review and adjustment for inflation. HST is not obligated to measure volume gains and losses per the terms of these agreements. Under the storage services agreements with both HST and WHC, the Partnership is obligated to make available to MPC, on a firm basis, the available storage capacity at the tank farms and butane and propane caverns and MPC pays the Partnership a per-barrel fee for such storage capacity, regardless of whether MPC fully utilizes the available capacity. MPLXT is a party to a terminal services agreement with MPC, dated March 1, 2017. Under this agreement, MPLXT provides terminal storage for refined petroleum products, as well as related services, for MPC. MPC pays MPLXT monthly for such services based on contractual fees relating to MPC product deliveries as well as any viscosity surcharges, loading, handling, transfers or other related charges. This agreement is set to expire on March 31, 2026 and automatically renews for two additional renewal terms of five years each unless terminated by either party. The Partnership is party to various employee services agreements with MPC under which the Partnership reimburses MPC for employee benefit expenses, along with the provision of operational and management services, including those in support of HST, WHC and MPLXT. Related Party Transactions Sales to related parties were as follows: Three Months Ended March 31, (In millions) 2017 2016 Service revenues MPC $ 255 $ 177 Rental income MPC $ 67 $ 38 Product sales (1) MPC $ 2 $ 3 (1) For the three months ended March 31, 2017 and 2016 , there were $57 million and $5 million , respectively, of 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. Related party sales to MPC consist of crude oil and refined products pipeline transportation services based on regulated tariff rates, storage services based on contracted rates and transportation services provided by HSM. 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 system 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 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 . The revenue received from related parties, included in Other income-related parties on the Consolidated Statements of Income, was as follows: Three Months Ended March 31, (In millions) 2017 2016 MPC $ 11 $ 14 MarkWest Utica EMG 4 2 Ohio Gathering 4 4 Other 3 1 Total $ 22 $ 21 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) 2017 2016 Purchases - related parties $ 15 $ 7 General and administrative expenses 8 10 Total $ 23 $ 17 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 were as follows: Three Months Ended March 31, (In millions) 2017 2016 MPC $ 10 $ 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 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. Employee services expenses from related parties were as follows: Three Months Ended March 31, (In millions) 2017 2016 Purchases - related parties $ 92 $ 71 General and administrative expenses 25 21 Total $ 117 $ 92 Receivables from related parties, which include reimbursements from the MarkWest Merger to be provided by MPC for the conversion of Class B units, were as follows: (In millions) March 31, 2017 December 31, 2016 MPC $ 177 $ 242 Sherwood Midstream 14 — Jefferson Gas Gathering 2 — MarkWest Utica EMG — 2 Ohio Gathering 3 2 Other 2 1 Total $ 198 $ 247 Long-term receivables with related parties, which includes straight line rental income, were as follows: (In millions) March 31, 2017 December 31, 2016 MPC $ 13 $ 11 Payables to related parties were as follows: (In millions) March 31, 2017 December 31, 2016 MPC $ 59 $ 63 MarkWest Utica EMG 29 24 Sherwood Midstream 3 — Total $ 91 $ 87 During the three months ended March 31, 2017 and the year ended December 31, 2016 , MPC did not ship its minimum committed volumes on certain pipeline systems. 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 transportation services agreements. The Deferred revenue-related parties balance associated with the minimum volume deficiencies and project reimbursements were as follows: (In millions) March 31, 2017 December 31, 2016 Minimum volume deficiencies - MPC $ 50 $ 48 Project reimbursements - MPC 14 9 Total $ 64 $ 57

Net Income Per Limited Partner

Net Income Per Limited Partner Unit	3 Months Ended
	Mar. 31, 2017
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, general partner units, Preferred units, certain equity-based compensation awards and incentive distribution rights (“IDRs”). As discussed in Note 1 , the HST, WHC and MPLXT acquisition was a transfer between entities under common control. As entities under common control with MPC, prior periods were retrospectively adjusted to furnish comparative information. Accordingly, the prior period earnings have been allocated to the general partner and do not affect the net income (loss) per unit calculation. The earnings for the entities acquired under common control will be included in the net income (loss) per unit calculation prospectively as described above. For the three months ended March 31, 2017 and 2016 , the Partnership had dilutive potential common units consisting of certain equity-based compensation awards and Class B units. Diluted net income (loss) per limited partner unit for the three months ended March 31, 2016 is the same as basic net income (loss) per limited partner unit since the inclusion of any potential common units would have been anti-dilutive. Potential common units omitted from the diluted earnings per unit calculation for the three months ended March 31, 2017 and 2016 were less than 1 million and approximately 10 million , respectively. Three Months Ended March 31, (In millions) 2017 2016 Net income (loss) attributable to MPLX LP $ 150 $ (60 ) Less: Limited partners’ distributions declared on Preferred units (1) 16 — General partner’s distributions declared (including IDRs) (1) 65 44 Limited partners’ distributions declared on common units (1) 198 156 Undistributed net loss attributable to MPLX LP $ (129 ) $ (260 ) (1) See Note 7 for distribution information. 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 Three Months Ended March 31, 2016 (In millions, except per unit data) General Partner Limited Partners’ Common Units Total Basic and diluted net income attributable to MPLX LP per unit: Net income (loss) attributable to MPLX LP: Distributions declared (including IDRs) $ 44 $ 156 $ 200 Undistributed net loss attributable to MPLX LP (5 ) (255 ) (260 ) Net income (loss) attributable to MPLX LP (1) $ 39 $ (99 ) $ (60 ) Weighted average units outstanding: Basic 7 300 307 Diluted 7 300 307 Net loss attributable to MPLX LP per limited partner unit: Basic $ (0.33 ) Diluted $ (0.33 ) (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, 2017
Equity [Abstract]
Equity	Equity The changes in the number of units outstanding from December 31, 2016 through March 31, 2017 are summarized below: (In units) Common Class B General Partner Total Balance at December 31, 2016 357,193,288 3,990,878 7,371,105 368,555,271 Unit-based compensation awards (1) 99,463 — 2,030 101,493 Issuance of units under the ATM Program (2) 4,151,258 — 84,720 4,235,978 Contribution of HST/WHC/MPLXT (3) 12,960,376 — 264,497 13,224,873 Balance at March 31, 2017 374,404,385 3,990,878 7,722,352 386,117,615 (1) As a result of the unit-based compensation awards issued during the period, MPLX GP contributed less than $1 million in exchange for 2,030 general partner units to maintain its two percent GP Interest. (2) As a result of common units issued under the ATM Program during the period, MPLX GP contributed $3 million in exchange for 84,720 general partner units to maintain its two percent GP Interest. (3) See Note 3 for information regarding the HST, WHC and MPLXT acquisition. 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 8 , and subsequently allocated to the general partner and limited partner unitholders. However, when distributions related to the incentive distribution rights are made, earnings equal to the amount of those distributions are first allocated to the general partner before the remaining earnings are 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: Three Months Ended March 31, (In millions) 2017 2016 Net income (loss) attributable to MPLX LP $ 150 $ (60 ) Less: Preferred unit distributions 16 — General partner's incentive distribution rights and other 61 41 Net income (loss) attributable to MPLX LP available to general and limited partners $ 73 $ (101 ) General partner's two percent GP Interest in net income (loss) attributable to MPLX LP $ 1 $ (2 ) General partner's incentive distribution rights and other 61 41 General partner's GP Interest in net income attributable to MPLX LP $ 62 $ 39 Cash distributions – The partnership agreement sets forth the calculation to be used to determine the amount and priority of cash distributions that the common and general partner will receive. In accordance with the partnership agreement, on April 26, 2017 , the Partnership declared a quarterly cash distribution, based on the results of the first quarter of 2017 , totaling $263 million , or $0.5400 per unit. These distributions will be paid on May 15, 2017 to unitholders of record on May 8, 2017 . The allocation of total quarterly cash distributions to general, limited and Preferred unitholders is as follows for the three months ended March 31, 2017 and 2016 . 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) 2017 2016 General partner's distributions: General partner's distributions on general partner units $ 5 $ 4 General partner's distributions on incentive distribution rights 60 40 Total distribution on general partner units and incentive distribution rights $ 65 $ 44 Limited partners' distributions: Common unitholders, includes common units of general partner $ 198 $ 156 Subordinated unitholders — — Total limited partners' distributions 198 156 Preferred unit distributions 16 — Total cash distributions declared $ 279 $ 200

Redeemable Preferred Units (Not

Redeemable Preferred Units (Notes)	3 Months Ended
	Mar. 31, 2017
Temporary Equity Disclosure [Abstract]
Temporary Equity Disclosure [Text Block]	8 . Redeemable Preferred Units Private Placement of Preferred Units – On May 13, 2016, MPLX LP completed the private placement of approximately 30.8 million 6.5 percent Series A Convertible Preferred units (the "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 common units. The changes in the redeemable preferred balance from December 31, 2016 through March 31, 2017 are summarized below: (In millions) Redeemable Preferred Units Balance at December 31, 2016 $ 1,000 Net income 16 Distributions received by Preferred unitholders (16 ) Balance at March 31, 2017 $ 1,000 The purchasers may convert their Preferred units into common units, at any time after the third anniversary of the issuance date or prior to liquidation, dissolution or winding up of the Partnership, in full or in part, subject to minimum conversion amounts and conditions. After the fourth anniversary of the issuance date, the Partnership may convert the Preferred units into common units at any time, in whole or in part, subject to certain minimum conversion amounts and conditions, if the closing price of MPLX LP common units is greater than $48.75 for the 20 day trading period immediately preceding the conversion notice date. The conversion rate for the Preferred units shall be the quotient of (a) the sum of (i) $32.50, plus (ii) any unpaid cash distributions on the applicable Preferred Unit, divided by (b) $32.50. The holders of the Preferred units are entitled to vote on an as-converted basis with the common unitholders and will have certain other class voting rights with respect to any amendment to the partnership agreement that would adversely affect any rights, preferences or privileges of the Preferred units. In addition, upon certain events involving a change in control the holders of Preferred units may elect, among other potential elections, to convert their Preferred units to common units at the then change of control conversion rate. 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 (Notes)

Segment Information (Notes)	3 Months Ended
	Mar. 31, 2017
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 is organized and managed based upon the nature of the products and services it offers. • L&S – transports, stores and distributes crude oil and refined petroleum products. Segment information for prior periods includes HST, WHC and MPLXT as they are entities under common control. Segment information for periods prior to the Ozark pipeline acquisition does not include amounts for these operations. See Note 3 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 4 ). However, the CEO views the Partnership-operated equity method investments’ financial information as if those investments were consolidated. Segment operating income represents income from operations attributable to the reportable segments. Corporate general and administrative expenses, unrealized derivative gains (losses), property, plant and equipment, goodwill impairment 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 the Predecessors of the HSM, 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, 2017 (In millions) L&S G&P Total Revenues and other income: Segment revenues $ 345 $ 597 $ 942 Segment other income 12 1 13 Total segment revenues and other income 357 598 955 Costs and expenses: Segment cost of revenues 148 253 401 Segment operating income before portion attributable to noncontrolling interest and Predecessor 209 345 554 Segment portion attributable to noncontrolling interest and Predecessor 53 36 89 Segment operating income attributable to MPLX LP $ 156 $ 309 $ 465 Three Months Ended March 31, 2016 (In millions) L&S G&P Total Revenues and other income: Segment revenues $ 231 $ 498 $ 729 Segment other income 16 — 16 Total segment revenues and other income 247 498 745 Costs and expenses: Segment cost of revenues 97 200 297 Segment operating income before portion attributable to noncontrolling interest and Predecessor 150 298 448 Segment portion attributable to noncontrolling interest and Predecessor 62 41 103 Segment operating income attributable to MPLX LP $ 88 $ 257 $ 345 Three Months Ended March 31, (In millions) 2017 2016 Reconciliation to Income from operations: L&S segment operating income attributable to MPLX LP $ 156 $ 88 G&P segment operating income attributable to MPLX LP 309 257 Segment operating income attributable to MPLX LP 465 345 Segment portion attributable to unconsolidated affiliates (40 ) (42 ) Segment portion attributable to Predecessor 53 62 Income from equity method investments 5 5 Other income - related parties 11 7 Unrealized derivative gains (losses) (1) 16 (9 ) Depreciation and amortization (187 ) (136 ) Impairment expense — (129 ) General and administrative expenses (58 ) (53 ) Income from operations $ 265 $ 50 Three Months Ended March 31, (In millions) 2017 2016 Reconciliation to Total revenues and other income: Total segment revenues and other income $ 955 $ 745 Revenue adjustment from unconsolidated affiliates (92 ) (104 ) Income from equity method investments 5 5 Other income - related parties 11 7 Unrealized derivative gains (losses) (1) 7 (8 ) Total revenues and other income $ 886 $ 645 Three Months Ended March 31, (In millions) 2017 2016 Reconciliation to Net income attributable to noncontrolling interests and Predecessor: Segment portion attributable to noncontrolling interest and Predecessor $ 89 $ 103 Portion of noncontrolling interests and Predecessor related to items below segment income from operations (36 ) (34 ) Portion of operating income attributable to noncontrolling interest of unconsolidated affiliates (16 ) (23 ) Net income attributable to noncontrolling interests and Predecessor $ 37 $ 46 (1) The Partnership makes a distinction between realized or unrealized gains and losses on derivatives. During the period when a derivative contract is outstanding, we record changes in the fair value of the derivative as an unrealized gain or loss. When a derivative contract matures or is settled, we reverse the previously recorded unrealized gain or loss and record the realized gain or loss of the contract. The following reconciles segment capital expenditures to total capital expenditures: Three Months Ended March 31, (In millions) 2017 2016 L&S segment capital expenditures $ 97 $ 75 G&P segment capital expenditures 298 273 Total segment capital expenditures 395 348 Less: Capital expenditures for Partnership-operated, non-wholly-owned subsidiaries in G&P segment 115 44 Total capital expenditures $ 280 $ 304 Total assets by reportable segment were: (In millions) March 31, 2017 December 31, 2016 Cash and cash equivalents $ 265 $ 234 L&S 3,713 2,978 G&P 14,307 14,297 Total assets $ 18,285 $ 17,509

Inventories (Notes)

Inventories (Notes)	3 Months Ended
	Mar. 31, 2017
Inventory Disclosure [Abstract]
Inventory Disclosure [Text Block]	Inventories Inventories consist of the following: (In millions) March 31, 2017 December 31, 2016 NGLs $ 1 $ 2 Line fill 8 9 Spare parts, materials and supplies 53 44 Total inventories $ 62 $ 55

Property, Plant and Equipment

Property, Plant and Equipment	3 Months Ended
	Mar. 31, 2017
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, 2017 December 31, 2016 Natural gas gathering and NGL transportation pipelines and facilities $ 4,843 $ 4,748 Processing, fractionation and storage facilities 3,654 3,467 Pipelines and related assets 2,005 1,799 Barges and towing vessels 528 479 Terminals and related assets 889 839 Land, building, office equipment and other 711 757 Construction-in-progress 780 1,013 Total 13,410 13,102 Less accumulated depreciation 1,999 1,694 Property, plant and equipment, net $ 11,411 $ 11,408

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2017
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 13 . Money market funds, which are included in Cash and cash equivalents on the Consolidated Balance Sheets, are measured at fair value and are included in Level 1 measurements of the valuation hierarchy. Level 2 instruments include crude oil and natural gas swap contracts. Level 3 instruments include all NGL transactions and embedded derivatives in commodity contracts. The following table presents the financial instruments carried at fair value classified by the valuation hierarchy: March 31, 2017 December 31, 2016 (In millions) Assets Liabilities Assets Liabilities Significant other observable inputs (Level 2) Commodity contracts $ — $ — $ — $ — Significant unobservable inputs (Level 3) Commodity contracts 1 (1 ) — (6 ) Embedded derivatives in commodity contracts — (44 ) — (54 ) Total carrying value in Consolidated Balance Sheets $ 1 $ (45 ) $ — $ (60 ) The following table provides additional information about the significant unobservable inputs used in the valuation of Level 3 instruments as of March 31, 2017 . The market approach is used for valuation of all instruments. Level 3 Instrument Balance Sheet Classification Unobservable Inputs Value Range Time Period Commodity contracts Assets Forward ethane prices (per gallon) (1) $0.24 - $0.28 Apr. 17 - Dec. 17 Forward isobutane prices (per gallon) (1) $0.71 - $0.81 Apr. 17 - Dec. 18 Forward normal butane prices (per gallon) (1) $0.64 - $0.77 Apr. 17 - Dec. 18 Commodity contracts Liabilities Forward propane prices (per gallon) (1) $0.56 - $0.64 Apr. 17 - Dec. 18 Forward natural gasoline prices (per gallon) (1) $1.08 - $1.16 Apr. 17 - Dec. 18 Embedded derivatives in commodity contracts Liabilities Forward propane prices (per gallon) (1) $0.53 - $0.64 Apr. 17 - Dec. 22 Forward isobutane prices (per gallon) (1) $0.68 - $0.81 Apr. 17 - Dec. 22 Forward normal butane prices (per gallon) (1) $0.64 - $0.77 Apr. 17 - Dec. 22 Forward natural gasoline prices (per gallon) (1) $1.04 - $1.16 Apr. 17 - Dec. 22 Forward natural gas prices (per mmbtu) (2) $2.22 - $3.35 Apr. 17 - Dec. 22 Probability of renewal (3) 50.0% Probability of renewal for second 5-yr term (3) 75.0% (1) NGL prices used in the valuations are lower in the early years and increase over time. (2) Natural gas prices used in the valuations are higher in the early years and decrease over time. (3) The producer counterparty to the embedded derivative has the option to renew the gas purchase agreement and the related keep-whole processing agreement for two successive five -year terms after 2022. The embedded gas purchase agreement cannot be renewed without the renewal of the related keep-whole processing agreement. Due to the significant number of years until the renewal options are exercisable and the high level of uncertainty regarding the counterparty’s future business strategy, the future commodity price environment, and the future competitive environment for midstream services in the Southern Appalachian region, management determined that a 50 percent probability of renewal for the first five-year term and 75 percent for the second five-year term are appropriate assumptions. Included in this assumption is a further extension of management’s estimates of future frac spreads through 2032. Fair Value Sensitivity Related to Unobservable Inputs Commodity contracts (assets and liabilities) – For the Partnership’s commodity 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 the derivative liabilities. The forward prices for the individual NGL products generally increase or decrease in a positive correlation with one another. Embedded derivatives in commodity contracts – The Partnership has one embedded derivative liability comprised of both the purchase of natural gas at prices impacted by the frac spread and the probability of contract renewal (the “Natural Gas Embedded Derivative”), as discussed further in Note 13 . Increases (decreases) in the frac spread result in an increase (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. Level 3 Valuation Process The Partnership’s Risk Management Department (the “Risk Department”) is responsible for the valuation of the Partnership’s commodity derivative contracts and embedded derivatives in commodity contracts, except for the Natural Gas Embedded Derivative. The Risk Department reports to the Chief Financial Officer and is responsible for the oversight of the Partnership’s commodity risk management program. The members of the Risk Department have the requisite experience, knowledge and day-to-day involvement in the energy commodity markets to ensure appropriate valuations and understand the changes in the valuations from period to period. The valuations of the Level 3 commodity derivative contracts are performed by a third-party pricing service and reviewed and validated on a quarterly basis by the Risk Department by comparing the pricing and option volatilities to actual market data and/or data provided by at least one other independent third-party pricing service. Management is responsible for the valuation of the Natural Gas Embedded Derivative discussed in Note 13 . Included in the valuation of the Natural Gas Embedded Derivative are assumptions about the forward price curves for NGLs and natural gas for periods in which price curves are not available from third-party pricing services due to insufficient market data. The Risk Department must develop forward price curves for NGLs and natural gas through the initial contract term (April 2017 through December 2022) for management’s use in determining the fair value of the Natural Gas Embedded Derivative. In developing the pricing curves for these periods, the Risk Department maximizes its use of the latest known market data and trends as well as its understanding of the historical relationships between forward NGL and natural gas prices and the forward market data that is available for the required period, such as crude oil pricing and natural gas pricing from other markets. However, there is very limited actual market data available to validate the Risk Department’s estimated price curves. Management also assesses the probability of the producer customer’s renewal of the contracts, which includes consideration of: • The estimated favorability of the contracts to the producer customer as compared to other options that would be available to them at the time and in the relative geographic area of their producing assets; • Extrapolated pricing curves, using a weighted average probability method that is based on historical frac spreads, which impact the calculation of favorability; and • The producer customer’s potential business strategy decision points that may exist at the time the counterparty would elect whether to renew the contracts. Changes in Level 3 Fair Value Measurements The tables below include a rollforward of the balance sheet amounts for the three months ended March 31, 2017 and 2016 , respectively (including the change in fair value), for assets and liabilities classified by the Partnership within Level 3 of the valuation hierarchy. Three Months Ended March 31, 2017 Three Months Ended March 31, 2016 (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 $ (6 ) $ (54 ) $ 7 $ (32 ) Total gain (loss) (realized and unrealized) included in earnings (1) 5 8 (1 ) (4 ) Settlements 1 2 (6 ) 2 Fair value at end of period $ — $ (44 ) $ — $ (34 ) The amount of total gains/(losses) for the period included in earnings attributable to the change in unrealized losses relating to liabilities still held at end of period $ 5 $ 8 $ (2 ) $ (3 ) (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 on Embedded Derivatives in Commodity Contracts are recorded in Purchased product costs . 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 13 ). 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, 2017 December 31, 2016 (In millions) Fair Value Carrying Value Fair Value Carrying Value Long-term debt $ 7,290 $ 6,675 $ 4,953 $ 4,422 SMR liability 107 95 108 96

