MPLX (MPLX) 10-Q 30 Oct 17 2017 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2017	Oct. 27, 2017
Document Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	Q3
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		407,066,320
General Partner Units
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		8,307,473

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) shares in Millions, $ in Millions		3 Months Ended			9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016		Sep. 30, 2017		Sep. 30, 2016
Revenues and other income:
Service revenue		$ 299	$ 250		$ 845		$ 712
Service revenue - related parties		276	253	[1]	801		676	[1]
Rental income		69	77		208		218
Rental income - related parties		70	68	[1]	207		172	[1]
Product sales		217	157		611		394
Product sales - related parties		2	2		6		8
Gain (loss) on sale of assets		0	1		1		1
Income (loss) from equity method investments		23	6		29	[2]	(72)	[2]
Other income		2	2		5		5
Other income - related parties		22	22	[1]	69		67	[1]
Total revenues and other income		980	838	[1]	2,782		2,181	[1]
Costs and expenses:
Cost of revenues (excludes items below)		129	122	[1]	381		329	[1]
Purchased product costs		170	117		441		310
Rental cost of sales		19	13	[1]	44		42	[1]
Rental cost of sales - related parties		0	0	[1]	1		1	[1]
Purchases - related parties		114	109	[1]	330		286	[1]
Depreciation and amortization		164	151	[1]	515		438	[1],[3]
Impairment expense		0	0		0		130
General and administrative expenses		59	56	[1]	174		172	[1]
Other taxes		14	12		40		37
Total costs and expenses		669	580	[1]	1,926		1,745	[1]
Income from operations		311	258	[1]	856		436	[1]
Related party interest and other financial costs		1	0		1		1
Interest expense (net of amounts capitalized)		77	51		217		158
Other financial costs		15	13		40		37
Income before income taxes		218	194	[1]	598		240	[1]
Provision (benefit) for income taxes		(1)	0		(3)		12
Net income		217	194	[1]	595		252	[1],[3]
Less: Net income (loss) attributable to noncontrolling interest		1	2		3		3
Less: Net income attributable to Predecessor		0	51	[1]	36		149	[1]
Net income (loss) attributable to MPLX LP	[4]	216	141		556		100
Less: Preferred unit distributions		16	16		49		25
Less: General partner’s interest in net income attributable to MPLX LP		86	51		222		136
Limited partners' interest in net income (loss) attributable to MPLX LP		$ 114	$ 74		$ 285		$ (61)
Weighted average limited partner units outstanding:
Common - basic (in shares)		433	379		417		347
Common - diluted (in shares)		434	384		420		347
Limited Partners Common Units
Costs and expenses:
Net income (loss) attributable to MPLX LP	[4]	$ 114	$ 74		$ 285		$ (61)
Net income (loss) attributable to MPLX LP per limited partner unit:
Common - basic (in USD per unit)		$ 0.29	$ 0.22		$ 0.75		$ (0.19)
Common - diluted (in USD per unit)		$ 0.29	$ 0.21		$ 0.75		$ (0.19)
Weighted average limited partner units outstanding:
Common - basic (in shares)		394	341		378		324
Common - diluted (in shares)		395	346		381		324
Cash distributions declared per limited partner common unit		$ 0.5875	$ 0.5150		$ 1.6900		$ 1.5300
[1]	Financial information has been retrospectively adjusted for the acquisition of HST, WHC and MPLXT from MPC. See Notes 1 and 3.
[2]	Income (loss) from equity method investments includes the impact of any basis differential amortization or accretion.
[3]	Financial information has been retrospectively adjusted for the acquisition of HST, WHC and MPLXT from MPC. See Notes 1 and 3.
[4]	Allocation of net income (loss) attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.

Consolidated Statements of Inc3

Consolidated Statements of Income Consolidated Statements of Income (Parenthetical) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Statement [Abstract]
Interest costs capitalized	$ 6	$ 7	$ 24	$ 21

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 3	$ 234
Receivables, net	320	299
Receivables - related parties	152	247
Inventories	64	55
Other current assets	32	33
Total current assets	571	868
Equity method investments	3,997	2,471
Property, plant and equipment, net	11,922	11,408
Intangibles, net	463	492
Goodwill	2,245	2,245
Long-term receivables - related parties	18	11
Other noncurrent assets	22	14
Total assets	19,238	17,509
Current liabilities:
Accounts payable	152	140
Accrued liabilities	202	232
Payables - related parties	317	87
Deferred revenue	3	2
Deferred revenue - related parties	42	38
Accrued property, plant and equipment	183	146
Accrued taxes	44	38
Accrued interest payable	64	53
Other current liabilities	41	27
Total current liabilities	1,048	763
Long-term deferred revenue	34	12
Long-term deferred revenue - related parties	40	19
Long-term debt	6,848	4,422
Deferred income taxes	7	6
Deferred credits and other liabilities	175	177
Total liabilities	8,152	5,399
Commitments and contingencies (see Note 17)
Redeemable preferred units	1,000	1,000
Equity
Total MPLX LP partners’ capital	9,941	11,092
Noncontrolling interests	145	18
Total equity	10,086	11,110
Total liabilities, preferred units and equity	19,238	17,509
MPC
Current assets:
Receivables - related parties	144	242
Long-term receivables - related parties	18	11
Current liabilities:
Payables - related parties	277	63
Limited Partners Common Units | MPC
Equity
Total MPLX LP partners’ capital	1,302	1,069
Limited Partners Class B Units
Equity
Total MPLX LP partners’ capital	0	133
GP Held Limited Partners Common Units | MPC
Equity
Total MPLX LP partners’ capital	822	0
General Partner | MPC
Equity
Total MPLX LP partners’ capital	(626)	1,013
Public | Limited Partners Common Units
Equity
Total MPLX LP partners’ capital	8,457	8,086
Predecessor | MPC
Equity
Total MPLX LP partners’ capital	0	791
Total equity	0	791	[1]
Loop Llc and Explorer Pipeline [Member]
Equity
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ (14)	$ 0
[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	Sep. 30, 2017	Dec. 31, 2016
Limited Partners Common Units | Public
Units issued	289	271
Units outstanding	289	271
Limited Partners Class B Units
Units issued	0	4
Units outstanding	0	4
MPC | Limited Partners Common Units
Units issued	95	86
Units outstanding	95	86
MPC | GP Held Limited Partners Common Units
Units issued	23	0
Units outstanding	23	0
MPC | General Partner
GP units issued	8	7
GP units outstanding	8	7