Derivative Financial Instrument

Derivative Financial Instruments (Notes)	3 Months Ended
	Mar. 31, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Instruments and Hedging Activities Disclosure [Text Block]	Derivative Financial Instruments Commodity Derivatives NGL and natural gas prices are volatile and are impacted by changes in fundamental supply and demand, as well as market uncertainty, availability of NGL transportation and fractionation capacity and a variety of additional factors that are beyond the Partnership’s control. The Partnership’s profitability is directly affected by prevailing commodity prices primarily as a result of processing or conditioning at its own or third-party processing plants, purchasing and selling or gathering and transporting volumes of natural gas at index-related prices and the cost of third-party transportation and fractionation services. To the extent that commodity prices influence the level of natural gas drilling by the Partnership’s producer customers, such prices also affect profitability. To protect itself financially against adverse price movements and to maintain more stable and predictable cash flows so that the Partnership can meet its cash distribution objectives, debt service and capital plans, the Partnership executes a strategy governed by its risk management policy. The Partnership has a committee comprised of senior management that oversees risk management activities, continually monitors the risk management program and adjusts its strategy as conditions warrant. The Partnership enters into certain derivative contracts to reduce the risks associated with unfavorable changes in the prices of natural gas and NGLs. Derivative contracts utilized are swaps traded on the OTC market and fixed price forward contracts. The risk management policy does not allow the Partnership to take speculative positions with its derivative contracts. To mitigate its cash flow exposure to fluctuations in the price of NGLs, the Partnership has entered into derivative financial instruments relating to the future price of NGLs and crude oil. The Partnership currently manages the majority of its NGL price risk using direct product NGL derivative contracts. The Partnership enters into NGL derivative contracts when adequate market liquidity exists and future prices are satisfactory. A portion of the Partnership’s NGL price exposure is managed by using crude oil contracts. In periods where NGL prices and crude oil prices are not consistent with the historical relationship, the crude oil contracts create increased risk and additional gains or losses. The Partnership may settle its crude oil contracts prior to the contractual settlement date in order to take advantage of favorable terms and reduce the future exposure resulting from the less effective crude oil contracts. Based on its current volume forecasts, the majority of its derivative positions used to manage the future commodity price exposure are expected to be direct product NGL derivative contracts. To mitigate its cash flow exposure to fluctuations in the price of natural gas, the Partnership primarily utilizes derivative financial instruments relating to the future price of natural gas and takes into account the partial offset of its long and short gas positions resulting from normal operating activities. As a result of its current derivative positions, the Partnership has mitigated a portion of its expected commodity price risk through the fourth quarter of 2018. The Partnership would be exposed to additional commodity risk in certain situations such as if producers under-deliver or over-deliver product or when processing facilities are operated in different recovery modes. In the event the Partnership has derivative positions in excess of the product delivered or expected to be delivered, the excess derivative positions may be terminated. Management conducts a standard credit review on counterparties to derivative contracts, and has provided the counterparties with a guaranty as credit support for its obligations. A separate agreement with certain counterparties allows MarkWest Liberty Midstream to enter into derivative positions without posting cash collateral. The Partnership uses standardized agreements that allow for offset of certain positive and negative exposures (“master netting arrangements”) in the event of default or other terminating events, including bankruptcy. The Partnership records derivative contracts at fair value in the Consolidated Balance Sheets and has not elected hedge accounting or the normal purchases and normal sales designation (except for electricity and certain other qualifying contracts, for which the normal purchases and normal sales designation has been elected). The Partnership’s accounting may cause volatility in the Consolidated Statements of Income as the Partnership recognizes in current earnings all unrealized gains and losses from the changes in fair value of derivatives. The Partnership makes a distinction between realized or unrealized gains and losses on derivatives. During the period when a derivative contract is outstanding, we record changes in the fair value of the derivative as an unrealized gain or loss. When a derivative contract matures or is settled, we reverse the previously recorded unrealized gain or loss and record the realized gain or loss of the contract. Volume of Commodity Derivative Activity As of March 31, 2017 , the Partnership had the following outstanding commodity contracts that were executed to manage the cash flow risk associated with future sales of NGLs: Derivative contracts not designated as hedging instruments Financial Position Notional Quantity (net) Crude Oil (bbl) Short 51,900 Natural Gas (MMBtu) Long 1,431,472 NGLs (gal) Short 81,257,382 Embedded Derivatives in Commodity Contracts The Partnership has a commodity contract with a producer customer in the Southern Appalachian region that creates a floor on the frac spread for gas purchases of 9,000 Dth/d. The commodity contract is a component of a broader regional arrangement that also includes a keep-whole processing agreement. For accounting purposes, these contracts have been aggregated into a single contract and are evaluated together. In February 2011, the Partnership executed agreements with the producer customer to extend the commodity contract and the related processing agreement from March 31, 2015 to December 31, 2022, with the producer customer’s option to extend the agreement for two successive five -year terms through December 31, 2032. The purchase of gas at prices based on the frac spread and the option to extend the agreements have been identified as a single embedded derivative, which is recorded at fair value. The probability of renewal is determined based on extrapolated pricing curves, a review of the overall expected favorability of the contracts based on such pricing curves, and assumptions about the counterparty’s potential business strategy decision points that may exist at the time the counterparty would elect whether to renew the contract. The changes in fair value of this 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, 2017 and December 31, 2016 , the estimated fair value of this contract was a liability of $44 million and $54 million , respectively. Financial Statement Impact of Derivative Contracts There were no material changes to the Partnership’s policy regarding the accounting for these instruments as previously disclosed in the Partnership’s Annual Report on Form 10-K for the year ended December 31, 2016 . The impact of the Partnership’s derivative instruments on its Consolidated Balance Sheets is summarized below: (In millions) March 31, 2017 December 31, 2016 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 $ 1 $ (7 ) $ — $ (13 ) Other noncurrent assets / deferred credits and other liabilities — (38 ) — (47 ) Total $ 1 $ (45 ) $ — $ (60 ) (1) Includes embedded derivatives in commodity contracts as discussed above. 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. The net amounts in the table below equal the balances presented in the Consolidated Balance Sheets: March 31, 2017 Assets Liabilities (In millions) Gross Amount Gross Amounts Offset in the Consolidated Balance Sheets Net Amount of Assets in the Consolidated Balance Sheets Gross Amount Gross Amounts Offset in the Consolidated Balance Sheets Net Amount of Liabilities in the Consolidated Balance Sheets Current Commodity contracts $ 2 $ (1 ) $ 1 $ (2 ) $ 1 $ (1 ) Embedded derivatives in commodity contracts — — — (6 ) — (6 ) Total current derivative instruments 2 (1 ) 1 (8 ) 1 (7 ) Non-current Commodity contracts — — — — — — Embedded derivatives in commodity contracts — — — (38 ) — (38 ) Total non-current derivative instruments — — — (38 ) — (38 ) Total derivative instruments $ 2 $ (1 ) $ 1 $ (46 ) $ 1 $ (45 ) In the table above, the Partnership does not offset a counterparty’s current derivative contracts with the counterparty’s non-current derivative contracts, although the Partnership’s master netting arrangements would allow current and non-current positions to be offset in the event of default. Additionally, in the event of a default, the Partnership’s master netting arrangements would allow for the offsetting of all transactions executed under the master netting arrangement. These types of transactions may include non-derivative instruments, derivatives qualifying for scope exceptions, receivables and payables arising from settled positions and other forms of non-cash collateral (such as letters of credit). The impact of the Partnership’s derivative contracts not designated as hedging instruments and the location of gain or (loss) recognized in the Consolidated Statements of Income is summarized below. Three Months Ended March 31, (In millions) 2017 2016 Product sales Realized (loss) gain $ (1 ) $ 7 Unrealized gain (loss) 7 (8 ) Total revenue: derivative gain (loss) from product sales 6 (1 ) Purchased product costs Realized loss (2 ) (1 ) Unrealized gain (loss) 9 (1 ) Total purchased product costs: derivative gain (loss) from product purchases 7 (2 ) Cost of Revenues Realized loss — (1 ) Total cost of revenues: derivative loss from cost of revenues — (1 ) Total derivative gains (losses) $ 13 $ (4 )

Debt

Debt	3 Months Ended
	Mar. 31, 2017
Debt Disclosure [Abstract]
Debt	Debt The Partnership’s outstanding borrowings consisted of the following: (In millions) March 31, 2017 December 31, 2016 MPLX LP: Bank revolving credit facility due 2020 $ — $ — Term loan facility due 2019 250 250 5.500% senior notes due February 2023 710 710 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 — 5.200% senior notes due March 2047 1,000 — Consolidated subsidiaries: MarkWest - 4.500% - 5.500% senior notes, due 2023 - 2025 63 63 MPL - capital lease obligations due 2020 8 8 Total 7,108 4,858 Unamortized debt issuance costs (28 ) (7 ) Unamortized discount (425 ) (428 ) Amounts due within one year (1 ) (1 ) Total long-term debt due after one year $ 6,654 $ 4,422 Credit Agreements During the three months ended March 31, 2017 , the Partnership had no borrowings under the bank revolving credit facility. At March 31, 2017 , the Partnership had no outstanding borrowings and $3 million letters of credit outstanding under this facility, resulting in total availability of $2.0 billion , or 99.9 percent of the borrowing capacity. The $250 million term loan facility was drawn in full on November 20, 2014. The borrowings under this facility during the three months ended March 31, 2017 were at an average interest rate of 2.261 percent . Senior Notes 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 “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.

Supplemental Cash Flow Informat

Supplemental Cash Flow Information	3 Months Ended
	Mar. 31, 2017
Supplemental Cash Flow Elements [Abstract]
Supplemental Cash Flow Information	Supplemental Cash Flow Information Three Months Ended March 31, (In millions) 2017 2016 Net cash provided by operating activities included: Interest paid (net of amounts capitalized) $ 49 $ 53 Non-cash investing and financing activities: Net transfers of property, plant and equipment from materials and supplies inventories $ 6 $ (4 ) Contribution of fixed assets to joint venture (1) 328 — (1) Contribution of assets to Sherwood Midstream and Sherwood Midstream Holdings. See Note 4 . 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) 2017 2016 Increase (decrease) in capital accruals $ 2 $ (23 ) In connection with the acquisition of HSM described in Note 3 , MPC agreed to waive first quarter 2016 distributions on the MPLX LP common units issued in connection with the transaction. MPC did not receive general partner distributions or incentive distribution rights that would have otherwise accrued on such MPLX LP common units with respect to the first quarter distributions. The value of these waived distributions was $15 million . In connection with the acquisition of HST, WHC and MPLXT described in Note 3 , MPC agreed to waive two-thirds of the first quarter 2017 distributions on the MPLX LP common units issued in connection with the transaction. MPC will not receive general partner distributions or incentive distribution rights that would have otherwise accrued on such MPLX LP common units with respect to the first quarter distributions. The value of these waived distributions is $6 million . 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, which has been prorated to $0.8 million from the acquisition date.

Equity-Based Compensation Plan

Equity-Based Compensation Plan	3 Months Ended
	Mar. 31, 2017
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, 2017 : Number Weighted Outstanding at December 31, 2016 1,173,411 $ 33.09 Granted 150,600 37.91 Settled (158,814 ) 33.62 Forfeited (18,055 ) 33.04 Outstanding at March 31, 2017 1,147,142 33.65 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. The performance units paying out in units are accounted for as equity awards. The performance units granted in 2017 are hybrid awards having a three-year performance period of January 1, 2017 through December 31, 2019. The payout of the award is dependent on two independent conditions, each constituting 50 percent of the overall target units granted. The awards have a performance condition based on MPLX LP’s distributable cash flow during the last twelve months of the performance period, and a market condition based on MPLX LP’s total unitholder return over the entire three year performance period. The market condition was valued using a Monte Carlo valuation, with the result being combined with the expected payout of the performance condition as of the grant date, resulting in a grant date fair value of $0.90 for the 2017 equity-classified performance units. The following is a summary of the equity-classified performance unit award activity for the three months ended March 31, 2017 : Number of Outstanding at December 31, 2016 1,799,249 Granted 1,365,937 Settled (464,500 ) Forfeited (14,375 ) Outstanding at March 31, 2017 2,686,311

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2017
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, 2017 and December 31, 2016 , accrued liabilities for remediation totaled $1 million . However, it is not presently possible to estimate the ultimate amount of all remediation costs that might be incurred or the penalties, if any, which may be imposed. At March 31, 2017 and December 31, 2016 , there was $4 million and less than $1 million , respectively, in receivables from MPC for indemnification of environmental costs related to incidents occurring prior to the Initial Offering. In July 2015, representatives from the EPA and the United States Department of Justice conducted a raid on a MarkWest Liberty Midstream pipeline launcher/receiver site utilized for pipeline maintenance operations in Washington County, Pennsylvania pursuant to a search warrant issued by a magistrate of the United States District Court for the Western District of Pennsylvania. As part of this initiative, the U.S. Attorney’s Office for the Western District of Pennsylvania, proceeded with an investigation of MarkWest Liberty Midstream’s launcher/receiver, pipeline and compressor station operations. In response to the investigation, MarkWest initiated independent studies which demonstrated that there was no risk to worker safety and no threat of public harm associated with MarkWest Liberty Midstream’s launcher/receiver operations. These findings were supported by a subsequent inspection and review by the Occupational Safety and Health Administration. After providing these studies, and other substantial documentation related to MarkWest Liberty Midstream's pipeline and compressor stations, and arranging site visits and conducting several meetings with the government’s representatives, on September 13, 2016, the U.S. Attorney’s Office for the Western District of Pennsylvania rendered a declination decision, dropping its criminal investigation and declining to pursue charges in this matter. MarkWest Liberty Midstream continues to discuss with the EPA and the State of Pennsylvania civil enforcement allegations associated with permitting or other related regulatory obligations for its launcher/receiver and compressor station facilities in the region. In connection with these discussions, MarkWest Liberty Midstream received an initial proposal from the EPA to settle all civil claims associated with this matter for the combination of a proposed cash penalty of approximately $2.4 million and proposed supplemental environmental projects with an estimated cost of approximately $3.6 million . MarkWest Liberty Midstream has submitted a response asserting that this action involves novel issues surrounding primarily minor source emissions from facilities that the agencies themselves considered de minimis and were not the subject of regulation and consequently that the settlement proposal is excessive. MarkWest Liberty Midstream will continue to negotiate with EPA regarding the amount and scope of the proposed settlement. 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 – 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 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. 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, 2017 , the Partnership’s contractual commitments to acquire property, plant and equipment totaled $409 million . These commitments at March 31, 2017 were primarily related to plant expansion projects for the Marcellus and Southwest Operations and the Cornerstone Pipeline project. 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, 2017 , 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.

Description of the Business a25

Description of the Business and Basis of Presentation Description of Business and Basis of Presentation (Policies)	3 Months Ended
	Mar. 31, 2017
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, Policy [Policy Text Block]	In preparing the Consolidated Statements of Equity, net income attributable to MPLX LP is allocated to preferred unitholders based on a fixed distribution schedule, as discussed in Note 8 , and subsequently allocated to the general partner and limited partner unitholders. Distributions, although earned, are not accrued for until declared. However, when distributions related to the incentive distribution rights are made, earnings equal to the amount of those distributions are first allocated to the general partner before the remaining earnings are allocated to the limited partner unitholders based on their respective ownership percentages. The allocation of net income attributable to MPLX LP for purposes of calculating net income per limited partner unit is described in Note 6 . Net income (loss) per unit applicable to common limited partner units is computed by dividing 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, general partner units, Preferred units, certain equity-based compensation awards and incentive distribution rights (“IDRs”).

Net Income Per Limited Partne26

Net Income Per Limited Partner Unit Earnings Per Share, Policy (Policies)	3 Months Ended
	Mar. 31, 2017
Earnings Per Share [Abstract]
Earnings Per Share, Policy [Policy Text Block]	In preparing the Consolidated Statements of Equity, net income attributable to MPLX LP is allocated to preferred unitholders based on a fixed distribution schedule, as discussed in Note 8 , and subsequently allocated to the general partner and limited partner unitholders. Distributions, although earned, are not accrued for until declared. However, when distributions related to the incentive distribution rights are made, earnings equal to the amount of those distributions are first allocated to the general partner before the remaining earnings are allocated to the limited partner unitholders based on their respective ownership percentages. The allocation of net income attributable to MPLX LP for purposes of calculating net income per limited partner unit is described in Note 6 . Net income (loss) per unit applicable to common limited partner units is computed by dividing 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, general partner units, Preferred units, certain equity-based compensation awards and incentive distribution rights (“IDRs”).

Acquisitions (Tables)

Acquisitions (Tables)	3 Months Ended
	Mar. 31, 2017
Business Acquisition [Line Items]
Schedule of Business Acquisitions, by Acquisition [Table Text Block]	The following table presents the Partnership’s previously reported unaudited Consolidated Statements of Income, retrospectively adjusted for the acquisition of HST and WHC: Three Months Ended March 31, 2016 (In millions, except per unit data) MPLX LP (Previously Reported) HST/WHC Eliminations (1) MPLX LP (Currently Reported) Revenues and other income: Service revenue $ 229 $ — $ — $ 229 Service revenue to related parties 150 27 — 177 Rental income 70 — — 70 Rental income to related parties 26 12 — 38 Product sales 100 — — 100 Product sales to related parties 3 — — 3 Income from equity method investments 5 — — 5 Other income 2 — — 2 Other income - related parties 24 — (3 ) 21 Total revenues and other income 609 39 (3 ) 645 Costs and expenses: Cost of revenues (excludes items below) 89 5 — 94 Purchased product costs 79 — — 79 Rental cost of sales 14 — — 14 Purchases from related parties 76 5 (3 ) 78 Depreciation and amortization 132 4 — 136 Impairment expense 129 — — 129 General and administrative expenses 52 1 — 53 Other taxes 11 1 — 12 Total costs and expenses 582 16 (3 ) 595 Income from operations 27 23 — 50 Related party interest and other financial income 1 — — 1 Interest expense (net of amounts capitalized) 55 — — 55 Other financial costs 12 — — 12 (Loss) income before income taxes (41 ) 23 — (18 ) Benefit for income taxes (4 ) — — (4 ) Net (loss) income (37 ) 23 — (14 ) Net income attributable to Predecessor 23 23 — 46 Net loss attributable to MPLX LP (60 ) — — (60 ) Less: General partner’s interest in net income attributable to MPLX LP 39 — — 39 Limited partners’ interest in net loss attributable to MPLX LP $ (99 ) $ — $ — $ (99 ) (1) Represents intercompany transactions eliminated during the consolidation process, in accordance with GAAP. The following table presents the Partnership’s previously reported Consolidated Balance Sheet, retrospectively adjusted for the acquisition of HST, WHC and MPLXT: December 31, 2016 (In millions) MPLX LP (Previously Reported) HST/WHC MPLXT Eliminations (1) MPLX LP (Currently Reported) Assets Current assets: Cash and cash equivalents $ 234 $ — $ — $ — $ 234 Receivables, net 297 1 1 — 299 Receivables from related parties 122 91 38 (4 ) 247 Inventories 54 1 — — 55 Other current assets 33 — — — 33 Total current assets 740 93 39 (4 ) 868 Equity method investments 2,467 — 4 — 2,471 Property, plant and equipment, net 10,730 265 413 — 11,408 Intangibles, net 492 — — — 492 Goodwill 2,199 25 21 — 2,245 Long-term receivables from related parties 4 — 7 — 11 Other noncurrent assets 14 — — — 14 Total assets $ 16,646 $ 383 $ 484 $ (4 ) $ 17,509 Liabilities Current liabilities: Accounts payable $ 123 $ 5 $ 12 $ — $ 140 Accrued liabilities 228 4 — — 232 Payables to related parties 75 4 12 (4 ) 87 Deferred revenue 2 — — — 2 Deferred revenue - related parties 34 4 — — 38 Accrued property, plant and equipment 132 9 5 — 146 Accrued taxes 33 2 3 — 38 Accrued interest payable 53 — — — 53 Other current liabilities 24 1 2 — 27 Total current liabilities 704 29 34 (4 ) 763 Long-term deferred revenue 12 — — — 12 Long-term deferred revenue - related parties 15 — 4 — 19 Long-term debt 4,422 — — — 4,422 Deferred income taxes 5 — 1 — 6 Deferred credits and other liabilities 169 2 6 — 177 Total liabilities 5,327 31 45 (4 ) 5,399 Redeemable preferred units 1,000 — — — 1,000 Equity Common unitholders - public 8,086 — — — 8,086 Class B unitholders 133 — — — 133 Common unitholder - MPC 1,069 — — — 1,069 General partner - MPC 1,013 — — — 1,013 Equity of Predecessor — 352 439 — 791 Total MPLX LP partners’ capital 10,301 352 439 — 11,092 Noncontrolling interest 18 — — — 18 Total equity 10,319 352 439 — 11,110 Total liabilities and equity $ 16,646 $ 383 $ 484 $ (4 ) $ 17,509 (1) Represents intercompany transactions eliminated during the consolidation process, in accordance with GAAP. The following table presents the Partnership’s previously reported unaudited Consolidated Statements of Cash Flows, retrospectively adjusted for the acquisition of HST and WHC: Three Months Ended March 31, 2016 (In millions) MPLX LP (Previously Reported) HST/WHC MPLX LP (Currently Reported) Increase (decrease) in cash and cash equivalents Operating activities: Net (loss) income $ (37 ) $ 23 $ (14 ) Adjustments to reconcile net income to net cash provided by operating activities: Amortization of deferred financing costs 11 — 11 Depreciation and amortization 132 4 136 Impairment expense 129 — 129 Deferred income taxes (4 ) — (4 ) Income from equity method investments (5 ) — (5 ) Distributions from unconsolidated affiliates 38 — 38 Changes in: Current receivables (5 ) — (5 ) Inventories 1 — 1 Change in fair value of derivatives 12 — 12 Current accounts payable and accrued liabilities (15 ) (3 ) (18 ) Receivables from / liabilities to related parties 23 — 23 All other, net 15 2 17 Net cash provided by operating activities 295 26 321 Investing activities: Additions to property, plant and equipment (291 ) (13 ) (304 ) Investments - loans from (to) related parties 77 (13 ) 64 Investments in unconsolidated affiliates (29 ) — (29 ) All other, net 3 — 3 Net cash used in investing activities (240 ) (26 ) (266 ) Financing activities: Long-term debt - borrowings 306 — 306 - repayments (857 ) — (857 ) Related party debt - borrowings 1,437 — 1,437 - repayments (1,007 ) — (1,007 ) Net proceeds from equity offerings 321 — 321 Distributions to unitholders and general partner (190 ) — (190 ) Distributions to noncontrolling interests (1 ) — (1 ) Contributions from noncontrolling interests 2 — 2 All other, net (1 ) — (1 ) Distributions to MPC from Predecessor (104 ) — (104 ) Net cash used in financing activities (94 ) — (94 ) Net decrease in cash and cash equivalents (39 ) — (39 ) Cash and cash equivalents at beginning of period 43 — 43 Cash and cash equivalents at end of period $ 4 $ — $ 4
Ozark Pipeline [Member]
Business Acquisition [Line Items]
Business Acquisition, Pro Forma Information [Table Text Block]	The amounts of revenue and income from operations associated with the acquisition included in the Consolidated Statements of Income from the acquisition date, March 1, 2017, to March 31, 2017 are as follows: (In millions) One Month Ended March 31, 2017 Revenues and other income $ 7 Income from operations 2

Equity Method Investments (Tabl

Equity Method Investments (Tables)	3 Months Ended
	Mar. 31, 2017
Equity Method Investments and Joint Ventures [Abstract]
Summarized Financial Information For Equity Method Investees Table [Text Block]	Summarized financial information for the Partnership’s equity method investments for the three months ended March 31, 2017 and 2016 is as follows: Three Months Ended March 31, 2017 (In millions) MarkWest Utica EMG Other VIEs Non-VIEs Total Revenue 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 Three Months Ended March 31, 2016 (In millions) MarkWest Utica EMG Other VIEs Non-VIEs Total Revenue and other income $ 61 $ 5 $ 29 $ 95 Costs and expenses 22 4 20 46 Income from operations 39 1 9 49 Net income 38 1 9 48 Income from equity method investments (1) 4 — 1 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, 2017 and December 31, 2016 is as follows: March 31, 2017 (In millions) MarkWest Utica EMG (1) Other VIEs Non-VIEs Total Current assets $ 63 $ 48 $ 34 $ 145 Noncurrent assets 2,145 832 379 3,356 Current liabilities 29 96 20 145 Noncurrent liabilities 2 10 — 12 December 31, 2016 (In millions) MarkWest Utica EMG (1) Other VIEs Non-VIEs Total Current assets $ 45 $ 2 $ 40 $ 87 Noncurrent assets 2,173 132 390 2,695 Current liabilities 30 4 26 60 Noncurrent liabilities 2 13 — 15 (1) MarkWest Utica EMG’s noncurrent assets includes its investment in its subsidiary Ohio Gathering, which does not appear elsewhere in this table. The investment was $792 million and $794 million as of March 31, 2017 and December 31, 2016 , respectively.

Related Party Agreements and 29

Related Party Agreements and Transactions (Tables)	3 Months Ended
	Mar. 31, 2017
Related Party Transactions [Abstract]
Sales to Related Parties	Sales to related parties were as follows: Three Months Ended March 31, (In millions) 2017 2016 Service revenues MPC $ 255 $ 177 Rental income MPC $ 67 $ 38 Product sales (1) MPC $ 2 $ 3
Summary of Fees Received for Operating Pipelines for Related Parties Included in Other Income - Related Parties	The revenue received from related parties, included in Other income-related parties on the Consolidated Statements of Income, was as follows: Three Months Ended March 31, (In millions) 2017 2016 MPC $ 11 $ 14 MarkWest Utica EMG 4 2 Ohio Gathering 4 4 Other 3 1 Total $ 22 $ 21
Summary of Charges for Services Included in Purchases from Related Parties	Three Months Ended March 31, (In millions) 2017 2016 Purchases - related parties $ 15 $ 7 General and administrative expenses 8 10 Total $ 23 $ 17
Summary of Related Party Costs Added to Property, Plant and Equipment	Three Months Ended March 31, (In millions) 2017 2016 MPC $ 10 $ 10
Employee Services Expenses from Related Parties	Employee services expenses from related parties were as follows: Three Months Ended March 31, (In millions) 2017 2016 Purchases - related parties $ 92 $ 71 General and administrative expenses 25 21 Total $ 117 $ 92
Receivables from Related Parties	Receivables from related parties, which include reimbursements from the MarkWest Merger to be provided by MPC for the conversion of Class B units, were as follows: (In millions) March 31, 2017 December 31, 2016 MPC $ 177 $ 242 Sherwood Midstream 14 — Jefferson Gas Gathering 2 — MarkWest Utica EMG — 2 Ohio Gathering 3 2 Other 2 1 Total $ 198 $ 247
Schedule of Long Term Receivables with Related Parties [Table Text Block]	Long-term receivables with related parties, which includes straight line rental income, were as follows: (In millions) March 31, 2017 December 31, 2016 MPC $ 13 $ 11
Payables to Related Parties	Payables to related parties were as follows: (In millions) March 31, 2017 December 31, 2016 MPC $ 59 $ 63 MarkWest Utica EMG 29 24 Sherwood Midstream 3 — Total $ 91 $ 87
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, 2017 December 31, 2016 Minimum volume deficiencies - MPC $ 50 $ 48 Project reimbursements - MPC 14 9 Total $ 64 $ 57

Net Income Per Limited Partne30

Net Income Per Limited Partner Unit (Tables)	3 Months Ended
	Mar. 31, 2017
Earnings Per Share [Abstract]
Schedule of Distributions By Partner By Class	For the three months ended March 31, 2017 and 2016 , the Partnership had dilutive potential common units consisting of certain equity-based compensation awards and Class B units. Diluted net income (loss) per limited partner unit for the three months ended March 31, 2016 is the same as basic net income (loss) per limited partner unit since the inclusion of any potential common units would have been anti-dilutive. Potential common units omitted from the diluted earnings per unit calculation for the three months ended March 31, 2017 and 2016 were less than 1 million and approximately 10 million , respectively. Three Months Ended March 31, (In millions) 2017 2016 Net income (loss) attributable to MPLX LP $ 150 $ (60 ) Less: Limited partners’ distributions declared on Preferred units (1) 16 — General partner’s distributions declared (including IDRs) (1) 65 44 Limited partners’ distributions declared on common units (1) 198 156 Undistributed net loss attributable to MPLX LP $ (129 ) $ (260 ) (1) See Note 7 for distribution information. 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
Schedule of Basic and Diluted Earnings Per Unit	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 Three Months Ended March 31, 2016 (In millions, except per unit data) General Partner Limited Partners’ Common Units Total Basic and diluted net income attributable to MPLX LP per unit: Net income (loss) attributable to MPLX LP: Distributions declared (including IDRs) $ 44 $ 156 $ 200 Undistributed net loss attributable to MPLX LP (5 ) (255 ) (260 ) Net income (loss) attributable to MPLX LP (1) $ 39 $ (99 ) $ (60 ) Weighted average units outstanding: Basic 7 300 307 Diluted 7 300 307 Net loss attributable to MPLX LP per limited partner unit: Basic $ (0.33 ) Diluted $ (0.33 ) (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, 2017
Equity [Abstract]
Schedule of Stockholders Equity [Table Text Block]	The changes in the number of units outstanding from December 31, 2016 through March 31, 2017 are summarized below: (In units) Common Class B General Partner Total Balance at December 31, 2016 357,193,288 3,990,878 7,371,105 368,555,271 Unit-based compensation awards (1) 99,463 — 2,030 101,493 Issuance of units under the ATM Program (2) 4,151,258 — 84,720 4,235,978 Contribution of HST/WHC/MPLXT (3) 12,960,376 — 264,497 13,224,873 Balance at March 31, 2017 374,404,385 3,990,878 7,722,352 386,117,615 (1) As a result of the unit-based compensation awards issued during the period, MPLX GP contributed less than $1 million in exchange for 2,030 general partner units to maintain its two percent GP Interest. (2) As a result of common units issued under the ATM Program during the period, MPLX GP contributed $3 million in exchange for 84,720 general partner units to maintain its two percent GP Interest. (3) See Note 3 for information regarding the HST, WHC and MPLXT acquisition.
Schedule Of Calculation Of Net Income Applicable to Partners [Table Text Block]	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 8 , and subsequently allocated to the general partner and limited partner unitholders. However, when distributions related to the incentive distribution rights are made, earnings equal to the amount of those distributions are first allocated to the general partner before the remaining earnings are 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: Three Months Ended March 31, (In millions) 2017 2016 Net income (loss) attributable to MPLX LP $ 150 $ (60 ) Less: Preferred unit distributions 16 — General partner's incentive distribution rights and other 61 41 Net income (loss) attributable to MPLX LP available to general and limited partners $ 73 $ (101 ) General partner's two percent GP Interest in net income (loss) attributable to MPLX LP $ 1 $ (2 ) General partner's incentive distribution rights and other 61 41 General partner's GP Interest in net income attributable to MPLX LP $ 62 $ 39
Distributions Made to Limited Partner, by Distribution [Table Text Block]	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) 2017 2016 General partner's distributions: General partner's distributions on general partner units $ 5 $ 4 General partner's distributions on incentive distribution rights 60 40 Total distribution on general partner units and incentive distribution rights $ 65 $ 44 Limited partners' distributions: Common unitholders, includes common units of general partner $ 198 $ 156 Subordinated unitholders — — Total limited partners' distributions 198 156 Preferred unit distributions 16 — Total cash distributions declared $ 279 $ 200

Redeemable Preferred Units (Tab

Redeemable Preferred Units (Tables)	3 Months Ended
	Mar. 31, 2017
Temporary Equity Disclosure [Abstract]
Temporary Equity [Table Text Block]	(In millions) Redeemable Preferred Units Balance at December 31, 2016 $ 1,000 Net income 16 Distributions received by Preferred unitholders (16 ) Balance at March 31, 2017 $ 1,000

Segment Information (Tables)