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016
Operating activities:
Net income		$ 595	$ 252	[1],[2]
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization of deferred financing costs		38	34
Depreciation and amortization		515	438	[1],[2]
Impairment expense		0	130
Deferred income taxes		2	(16)
Asset retirement expenditures		2	4	[2]
(Gain) loss on disposal of assets		1	1
(Income) loss from equity method investments	[3]	29	(72)
Distributions from unconsolidated affiliates		136	111
Changes in:
Current receivables		(20)	(43)	[2]
Inventories		3	4
Fair value of derivatives		3	(28)
Current accounts payable and accrued liabilities		6	64	[2]
Receivables from / liabilities to related parties		61	(104)	[2]
All other, net		43	18	[2]
Net cash provided by operating activities		1,338	975	[2]
Investing activities:
Additions to property, plant and equipment		(1,004)	(943)	[2]
Acquisitions, net of cash acquired		(249)	0
Disposal of assets		4	0
Investments - net related party loans		80	103	[2]
Investments in unconsolidated affiliates		690	56
Distributions from unconsolidated affiliates - return of capital		24	0
All other, net		(2)	4
Net cash used in investing activities		(1,837)	(892)	[2]
Financing activities:
Long-term debt - borrowings		2,661	434
Long-term debt - repayments		(251)	(1,312)
Related party debt - borrowings		829	2,215
Related party debt - repayments		627	2,223
Debt issuance costs		25	0
Net proceeds from equity offerings		483	510
Issuance of redeemable preferred units		0	984
Distributions to preferred unitholders		(49)	(9)
Distributions to unitholders and general partner		(800)	(612)
Distributions to noncontrolling interests		(4)	(3)
Contributions from noncontrolling interests		128	4
Consideration payment to Class B unitholders		25	25
All other, net		(8)	(2)
Contribution from MPC		0	225
Net cash provided by financing activities		268	82
Net (decrease) increase in cash and cash equivalents		(231)	165
Cash and cash equivalents at beginning of period		234	43
Cash and cash equivalents at end of period		3	208
MPC
Financing activities:
Distributions to MPC		1,931	0
Predecessor | MPC
Financing activities:
Distributions to MPC		$ 113	$ 104
[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.
[3]	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	Common UnitholdersPublic	Common UnitholdersMPC	Class B UnitholdersPublic	Common Unitholder GPMPC	General PartnerMPC	AOCI Attributable to Parent [Member]	Noncontrolling Interests	LOOP LOCAP SAX and Explorer Llc [Member]MPC	LOOP LOCAP SAX and Explorer Llc [Member]General PartnerMPC
Beginning Balance (Predecessor) at Dec. 31, 2015	[1]		$ 692
Beginning Balance at Dec. 31, 2015		$ 9,946		$ 7,691	$ 465	$ 266		$ 819		$ 13
Distributions to MPC | Predecessor			104
Distributions to MPC			0
Issuance of units under ATM Program		510		499				11
Net income (loss) | Predecessor	[1]		149
Net income (loss)		227		(51)	(10)			136		3
Contribution from MPC		225			84			141
Contribution of MarkWest Hydrocarbon from MPC		(188)						(188)
Distribution of MarkWest Hydrocarbon to MPC		565						565
Allocation of MPC's net investment at acquisition | Predecessor			(332)
Allocation of MPC's net investment at acquisition					669			(337)
Distributions to unitholders and general partner		(612)		(378)	(98)			(136)
Distributions to noncontrolling interests		(3)								(3)
Contributions from noncontrolling interests		4								4
Class B unit conversion				(133)		(133)
Non-cash contribution from MPC | Predecessor			334
Non-cash contribution from MPC		334
Equity-based compensation		6		6
Deferred income tax impact from changes in equity		(17)		(2)	(13)			(2)
Ending Balance (Predecessor) at Sep. 30, 2016			739
Ending Balance at Sep. 30, 2016		10,893		7,898	1,097	133		1,009		17
Beginning Balance (Predecessor) at Dec. 31, 2016	[1]		791
Beginning Balance at Dec. 31, 2016		11,110		8,086	1,069	133		1,013		18
Distributions to MPC | Predecessor			113
Distributions to MPC			1,931		537			1,394			$ 13	$ 13
Issuance of units under ATM Program		483		473				10
Net income (loss) | Predecessor			36
Net income (loss)		546		212	68		$ 5	222		3
Contribution from MPC | Predecessor			689
Contribution from MPC		675							$ (14)
Allocation of MPC's net investment at acquisition | Predecessor			(1,403)
Allocation of MPC's net investment at acquisition		0			845		824	(266)
Distributions to unitholders and general partner		(800)		(452)	(143)		(7)	(198)
Distributions to noncontrolling interests		(4)								(4)
Contributions from noncontrolling interests		128								128
Class B unit conversion				(133)		(133)
Equity-based compensation		5		5
Ending Balance (Predecessor) at Sep. 30, 2017			$ 0
Ending Balance at Sep. 30, 2017		$ 10,086		$ 8,457	$ 1,302	$ 0	$ 822	$ (626)	$ (14)	$ 145
[1]	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	9 Months Ended
	Sep. 30, 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. References to “MPC” refer collectively to Marathon Petroleum Corporation and its subsidiaries, other than the Partnership. The Partnership’s business consists of two segments based on the nature of services it offers: Logistics and Storage (“L&S”) which is focused on crude oil and refined petroleum products and Gathering and Processing (“G&P”) which is focused on natural gas and NGLs. See Note 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 interests in the accompanying Consolidated Balance Sheets. Intercompany investments, accounts and transactions have been eliminated. The Partnership’s investments in which the Partnership exercises significant influence but does not control and does not have a controlling financial interest are accounted for using the equity method. The Partnership’s investments in a VIE in which the Partnership exercises significant influence but does not control and is not the primary beneficiary are also accounted for using the equity method. Effective March 1, 2017 , the Partnership acquired pipeline, storage and terminal businesses that are operated through Hardin Street Transportation LLC (“HST”), Woodhaven Cavern LLC (“WHC”) and MPLX Terminals LLC (“MPLXT”) (collectively with Hardin Street Marine LLC (“HSM”), “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 businesses 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 Predecessor on a historical basis. The financial statements of 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 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 until declared. However, when distributions related to the IDRs 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, as updated by our Current Report on Form 8-K filed on May 1, 2017. The results of operations for the three and nine months ended September 30, 2017 are not necessarily indicative of the results to be expected for the full year.