Segment Information (Tables)	3 Months Ended
	Mar. 31, 2017
Segment Reporting [Abstract]
Schedule of Segment Reporting Information, by Segment [Table Text Block]	Three Months Ended March 31, 2017 (In millions) L&S G&P Total Revenues and other income: Segment revenues $ 345 $ 597 $ 942 Segment other income 12 1 13 Total segment revenues and other income 357 598 955 Costs and expenses: Segment cost of revenues 148 253 401 Segment operating income before portion attributable to noncontrolling interest and Predecessor 209 345 554 Segment portion attributable to noncontrolling interest and Predecessor 53 36 89 Segment operating income attributable to MPLX LP $ 156 $ 309 $ 465 Three Months Ended March 31, 2016 (In millions) L&S G&P Total Revenues and other income: Segment revenues $ 231 $ 498 $ 729 Segment other income 16 — 16 Total segment revenues and other income 247 498 745 Costs and expenses: Segment cost of revenues 97 200 297 Segment operating income before portion attributable to noncontrolling interest and Predecessor 150 298 448 Segment portion attributable to noncontrolling interest and Predecessor 62 41 103 Segment operating income attributable to MPLX LP $ 88 $ 257 $ 345 Three Months Ended March 31, (In millions) 2017 2016 Reconciliation to Income from operations: L&S segment operating income attributable to MPLX LP $ 156 $ 88 G&P segment operating income attributable to MPLX LP 309 257 Segment operating income attributable to MPLX LP 465 345 Segment portion attributable to unconsolidated affiliates (40 ) (42 ) Segment portion attributable to Predecessor 53 62 Income from equity method investments 5 5 Other income - related parties 11 7 Unrealized derivative gains (losses) (1) 16 (9 ) Depreciation and amortization (187 ) (136 ) Impairment expense — (129 ) General and administrative expenses (58 ) (53 ) Income from operations $ 265 $ 50
Reconciliation of Operating Profit (Loss) from Segments to Consolidated [Table Text Block]	Three Months Ended March 31, (In millions) 2017 2016 Reconciliation to Income from operations: L&S segment operating income attributable to MPLX LP $ 156 $ 88 G&P segment operating income attributable to MPLX LP 309 257 Segment operating income attributable to MPLX LP 465 345 Segment portion attributable to unconsolidated affiliates (40 ) (42 ) Segment portion attributable to Predecessor 53 62 Income from equity method investments 5 5 Other income - related parties 11 7 Unrealized derivative gains (losses) (1) 16 (9 ) Depreciation and amortization (187 ) (136 ) Impairment expense — (129 ) General and administrative expenses (58 ) (53 ) Income from operations $ 265 $ 50
Reconciliation of Revenue from Segments to Consolidated [Table Text Block]	Three Months Ended March 31, (In millions) 2017 2016 Reconciliation to Total revenues and other income: Total segment revenues and other income $ 955 $ 745 Revenue adjustment from unconsolidated affiliates (92 ) (104 ) Income from equity method investments 5 5 Other income - related parties 11 7 Unrealized derivative gains (losses) (1) 7 (8 ) Total revenues and other income $ 886 $ 645
Reconciliation of Net Income Attributable to Noncontrolling Interests [Table Text Block]	Three Months Ended March 31, (In millions) 2017 2016 Reconciliation to Net income attributable to noncontrolling interests and Predecessor: Segment portion attributable to noncontrolling interest and Predecessor $ 89 $ 103 Portion of noncontrolling interests and Predecessor related to items below segment income from operations (36 ) (34 ) Portion of operating income attributable to noncontrolling interest of unconsolidated affiliates (16 ) (23 ) Net income attributable to noncontrolling interests and Predecessor $ 37 $ 46
Reconciliation of Other Significant Reconciling Items from Segments to Consolidated [Table Text Block]	The following reconciles segment capital expenditures to total capital expenditures: Three Months Ended March 31, (In millions) 2017 2016 L&S segment capital expenditures $ 97 $ 75 G&P segment capital expenditures 298 273 Total segment capital expenditures 395 348 Less: Capital expenditures for Partnership-operated, non-wholly-owned subsidiaries in G&P segment 115 44 Total capital expenditures $ 280 $ 304
Reconciliation of Assets from Segment to Consolidated [Table Text Block]	Total assets by reportable segment were: (In millions) March 31, 2017 December 31, 2016 Cash and cash equivalents $ 265 $ 234 L&S 3,713 2,978 G&P 14,307 14,297 Total assets $ 18,285 $ 17,509

Inventories (Tables)

Inventories (Tables)	3 Months Ended
	Mar. 31, 2017
Inventory Disclosure [Abstract]
Schedule of Inventory, Current [Table Text Block]	Inventories consist of the following: (In millions) March 31, 2017 December 31, 2016 NGLs $ 1 $ 2 Line fill 8 9 Spare parts, materials and supplies 53 44 Total inventories $ 62 $ 55

Property, Plant and Equipment (

Property, Plant and Equipment (Tables)	3 Months Ended
	Mar. 31, 2017
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment	Property, plant and equipment with associated accumulated depreciation is shown below: (In millions) March 31, 2017 December 31, 2016 Natural gas gathering and NGL transportation pipelines and facilities $ 4,843 $ 4,748 Processing, fractionation and storage facilities 3,654 3,467 Pipelines and related assets 2,005 1,799 Barges and towing vessels 528 479 Terminals and related assets 889 839 Land, building, office equipment and other 711 757 Construction-in-progress 780 1,013 Total 13,410 13,102 Less accumulated depreciation 1,999 1,694 Property, plant and equipment, net $ 11,411 $ 11,408

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2017
Fair Value Disclosures [Abstract]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block]	The tables below include a rollforward of the balance sheet amounts for the three months ended March 31, 2017 and 2016 , respectively (including the change in fair value), for assets and liabilities classified by the Partnership within Level 3 of the valuation hierarchy. Three Months Ended March 31, 2017 Three Months Ended March 31, 2016 (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 $ (6 ) $ (54 ) $ 7 $ (32 ) Total gain (loss) (realized and unrealized) included in earnings (1) 5 8 (1 ) (4 ) Settlements 1 2 (6 ) 2 Fair value at end of period $ — $ (44 ) $ — $ (34 ) The amount of total gains/(losses) for the period included in earnings attributable to the change in unrealized losses relating to liabilities still held at end of period $ 5 $ 8 $ (2 ) $ (3 ) (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 on Embedded Derivatives in Commodity Contracts are recorded in Purchased product costs .
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block]	The following table presents the financial instruments carried at fair value classified by the valuation hierarchy: March 31, 2017 December 31, 2016 (In millions) Assets Liabilities Assets Liabilities Significant other observable inputs (Level 2) Commodity contracts $ — $ — $ — $ — Significant unobservable inputs (Level 3) Commodity contracts 1 (1 ) — (6 ) Embedded derivatives in commodity contracts — (44 ) — (54 ) Total carrying value in Consolidated Balance Sheets $ 1 $ (45 ) $ — $ (60 )
Fair 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, 2017 December 31, 2016 (In millions) Fair Value Carrying Value Fair Value Carrying Value Long-term debt $ 7,290 $ 6,675 $ 4,953 $ 4,422 SMR liability 107 95 108 96
Fair Value Inputs Assets and Liabilities Quantitative Information [Table Text Block]	The following table provides additional information about the significant unobservable inputs used in the valuation of Level 3 instruments as of March 31, 2017 . The market approach is used for valuation of all instruments. Level 3 Instrument Balance Sheet Classification Unobservable Inputs Value Range Time Period Commodity contracts Assets Forward ethane prices (per gallon) (1) $0.24 - $0.28 Apr. 17 - Dec. 17 Forward isobutane prices (per gallon) (1) $0.71 - $0.81 Apr. 17 - Dec. 18 Forward normal butane prices (per gallon) (1) $0.64 - $0.77 Apr. 17 - Dec. 18 Commodity contracts Liabilities Forward propane prices (per gallon) (1) $0.56 - $0.64 Apr. 17 - Dec. 18 Forward natural gasoline prices (per gallon) (1) $1.08 - $1.16 Apr. 17 - Dec. 18 Embedded derivatives in commodity contracts Liabilities Forward propane prices (per gallon) (1) $0.53 - $0.64 Apr. 17 - Dec. 22 Forward isobutane prices (per gallon) (1) $0.68 - $0.81 Apr. 17 - Dec. 22 Forward normal butane prices (per gallon) (1) $0.64 - $0.77 Apr. 17 - Dec. 22 Forward natural gasoline prices (per gallon) (1) $1.04 - $1.16 Apr. 17 - Dec. 22 Forward natural gas prices (per mmbtu) (2) $2.22 - $3.35 Apr. 17 - Dec. 22 Probability of renewal (3) 50.0% Probability of renewal for second 5-yr term (3) 75.0% (1) NGL prices used in the valuations are lower in the early years and increase over time. (2) Natural gas prices used in the valuations are higher in the early years and decrease over time. (3) The producer counterparty to the embedded derivative has the option to renew the gas purchase agreement and the related keep-whole processing agreement for two successive five -year terms after 2022. The embedded gas purchase agreement cannot be renewed without the renewal of the related keep-whole processing agreement. Due to the significant number of years until the renewal options are exercisable and the high level of uncertainty regarding the counterparty’s future business strategy, the future commodity price environment, and the future competitive environment for midstream services in the Southern Appalachian region, management determined that a 50 percent probability of renewal for the first five-year term and 75 percent for the second five-year term are appropriate assumptions. Included in this assumption is a further extension of management’s estimates of future frac spreads through 2032.

Derivative Financial Instrume37

Derivative Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Schedule of Derivative Instruments in Statement of Financial Position, Fair Value [Table Text Block]	The impact of the Partnership’s derivative instruments on its Consolidated Balance Sheets is summarized below: (In millions) March 31, 2017 December 31, 2016 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 $ 1 $ (7 ) $ — $ (13 ) Other noncurrent assets / deferred credits and other liabilities — (38 ) — (47 ) Total $ 1 $ (45 ) $ — $ (60 ) (1) Includes embedded derivatives in commodity contracts as discussed above.
Schedule of Notional Amounts of Outstanding Derivative Positions [Table Text Block]	As of March 31, 2017 , the Partnership had the following outstanding commodity contracts that were executed to manage the cash flow risk associated with future sales of NGLs: Derivative contracts not designated as hedging instruments Financial Position Notional Quantity (net) Crude Oil (bbl) Short 51,900 Natural Gas (MMBtu) Long 1,431,472 NGLs (gal) Short 81,257,382
Offsetting Assets and Liabilities [Table Text Block]	The net amounts in the table below equal the balances presented in the Consolidated Balance Sheets: March 31, 2017 Assets Liabilities (In millions) Gross Amount Gross Amounts Offset in the Consolidated Balance Sheets Net Amount of Assets in the Consolidated Balance Sheets Gross Amount Gross Amounts Offset in the Consolidated Balance Sheets Net Amount of Liabilities in the Consolidated Balance Sheets Current Commodity contracts $ 2 $ (1 ) $ 1 $ (2 ) $ 1 $ (1 ) Embedded derivatives in commodity contracts — — — (6 ) — (6 ) Total current derivative instruments 2 (1 ) 1 (8 ) 1 (7 ) Non-current Commodity contracts — — — — — — Embedded derivatives in commodity contracts — — — (38 ) — (38 ) Total non-current derivative instruments — — — (38 ) — (38 ) Total derivative instruments $ 2 $ (1 ) $ 1 $ (46 ) $ 1 $ (45 )
Derivative Instruments, Gain (Loss) [Table Text Block]	The impact of the Partnership’s derivative contracts not designated as hedging instruments and the location of gain or (loss) recognized in the Consolidated Statements of Income is summarized below. Three Months Ended March 31, (In millions) 2017 2016 Product sales Realized (loss) gain $ (1 ) $ 7 Unrealized gain (loss) 7 (8 ) Total revenue: derivative gain (loss) from product sales 6 (1 ) Purchased product costs Realized loss (2 ) (1 ) Unrealized gain (loss) 9 (1 ) Total purchased product costs: derivative gain (loss) from product purchases 7 (2 ) Cost of Revenues Realized loss — (1 ) Total cost of revenues: derivative loss from cost of revenues — (1 ) Total derivative gains (losses) $ 13 $ (4 )

Debt (Tables)

Debt (Tables)	3 Months Ended
	Mar. 31, 2017
Debt Disclosure [Abstract]
Summary of Outstanding Borrowings	The Partnership’s outstanding borrowings consisted of the following: (In millions) March 31, 2017 December 31, 2016 MPLX LP: Bank revolving credit facility due 2020 $ — $ — Term loan facility due 2019 250 250 5.500% senior notes due February 2023 710 710 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 — 5.200% senior notes due March 2047 1,000 — Consolidated subsidiaries: MarkWest - 4.500% - 5.500% senior notes, due 2023 - 2025 63 63 MPL - capital lease obligations due 2020 8 8 Total 7,108 4,858 Unamortized debt issuance costs (28 ) (7 ) Unamortized discount (425 ) (428 ) Amounts due within one year (1 ) (1 ) Total long-term debt due after one year $ 6,654 $ 4,422

Supplemental Cash Flow Inform39

Supplemental Cash Flow Information (Tables)	3 Months Ended
	Mar. 31, 2017
Supplemental Cash Flow Elements [Abstract]
Summary of Supplemental Cash Flow Information	Three Months Ended March 31, (In millions) 2017 2016 Net cash provided by operating activities included: Interest paid (net of amounts capitalized) $ 49 $ 53 Non-cash investing and financing activities: Net transfers of property, plant and equipment from materials and supplies inventories $ 6 $ (4 ) Contribution of fixed assets to joint venture (1) 328 —
Summary of Reconciliation of Additions to Property, Plant and Equipment to Total Capital Expenditures	Three Months Ended March 31, (In millions) 2017 2016 Increase (decrease) in capital accruals $ 2 $ (23 )

Equity-Based Compensation Plan

Equity-Based Compensation Plan (Tables)	3 Months Ended
	Mar. 31, 2017
Phantom Units [Member]
Equity Transactions And Share Based Compensation [Line Items]
Schedule of Share-based Compensation, Restricted Stock Units Award Activity [Table Text Block]	The following is a summary of phantom unit award activity of MPLX LP common units for the three months ended March 31, 2017 : Number Weighted Outstanding at December 31, 2016 1,173,411 $ 33.09 Granted 150,600 37.91 Settled (158,814 ) 33.62 Forfeited (18,055 ) 33.04 Outstanding at March 31, 2017 1,147,142 33.65
Performance Shares [Member]
Equity Transactions And Share Based Compensation [Line Items]
Schedule of Share-based Compensation, Restricted Stock Units Award Activity [Table Text Block]	The following is a summary of the equity-classified performance unit award activity for the three months ended March 31, 2017 : Number of Outstanding at December 31, 2016 1,799,249 Granted 1,365,937 Settled (464,500 ) Forfeited (14,375 ) Outstanding at March 31, 2017 2,686,311

Description of Business and Bas

Description of Business and Basis of Presentation - Additional Information (Detail)	Mar. 01, 2017	Mar. 31, 2017
Description Of Business And Basis Of Presentation [Line Items]
Number of Reportable Segments		2
HST & WHC & MPLXT [Member]
Description Of Business And Basis Of Presentation [Line Items]
Business Acquisition, Effective Date of Acquisition	Mar. 1, 2017

Acquisitions Acquisition of HST

Acquisitions Acquisition of HST, WHC & MPLXT (Details) bbl in Millions, $ in Millions		Mar. 01, 2017USD ($)misharesbbl	Mar. 31, 2016USD ($)	Mar. 31, 2017USD ($)shares
Business Acquisition [Line Items]
Contribution from MPC | $				$ 12
Number of natural gas liquids storage caverns		9
Number of light product terminals		62
Limited Partners Common Units [Member]
Business Acquisition [Line Items]
Contribution from MPC | $		$ 6	$ 15
HST & WHC & MPLXT [Member] | Cash and Cash Equivalents [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross | $		$ 1,500
HST & WHC & MPLXT [Member] | General Partner Units [Member]
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares	[1]	264,497		264,497
HST & WHC & MPLXT [Member] | Limited Partner [Member]
Business Acquisition [Line Items]
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned | $		$ 504
Business Acquisition, Equity Interest Issued or Issuable, Fair Value Assigned | $		$ 503
HST & WHC & MPLXT [Member] | Limited Partners Common Units [Member]
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares	[1]			12,960,376
HST & WHC & MPLXT [Member] | General Partner Common Units [Member]
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares	[1]	9,197,900
MPLX Logistics LLC [Member] | HST & WHC & MPLXT [Member] | Limited Partners Common Units [Member]
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares	[1]	2,630,427
MPLX Holdings Inc [Member] | HST & WHC & MPLXT [Member] | Limited Partners Common Units [Member]
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions | shares	[1]	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
[1]	(3)See Note 3 for information regarding the HST, WHC and MPLXT acquisition.

Acquisitions Acquisition of H43

Acquisitions Acquisition of HST, WHC & MPLXT - Income Statement (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Business Acquisition [Line Items]
Service revenue		$ 260	$ 229
Service revenue - related parties		255	177	[1]
Rental income		69	70
Rental income - related parties		67	38	[1]
Product sales		203	100
Product sales - related parties		2	3
Income from equity method investments	[2]	5	5
Other income		2	2
Other income - related parties		22	21	[1]
Total revenues and other income		886	645	[1]
Cost of revenues (excludes items below)		113	94	[1]
Purchased product costs		131	79
Rental cost of sales		12	14
Purchases - related parties		107	78	[1]
Depreciation and amortization		187	136	[1],[3]
Impairment expense		0	129
General and administrative expenses		58	53	[1]
Other taxes		13	12	[1]
Costs and Expenses		621	595	[1]
Income from operations		265	50	[1]
Related party interest and other financial costs		0	1
Interest expense (net of amounts capitalized)		66	55
Other Nonoperating Income (Expense)		(12)	(12)
Income (loss) before income taxes		187	(18)	[1]
Benefit for income taxes		0	4
Net income (loss)		187	(14)	[1],[3]
Net income attributable to noncontrolling interest		1	0
Less: Net income attributable to Predecessor		36	46	[1]
Net Income (Loss) Attributable to Parent	[4]	150	(60)
Less: General partner’s interest in net income attributable to MPLX LP		62	39
Net Income (Loss) Allocated to Limited Partners		$ 72	(99)
Scenario, Previously Reported [Member]
Business Acquisition [Line Items]
Service revenue			229
Service revenue - related parties			150
Rental income			70
Rental income - related parties			26
Product sales			100
Product sales - related parties			3
Income from equity method investments			5
Other income			2
Other income - related parties			24
Total revenues and other income			609
Cost of revenues (excludes items below)			89
Purchased product costs			79
Rental cost of sales			14
Purchases - related parties			76
Depreciation and amortization			132
Impairment expense			129
General and administrative expenses			52
Other taxes			11
Costs and Expenses			582
Income from operations			27
Related party interest and other financial costs			1
Interest expense (net of amounts capitalized)			55
Other Nonoperating Income (Expense)			(12)
Income (loss) before income taxes			(41)
Benefit for income taxes			4
Net income (loss)			(37)
Less: Net income attributable to Predecessor			23
Net Income (Loss) Attributable to Parent			(60)
Less: General partner’s interest in net income attributable to MPLX LP			39
Net Income (Loss) Allocated to Limited Partners			(99)
Scenario, Adjustment [Member]
Business Acquisition [Line Items]
Service revenue			0
Service revenue - related parties			0
Rental income			0
Rental income - related parties			0
Product sales			0
Product sales - related parties			0
Income from equity method investments			0
Other income			0
Other income - related parties	[5]		(3)
Total revenues and other income	[5]		(3)
Cost of revenues (excludes items below)			0
Purchased product costs			0
Rental cost of sales			0
Purchases - related parties	[5]		(3)
Depreciation and amortization			0
Impairment expense			0
General and administrative expenses			0
Other taxes			0
Costs and Expenses	[5]		(3)
Income from operations			0
Related party interest and other financial costs			0
Interest expense (net of amounts capitalized)			0
Other Nonoperating Income (Expense)			0
Income (loss) before income taxes			0
Benefit for income taxes			0
Net income (loss)			0
Less: Net income attributable to Predecessor			0
Net Income (Loss) Attributable to Parent			0
Less: General partner’s interest in net income attributable to MPLX LP			0
Net Income (Loss) Allocated to Limited Partners			0
HST & WHC [Member] | Scenario, Adjustment [Member]
Business Acquisition [Line Items]
Service revenue			0
Service revenue - related parties			27
Rental income			0
Rental income - related parties			12
Product sales			0
Product sales - related parties			0
Income from equity method investments			0
Other income			0
Other income - related parties			0
Total revenues and other income			39
Cost of revenues (excludes items below)			5
Purchased product costs			0
Rental cost of sales			0
Purchases - related parties			5
Depreciation and amortization			4
Impairment expense			0
General and administrative expenses			1
Other taxes			1
Costs and Expenses			16
Income from operations			23
Related party interest and other financial costs			0
Interest expense (net of amounts capitalized)			0
Other Nonoperating Income (Expense)			0
Income (loss) before income taxes			23
Benefit for income taxes			0
Net income (loss)			23
Less: Net income attributable to Predecessor			23
Net Income (Loss) Attributable to Parent			0
Less: General partner’s interest in net income attributable to MPLX LP			0
Net Income (Loss) Allocated to Limited Partners			$ 0
[1]	(1)Financial information has been retrospectively adjusted for the acquisition of HST and WHC from MPC. See Notes 1 and 3.
[2]	(1)Income (loss) from equity method investments includes the impact of any basis differential amortization or accretion.
[3]	(1)Financial information has been retrospectively adjusted for the acquisition of HST and WHC from MPC. See Notes 1 and 3.
[4]	llocation of net income (loss) attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.
[5]	(1)

Acquisitions Acquisition of H44

Acquisitions Acquisition of HST, WHC & MPLXT - Balance Sheet (Details) - USD ($) $ in Millions		Mar. 31, 2017	Dec. 31, 2016		Mar. 31, 2016		Dec. 31, 2015
Business Acquisition [Line Items]
Cash and cash equivalents		$ 265	$ 234		$ 4		$ 43
Receivables, net		255	299	[1]
Receivables - related parties		198	247	[1]
Inventories		62	55	[1]
Other current assets		31	33
Assets, Current		811	868	[1]
Equity method investments		3,306	2,471	[1]
Property, plant and equipment, net		11,411	11,408	[1]
Intangibles, net		482	492
Goodwill		2,245	2,245	[1]
Long-term receivables - related parties		13	11	[1]
Other noncurrent assets		17	14
Assets		18,285	17,509	[1]
Accounts payable		121	140	[1]
Accrued liabilities		194	232	[1]
Payables - related parties		91	87	[1]
Deferred Revenue, Current		3	2
Deferred revenue - related parties		38	38	[1]
Accrued property, plant and equipment		138	146	[1]
Accrued taxes		31	38	[1]
Accrued interest payable		70	53
Other current liabilities		26	27	[1]
Liabilities, Current		712	763	[1]
Long-term deferred revenue		19	12
Long-term deferred revenue - related parties		26	19	[1]
Long-term debt		6,654	4,422
Deferred income taxes		6	6	[1]
Deferred credits and other liabilities		168	177	[1]
Liabilities		7,585	5,399	[1]
Redeemable preferred units		1,000	1,000
Total MPLX LP partners’ capital		9,557	11,092	[1]
Noncontrolling interest		143	18
Partners' Capital, Including Portion Attributable to Noncontrolling Interest		9,700	11,110	[1],[2]	9,957	[2]	9,946	[2]
Liabilities and Equity		18,285	17,509	[1]
Scenario, Adjustment [Member]
Business Acquisition [Line Items]
Cash and cash equivalents			0
Receivables, net			0
Receivables - related parties	[3]		(4)
Inventories			0
Other current assets			0
Assets, Current	[3]		(4)
Equity method investments			0
Property, plant and equipment, net			0
Intangibles, net			0
Goodwill			0
Long-term receivables - related parties			0
Other noncurrent assets			0
Assets	[3]		(4)
Accounts payable			0
Accrued liabilities			0
Payables - related parties	[3]		(4)
Deferred Revenue, Current			0
Deferred revenue - related parties			0
Accrued property, plant and equipment			0
Accrued taxes			0
Accrued interest payable			0
Other current liabilities			0
Liabilities, Current	[3]		(4)
Long-term deferred revenue			0
Long-term deferred revenue - related parties			0
Long-term debt			0
Deferred income taxes			0
Deferred credits and other liabilities			0
Liabilities	[3]		(4)
Redeemable preferred units			0
Total MPLX LP partners’ capital			0
Noncontrolling interest			0
Partners' Capital, Including Portion Attributable to Noncontrolling Interest			0
Liabilities and Equity	[3]		(4)
Scenario, Previously Reported [Member]
Business Acquisition [Line Items]
Cash and cash equivalents			234		4		43
Receivables, net			297
Receivables - related parties			122
Inventories			54
Other current assets			33
Assets, Current			740
Equity method investments			2,467
Property, plant and equipment, net			10,730
Intangibles, net			492
Goodwill			2,199
Long-term receivables - related parties			4
Other noncurrent assets			14
Assets			16,646
Accounts payable			123
Accrued liabilities			228
Payables - related parties			75
Deferred Revenue, Current			2
Deferred revenue - related parties			34
Accrued property, plant and equipment			132
Accrued taxes			33
Accrued interest payable			53
Other current liabilities			24
Liabilities, Current			704
Long-term deferred revenue			12
Long-term deferred revenue - related parties			15
Long-term debt			4,422
Deferred income taxes			5
Deferred credits and other liabilities			169
Liabilities			5,327
Redeemable preferred units			1,000
Total MPLX LP partners’ capital			10,301
Noncontrolling interest			18
Partners' Capital, Including Portion Attributable to Noncontrolling Interest			10,319
Liabilities and Equity			16,646
MPLX Terminals [Member] | Scenario, Adjustment [Member]
Business Acquisition [Line Items]
Cash and cash equivalents			0
Receivables, net			1
Receivables - related parties			38
Inventories			0
Other current assets			0
Assets, Current			39
Equity method investments			4
Property, plant and equipment, net			413
Intangibles, net			0
Goodwill			21
Long-term receivables - related parties			7
Other noncurrent assets			0
Assets			484
Accounts payable			12
Accrued liabilities			0
Payables - related parties			12
Deferred Revenue, Current			0
Deferred revenue - related parties			0
Accrued property, plant and equipment			5
Accrued taxes			3
Accrued interest payable			0
Other current liabilities			2
Liabilities, Current			34
Long-term deferred revenue			0
Long-term deferred revenue - related parties			4
Long-term debt			0
Deferred income taxes			1
Deferred credits and other liabilities			6
Liabilities			45
Redeemable preferred units			0
Total MPLX LP partners’ capital			439
Noncontrolling interest			0
Partners' Capital, Including Portion Attributable to Noncontrolling Interest			439
Liabilities and Equity			484
HST & WHC [Member] | Scenario, Adjustment [Member]
Business Acquisition [Line Items]
Cash and cash equivalents			0		0		0
Receivables, net			1
Receivables - related parties			91
Inventories			1
Other current assets			0
Assets, Current			93
Equity method investments			0
Property, plant and equipment, net			265
Intangibles, net			0
Goodwill			25
Long-term receivables - related parties			0
Other noncurrent assets			0
Assets			383
Accounts payable			5
Accrued liabilities			4
Payables - related parties			4
Deferred Revenue, Current			0
Deferred revenue - related parties			4
Accrued property, plant and equipment			9
Accrued taxes			2
Accrued interest payable			0
Other current liabilities			1
Liabilities, Current			29
Long-term deferred revenue			0
Long-term deferred revenue - related parties			0
Long-term debt			0
Deferred income taxes			0
Deferred credits and other liabilities			2
Liabilities			31
Redeemable preferred units			0
Total MPLX LP partners’ capital			352
Noncontrolling interest			0
Partners' Capital, Including Portion Attributable to Noncontrolling Interest			352
Liabilities and Equity			383
Limited Partners Class B Units [Member]
Business Acquisition [Line Items]
Total MPLX LP partners’ capital		133	133
Limited Partners Class B Units [Member] | Scenario, Adjustment [Member]
Business Acquisition [Line Items]
Total MPLX LP partners’ capital			0
Limited Partners Class B Units [Member] | Scenario, Previously Reported [Member]
Business Acquisition [Line Items]
Total MPLX LP partners’ capital			133
Limited Partners Class B Units [Member] | MPLX Terminals [Member] | Scenario, Adjustment [Member]
Business Acquisition [Line Items]
Total MPLX LP partners’ capital			0
Limited Partners Class B Units [Member] | HST & WHC [Member] | Scenario, Adjustment [Member]
Business Acquisition [Line Items]
Total MPLX LP partners’ capital			0
Limited Partners Common Units [Member] | Public [Member]
Business Acquisition [Line Items]
Total MPLX LP partners’ capital		8,147	8,086
Limited Partners Common Units [Member] | Public [Member] | Scenario, Adjustment [Member]
Business Acquisition [Line Items]
Total MPLX LP partners’ capital			0
Limited Partners Common Units [Member] | Public [Member] | Scenario, Previously Reported [Member]
Business Acquisition [Line Items]
Total MPLX LP partners’ capital			8,086
Limited Partners Common Units [Member] | Public [Member] | MPLX Terminals [Member] | Scenario, Adjustment [Member]
Business Acquisition [Line Items]
Total MPLX LP partners’ capital			0
Limited Partners Common Units [Member] | Public [Member] | HST & WHC [Member] | Scenario, Adjustment [Member]
Business Acquisition [Line Items]
Total MPLX LP partners’ capital			0
MPC [Member]
Business Acquisition [Line Items]
Receivables - related parties		177	242
Long-term receivables - related parties		13	11
Payables - related parties		59	63
MPC [Member] | Scenario, Adjustment [Member]
Business Acquisition [Line Items]
Total MPLX LP partners’ capital			0
MPC [Member] | Predecessor [Member]
Business Acquisition [Line Items]
Total MPLX LP partners’ capital		0	791	[1]
Partners' Capital, Including Portion Attributable to Noncontrolling Interest	[2]		791		$ 302		$ 692
MPC [Member] | Scenario, Previously Reported [Member]
Business Acquisition [Line Items]
Total MPLX LP partners’ capital			0
MPC [Member] | MPLX Terminals [Member] | Scenario, Adjustment [Member]
Business Acquisition [Line Items]
Total MPLX LP partners’ capital			439
MPC [Member] | HST & WHC [Member] | Scenario, Adjustment [Member]
Business Acquisition [Line Items]
Total MPLX LP partners’ capital			352
MPC [Member] | General Partner [Member]
Business Acquisition [Line Items]
Total MPLX LP partners’ capital		(257)	1,013
MPC [Member] | General Partner [Member] | Scenario, Adjustment [Member]
Business Acquisition [Line Items]
Total MPLX LP partners’ capital			0
MPC [Member] | General Partner [Member] | Scenario, Previously Reported [Member]
Business Acquisition [Line Items]
Total MPLX LP partners’ capital			1,013
MPC [Member] | General Partner [Member] | MPLX Terminals [Member] | Scenario, Adjustment [Member]
Business Acquisition [Line Items]
Total MPLX LP partners’ capital			0
MPC [Member] | General Partner [Member] | HST & WHC [Member] | Scenario, Adjustment [Member]
Business Acquisition [Line Items]
Total MPLX LP partners’ capital			0
MPC [Member] | Limited Partners Common Units [Member]
Business Acquisition [Line Items]
Total MPLX LP partners’ capital		$ 1,184	1,069
MPC [Member] | Limited Partners Common Units [Member] | Scenario, Adjustment [Member]
Business Acquisition [Line Items]
Total MPLX LP partners’ capital			0
MPC [Member] | Limited Partners Common Units [Member] | Scenario, Previously Reported [Member]
Business Acquisition [Line Items]
Total MPLX LP partners’ capital			1,069
MPC [Member] | Limited Partners Common Units [Member] | MPLX Terminals [Member] | Scenario, Adjustment [Member]
Business Acquisition [Line Items]
Total MPLX LP partners’ capital			0
MPC [Member] | Limited Partners Common Units [Member] | HST & WHC [Member] | Scenario, Adjustment [Member]
Business Acquisition [Line Items]
Total MPLX LP partners’ capital			$ 0
[1]	Financial information has been retrospectively adjusted for the acquisition of HST, WHC and MPLXT from MPC. See Notes 1 and 3.
[2]	(1)Financial information has been retrospectively adjusted for the acquisition of HST, WHC and MPLXT from MPC. See Notes 1 and 3.
[3]	(1)Represents intercompany transactions eliminated during the consolidation process, in accordance with GAAP.

Acquisitions Acquisition of H45

Acquisitions Acquisition of HST, WHC & MPLXT - Cash Flow (Details) - USD ($) $ in Millions		3 Months Ended			12 Months Ended
		Mar. 31, 2017	Mar. 31, 2016		Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Net income (loss)		$ 187	$ (14)	[1],[2]
Amortization of deferred financing costs		12	11
Depreciation and amortization		187	136	[1],[2]
Impairment expense		0	129
Deferred income taxes		0	(4)
Asset retirement expenditures		(1)	0
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property		(1)	0
Income from equity method investments	[3]	(5)	(5)
Distributions from unconsolidated affiliates		33	38
Increase (Decrease) in Accounts Receivable		44	(5)
Increase (Decrease) in Inventories		0	1
Increase (Decrease) in Derivative Assets and Liabilities		(18)	12
Current accounts payable and accrued liabilities		(59)	(18)	[2]
Increase (Decrease) in Due from Related Parties, Current		(18)	23
All other, net		16	17	[2]
Net Cash Provided by (Used in) Operating Activities		377	321	[2]
Additions to property, plant and equipment		(280)	(304)
Acquisitions, net of cash acquired		(220)
Investments - loans from (to) related parties		80	64	[2]
Payments to Acquire Equity Method Investments		(554)	(29)
All other, net		2	3
Net cash used in investing activities		(953)	(266)	[2]
Long-term debt - borrowings		2,241	306
Long-term debt - repayments		(1)	(857)
Related party debt - borrowings		12	1,437
Repayments of Related Party Debt		(12)	(1,007)
Payments of Debt Issuance Costs		(21)	0
Net proceeds from equity offerings		151	321
Distribution Made to Limited Partner, Cash Distributions Paid		(242)	(190)
Payments to Noncontrolling Interests		(2)	(1)
Contributions from noncontrolling interests		126	2		$ 2
All other, net		(5)	(1)
Net cash provided by (used in) financing activities		607	(94)
Cash and Cash Equivalents, Period Increase (Decrease)		31	(39)
Cash and cash equivalents		265	4		234	$ 43
Scenario, Adjustment [Member]
Business Acquisition [Line Items]
Net income (loss)			0
Depreciation and amortization			0
Impairment expense			0
Income from equity method investments			0
Cash and cash equivalents					0
Scenario, Previously Reported [Member]
Business Acquisition [Line Items]
Net income (loss)			(37)
Amortization of deferred financing costs			11
Depreciation and amortization			132
Impairment expense			129
Deferred income taxes			(4)
Income from equity method investments			(5)
Distributions from unconsolidated affiliates			38
Increase (Decrease) in Accounts Receivable			(5)
Increase (Decrease) in Inventories			1
Increase (Decrease) in Derivative Assets and Liabilities			12
Current accounts payable and accrued liabilities			(15)
Increase (Decrease) in Due from Related Parties, Current			23
All other, net			15
Net Cash Provided by (Used in) Operating Activities			295
Additions to property, plant and equipment			(291)
Investments - loans from (to) related parties			77
Payments to Acquire Equity Method Investments			(29)
All other, net			3
Net cash used in investing activities			(240)
Long-term debt - borrowings			306
Long-term debt - repayments			(857)
Related party debt - borrowings			1,437
Repayments of Related Party Debt			(1,007)
Net proceeds from equity offerings			321
Distribution Made to Limited Partner, Cash Distributions Paid			(190)
Payments to Noncontrolling Interests			(1)
Contributions from noncontrolling interests			2
All other, net			(1)
Net cash provided by (used in) financing activities			(94)
Cash and Cash Equivalents, Period Increase (Decrease)			(39)
Cash and cash equivalents			4		234	43
HST & WHC [Member] | Scenario, Adjustment [Member]
Business Acquisition [Line Items]
Net income (loss)			23
Amortization of deferred financing costs			0
Depreciation and amortization			4
Impairment expense			0
Deferred income taxes			0
Income from equity method investments			0
Distributions from unconsolidated affiliates			0
Increase (Decrease) in Accounts Receivable			0
Increase (Decrease) in Inventories			0
Increase (Decrease) in Derivative Assets and Liabilities			0
Current accounts payable and accrued liabilities			(3)
Increase (Decrease) in Due from Related Parties, Current			0
All other, net			2
Net Cash Provided by (Used in) Operating Activities			26
Additions to property, plant and equipment			(13)
Investments - loans from (to) related parties			(13)
Payments to Acquire Equity Method Investments			0
All other, net			0
Net cash used in investing activities			(26)
Long-term debt - borrowings			0
Long-term debt - repayments			0
Related party debt - borrowings			0
Repayments of Related Party Debt			0
Net proceeds from equity offerings			0
Distribution Made to Limited Partner, Cash Distributions Paid			0
Payments to Noncontrolling Interests			0
Contributions from noncontrolling interests			0
All other, net			0
Net cash provided by (used in) financing activities			0
Cash and Cash Equivalents, Period Increase (Decrease)			0
Cash and cash equivalents			0		0	$ 0
MPC [Member]
Business Acquisition [Line Items]
Distributions to MPC from Predecessor		(1,511)			(104)
MPC [Member] | Scenario, Previously Reported [Member]
Business Acquisition [Line Items]
Distributions to MPC from Predecessor			(104)
MPC [Member] | Predecessor [Member]
Business Acquisition [Line Items]
Distributions to MPC from Predecessor		$ (113)	(104)		$ (104)
MPC [Member] | HST & WHC [Member] | Scenario, Adjustment [Member]
Business Acquisition [Line Items]
Distributions to MPC from Predecessor			$ 0
[1]	(1)Financial information has been retrospectively adjusted for the acquisition of HST and WHC from MPC. See Notes 1 and 3.
[2]	(1)Financial information has been retrospectively adjusted for the acquisition of HST and WHC from MPC. See Notes 1 and 3.
[3]	(1)Income (loss) from equity method investments includes the impact of any basis differential amortization or accretion.