Accounting Standards

Accounting Standards	9 Months Ended
	Sep. 30, 2017
Accounting Standards [Abstract]
Accounting Standards	Accounting Standards Recently Adopted In October 2016, the FASB issued an accounting standards 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 standards 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 standards update on the accounting for employee share-based payments. This update requires the recognition of income tax effects of awards through the income statement when awards vest or are settled. It also increases 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 standards update in the first quarter of 2017 and it did not have a material impact on the consolidated financial statements. Not Yet Adopted In August 2017, the FASB issued an accounting standards update to amend the hedge accounting rules to simplify the application of hedge accounting guidance and better portray the economic results of risk management activities in the financial statements. The guidance expands the ability to hedge nonfinancial and financial risk components, reduces complexity in fair value hedges of interest rate risk, eliminates the requirement to separately measure and report hedge ineffectiveness, as well as eases certain hedge effectiveness assessment requirements. The guidance is effective beginning in 2019 with early adoption permitted. The Partnership is in the process of determining the impact of this guidance, including transition elections and required disclosures, on the consolidated financial statements and the timing of adoption. In May 2017, the FASB issued an accounting standards update to provide guidance about when changes to the terms or conditions of a share-based payment award require an entity to apply modification accounting. An entity should account for the effects of a modification unless the fair value, vesting conditions and balance sheet classification of the modified award is the same as the original award immediately before the original award is modified. The Partnership will adopt the new standard on a prospective basis beginning on January 1, 2018. The application of this new accounting standard is not expected to have a material impact on the consolidated financial statements. In February 2017, the FASB issued an accounting standards 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 of the revenue recognition accounting standards update, which the Partnership will implement January 1, 2018. The Partnership plans to adopt the new standard using the modified retrospective method and does not expect the application of this accounting standards update to have a material impact on the consolidated financial statements. In January 2017, the FASB issued an accounting standards update which simplifies the subsequent measurement of goodwill by eliminating Step 2 from the goodwill impairment test. Under the new guidance, the recognition of an impairment charge is calculated based on the amount by which the carrying amount exceeds the reporting unit’s fair value, which could be different from the amount calculated under the current method using the implied fair value of the goodwill; however, the loss recognized should not exceed the total amount of goodwill allocated to that reporting unit. The guidance should be applied on a prospective basis, and is effective for annual or 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 standards update on the consolidated financial statements. In January 2017, the FASB issued an accounting standards 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 evaluating this accounting standards update and determining whether it will early adopt. In November 2016, the FASB issued an accounting standards 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 standards update will not have a material impact on the Consolidated Statements of Cash Flows. In August 2016, the FASB issued an accounting standards update related to the classification of certain cash flows. The accounting standards update provides specific guidance on eight cash flow classification issues, including debt prepayment or debt extinguishment costs and distributions received from equity method investees, to reduce diversity in practice. 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 Partnership does not expect application of this accounting standards update to have a material impact on the Consolidated Statements of Cash Flows. In June 2016, the FASB issued an accounting standards update related to the accounting for credit losses on certain financial instruments. The guidance requires that for most financial assets, losses 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 standards update to have a material impact on the consolidated financial statements. In February 2016, the FASB issued an accounting standards update requiring lessees to record virtually all leases on their balance sheets. The accounting standards update also requires expanded disclosures to help financial statement users better understand the amount, timing and uncertainty of cash flows arising from leases. For lessors, this amended guidance modifies the classification criteria and the accounting for sales-type and direct financing leases. The change will be effective on a modified retrospective basis for fiscal years beginning after December 15, 2018, and interim periods within those 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. The Partnership believes the impact will be material on the consolidated financial statements as all operating leases will be recognized as a right of use asset and lease obligation. Based on results of the evaluation process to date, the Partnership also believes the impact on existing processes, controls and information systems may be material. In January 2016, the FASB issued an accounting standards 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 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 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 standards 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 Partnership does not expect application of this accounting standards update to have a material impact on the consolidated financial statements. In May 2014, the FASB issued Accounting Standards Update 2014-09 which created Accounting Standards Codification Topic 606, “Revenue from Contracts with Customers” (“ASC 606”). The guidance in ASC 606 states that revenue is recognized when a customer obtains control of a good or service. Recognition of the revenue 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 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 fiscal years, with early adoption permitted no earlier than January 1, 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 after adoption as compared with the guidance that was in effect before adoption. The Partnership is currently evaluating the impact of the revenue recognition standard on its consolidated 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. This phase was completed as of September 30, 2017. Based on the results of the first phase assessment, the Partnership has reached conclusions for all material contract types. Revenue recognition patterns will not change for fee-based or percent-of-proceeds contracts. The Partnership does expect certain amounts to be grossed up in revenue and cost of revenues as a result of implementation, specifically related to third-party reimbursements from customers and commodities received as consideration in service agreements, such as keep-whole arrangements. In the third quarter of 2017, the Partnership finalized a conclusion on the valuation of non-cash consideration received in the form of a commodity product, with the valuation being performed on the date the service performance obligation is completed. The Partnership is in the process of finalizing the second phase of implementation, which includes the calculation of the impact of the new standard on results and the development of new policies, procedures and disclosures related to the application upon adoption. The Partnership believes third-party reimbursements included in the transaction price would have resulted in a gross up in 2016 and 2017 service revenue and cost of revenues between $300 million and $350 million annually, with no impact to net income. The Partnership will provide a summary of the total ASC 606 impact in the Annual Report on Form 10-K for the year ended December 31, 2017.