Acquisitions Acquisition of Oza

Acquisitions Acquisition of Ozark Pipeline (Details) bbl / d in Thousands, $ in Millions	Mar. 01, 2017USD ($)bbl / dinmi	Mar. 31, 2017USD ($)	Mar. 31, 2017USD ($)	Mar. 31, 2016USD ($)
Business Acquisition [Line Items]
Additions to property, plant and equipment			$ 280	$ 304
Ozark Pipeline [Member]
Business Acquisition [Line Items]
Additions to property, plant and equipment	$ 220
Pipeline length | mi	433
Pipeline diameter | in	22
Pipeline barrels capable of transportation | bbl / d	0
Revenues and other income		$ 7
Income from operations		$ 2

Acquisitions - Acquisition of H

Acquisitions - Acquisition of Hardin Street Marine LLC (Details) $ in Millions	Mar. 31, 2016USD ($)shares	Mar. 31, 2017USD ($)	Mar. 31, 2016USD ($)
Business Acquisition [Line Items]
Contribution from MPC		$ 12
Number of Tow Boats	18		18
Number of barges	205		205
Hardin Street Marine [Member]
Business Acquisition [Line Items]
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned	$ 600		$ 600
Business Acquisition, Effective Date of Acquisition	Mar. 31, 2016
Limited Partner [Member] | Hardin Street Marine [Member]
Business Acquisition [Line Items]
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares | shares	22,534,002
Business Acquisition, Equity Interest Issued or Issuable, Fair Value Assigned	$ 669		669
General Partner [Member] | Hardin Street Marine [Member]
Business Acquisition [Line Items]
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares | shares	459,878
Business Acquisition, Equity Interest Issued or Issuable, Fair Value Assigned	$ 14		$ 14
MPC [Member]
Business Acquisition [Line Items]
Contribution from MPC	$ 15
Percent of Volumes Shipped			60.00%

Equity Method Investments - Sum

Equity Method Investments - Summary of Equity Method Investment Financial Information (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Revenue		$ 103	$ 95
Costs and expenses		66	46
Income from operations		37	49
Net income		33	48
Income from equity method investments	[1]	5	5
Current assets		145		$ 87
Noncurrent assets		3,356		2,695
Current liabilities		145		60
Noncurrent liabilities		12		15
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity		1,100		1,100
MarkWest Utica EMG [Member]
Schedule of Equity Method Investments [Line Items]
Revenue		50	61
Costs and expenses		25	22
Income from operations		25	39
Net income		25	38
Income from equity method investments	[1]	4	4
Current assets		63		45
Noncurrent assets	[2]	2,145		2,173
Current liabilities		29		30
Noncurrent liabilities		2		2
Equity Method Investment Difference Between Carrying Amount And Underlying Equity Portion Related To Goodwill Not Amortized		459
Other VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Revenue		8	5
Costs and expenses		8	4
Income from operations		0	1
Net income		0	1
Income from equity method investments	[1]	(1)	0
Current assets		48		2
Noncurrent assets		832		132
Current liabilities		96		4
Noncurrent liabilities		10		13
Non-VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Revenue		45	29
Costs and expenses		33	20
Income from operations		12	9
Net income		8	9
Income from equity method investments	[1]	2	$ 1
Current assets		34		40
Noncurrent assets		379		390
Current liabilities		20		26
Noncurrent liabilities		0		0
Utica Condensate [Member]
Schedule of Equity Method Investments [Line Items]
Investments in subsidiary		$ 792		$ 794
[1]	(1)Income (loss) from equity method investments includes the impact of any basis differential amortization or accretion.
[2]	(1)MarkWest Utica EMG’s noncurrent assets includes its investment in its subsidiary Ohio Gathering, which does not appear elsewhere in this table. The investment was $792 million and $794 million as of March 31, 2017 and December 31, 2016, respectively.

Equity Method Investments (Deta

Equity Method Investments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016		Dec. 31, 2016		Feb. 28, 2013
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 3,306			$ 2,471	[1]
Other income - related parties	$ 22	$ 21	[2]
MarkWest Utica EMG [Member]
Schedule of Equity Method Investments [Line Items]
Minimum Capital Contributed by Noncontrolling Owners						$ 950
Percentage of Required Capital Contribution by Reporting Entity after NCI						100.00%
Aggregate Contributions to VIE Threshold						$ 2,000
Percentage of Ownership Interest in Joint Venture, Maximum	70.00%
Threshold Percentage of Noncontrolling Ownership Interest in Joint Venture	30.00%
Maximum Percentage of Additional Noncontrolling Capital Contribution before Threshold Ownership	10.00%
Actual Capital Contributed by Noncontrolling Owners	$ 1,200
Actual Capital Contribution to Date	$ 1,500
Noncontrolling Owners Preference Threshold						$ 500
Noncontrolling Owners Accrual of Preference Amount		4
Percentage of aggregate investment balance	56.00%
Equity method investments	$ 2,200			$ 2,200
Other income - related parties	$ 4	2
Equity Method Investment, Ownership Percentage	56.00%
Ohio Gathering Company, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Other income - related parties	$ 4	$ 4
Equity Method Investment, Ownership Percentage	34.00%
[1]	Financial information has been retrospectively adjusted for the acquisition of HST, WHC and MPLXT from MPC. See Notes 1 and 3.
[2]	(1)Financial information has been retrospectively adjusted for the acquisition of HST and WHC from MPC. See Notes 1 and 3.

Equity Method Investments Sherw

Equity Method Investments Sherwood Midstream (Details) bbl / d in Thousands, $ in Millions	Jan. 01, 2017USD ($)abbl / d	Mar. 31, 2017USD ($)		Mar. 31, 2016USD ($)		Dec. 31, 2016USD ($)	[2]
Schedule of Equity Method Investments [Line Items]
Contribution of Property		$ 328	[1]	$ 0
Assets, Current		811				$ 868
Liabilities, Current		712				763
Equity method investments		3,306				$ 2,471
Other income - related parties		$ 22		$ 21	[3]
Sherwood Midstream LLC [Member]
Schedule of Equity Method Investments [Line Items]
Area of Real Estate Property | a	195,000
Equity Method Investment, Ownership Percentage		50.00%
Contribution of Property	$ 134
Payments to Acquire Interest in Joint Venture	$ 20
Equity method investments		$ 147
Other income - related parties		1
MarkWest Ohio Fractionation Company, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Capacity | bbl / d	0
Assets, Current		12
Assets, Noncurrent		23
Liabilities, Current		$ 31
Antero Midstream Partners L.P. [Member] | Sherwood Midstream LLC [Member]
Schedule of Equity Method Investments [Line Items]
Payments to Acquire Interest in Joint Venture	$ 154
Sherwood Midstream LLC [Member] | MarkWest Ohio Fractionation Company, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Payments to Acquire Interest in Joint Venture	$ 126
[1]	(1)Contribution of assets to Sherwood Midstream and Sherwood Midstream Holdings. See Note 4.
[2]	Financial information has been retrospectively adjusted for the acquisition of HST, WHC and MPLXT from MPC. See Notes 1 and 3.
[3]	(1)Financial information has been retrospectively adjusted for the acquisition of HST and WHC from MPC. See Notes 1 and 3.

Equity Method Investments She51

Equity Method Investments Sherwood Midstream Holdings (Details) - USD ($) $ in Millions	Jan. 01, 2017	Mar. 31, 2017		Mar. 31, 2016	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Equity method investments		$ 3,306			$ 2,471	[1]
Contribution of Property		328	[2]	$ 0
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity		1,100			$ 1,100
Distributions from unconsolidated affiliates - return of capital		20
Sherwood Midstream Holdings [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments		$ 166
Fair Value Of Assets Contributed	$ 209
Equity Method Investment, Ownership Percentage		90.00%
Contribution of Property	194
Gain (Loss) on Disposition of Assets	$ 2
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity		$ 0
Distributions from unconsolidated affiliates - return of capital		$ 41
Direct Ownership Interest [Member] | Sherwood Midstream Holdings [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage		7900.00%
Indirect Ownership Interest [Member] | Sherwood Midstream Holdings [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage		1050.00%
Sherwood Midstream LLC [Member] | Sherwood Midstream Holdings [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	2100.00%
Payments to Acquire Interest in Joint Venture	$ 44
[1]	Financial information has been retrospectively adjusted for the acquisition of HST, WHC and MPLXT from MPC. See Notes 1 and 3.
[2]	(1)Contribution of assets to Sherwood Midstream and Sherwood Midstream Holdings. See Note 4.

Equity Method Investments MarEn

Equity Method Investments MarEn Bakken (Details) bbl / d in Thousands, $ in Millions	Feb. 15, 2017USD ($)	Mar. 31, 2017USD ($)bbl / d	Mar. 31, 2016USD ($)	Dec. 31, 2016USD ($)	[1]
Schedule of Equity Method Investments [Line Items]
Investments in unconsolidated affiliates		$ 554	$ 29
Expected delivery of crude oil barrels per day | bbl / d		470
Equity method investments		$ 3,306		$ 2,471
MarEn Bakken Company LLC [Member]
Schedule of Equity Method Investments [Line Items]
Percentage of Ownership Interest in Joint Venture Acquired		36.75%
Equity Method Investment, Ownership Percentage		25.00%
Bakken Pipeline System [Member]
Schedule of Equity Method Investments [Line Items]
Percentage of Ownership Interest in Joint Venture Acquired	3675.00%
Equity Method Investment, Ownership Percentage		9.1875%
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%
Equity method investments		$ 503
MPC [Member]
Schedule of Equity Method Investments [Line Items]
Incentive Distribution Rights Forfeited per Quarter	$ 1.6
Number of Quarters IDRs Forfeited	12
Prorated Incentive Distribution Rights Forfeited		$ 0.8
MarEn Bakken Company LLC [Member]
Schedule of Equity Method Investments [Line Items]
Investments in unconsolidated affiliates	$ 2,000
[1]	Financial information has been retrospectively adjusted for the acquisition of HST, WHC and MPLXT from MPC. See Notes 1 and 3.

Related Party Agreements and 53

Related Party Agreements and Transactions - Additional Information (Detail) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Centennial [Member] | MPC [Member]
Related Party Transaction [Line Items]
Equity Method Investment, Ownership Percentage	50.00%
Muskegon [Member] | MPC [Member]
Related Party Transaction [Line Items]
Equity Method Investment, Ownership Percentage	60.00%
MarkWest Utica EMG [Member]
Related Party Transaction [Line Items]
Equity Method Investment, Ownership Percentage	56.00%
Ohio Gathering Company, LLC [Member]
Related Party Transaction [Line Items]
Equity Method Investment, Ownership Percentage	34.00%
Sherwood Midstream LLC [Member]
Related Party Transaction [Line Items]
Equity Method Investment, Ownership Percentage	50.00%
Sherwood Midstream Holdings [Member]
Related Party Transaction [Line Items]
Equity Method Investment, Ownership Percentage	90.00%
MarEn Bakken Company LLC [Member]
Related Party Transaction [Line Items]
Equity Method Investment, Ownership Percentage	25.00%
Percentage of Ownership Interest in Joint Venture Acquired	36.75%
Transportation Services Agreements [Member] | Transportation of crude and refined product services [Member] | Commercial Agreements [Member] | MPC [Member]
Related Party Transaction [Line Items]
Term Of Agreements	4 years
Number of Renewals	2
Terminal Services Agreements [Member] | Terminal storage and related services [Member] | Commercial Agreements [Member] | MPC [Member]
Related Party Transaction [Line Items]
Term Of Agreements	5 years
Number of Renewals	2
Ten Year Storage Services Agreement [Member] | Storage Services Butane and Propane Caverns [Member] | Commercial Agreements [Member] | MPC [Member]
Related Party Transaction [Line Items]
Term Of Agreements	10 years
Three Year Storage Services Agreement [Member] | Storage Services Tank Farm [Member] | Commercial Agreements [Member] | MPC [Member]
Related Party Transaction [Line Items]
Term Of Agreements	3 years
Related Party Revolving Credit Agreement [Member] | MPC Investment [Member]
Related Party Transaction [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 500
Line of Credit Facility, Expiration Date	Dec. 4, 2020
Debt Instrument, Description of Variable Rate Basis	LIBOR plus 1.50 percent
Proceeds from Long-term Lines of Credit	$ 0	$ 2,500
Repayments of Long-term Lines of Credit	0	2,500
Long-term Debt, Gross	$ 0	$ 0
Debt Instrument, Interest Rate, Effective Percentage	0.00%	1.939%

Related Party Agreements and 54

Related Party Agreements and Transactions - Sales to Related Parties (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Related Party Transaction [Line Items]
Service revenue - related parties		$ 255	$ 177	[1]
Rental income - related parties		67	38	[1]
Product sales - related parties		2	3
MPC [Member]
Related Party Transaction [Line Items]
Service revenue - related parties		255	177
Rental income - related parties		67	38
Product sales - related parties	[2]	2	3
Sales Revenue, Goods, Related Party, Net Zero		$ 57	$ 5
[1]	(1)Financial information has been retrospectively adjusted for the acquisition of HST and WHC from MPC. See Notes 1 and 3.
[2]	(1)For the three months ended March 31, 2017 and 2016, there were $57 million and $5 million, respectively, of 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.

Related Party Agreements and 55

Related Party Agreements and Transactions - Other Income - Related Parties (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Related Party Transaction [Line Items]
Other income - related parties	$ 22	$ 21	[1]
MPC [Member]
Related Party Transaction [Line Items]
Other income - related parties	11	14
MarkWest Utica EMG [Member]
Related Party Transaction [Line Items]
Other income - related parties	4	2
Ohio Gathering Company, LLC [Member]
Related Party Transaction [Line Items]
Other income - related parties	4	4
Other [Member]
Related Party Transaction [Line Items]
Other income - related parties	$ 3	$ 1
[1]	(1)Financial information has been retrospectively adjusted for the acquisition of HST and WHC from MPC. See Notes 1 and 3.

Related Party Agreements and 56

Related Party Agreements and Transaction - Summary of Charges for Omnibus Agreement (Detail) - MPC [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Related Party Transaction [Line Items]
Charges for services included in purchases from related party	$ 23	$ 17
Purchases From Related Parties [Member]
Related Party Transaction [Line Items]
Charges for services included in purchases from related party	15	7
General and Administrative Expense [Member]
Related Party Transaction [Line Items]
Charges for services included in purchases from related party	$ 8	$ 10

Related Party Agreements and 57

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, 2017	Mar. 31, 2016
MPC [Member] | Assets Under Construction [Member]
Property, Plant and Equipment [Line Items]
Costs added to property, plant and equipment	$ 10	$ 10

Related Party Agreements and 58

Related Party Agreements and Transactions - Employee Services Agreements (Detail) - MPC [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	$ 117	$ 92
Purchases From Related Parties [Member]
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	92	71
General and Administrative Expense [Member]
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	$ 25	$ 21

Related Party Agreements and 59

Related Party Agreements and Transactions - Receivables from Related Parties (Detail) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Related Party Transaction [Line Items]
Receivables - related parties	$ 198	$ 247	[1]
MPC [Member]
Related Party Transaction [Line Items]
Receivables - related parties	177	242
Sherwood Midstream [Member]
Related Party Transaction [Line Items]
Receivables - related parties	14	0
MarkWest EMG Jefferson Dry Gas Gathering Company, L.L.C. [Member]
Related Party Transaction [Line Items]
Receivables - related parties	2	0
MarkWest Utica EMG [Member]
Related Party Transaction [Line Items]
Receivables - related parties	0	2
Ohio Gathering Company, LLC [Member]
Related Party Transaction [Line Items]
Receivables - related parties	3	2
Other [Member]
Related Party Transaction [Line Items]
Receivables - related parties	$ 2	$ 1
[1]	Financial information has been retrospectively adjusted for the acquisition of HST, WHC and MPLXT from MPC. See Notes 1 and 3.

Related Party Agreements and 60

Related Party Agreements and Transactions Related Party Agreements and Transactions - Long-term Receivables from Related Parties (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Related Party Transaction [Line Items]
Long-term receivables - related parties	$ 13	$ 11	[1]
MPC [Member]
Related Party Transaction [Line Items]
Long-term receivables - related parties	$ 13	$ 11
[1]	Financial information has been retrospectively adjusted for the acquisition of HST, WHC and MPLXT from MPC. See Notes 1 and 3.

Related Party Agreements and 61

Related Party Agreements and Transactions - Payables to Related Parties (Detail) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Related Party Transaction [Line Items]
Payables - related parties	$ 91	$ 87	[1]
MPC [Member]
Related Party Transaction [Line Items]
Payables - related parties	59	63
MarkWest Utica EMG [Member]
Related Party Transaction [Line Items]
Payables - related parties	29	24
Sherwood Midstream [Member]
Related Party Transaction [Line Items]
Payables - related parties	$ 3	$ 0
[1]	Financial information has been retrospectively adjusted for the acquisition of HST, WHC and MPLXT from MPC. See Notes 1 and 3.