Acquisitions

Acquisitions	9 Months Ended
	Sep. 30, 2017
Equity [Abstract]
Acquisitions	Acquisitions Joint-Interest Acquisition On September 1, 2017 , the Partnership entered into a Membership Interests and Shares Contributions Agreement (the “September 2017 Contributions Agreement”) with MPLX GP 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, whereby the Partnership agreed to acquire certain ownership interests in joint venture entities indirectly held by MPC. Pursuant to the September 2017 Contributions Agreement, MPC Investment agreed to contribute: all of the membership interests of Lincoln Pipeline LLC, which holds a 35 percent interest in Illinois Extension Pipeline Company, L.L.C. (“Illinois Extension”); all of the membership interests of MPL Louisiana Holdings LLC, which holds a 40.7 percent interest in LOOP LLC (“LOOP”); a 58.52 percent interest in LOCAP LLC (“LOCAP”); and a 24.51 percent interest in Explorer Pipeline Company (“Explorer”), through a series of intercompany contributions to the Partnership for an agreed upon purchase price of approximately $420 million in cash and equity consideration valued at approximately $630 million for total consideration of $1.05 billion (collectively, the “Joint-Interest Acquisition”). The number of common units representing the equity consideration was then determined by dividing the contribution amount by the simple average of the ten day trading volume weighted average NYSE price of a common unit for the ten trading days ending at market close on August 31, 2017. The fair value of the common and general partner units issued was approximately $653 million based on the closing common unit price as of September 1, 2017, as recorded on the Consolidated Statements of Equity, for a total purchase price of $1.07 billion . The equity issued consisted of: (i) 13,719,017 common units to MPLX GP, (ii) 3,350,893 common units to MPLX Logistics and (iii) 1,441,224 common units to MPLX Holdings. The Partnership also issued 377,778 general partner units to MPLX GP in order to maintain its two percent general partner interest (“GP Interest”) in the Partnership. Illinois Extension operates the 168 -mile, 24 -inch diameter Southern Access Extension (“SAX”) crude oil pipeline from Flanagan, Illinois to Patoka, Illinois, as well as additional tankage and two pump stations. LOOP owns and operates midstream crude oil infrastructure, including a deep water oil port offshore of Louisiana, pipelines and onshore storage facilities. LOOP also manages the operations of LOCAP, an affiliate pipeline system. LOCAP owns and operates a crude oil pipeline and tank facility in St. James, Louisiana, that distributes oil received from LOOP’s storage facilities and other connecting pipelines to nearby refineries and into the mid-continent region of the United States. Explorer owns and operates an approximate 1,830 -mile common carrier pipeline that primarily transports gasoline, diesel, diluent and jet fuel from the Gulf Coast refining complex to the Midwest United States. The Partnership accounts for the Joint-Interest Acquisition entities as equity method investments within its L&S segment. As a transfer between entities under common control, the Partnership recorded the Joint-Interest Acquisition on its Consolidated Balance Sheets at MPC’s historical basis, which included accumulated other comprehensive loss. The Partnership recognizes an accumulated other comprehensive loss on its Consolidated Balance Sheets relating to pension and other post-retirement benefits provided by the LOOP and Explorer joint-interests to their employees. MPLX LP is not a sponsor of these benefit plans. There were no changes to Accumulated other comprehensive loss during the period September 1, 2017 through September 30, 2017. Distributions of cash received from the entities and interests acquired in the Joint-Interest Acquisition related to periods prior to the acquisition will be prorated on a daily basis with MPLX LP retaining the portion of distributions beginning on the closing date. All amounts distributed to MPLX LP related to periods before the acquisition will be paid to MPC. Additionally, MPLX LP has agreed to pay MPC for any distributions of cash from LOOP related to the sale of LOOP’s excess crude oil inventory. Because the future distributions or payments cannot be reasonably quantified, a liability was not recorded in connection with the acquisition. MPLX LP subsequently received distributions related to the third quarter 2017 and recorded a liability to MPC and a corresponding decrease to the general partner’s equity for $13 million , as shown on the Consolidated Statements of Equity. There is no income associated with the Joint-Interest Acquisition included in the Consolidated Statements of Income since the September 1, 2017 acquisition date, as the Partnership accounts for these investments in arrears using the most recently available information. The Partnership’s investment balance at September 30, 2017 related to the acquired interests is approximately $645 million and reported under the caption Equity method investments on the Consolidated Balance Sheets. MPC agreed to waive approximately two-thirds of the third quarter 2017 distributions on the common units issued in connection with the Joint-Interest Acquisition. As a result of this waiver, MPC did not receive approximately two-thirds of the distributions or IDRs that would have otherwise accrued on such common units with respect to the third quarter 2017 distributions. The value of these waived distributions was $10 million . Acquisition of Hardin Street Transportation LLC, Woodhaven Cavern LLC and MPLX Terminals LLC MPC contributed the assets of HST, WHC and MPLXT to newly created and wholly-owned subsidiaries and entered into commercial agreements related to services provided by these new entities to MPC on January 1, 2015 for HST and WHC and April 1, 2016 for MPLXT. Pursuant to a Membership Interests Contributions Agreement entered into on March 1, 2017 by the Partnership with MPLX GP, MPLX Logistics, MPLX Holdings and MPC Investment, each a wholly-owned subsidiary of MPC, MPC Investment agreed to contribute the outstanding membership interests in HST, WHC and MPLXT through a series of intercompany contributions to the Partnership for approximately $1.5 billion in cash and equity consideration valued at approximately $504 million (the “Transaction”). The number of common units representing the equity consideration was determined by dividing the contribution amount by the simple average of the ten day trailing volume weighted average NYSE price of a common unit for the ten trading days ending at market close on February 28, 2017. The fair value of the common and general partner units issued was approximately $503 million , as recorded on the Consolidated Statements of Equity, and consisted of (i) 9,197,900 common units to MPLX GP, (ii) 2,630,427 common units to MPLX Logistics and (iii) 1,132,049 common units to MPLX Holdings. The Partnership also issued 264,497 general partner units to MPLX GP in order to maintain its two percent GP Interest in the Partnership. MPC agreed to waive two-thirds of the first quarter 2017 distributions on the common units issued in connection with the Transaction. As a result of this waiver, MPC did not receive two-thirds of the general partner distributions or IDRs that would have otherwise accrued on such common units with respect to the first quarter 2017 distributions. The value of these waived distributions was $6 million . HST owns and operates various private crude oil and refined product pipeline systems and associated storage tanks. As of the acquisition date, these pipeline systems consisted of 174 miles of crude oil pipelines and 430 miles of refined products pipelines. WHC owns and operates nine butane and propane storage caverns located in Michigan with approximately 1.8 million barrels of NGL storage capacity. As of the acquisition date, MPLXT owned and operated 59 terminals for the receipt, storage, blending, additization, handling and redelivery of refined petroleum products. Additionally, MPLXT operated one leased terminal and had partial ownership interest in two terminals. Collectively, these 62 terminals have a combined shell capacity of approximately 23.6 million barrels. The terminal facilities are located primarily in the Midwest, Gulf Coast and Southeast regions of the United States. The Partnership accounts for these businesses within its L&S segment. 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. Prior to these dates, these entities were not considered businesses and, therefore, there are no financial results from which to recast. The following tables present the Partnership’s previously reported unaudited Consolidated Statements of Income for the three and nine months ended September 30, 2016 , retrospectively adjusted for the acquisition of HST, WHC and MPLXT: Three Months Ended September 30, 2016 (In millions, except per unit data) MPLX LP (Previously Reported) HST/WHC MPLXT Eliminations (1) MPLX LP (Currently Reported) Revenues and other income: Service revenue $ 250 $ — $ — $ — $ 250 Service revenue - related parties 153 28 72 — 253 Rental income 77 — — — 77 Rental income - related parties 29 13 26 — 68 Product sales 157 — — — 157 Product sales - related parties 2 — — — 2 Income from equity method investments 6 — — — 6 Gain on sale of assets 1 — — — 1 Other income 2 — — — 2 Other income - related parties 26 — — (4 ) 22 Total revenues and other income 703 41 98 (4 ) 838 Costs and expenses: Cost of revenues (excludes items below) 90 10 22 — 122 Purchased product costs 117 — — — 117 Rental cost of sales 11 2 — — 13 Rental cost of sales - related parties — 1 — (1 ) — Purchases - related parties 84 4 24 (3 ) 109 Depreciation and amortization 138 4 9 — 151 General and administrative expenses 46 2 8 — 56 Other taxes 10 — 2 — 12 Total costs and expenses 496 23 65 (4 ) 580 Income from operations 207 18 33 — 258 Interest expense (net of amounts capitalized) 51 — — — 51 Other financial costs 13 — — — 13 Income before income taxes 143 18 33 — 194 Net income 143 18 33 — 194 Less: Net income attributable to noncontrolling interests 2 — — — 2 Less: Net income attributable to Predecessor — 18 33 — 51 Net income attributable to MPLX LP 141 — — — 141 Less: Preferred unit distributions 16 — — — 16 Less: General partner’s interest in net income attributable to MPLX LP 51 — — — 51 Limited partners’ interest in net income attributable to MPLX LP $ 74 $ — $ — $ — $ 74 (1) Represents intercompany transactions eliminated during the consolidation process, in accordance with GAAP. Nine Months Ended September 30, 2016 (In millions, except per unit data) MPLX LP (Previously Reported) HST/WHC MPLXT Eliminations (1) MPLX LP (Currently Reported) Revenues and other income: Service revenue $ 712 $ — $ — $ — $ 712 Service revenue - related parties 448 82 146 — 676 Rental income 218 — — — 218 Rental income - related parties 84 36 52 — 172 Product sales 394 — — — 394 Product sales - related parties 8 — — — 8 Loss from equity method investments (72 ) — — — (72 ) Gain on sale of assets 1 — — — 1 Other income 5 — — — 5 Other income - related parties 78 — — (11 ) 67 Total revenues and other income 1,876 118 198 (11 ) 2,181 Costs and expenses: Cost of revenues (excludes items below) 263 24 42 — 329 Purchased product costs 310 — — — 310 Rental cost of sales 39 3 — — 42 Rental cost of sales - related parties — 2 — (1 ) 1 Purchases - related parties 238 13 45 (10 ) 286 Depreciation and amortization 407 12 19 — 438 Impairment expense 130 — — — 130 General and administrative expenses 147 5 20 — 172 Other taxes 32 2 3 — 37 Total costs and expenses 1,566 61 129 (11 ) 1,745 Income from operations 310 57 69 — 436 Related party interest and other financial income 1 — — — 1 Interest expense (net of amounts capitalized) 158 — — — 158 Other financial costs 37 — — — 37 Income before income taxes 114 57 69 — 240 Benefit for income taxes (12 ) — — — (12 ) Net income 126 57 69 — 252 Less: Net income attributable to noncontrolling interests 3 — — — 3 Less: Net income attributable to Predecessor 23 57 69 — 149 Net income attributable to MPLX LP 100 — — — 100 Less: Preferred unit distributions 25 — — — 25 Less: General partner’s interest in net income attributable to MPLX LP 136 — — — 136 Limited partners’ interest in net loss attributable to MPLX LP $ (61 ) $ — $ — $ — $ (61 ) (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, WHC and MPLXT: Nine Months Ended September 30, 2016 (In millions) MPLX LP (Previously Reported) HST/WHC MPLXT MPLX LP (Currently Reported) Increase (decrease) in cash and cash equivalents Operating activities: Net income $ 126 $ 57 $ 69 $ 252 Adjustments to reconcile net income to net cash provided by (used in) operating activities: Amortization of deferred financing costs 34 — — 34 Depreciation and amortization 407 12 19 438 Impairment expense 130 — — 130 Deferred income taxes (16 ) — — (16 ) Asset retirement expenditures (3 ) (1 ) — (4 ) Gain on disposal of assets (1 ) — — (1 ) Loss from equity method investments 72 — — 72 Distributions from unconsolidated affiliates 111 — — 111 Changes in: Current receivables (44 ) 1 — (43 ) Inventories (4 ) — — (4 ) Fair value of derivatives 28 — — 28 Current accounts payable and accrued liabilities 59 (1 ) 6 64 Receivables from / liabilities to related parties 15 3 (122 ) (104 ) All other, net 18 2 (2 ) 18 Net cash provided by (used in) operating activities 932 73 (30 ) 975 Investing activities: Additions to property, plant and equipment (874 ) (36 ) (33 ) (943 ) Investments - net related party loans 77 (37 ) 63 103 Investments in unconsolidated affiliates (56 ) — — (56 ) All other, net 4 — — 4 Net cash (used in) provided by investing activities (849 ) (73 ) 30 (892 ) Financing activities: Long-term debt - borrowings 434 — — 434 - repayments (1,312 ) — — (1,312 ) Related party debt - borrowings 2,215 — — 2,215 - repayments (2,223 ) — — (2,223 ) Net proceeds from equity offerings 510 — — 510 Issuance of redeemable preferred units 984 — — 984 Distributions to preferred unitholders (9 ) — — (9 ) Distributions to unitholders and general partner (612 ) — — (612 ) Distributions to noncontrolling interests (3 ) — — (3 ) Contributions from noncontrolling interests 4 — — 4 Consideration payment to Class B unitholders (25 ) — — (25 ) All other, net (2 ) — — (2 ) Contribution from MPC 225 — — 225 Distributions to MPC from Predecessor (104 ) — — (104 ) Net cash provided by financing activities 82 — — 82 Net increase in cash and cash equivalents 165 — — 165 Cash and cash equivalents at beginning of period 43 — — 43 Cash and cash equivalents at end of period $ 208 $ — $ — $ 208 Acquisition of Ozark Pipeline On March 1, 2017, the Partnership acquired the Ozark pipeline from Enbridge Pipelines (Ozark) LLC for approximately $219 million , including purchase price adjustments made in the second quarter of 2017. Based on the final fair value estimates of assets acquired and liabilities assumed at the acquisition date, the purchase price was primarily allocated to property, plant and equipment. The Ozark pipeline is a 433 -mile, 22 -inch crude oil pipeline originating in Cushing, Oklahoma, and terminating in Wood River, Illinois, capable of transporting approximately 230 mbpd. The Partnership accounts for the Ozark pipeline within its L&S segment. The amounts of revenue and income from operations associated with the acquisition included in the Consolidated Statements of Income, since the March 1, 2017 acquisition date, are as follows: (In millions) Three Months Ended September 30, 2017 Seven Months Ended September 30, 2017 Revenues and other income $ 19 $ 45 Income from operations 6 17 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. MarEn Bakken On February 15, 2017, the Partnership closed on a joint venture, MarEn Bakken Company, LLC (“MarEn Bakken”), with Enbridge Energy Partners L.P. in which MPLX LP acquired a partial, indirect interest in the Dakota Access Pipeline and Energy Transfer Crude Oil Company Pipeline projects, collectively referred to as the Bakken Pipeline system, from Energy Transfer Partners, L.P. and Sunoco Logistics Partners, L.P. The Partnership 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. The Partnership holds, through a subsidiary, a 25 percent interest in MarEn Bakken, which equates to a 9.1875 percent indirect interest in the Bakken Pipeline system. The Partnership accounts for its investment in MarEn Bakken as an equity method investment and bases the equity method accounting for this joint venture in arrears using the most recently available information. The Partnership’s investment balance at September 30, 2017 is approximately $520 million and reported under the caption Equity method investments on the Consolidated Balance Sheets. In connection with the Partnership’s acquisition of a partial, indirect equity interest in the Bakken Pipeline system, MPC agreed to waive its right to receive incentive distributions of $1.6 million per quarter for twelve consecutive quarters, beginning with distributions declared in the first quarter of 2017 and paid to MPC in the second quarter of 2017, which was prorated to $0.8 million from the acquisition date. 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 common units issued in connection with this transaction. As a result of this waiver, MPC did not receive general partner distributions or IDRs that would have otherwise accrued on such 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 219 owned and leased barges as of the acquisition date, which transport light products, heavy oils, crude oil, renewable fuels, chemicals and feedstocks in the Midwest and Gulf Coast regions of the United States, 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 within its L&S segment.