Related Party Agreements and 62

Related Party Agreements and Transactions - Summary of Deferred Revenue - Related Parties (Detail) - MPC [Member] - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Related Party Transaction [Line Items]
Deferred Revenue Related Parties	$ 64	$ 57
Minimum volume deficiencies [Member]
Related Party Transaction [Line Items]
Deferred Revenue Related Parties	50	48
Project Reimbursements [Member]
Related Party Transaction [Line Items]
Deferred Revenue Related Parties	$ 14	$ 9

Net Income Per Limited Partne63

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, 2017	Mar. 31, 2016
Net Income Per Share [Line Items]
Potentially Dilutive Units		1	10
Net income attributable to MPLX LP	[1]	$ 150	$ (60)
Less: Distribution declared		279	200
Undistributed net income attributable to MPLX LP		(129)	(260)
Preferred Units [Member]
Net Income Per Share [Line Items]
Net income attributable to MPLX LP	[1]	16
Less: Distribution declared	[2]	16	0
Undistributed net income attributable to MPLX LP		0
General Partner [Member] | MPC [Member]
Net Income Per Share [Line Items]
Net income attributable to MPLX LP	[1]	62	39
Less: Distribution declared	[2]	65	44
Undistributed net income attributable to MPLX LP		(3)	(5)
Limited Partners Common Units [Member]
Net Income Per Share [Line Items]
Net income attributable to MPLX LP	[1]	72	(99)
Less: Distribution declared	[2]	198	156
Undistributed net income attributable to MPLX LP		$ (126)	$ (255)
[1]	llocation 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 7 for distribution information. Three Months Ended March 31, 2017(In millions, except per unit data)GeneralPartner LimitedPartners’CommonUnits Redeemable Preferred Units TotalBasic and diluted net income attributable to MPLX LP per unit: Net income attributable to MPLX LP: Distributions declared (including IDRs)$65 $198 $16 $279Undistributed net loss attributable to MPLX LP (3) (126) — (129)Net income attributable to MPLX LP (1)$62 $72 $16 $150Weighted average units outstanding: Basic7 362 31 400Diluted7 367 31 405Net income attributable to MPLX LP per limited partner unit: Basic $0.20 Diluted $0.19

Net Income Per Limited Partne64

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, 2017	Mar. 31, 2016
Net income attributable to MPLX LP:
Distribution Made to Limited Partner, Cash Distributions Declared		$ 279	$ 200
Undistributed net income attributable to MPLX LP		(129)	(260)
Net income attributable to MPLX LP	[1]	$ 150	$ (60)
Weighted average units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic		400	307
Weighted Average Limited Partnership Units Outstanding, Diluted		405	307
Limited Partners Common Units [Member]
Net income attributable to MPLX LP:
Distribution Made to Limited Partner, Cash Distributions Declared	[2]	$ 198	$ 156
Undistributed net income attributable to MPLX LP		(126)	(255)
Net income attributable to MPLX LP	[1]	$ 72	$ (99)
Weighted average units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic		362	300
Weighted Average Limited Partnership Units Outstanding, Diluted		367	300
Net income attributable to MPLX LP per limited partner unit:
Basic (in USD per unit)		$ 0.20	$ (0.33)
Diluted (in USD per unit)		$ 0.19	$ (0.33)
Preferred Units [Member]
Net income attributable to MPLX LP:
Distribution Made to Limited Partner, Cash Distributions Declared	[2]	$ 16	$ 0
Undistributed net income attributable to MPLX LP		0
Net income attributable to MPLX LP	[1]	$ 16
Weighted average units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic		31
Weighted Average Limited Partnership Units Outstanding, Diluted		31
MPC [Member] | General Partner [Member]
Net income attributable to MPLX LP:
Distribution Made to Limited Partner, Cash Distributions Declared	[2]	$ 65	44
Undistributed net income attributable to MPLX LP		(3)	(5)
Net income attributable to MPLX LP	[1]	$ 62	$ 39
Weighted average units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic		7	7
Weighted Average Limited Partnership Units Outstanding, Diluted		7	7
[1]	llocation 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 7 for distribution information. Three Months Ended March 31, 2017(In millions, except per unit data)GeneralPartner LimitedPartners’CommonUnits Redeemable Preferred Units TotalBasic and diluted net income attributable to MPLX LP per unit: Net income attributable to MPLX LP: Distributions declared (including IDRs)$65 $198 $16 $279Undistributed net loss attributable to MPLX LP (3) (126) — (129)Net income attributable to MPLX LP (1)$62 $72 $16 $150Weighted average units outstanding: Basic7 362 31 400Diluted7 367 31 405Net income attributable to MPLX LP per limited partner unit: Basic $0.20 Diluted $0.19

Equity (Details)

Equity (Details) - USD ($) $ / shares in Units, $ in Millions		May 15, 2017	May 08, 2017	Apr. 26, 2017	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Stockholders Equity Note [Line Items]
Limited Liability Company (LLC) or Limited Partnership (LP), Managing Member or General Partner, Ownership Interest					2.00%
Unit-based compensation awards(1)	[1]				101,493
Issuance of units under ATM Program					$ 151		$ 321
Less: Distribution declared					$ 279	$ 200
Subsequent Event [Member]
Stockholders Equity Note [Line Items]
Distribution Made to Limited Partner, Declaration Date				Apr. 26, 2017
Cash distributions declared per limited partner common unit				$ 0.5400
Less: Distribution declared				$ 263
Distribution Made to Limited Partner, Distribution Date		May 15, 2017
Distribution Made to Limited Partner, Date of Record			May 8, 2017
ATM Program [Member]
Stockholders Equity Note [Line Items]
Issuance of units under the ATM Program(2)	[2]				4,235,978
Limited Partners Common Units [Member]
Stockholders Equity Note [Line Items]
Unit-based compensation awards(1)	[1]				99,463
Cash distributions declared per limited partner common unit					$ 0.5400	$ 0.5050
Less: Distribution declared	[3]				$ 198	$ 156
Limited Partners Common Units [Member] | ATM Program [Member]
Stockholders Equity Note [Line Items]
Issuance of units under the ATM Program(2)	[2]				4,151,258
General Partner Units [Member]
Stockholders Equity Note [Line Items]
Unit-based compensation awards(1)	[1]				2,030
Issuance of units under ATM Program					$ 1
General Partner Units [Member] | ATM Program [Member]
Stockholders Equity Note [Line Items]
Issuance of units under ATM Program					$ 3
Issuance of units under the ATM Program(2)	[2]				84,720
[1]	(1)As a result of the unit-based compensation awards issued during the period, MPLX GP contributed less than $1 million in exchange for 2,030 general partner units to maintain its two percent GP Interest.
[2]	(2)As a result of common units issued under the ATM Program during the period, MPLX GP contributed $3 million in exchange for 84,720 general partner units to maintain its two percent GP Interest.
[3]	See Note 7 for distribution information. Three Months Ended March 31, 2017(In millions, except per unit data)GeneralPartner LimitedPartners’CommonUnits Redeemable Preferred Units TotalBasic and diluted net income attributable to MPLX LP per unit: Net income attributable to MPLX LP: Distributions declared (including IDRs)$65 $198 $16 $279Undistributed net loss attributable to MPLX LP (3) (126) — (129)Net income attributable to MPLX LP (1)$62 $72 $16 $150Weighted average units outstanding: Basic7 362 31 400Diluted7 367 31 405Net income attributable to MPLX LP per limited partner unit: Basic $0.20 Diluted $0.19

Equity - Changes in Partners Ca

Equity - Changes in Partners Capital, Unit Rollforward (Details) - shares		Mar. 01, 2017	Mar. 31, 2017
Stockholders Equity [Line Items]
Balance at December 31, 2016			368,555,271
Units converted	[1]		(101,493)
Balance at March 31, 2017			386,117,615
Limited Partners Common Units [Member]
Stockholders Equity [Line Items]
Balance at December 31, 2016			357,193,288
Units converted	[1]		(99,463)
Balance at March 31, 2017			374,404,385
Class B Units [Member]
Stockholders Equity [Line Items]
Balance at December 31, 2016			3,990,878
Balance at March 31, 2017			3,990,878
General Partner Units [Member]
Stockholders Equity [Line Items]
Balance at December 31, 2016			7,371,105
Units converted	[1]		(2,030)
Balance at March 31, 2017			7,722,352
ATM Program [Member]
Stockholders Equity [Line Items]
Issuance of units under the ATM Program(2)	[2]		4,235,978
ATM Program [Member] | Limited Partners Common Units [Member]
Stockholders Equity [Line Items]
Issuance of units under the ATM Program(2)	[2]		4,151,258
ATM Program [Member] | General Partner Units [Member]
Stockholders Equity [Line Items]
Issuance of units under the ATM Program(2)	[2]		84,720
HST & WHC & MPLXT [Member] | Limited Partners Common Units [Member]
Stockholders Equity [Line Items]
Partners' Capital Account, Units, Acquisitions	[3]		12,960,376
HST & WHC & MPLXT [Member] | General Partner Units [Member]
Stockholders Equity [Line Items]
Partners' Capital Account, Units, Acquisitions	[3]	264,497	264,497
Hardin Street Marine [Member]
Stockholders Equity [Line Items]
Partners' Capital Account, Units, Acquisitions	[3]		13,224,873
[1]	(1)As a result of the unit-based compensation awards issued during the period, MPLX GP contributed less than $1 million in exchange for 2,030 general partner units to maintain its two percent GP Interest.
[2]	(2)As a result of common units issued under the ATM Program during the period, MPLX GP contributed $3 million in exchange for 84,720 general partner units to maintain its two percent GP Interest.
[3]	(3)See Note 3 for information regarding the HST, WHC and MPLXT acquisition.

Equity - Net Income Allocation

Equity - Net Income Allocation (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Stockholders Equity [Line Items]
Net income attributable to MPLX LP	[1]	$ 150	$ (60)
Preferred unit distributions		(279)	(200)
General partner's incentive distribution rights and other		61	41
Net income (loss) attributable to MPLX LP available to general and limited partners		73	(101)
General partner's two percent GP Interest in net income (loss) attributable to MPLX LP		1	(2)
Less: General partner’s interest in net income attributable to MPLX LP		62	39
Preferred Units [Member]
Stockholders Equity [Line Items]
Net income attributable to MPLX LP	[1]	16
Preferred unit distributions	[2]	$ (16)	$ 0
[1]	llocation 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 7 for distribution information. Three Months Ended March 31, 2017(In millions, except per unit data)GeneralPartner LimitedPartners’CommonUnits Redeemable Preferred Units TotalBasic and diluted net income attributable to MPLX LP per unit: Net income attributable to MPLX LP: Distributions declared (including IDRs)$65 $198 $16 $279Undistributed net loss attributable to MPLX LP (3) (126) — (129)Net income attributable to MPLX LP (1)$62 $72 $16 $150Weighted average units outstanding: Basic7 362 31 400Diluted7 367 31 405Net income attributable to MPLX LP per limited partner unit: Basic $0.20 Diluted $0.19

Equity - Cash Distributions (De

Equity - Cash Distributions (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Incentive Distribution Made to Managing Member or General Partner [Line Items]
General partner's distributions	$ 5	$ 4
General partner's incentive distribution rights	60	40
Total general partner's distributions	65	44
Partners' distributions	279	200
Limited Partners Common Units [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Partners' distributions	198	156
Subordinated Units [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Partners' distributions	0	0
Preferred Units [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Partners' distributions	16	0
Limited Partner [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Partners' distributions	$ 198	$ 156

Redeemable Preferred Units (Det

Redeemable Preferred Units (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	May 13, 2016	Mar. 31, 2017
Temporary Equity Disclosure [Abstract]
Temporary Equity [Table Text Block]		(In millions) Redeemable Preferred Units Balance at December 31, 2016 $ 1,000 Net income 16 Distributions received by Preferred unitholders (16 ) Balance at March 31, 2017 $ 1,000
Series A Convertible Preferred Units [Member]
Redeemable Noncontrolling Interest [Line Items]
Issuance of units under the ATM Program(2)	30.8
Preferred Stock, Dividend Rate, Percentage	6.50%
Issuance of redeemable preferred units	$ 984
Preferred Stock, Dividend Rate, Per-Dollar-Amount	$ 0.528125
Preferred Units, Description	The purchasers may convert their Preferred units into common units, at any time after the third anniversary of the issuance date or prior to liquidation, dissolution or winding up of the Partnership, in full or in part, subject to minimum conversion amounts and conditions. After the fourth anniversary of the issuance date, the Partnership may convert the Preferred units into common units at any time, in whole or in part, subject to certain minimum conversion amounts and conditions, if the closing price of MPLX LP common units is greater than $48.75 for the 20 day trading period immediately preceding the conversion notice date. The conversion rate for the Preferred units shall be the quotient of (a) the sum of (i) $32.50, plus (ii) any unpaid cash distributions on the applicable Preferred Unit, divided by (b) $32.50. The holders of the Preferred units are entitled to vote on an as-converted basis with the common unitholders and will have certain other class voting rights with respect to any amendment to the partnership agreement that would adversely affect any rights, preferences or privileges of the Preferred units. In addition, upon certain events involving a change in control the holders of Preferred units may elect, among other potential elections, to convert their Preferred units to common units at the then change of control conversion rate.
Preferred Units [Member] | Series A Convertible Preferred Units [Member]
Redeemable Noncontrolling Interest [Line Items]
Shares Issued, Price Per Share	$ 32.50

Redeemable Preferred Units Rede

Redeemable Preferred Units Redeemable Preferred Units Rollforward (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	May 13, 2016	Mar. 31, 2017	Mar. 31, 2016
Redeemable Noncontrolling Interest [Line Items]
Net income		$ 16
Distributions to unitholders and general partner		(242)	$ (190)
Balance at March 31, 2017		1,000
Series A Convertible Preferred Units [Member]
Redeemable Noncontrolling Interest [Line Items]
Issuance of units under the ATM Program(2)	30.8
Preferred Stock, Dividend Rate, Percentage	6.50%
Preferred Stock, Dividend Rate, Per-Dollar-Amount	$ 0.528125
Preferred Units [Member]
Redeemable Noncontrolling Interest [Line Items]
Distributions to unitholders and general partner		$ 16
Preferred Units [Member] | Series A Convertible Preferred Units [Member]
Redeemable Noncontrolling Interest [Line Items]
Shares Issued, Price Per Share	$ 32.50

Segment Information (Details)

Segment Information (Details)	3 Months Ended
	Mar. 31, 2017
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, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Segment revenues	$ 203	$ 100
Segment other income	2	2
Total segment revenues and other income	886	645	[1]
Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment revenues	942	729
Segment other income	13	16
Total segment revenues and other income	955	745
Segment cost of revenues	401	297
Segment operating income before portion attributable to noncontrolling interest and Predecessor	554	448
Segment portion attributable to Predecessor	89	103
Segment operating income attributable to MPLX LP	465	345
Operating Segments [Member] | L&S [Member]
Segment Reporting Information [Line Items]
Segment revenues	345	231
Segment other income	12	16
Total segment revenues and other income	357	247
Segment cost of revenues	148	97
Segment operating income before portion attributable to noncontrolling interest and Predecessor	209	150
Segment portion attributable to Predecessor	53	62
Segment operating income attributable to MPLX LP	156	88
Operating Segments [Member] | G&P [Member]
Segment Reporting Information [Line Items]
Segment revenues	597	498
Segment other income	1	0
Total segment revenues and other income	598	498
Segment cost of revenues	253	200
Segment operating income before portion attributable to noncontrolling interest and Predecessor	345	298
Segment portion attributable to Predecessor	36	41
Segment operating income attributable to MPLX LP	$ 309	$ 257
[1]	(1)Financial information has been retrospectively adjusted for the acquisition of HST and WHC from MPC. See Notes 1 and 3.

Segment Information - Reconcili

Segment Information - Reconciliation to Income from Operations (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Segment Reporting, Revenue Reconciling Item [Line Items]
Less: Net income attributable to Predecessor		$ 36	$ 46	[1]
Income from equity method investments	[2]	5	5
Other income - related parties		22	21	[1]
Depreciation and amortization		(187)	(136)	[1],[3]
Impairment expense		0	(129)
General and administrative expenses		(58)	(53)	[1]
Income from operations		265	50	[1]
Operating Segments [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Segment operating income attributable to MPLX LP		465	345
Segment portion attributable to Predecessor		89	103
Segment Reconciling Items [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Segment portion attributable to unconsolidated affiliates		(40)	(42)
Less: Net income attributable to Predecessor		53	62
Income from equity method investments		5	5
Other income - related parties		11	7
Unrealized derivative gains (losses)(1)	[4]	16	(9)
L&S [Member] | Operating Segments [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Segment operating income attributable to MPLX LP		156	88
Segment portion attributable to Predecessor		53	62
G&P [Member] | Operating Segments [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Segment operating income attributable to MPLX LP		309	257
Segment portion attributable to Predecessor		$ 36	$ 41
[1]	(1)Financial information has been retrospectively adjusted for the acquisition of HST and WHC from MPC. See Notes 1 and 3.
[2]	(1)Income (loss) from equity method investments includes the impact of any basis differential amortization or accretion.
[3]	(1)Financial information has been retrospectively adjusted for the acquisition of HST and WHC from MPC. See Notes 1 and 3.
[4]	(1)The Partnership makes a distinction between realized or unrealized gains and losses on derivatives. During the period when a derivative contract is outstanding, we record changes in the fair value of the derivative as an unrealized gain or loss. When a derivative contract matures or is settled, we reverse the previously recorded unrealized gain or loss and record the realized gain or loss of the contract.

Segment Information - Reconci74

Segment Information - Reconciliation to Total Revenues and Other Income (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues and other income		$ 886	$ 645	[1]
Income from equity method investments	[2]	5	5
Other income - related parties		22	21	[1]
Operating Segments [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues and other income		955	745
Segment Reconciling Items [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue adjustment from unconsolidated affiliates		(92)	(104)
Income from equity method investments		5	5
Other income - related parties		11	7
Unrealized derivative gains (losses)(1)	[3]	$ 7	$ (8)
[1]	(1)Financial information has been retrospectively adjusted for the acquisition of HST and WHC from MPC. See Notes 1 and 3.
[2]	(1)Income (loss) from equity method investments includes the impact of any basis differential amortization or accretion.
[3]	(1)The Partnership makes a distinction between realized or unrealized gains and losses on derivatives. During the period when a derivative contract is outstanding, we record changes in the fair value of the derivative as an unrealized gain or loss. When a derivative contract matures or is settled, we reverse the previously recorded unrealized gain or loss and record the realized gain or loss of the contract.

Segment Information - Reconci75

Segment Information - Reconciliation to Net Income Attributable to Noncontrolling Interests (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Reconciliation to Net Income Attributable to Noncontrolling Interests [Line Items]
Net income attributable to noncontrolling interests and Predecessor	$ 37	$ 46
Operating Segments [Member]
Reconciliation to Net Income Attributable to Noncontrolling Interests [Line Items]
Segment portion attributable to noncontrolling interest and Predecessor	89	103
Segment Reconciling Items [Member]
Reconciliation to Net Income Attributable to Noncontrolling Interests [Line Items]
Portion of noncontrolling interests and Predecessor related to items below segment income from operations	(36)	(34)
Portion of operating income attributable to noncontrolling interest of unconsolidated affiliates	$ (16)	$ (23)

Segment Information - Reconci76

Segment Information - Reconciliation of Capital Expenditures (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Segment Reporting, Other Significant Reconciling Item [Line Items]
Additions to property, plant and equipment	$ (280)	$ (304)
Operating Segments [Member]
Segment Reporting, Other Significant Reconciling Item [Line Items]
Additions to property, plant and equipment	(395)	(348)
Operating Segments [Member] | L&S [Member]
Segment Reporting, Other Significant Reconciling Item [Line Items]
Additions to property, plant and equipment	(97)	(75)
Operating Segments [Member] | G&P [Member]
Segment Reporting, Other Significant Reconciling Item [Line Items]
Additions to property, plant and equipment	(298)	(273)
Segment Reconciling Items [Member]
Segment Reporting, Other Significant Reconciling Item [Line Items]
Additions to property, plant and equipment	$ 115	$ 44

Segment Information - Assets by

Segment Information - Assets by Segment (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016		Mar. 31, 2016	Dec. 31, 2015
Segment Reporting, Other Significant Reconciling Item [Line Items]
Cash and cash equivalents	$ 265	$ 234		$ 4	$ 43
Assets	18,285	17,509	[1]
L&S [Member] | Operating Segments [Member]
Segment Reporting, Other Significant Reconciling Item [Line Items]
Assets	3,713	2,978
G&P [Member] | Operating Segments [Member]
Segment Reporting, Other Significant Reconciling Item [Line Items]
Assets	$ 14,307	$ 14,297
[1]	Financial information has been retrospectively adjusted for the acquisition of HST, WHC and MPLXT from MPC. See Notes 1 and 3.