Equity Method Investments (Note

Equity Method Investments (Notes)	9 Months Ended
	Sep. 30, 2017
Equity Method Investments and Joint Ventures [Abstract]
Equity Method Investments and Joint Ventures Disclosure [Text Block]	Investments and Noncontrolling Interests Summarized financial information for the Partnership’s equity method investments for the nine months ended September 30, 2017 and 2016 is as follows: Nine Months Ended September 30, 2017 (In millions) MarkWest Utica EMG Other VIEs Non-VIEs Total Revenues and other income $ 137 $ 49 $ 178 $ 364 Costs and expenses 72 29 115 216 Income from operations 65 20 63 148 Net income 65 19 28 112 Income from equity method investments (1) 6 7 16 29 Nine Months Ended September 30, 2016 (In millions) MarkWest Utica EMG Other VIEs (2) Non-VIEs Total Revenues and other income $ 165 $ 13 $ 108 $ 286 Costs and expenses 70 107 80 257 Income (loss) from operations 95 (94 ) 28 29 Net income (loss) 94 (94 ) 28 28 Income (loss) from equity method investments (1) 10 (88 ) 6 (72 ) (1) Income (loss) from equity method investments includes the impact of any basis differential amortization or accretion. (2) Includes an impairment charge of $89 million for the nine months ended September 30, 2016 related to the Partnership’s investment in Ohio Condensate Company, L.L.C., which does not appear separately in this table. Summarized balance sheet information for the Partnership’s equity method investments as of September 30, 2017 and December 31, 2016 is as follows: September 30, 2017 (In millions) MarkWest Utica EMG (1) Other VIEs Non-VIEs Total Current assets $ 72 $ 47 $ 379 $ 498 Noncurrent assets 2,092 878 4,614 7,584 Current liabilities 37 55 492 584 Noncurrent liabilities 2 12 562 576 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, L.L.C.’s (“MarkWest Utica EMG”) noncurrent assets include its investment in its subsidiary Ohio Gathering Company, L.L.C. (“Ohio Gathering”), which does not appear elsewhere in this table. The investment was $794 million as of September 30, 2017 and December 31, 2016 . As of September 30, 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 Energy Partners, L.P. (“MarkWest”), and EMG Utica, LLC (“EMG Utica” and together with Utica Operating, the “Members”) executed agreements to form a joint venture, MarkWest Utica EMG, to develop significant natural gas gathering, processing and NGL fractionation, transportation and marketing infrastructure in eastern Ohio. 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 . 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 September 30, 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 and $12 million for the three and nine months ended September 30, 2016 , respectively. Under the Amended LLC Agreement, after December 31, 2016, cash generated by MarkWest Utica EMG that is available for distribution will be allocated to the Members in proportion to their respective investment balances. As of September 30, 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 investment in MarkWest Utica EMG’s, which was $2.2 billion at September 30, 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 and nine months ended September 30, 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”). Operational Service revenue is reported as Other income-related parties in the Consolidated Statements of Income. The amount of Operational Service revenue related to MarkWest Utica EMG for the three and nine months ended September 30, 2017 , totaled $5 million and $13 million , respectively. The amount of Operational Service revenue related to MarkWest Utica EMG for the three and nine months ended September 30, 2016 , totaled approximately $5 million and $12 million , respectively. Ohio Gathering Ohio Gathering is a subsidiary of MarkWest Utica EMG and is engaged in providing natural gas gathering services in the Utica Shale in eastern Ohio. Ohio Gathering is a joint venture between MarkWest Utica EMG and Summit Midstream Partners, LLC. As of September 30, 2017 , the Partnership has an approximate 34 percent indirect ownership interest in Ohio Gathering. As Ohio Gathering is a subsidiary of MarkWest Utica EMG, which is accounted for as an equity method investment, the Partnership reports its portion of Ohio Gathering’s net assets as a component of its investment in MarkWest Utica EMG. The Partnership receives Operational Service revenue for operating Ohio Gathering which is reported as Other income-related parties in the Consolidated Statements of Income. The amount of Operational Service revenue related to Ohio Gathering for the three and nine months ended September 30, 2017 , totaled $4 million and $12 million , respectively. The amount of Operational Service revenue related to Ohio Gathering for the three and nine months ended September 30, 2016 , totaled approximately $5 million and $12 million , respectively. Sherwood Midstream Effective January 1, 2017, MarkWest Liberty Midstream & Resources, L.L.C. (“MarkWest Liberty Midstream”), a wholly-owned and consolidated subsidiary of MarkWest, and Antero Midstream Partners, LP (“Antero Midstream”) formed a joint venture, Sherwood Midstream LLC (“Sherwood Midstream”), to support Antero Resources Corporation’s development in the Marcellus Shale. 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 then under construction with a fair value of approximately $134 million and cash of approximately $20 million . Antero Midstream made an initial capital contribution of approximately $154 million . Also effective January 1, 2017, MarkWest Liberty Midstream converted all of its ownership interests in MarkWest Ohio Fractionation Company, L.L.C. (“Ohio Fractionation”), a previously wholly-owned subsidiary, to Class A Interests and amended its LLC Agreement to create Class B-3 Interests, which were sold to Sherwood Midstream for $126 million in cash. The Class B-3 Interests provide Sherwood Midstream with the right to fractionation revenue and the obligation to pay expenses related to 20 mbpd of capacity in the Hopedale 3 fractionator. Sherwood Midstream accounts for its investment in Ohio Fractionation, which is a VIE, as an equity method investment as Sherwood Midstream does not control Ohio Fractionation. MarkWest Liberty Midstream has been deemed to be the primary beneficiary of Ohio Fractionation because it has control over the decisions that could significantly impact its financial performance, and as a result, consolidates Ohio Fractionation. The carrying amounts of assets and liabilities included in the Partnership’s Consolidated Balance Sheets pertaining to Ohio Fractionation at September 30, 2017 , were current assets of $51 million , non-current assets of $406 million and current liabilities of $26 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 interests 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 will be allocated to the members in proportion to their respective investment balances. 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 investment in Sherwood Midstream, which was approximately $220 million at September 30, 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 nine months ended September 30, 2017 . The Partnership receives Operational Service revenue for operating Sherwood Midstream. The amount of Operational Service revenue related to Sherwood Midstream for the three and nine months ended September 30, 2017 , totaled approximately $2 million and $6 million , respectively, 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 LLC (“Sherwood Midstream Holdings”), for the purpose of owning, operating and maintaining all of the shared assets that support the operations of the gas plants and other assets owned by Sherwood Midstream and the gas plants and deethanization facilities owned by MarkWest Liberty Midstream. MarkWest Liberty Midstream initially contributed certain real property, equipment and facilities with a fair value of approximately $209 million to Sherwood Midstream Holdings in exchange for a 79 percent initial ownership interest. Sherwood Midstream contributed cash of approximately $44 million to Sherwood Midstream Holdings in exchange for a 21 percent ownership interest. During the second quarter ended June 30, 2017, true-ups to the initial contributions were finalized. MarkWest Liberty Midstream contributed certain additional real property, equipment and facilities with a fair value of approximately $10 million to Sherwood Midstream Holdings and Sherwood Midstream contributed cash of approximately $4 million to Sherwood Midstream Holdings. 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 $203 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 $14 million is included in its investment in Sherwood Midstream Holdings and is reported under the caption Equity method investments on the Consolidated Balance Sheets. In connection with the initial contributions, MarkWest Liberty Midstream received a special distribution of approximately $45 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 Assets, as defined in the 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 investment in Sherwood Midstream Holdings, which was approximately $163 million at September 30, 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 nine months ended September 30, 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 September 30, 2017 , the Partnership has a 14.7 percent indirect ownership interest in Sherwood Midstream Holdings through Sherwood Midstream.