Inventories (Details)

Inventories (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Inventory Disclosure [Abstract]
NGLs	$ 1	$ 2
Line fill	8	9
Spare parts, materials and supplies	53	44
Inventories	$ 62	$ 55	[1]
[1]	Financial information has been retrospectively adjusted for the acquisition of HST, WHC and MPLXT from MPC. See Notes 1 and 3.

Property, Plant and Equipment -

Property, Plant and Equipment - Property, Plant and Equipment (Detail) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	$ 13,410	$ 13,102
Less accumulated depreciation	1,999	1,694
Net property, plant and equipment	11,411	11,408	[1]
Gas Gathering And Transmission Equipment And Facilities [Member]
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	4,843	4,748
Processing, Fractionation And Storage Facilities [Member]
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	3,654	3,467
Pipelines and Related Assets [Member]
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	2,005	1,799
Barges and towing vessels [Member]
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	528	479
Terminals and related assets [Member]
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	889	839
Land Building Office Equipment And Other [Member]
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	711	757
Assets Under Construction [Member]
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	$ 780	$ 1,013
[1]	Financial information has been retrospectively adjusted for the acquisition of HST, WHC and MPLXT from MPC. See Notes 1 and 3.

Fair Value Measurements - Recur

Fair Value Measurements - Recurring - Financial Instruments by Valuation Hierarchy (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross Amount	$ 2
Gross Amount	(46)
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross Amount	1	$ 0
Gross Amount	(45)	(60)
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]
Gross Amount	0	0
Gross Amount	(44)	(54)
Commodity contracts | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross Amount	0	0
Gross Amount	0	0
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]
Gross Amount	1	0
Gross Amount	$ (1)	$ (6)

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, 2017
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Number Of Renewal Periods	2
Embedded Derivative Renewal Term	5 years
Liability [Member] | Embedded derivatives in commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Probability of Renewal	50.00%	[1]
Fair Value Inputs Probability of Renewal Second Term	75.00%	[1]
[1]	(3)The producer counterparty to the embedded derivative has the option to renew the gas purchase agreement and the related keep-whole processing agreement for two successive five-year terms after 2022. The embedded gas purchase agreement cannot be renewed without the renewal of the related keep-whole processing agreement. Due to the significant number of years until the renewal options are exercisable and the high level of uncertainty regarding the counterparty’s future business strategy, the future commodity price environment, and the future competitive environment for midstream services in the Southern Appalachian region, management determined that a 50 percent probability of renewal for the first five-year term and 75 percent for the second five-year term are appropriate assumptions. Included in this assumption is a further extension of management’s estimates of future frac spreads through 2032.

Fair Value Measurements - Rec82

Fair Value Measurements - Recurring - Changes in Level 3 Measurements (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
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	$ (6)	$ 7
Total gain (loss) (realized and unrealized) included in earnings(1)	5	(1)
Settlements	1	(6)
Fair value at end of period	0	0
Fair Value Measurement with Unobservable Inputs Reconciliation, Recurring Basis Asset and Liabilities Net, Change in Unrealized Gain (Loss) Included in Earnings Contract Held	5	(2)
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	(54)	(32)
Total gain (loss) (realized and unrealized) included in earnings(1)	8	(4)
Settlements	2	2
Fair value at end of period	(44)	(34)
Fair Value Measurement with Unobservable Inputs Reconciliation, Recurring Basis Asset and Liabilities Net, Change in Unrealized Gain (Loss) Included in Earnings Contract Held	$ 8	$ (3)

Fair Value Measurements Fair Va

Fair Value Measurements Fair Value Measurements - Recurring (Details)	Mar. 31, 2017
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 Liability, Number of Instruments Held	1

Fair Value Measurements - Repor

Fair Value Measurements - Reported (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	$ 6,675	$ 4,422
Other Liabilities, Fair Value Disclosure	95	96
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	7,290	4,953
Other Liabilities, Fair Value Disclosure	$ 107	$ 108

Derivative Financial Instrume85

Derivative Financial Instruments - Volume of Commodity Derivative Activity (Details) - Not Designated as Hedging Instrument [Member] - Commodity contracts	Mar. 31, 2017MMBTUbblgal
Short [Member]
Derivative [Line Items]
Nonmonetary Notional Amount of Price Risk Derivatives of Crude Oil | bbl	51,900
Nonmonetary Notional Amount of Price Risk Derivatives of Natural Gas Liquids | gal	81,257,382
Long [Member]
Derivative [Line Items]
Nonmonetary Notional Amount Of Price Risk Derivatives Of Natural Gas | MMBTU	1,431,472

Derivative Financial Instrume86

Derivative Financial Instruments - Embedded Derivatives in Commodity Contracts (Details) - Natural Gas [Member] - Embedded derivatives in commodity contracts $ in Millions	3 Months Ended
	Mar. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Derivative [Line Items]
Notional amount for embedded derivative in commodity contract (in Dth per day)	9,000
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	$ 44	$ 54

Derivative Financial Instrume87

Derivative Financial Instruments - Derivatives Balance Sheet Location (Details) - Commodity contracts - USD ($) $ in Millions		Mar. 31, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	$ 1	$ 0
Derivative Liability, Fair Value, Gross Liability	[1]	45	60
Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	1	0
Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	[1]	7	13
Other Noncurrent Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	0	0
Deferred credits and other liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	[1]	$ 38	$ 47
[1]	(1)Includes embedded derivatives in commodity contracts as discussed above.

Derivative Financial Instrume88

Derivative Financial Instruments - Net Derivatives (Details) $ in Millions	Mar. 31, 2017USD ($)
Assets
Gross Amount	$ 2
Gross Amounts Offset in the Consolidated Balance Sheets	(1)
Net Amount of Assets in the Consolidated Balance Sheets	1
Liabilities
Gross Amount	(46)
Gross Amounts Offset in the Consolidated Balance Sheets	1
Net Amount of Liabilities in the Consolidated Balance Sheets	(45)
Current Assets [Member] | Commodity contracts
Assets
Gross Amount	2
Gross Amounts Offset in the Consolidated Balance Sheets	(1)
Net Amount of Assets in the Consolidated Balance Sheets	1
Current Liabilities [Member] | Commodity contracts
Liabilities
Gross Amount	(2)
Gross Amounts Offset in the Consolidated Balance Sheets	1
Net Amount of Liabilities in the Consolidated Balance Sheets	(1)
Non Current Assets [Member] | Commodity contracts
Assets
Gross Amount	0
Gross Amounts Offset in the Consolidated Balance Sheets	0
Net Amount of Assets in the Consolidated Balance Sheets	0
Non Current Liabilities [Member] | Commodity contracts
Liabilities
Gross Amount	0
Gross Amounts Offset in the Consolidated Balance Sheets	0
Net Amount of Liabilities in the Consolidated Balance Sheets	0
Not Designated as Hedging Instrument [Member] | Current Assets [Member]
Assets
Gross Amount	2
Gross Amounts Offset in the Consolidated Balance Sheets	(1)
Net Amount of Assets in the Consolidated Balance Sheets	1
Not Designated as Hedging Instrument [Member] | Current Assets [Member] | Embedded derivatives in commodity contracts
Assets
Gross Amount	0
Gross Amounts Offset in the Consolidated Balance Sheets	0
Net Amount of Assets in the Consolidated Balance Sheets	0
Not Designated as Hedging Instrument [Member] | Current Liabilities [Member]
Liabilities
Gross Amount	(8)
Gross Amounts Offset in the Consolidated Balance Sheets	1
Net Amount of Liabilities in the Consolidated Balance Sheets	(7)
Not Designated as Hedging Instrument [Member] | Current Liabilities [Member] | Embedded derivatives in commodity contracts
Liabilities
Gross Amount	(6)
Gross Amounts Offset in the Consolidated Balance Sheets	0
Net Amount of Liabilities in the Consolidated Balance Sheets	(6)
Not Designated as Hedging Instrument [Member] | Non Current Assets [Member]
Assets
Gross Amount	0
Gross Amounts Offset in the Consolidated Balance Sheets	0
Net Amount of Assets in the Consolidated Balance Sheets	0
Not Designated as Hedging Instrument [Member] | Non Current Assets [Member] | Embedded derivatives in commodity contracts
Assets
Gross Amount	0
Gross Amounts Offset in the Consolidated Balance Sheets	0
Net Amount of Assets in the Consolidated Balance Sheets	0
Not Designated as Hedging Instrument [Member] | Non Current Liabilities [Member]
Liabilities
Gross Amount	(38)
Gross Amounts Offset in the Consolidated Balance Sheets	0
Net Amount of Liabilities in the Consolidated Balance Sheets	(38)
Not Designated as Hedging Instrument [Member] | Non Current Liabilities [Member] | Embedded derivatives in commodity contracts
Liabilities
Gross Amount	(38)
Gross Amounts Offset in the Consolidated Balance Sheets	0
Net Amount of Liabilities in the Consolidated Balance Sheets	$ (38)

Derivatives Financial Instrumen

Derivatives Financial Instruments - Derivative Income Statement Location (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Derivative [Line Items]
Total gain (loss)	$ 13	$ (4)
Product Sales [Member]
Derivative [Line Items]
Realized gain (loss)	(1)	7
Unrealized gain (loss)	(7)	(8)
Total gain (loss)	6	(1)
Purchased product costs [Member]
Derivative [Line Items]
Realized gain (loss)	(2)	(1)
Unrealized gain (loss)	9	(1)
Total gain (loss)	7	(2)
Cost of Sales [Member]
Derivative [Line Items]
Realized gain (loss)	0	(1)
Total gain (loss)	$ 0	$ (1)

Debt - Summary of Outstanding B

Debt - Summary of Outstanding Borrowings (Detail) - USD ($) $ in Millions		Mar. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Total		$ 7,108	$ 4,858
Unamortized debt issuance costs		(28)	(7)
Unamortized discount	[1]	(425)	(428)
Amounts due within one year		(1)	(1)
Total long-term debt due after one year		6,654	4,422
Capital Lease Obligations [Member] | Marathon Pipe Line Llc [Member]
Debt Instrument [Line Items]
MPL - capital lease obligations due 2020		8	8
Bank revolving credit facility due 2020 | Line of Credit [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		0	0
MPLX Term Loan [Member] | Unsecured Debt [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		250	250
5.500% senior notes due February 2023 | Senior Notes [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		710	710
4.500% senior notes due July 2023 | Senior Notes [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		989	989
4.875% senior notes due December 2024 | Senior Notes [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		1,149	1,149
4.000% senior notes due February 2025 | Senior Notes [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		500	500
4.875% senior notes due June 2025 | Senior Notes [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		1,189	1,189
4.125% senior notes due 2027 | Senior Notes [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		1,250	0
5.200% senior notes due 2047 | Senior Notes [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		1,000	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
[1]	{F|ahBzfndlYmZpbGluZ3MtaHJkcmoLEgZYTUxEb2MiXlhCUkxEb2NHZW5JbmZvOmNlNmU1ZGJjNmRkMDQ0YTZiNjM5MTZjNDQ5YzIwZWQwfFRleHRTZWxlY3Rpb246MDlEOEQ2REFGRkUzNUJDQ0I2RjYwNjkyRUQxMzlFM0EM}

Debt - Summary of Outstanding91

Debt - Summary of Outstanding Borrowings Interest Rates and Table Due Dates (Details)	3 Months Ended
	Mar. 31, 2017
Mplx Lp [Member] | Line of Credit [Member] | MPLX Revolver [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Expiration Date	Dec. 4, 2020
Mplx Lp [Member] | Unsecured Debt [Member] | MPLX Term Loan [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Expiration Date	Nov. 20, 2019
Mplx Lp [Member] | Senior Notes [Member] | Senior Notes Due February 2023 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.50%
Debt Instrument, Maturity Date	Feb. 15, 2023
Mplx Lp [Member] | Senior Notes [Member] | Senior Notes Due July 2023 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.50%
Debt Instrument, Maturity Date	Jul. 15, 2023
Mplx Lp [Member] | Senior Notes [Member] | Senior Notes Due December 2024 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.875%
Debt Instrument, Maturity Date	Dec. 1, 2024
Mplx Lp [Member] | Senior Notes [Member] | Senior Notes Due February 2025 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.00%
Debt Instrument, Maturity Date	Feb. 15, 2025
Mplx Lp [Member] | Senior Notes [Member] | Senior Notes Due June 2025 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.875%
Debt Instrument, Maturity Date	Jun. 1, 2025
MarkWest [Member] | Senior Notes [Member] | Senior Notes Due February 2023 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.50%
Debt Instrument, Maturity Date	Feb. 15, 2023
MarkWest [Member] | Senior Notes [Member] | Senior Notes Due July 2023 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.50%
Debt Instrument, Maturity Date	Jul. 15, 2023
MarkWest [Member] | Senior Notes [Member] | Senior Notes Due December 2024 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.875%
Debt Instrument, Maturity Date	Dec. 1, 2024
MarkWest [Member] | Senior Notes [Member] | Senior Notes Due June 2025 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.875%
Debt Instrument, Maturity Date	Jun. 1, 2025
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 [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
MPLX Revolver [Member]
Debt Instrument [Line Items]
Proceeds from Lines of Credit	$ 0
Letters of Credit Outstanding, Amount	3
Line of Credit Facility, Remaining Borrowing Capacity	$ 2,000
Line of Credit Facility, Remaining Borrowing Capacity, Percentage	99.90%
MPLX Term Loan [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage	2.261%
Line of Credit [Member] | MPLX Revolver [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 0	$ 0
Senior Notes [Member]
Debt Instrument [Line Items]
Proceeds from (Repayments of) Debt	2,220
Debt Instrument, Face Amount	2,250
Senior Notes [Member] | 4.125% senior notes due 2027
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 1,250	0
Debt Instrument, Interest Rate, Stated Percentage	4.125%
Senior Notes [Member] | 5.200% senior notes due 2047
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 1,000	$ 0
Debt Instrument, Interest Rate, Stated Percentage	5.20%
Maximum [Member] | MPLX Term Loan [Member] | Letter of Credit [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 250

Supplemental Cash Flow Inform93

Supplemental Cash Flow Information - Summary of Supplemental Cash Flow Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017		Mar. 31, 2016
Net cash provided by operating activities included:
Interest paid (net of amounts capitalized)	$ 49		$ 53
Non-cash investing and financing activities:
Net transfers of property, plant and equipment from materials and supplies inventories	6		(4)
Contribution of Property	$ 328	[1]	$ 0
[1]	(1)Contribution of assets to Sherwood Midstream and Sherwood Midstream Holdings. See Note 4.

Supplemental Cash Flow Inform94

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, 2017	Mar. 31, 2016
Supplemental Cash Flow Elements [Abstract]
Increase (decrease) in capital accruals	$ 2	$ (23)

Supplemental Cash Flow Inform95

Supplemental Cash Flow Information Supplemental Cash Flow Information - Narrative (Details) $ in Millions	Mar. 01, 2017USD ($)	Feb. 15, 2017USD ($)	Mar. 31, 2016USD ($)	Mar. 31, 2017USD ($)
Suppplemental Cash Flow [Table] [Line Items]
Contribution from MPC				$ 12
Limited Partners Common Units [Member]
Suppplemental Cash Flow [Table] [Line Items]
Contribution from MPC	$ 6		$ 15
MPC [Member]
Suppplemental Cash Flow [Table] [Line Items]
Contribution from MPC			$ 15
Incentive Distribution Rights Forfeited per Quarter		$ 1.6
Number of Quarters IDRs Forfeited		12
Prorated Incentive Distribution Rights Forfeited				0.8
MPC [Member] | Limited Partners Common Units [Member]
Suppplemental Cash Flow [Table] [Line Items]
Contribution from MPC				$ 0

Equity-Based Compensation Pla96

Equity-Based Compensation Plan - Summary of Phantom Unit Award Activity (Details) - Phantom Units [Member]	3 Months Ended
	Mar. 31, 2017$ / sharesshares
Number of Units
Outstanding at December 31, 2016 | shares	1,173,411
Granted | shares	150,600
Settled | shares	158,814
Forfeited | shares	18,055
Outstanding at March 31, 2017 | shares	1,147,142
Weighted Average Fair Value
Outstanding at December 31, 2016 | $ / shares	$ 33.09
Granted (in USD per unit) | $ / shares	37.91
Settled (in USD per unit) | $ / shares	33.62
Forfeited (in USD per unit) | $ / shares	33.04
Outstanding at March 31, 2017 | $ / shares	$ 33.65

Equity-Based Compensation Pla97

Equity-Based Compensation Plan - Additional Information (Detail) - Performance Shares [Member] - MPLX LP 2012 Incentive Compensation Plan [Member]	3 Months Ended
	Mar. 31, 2017$ / shares
Equity Transactions And Share Based Compensation [Line Items]
Performance units grant date fair value (in USD per unit)	$ 0.90
Officer [Member]
Equity Transactions And Share Based Compensation [Line Items]
Share Based Compensation Arrangement By Share Based Payment Award Percentage Paid Out In Cash	75.00%
Share Based Compensation Arrangement By Share Based Payment Award Percentage Paid Out In Stock	25.00%

Equity-Based Compensation Pla98

Equity-Based Compensation Plan - Summary of Performance Unit Award Activity (Detail) - Performance Shares [Member]	3 Months Ended
	Mar. 31, 2017shares
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Outstanding at December 31, 2016	1,799,249
Granted	1,365,937
Settled	464,500
Forfeited	(14,375)
Outstanding at March 31, 2017	2,686,311

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	Sep. 06, 2016	Mar. 31, 2017	Dec. 31, 2016
Commitments And Contingencies [Line Items]
Accrued liabilities for remediation		$ 1	$ 1
Estimated cost of proposed supplemental environmental projects		3.6
Contractual commitments to acquire property, plant and equipment		409
Environmental Loss Contingency [Member] | MPC [Member]
Commitments And Contingencies [Line Items]
Receivables from MPC for indemnification of environmental costs		4	$ 1
Apex [Member]
Commitments And Contingencies [Line Items]
Apex litigation settlement amount	$ 10
Markwest Liberty Midstream [Member]
Commitments And Contingencies [Line Items]
EPA proposed penalty		$ 2.4