Related Party Agreements and Tr

Related Party Agreements and Transactions	9 Months Ended
	Sep. 30, 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 September 30, 2017 . Centennial owns a products pipeline and storage facility. • Muskegon Pipeline LLC (“Muskegon”), in which MPC has a 60 percent interest as of September 30, 2017 . Muskegon owns a common carrier products pipeline. • MarkWest Utica EMG, in which MPLX LP has a 56 percent interest as of September 30, 2017 . MarkWest Utica EMG is engaged in natural gas processing and NGL fractionation, transportation and marketing in Ohio. • Ohio Gathering, in which MPLX LP has a 34 percent indirect interest as of September 30, 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 September 30, 2017 . Sherwood Midstream supports the development of Antero Resources Corporation’s Marcellus Shale acreage in the rich-gas corridor of West Virginia. • Sherwood Midstream Holdings, in which MPLX LP has an 86 percent total direct and indirect interest as of September 30, 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. • Illinois Extension, in which MPLX LP has a 35 percent interest as of September 30, 2017 . Illinois Extension operates the SAX crude oil pipeline from Flanagan, Illinois to Patoka, Illinois, as well as additional tankage and two pump stations. • LOOP, in which MPLX LP has a 40.7 percent interest as of September 30, 2017 . LOOP owns and operates midstream crude oil infrastructure, including a deep water oil port offshore of Louisiana, pipelines, and onshore storage facilities, and manages operations of LOCAP, an affiliate pipeline system. • LOCAP, in which MPLX LP has a 58.52 percent interest as of September 30, 2017 . LOCAP owns and operates a crude oil pipeline and tank facility in St. James, Louisiana, that distributes oil received from LOOP’s storage facilities and other connecting pipelines to nearby refineries and into the midcontinent region of the United States. • Explorer, in which MPLX LP has a 24.51 percent interest as of September 30, 2017 . Explorer owns and operates a common carrier pipeline that primarily transports gasoline, diesel, diluent and jet fuel from the Gulf Coast refining complex to the Midwestern United States. 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 nine months ended September 30, 2017 , the Partnership borrowed $829 million and repaid $627 million , resulting in $202 million outstanding balance at September 30, 2017 , which is included in Payables-related parties on the Consolidated Balance Sheets. Borrowings were at an average interest rate of 2.721 percent , per annum, for the nine months ended September 30, 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, 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. 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. 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 September 30, Nine Months Ended September 30, (In millions) 2017 2016 2017 2016 Service revenues MPC $ 276 $ 253 $ 801 $ 676 Rental income MPC $ 70 $ 68 $ 207 $ 172 Product sales (1) MPC $ 2 $ 2 $ 6 $ 8 (1) There were additional product sales to MPC that net to zero within the consolidated financial statements as the transactions are recorded net due to the terms of the agreements under which such product was sold. For the three and nine months ended September 30, 2017 , these sales totaled $63 million and $173 million , respectively. For the three and nine months ended September 30, 2016 , these sales totaled $13 million and $25 million , respectively. 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 September 30, Nine Months Ended September 30, (In millions) 2017 2016 2017 2016 MPC $ 9 $ 10 $ 30 $ 36 MarkWest Utica EMG 5 5 13 12 Ohio Gathering 4 5 12 12 Other 4 2 14 7 Total $ 22 $ 22 $ 69 $ 67 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 September 30, Nine Months Ended September 30, (In millions) 2017 2016 2017 2016 Purchases - related parties $ 17 $ 11 $ 50 $ 29 General and administrative expenses 9 11 28 33 Total $ 26 $ 22 $ 78 $ 62 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 September 30, Nine Months Ended September 30, (In millions) 2017 2016 2017 2016 MPC $ 11 $ 14 $ 33 $ 36 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 September 30, Nine Months Ended September 30, (In millions) 2017 2016 2017 2016 Purchases - related parties $ 97 $ 98 $ 280 $ 257 General and administrative expenses 25 27 74 75 Total $ 122 $ 125 $ 354 $ 332 Receivables from related parties, which for December 31, 2016 included reimbursements from the MarkWest Merger to be provided by MPC for the conversion of Class B units, were as follows: (In millions) September 30, 2017 December 31, 2016 MPC $ 144 $ 242 MarkWest Utica EMG 2 2 Ohio Gathering 2 2 Other 4 1 Total $ 152 $ 247 Long-term receivables with related parties, which includes straight-line rental income, were as follows: (In millions) September 30, 2017 December 31, 2016 MPC $ 18 $ 11 Payables to related parties were as follows: (In millions) September 30, 2017 December 31, 2016 MPC (1) $ 277 $ 63 MarkWest Utica EMG 30 24 Other 10 — Total $ 317 $ 87 (1) Balance includes approximately $202 million related to the loan with MPC Investment as discussed above. During the nine months ended September 30, 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 agreements. The Deferred revenue-related parties balance associated with the minimum volume deficiencies and project reimbursements were as follows: (In millions) September 30, 2017 December 31, 2016 Minimum volume deficiencies - MPC $ 55 $ 48 Project reimbursements - MPC 27 9 Total $ 82 $ 57

Net Income Per Limited Partner

Net Income Per Limited Partner Unit	9 Months Ended
	Sep. 30, 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 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 September 30, 2017 , the Partnership had dilutive potential common units consisting of certain equity-based compensation awards. For the three months ended September 30, 2016 and the nine months ended September 30, 2017 and 2016 , the Partnership had dilutive potential common units consisting of certain equity-based compensation awards and Class B units. Potential common units omitted from the diluted earnings per unit calculation for the three and nine months ended September 30, 2017 were less than one million and for the three and nine months ended September 30, 2016 were less than one million and approximately eight million , respectively. Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2017 2016 2017 2016 Net income attributable to MPLX LP $ 216 $ 141 $ 556 $ 100 Less: Limited partners’ distributions declared on Preferred units (1) 16 16 49 25 General partner’s distributions declared (including IDRs) (1) 88 54 229 148 Limited partners’ distributions declared on common units (1) 232 179 648 507 Undistributed net loss attributable to MPLX LP $ (120 ) $ (108 ) $ (370 ) $ (580 ) (1) See Note 7 for distribution information. Three Months Ended September 30, 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) $ 88 $ 232 $ 16 $ 336 Undistributed net loss attributable to MPLX LP (2 ) (118 ) — (120 ) Net income attributable to MPLX LP (1) $ 86 $ 114 $ 16 $ 216 Weighted average units outstanding: Basic 8 394 31 433 Diluted 8 395 31 434 Net income attributable to MPLX LP per limited partner unit: Basic $ 0.29 Diluted $ 0.29 Three Months Ended September 30, 2016 (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) $ 54 $ 179 $ 16 $ 249 Undistributed net loss attributable to MPLX LP (3 ) (105 ) — (108 ) Net income attributable to MPLX LP (1) $ 51 $ 74 $ 16 $ 141 Weighted average units outstanding: Basic 7 341 31 379 Diluted 7 346 31 384 Net income attributable to MPLX LP per limited partner unit: Basic $ 0.22 Diluted $ 0.21 Nine Months Ended September 30, 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) $ 229 $ 648 $ 49 $ 926 Undistributed net loss attributable to MPLX LP (7 ) (363 ) — (370 ) Net income attributable to MPLX LP (1) $ 222 $ 285 $ 49 $ 556 Weighted average units outstanding: Basic 8 378 31 417 Diluted 8 381 31 420 Net income attributable to MPLX LP per limited partner unit: Basic $ 0.75 Diluted $ 0.75 Nine Months Ended September 30, 2016 (In millions, except per unit data) General Partner Limited Partners’ Common Units Redeemable Preferred Units Total Basic and diluted net loss attributable to MPLX LP per unit: Net income (loss) attributable to MPLX LP: Distributions declared (including IDRs) $ 148 $ 507 $ 25 $ 680 Undistributed net loss attributable to MPLX LP (12 ) (568 ) — (580 ) Net income (loss) attributable to MPLX LP (1) $ 136 $ (61 ) $ 25 $ 100 Weighted average units outstanding: Basic 7 324 16 347 Diluted 7 324 16 347 Net loss attributable to MPLX LP per limited partner unit: Basic $ (0.19 ) Diluted $ (0.19 ) (1) Allocation of net income (loss) attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.

Equity

Equity	9 Months Ended
	Sep. 30, 2017
Equity [Abstract]
Equity	Equity The changes in the number of units outstanding during the nine months ended September 30, 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) 183,509 — 3,745 187,254 Issuance of units under the ATM Program (2) 13,846,998 — 282,591 14,129,589 Contribution of HST/WHC/MPLXT (3) 12,960,376 — 264,497 13,224,873 Contribution of the Joint-Interest Acquisition (3) 18,511,134 — 377,778 18,888,912 Class B conversion (4) 4,350,057 (3,990,878 ) 7,330 366,509 Balance at September 30, 2017 407,045,362 — 8,307,046 415,352,408 (1) As a result of the unit-based compensation awards issued during the period, MPLX GP contributed less than $1 million in exchange for 3,745 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 $10 million in exchange for 282,591 general partner units to maintain its two percent GP Interest. (3) See Note 3 for information regarding this acquisition. (4) On July 1, 2017 , 3,990,878 Class B units converted to 4,350,057 common units and were eligible to receive the second quarter 2017 distribution. As a result of the Class B conversion, MPLX GP contributed less than $1 million in exchange for 7,330 general partner units to maintain its two percent GP Interest. Reorganization Transactions – On September 1, 2016, the Partnership and various affiliates initiated a series of reorganization transactions in order to simplify the Partnership’s ownership structure and its financial and tax reporting requirements. In connection with these transactions, the issued and outstanding MPLX LP Class A units, all of which were held by MarkWest Hydrocarbon, were either distributed to or purchased by MPC in exchange for $84 million in cash, 21,401,137 MPLX LP common units and 436,758 MPLX LP general partner units. Following these initial transactions, all of the MPLX LP Class A units were exchanged on a one-for-one basis for newly issued common units representing limited partner interests in MPLX LP. MPC also contributed $141 million to facilitate the repayment of intercompany debt between MarkWest Hydrocarbon and MarkWest. As a result of these transactions, the MPLX LP Class A units were eliminated, are no longer outstanding and no longer participate in distributions of cash from the Partnership. Cash that is derived from or attributable to MarkWest Hydrocarbon’s operations is now treated in the same manner as cash derived from or attributable to other operations of the Partnership and its subsidiaries. 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 IDRs 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 September 30, Nine Months Ended September 30, (In millions) 2017 2016 2017 2016 Net income attributable to MPLX LP $ 216 $ 141 $ 556 $ 100 Less: Preferred unit distributions 16 16 49 25 General partner's IDRs and other 83 49 216 137 Net income (loss) attributable to MPLX LP available to general and limited partners $ 117 $ 76 $ 291 $ (62 ) General partner's two percent GP Interest in net income (loss) attributable to MPLX LP $ 3 $ 2 $ 6 $ (1 ) General partner's IDRs and other 83 49 216 137 General partner's GP Interest in net income attributable to MPLX LP $ 86 $ 51 $ 222 $ 136 Cash distributions – The Partnership Agreement sets forth the calculation to be used to determine the amount and priority of cash distributions that the common unitholders, Preferred unitholders and general partner will receive. In accordance with the Partnership Agreement, on October 25, 2017 , the Partnership declared a quarterly cash distribution, based on the results of the third quarter of 2017 , totaling $320 million , or $0.5875 per common unit. These distributions will be paid on November 14, 2017 to common unitholders of record on November 6, 2017 . The allocation of total quarterly cash distributions to general, limited and Preferred unitholders is as follows for the three and nine months ended September 30, 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 September 30, Nine Months Ended September 30, (In millions) 2017 2016 2017 2016 General partner's distributions: General partner's distributions on general partner units $ 7 $ 5 $ 18 $ 13 General partner's distributions on IDRs 81 49 211 135 Total distribution on general partner units and IDRs $ 88 $ 54 $ 229 $ 148 Common and preferred unit distributions: Common unitholders, includes common units of general partner $ 232 $ 179 $ 648 $ 507 Preferred unit distributions 16 16 49 25 Total cash distributions declared $ 336 $ 249 $ 926 $ 680

Redeemable Preferred Units (Not

Redeemable Preferred Units (Notes)	9 Months Ended
	Sep. 30, 2017
Temporary Equity Disclosure [Abstract]
Temporary Equity Disclosure [Text Block]	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 holders of MPLX LP common units. The changes in the redeemable preferred balance from December 31, 2016 through September 30, 2017 are summarized below: (In millions) Redeemable Preferred Units Balance at December 31, 2016 $ 1,000 Net income 49 Distributions received by Preferred unitholders (49 ) Balance at September 30, 2017 $ 1,000 The holders may convert their Preferred units into common units at any time after the third anniversary of the issuance date or prior to liquidation, dissolution or winding up of the Partnership, in full or in part, subject to minimum conversion amounts and conditions. After the fourth anniversary of the issuance date, the Partnership may convert the Preferred units into common units at any time, in whole or in part, subject to certain minimum conversion amounts and conditions, if the closing price of MPLX LP common units is greater than $48.75 for the 20 day trading period immediately preceding the conversion notice date. The conversion rate for the Preferred units shall be the quotient of (a) the sum of (i) $32.50, plus (ii) any unpaid cash distributions on the applicable Preferred unit, divided by (b) $32.50. The holders of the Preferred units are entitled to vote on an as-converted basis with the common unitholders and will have certain other class voting rights with respect to any amendment to the Partnership Agreement that would adversely affect any rights, preferences or privileges of the Preferred units. In addition, upon certain events involving a change of control the holders of Preferred units may elect, among other potential elections, to convert their Preferred units to common units at the then-change of control conversion rate. The Preferred units are considered redeemable securities under GAAP due to the existence of redemption provisions upon a deemed liquidation event which is outside the Partnership’s control. Therefore, they are presented as temporary equity in the mezzanine section of the Consolidated Balance Sheets. The Preferred units have been recorded at their issuance date fair value, net of issuance costs. Income allocations increase the carrying value, and declared distributions decreased the carrying value of the Preferred units. As the Preferred units are not currently redeemable and not probable of becoming redeemable, adjustment to the initial carrying amount is not necessary and would only be required if it becomes probable that the Preferred units would become redeemable.

Segment Information (Notes)

Segment Information (Notes)	9 Months Ended
	Sep. 30, 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 are 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 retrospective adjustments in connection with the acquisition of HST, WHC and MPLXT. Segment information is not included for periods prior to the Joint-Interest Acquisition and the Ozark pipeline acquisition. 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), goodwill impairment, certain management fees and depreciation and amortization are not allocated to the reportable segments. Management does not consider these items allocable to or controllable by any individual segment and, therefore, excludes these items when evaluating segment performance. Segment results are also adjusted to exclude the portion of income from operations attributable to the noncontrolling interests related to partially-owned entities that are either consolidated or accounted for as equity method investments. Segment operating income attributable to MPLX LP excludes the operating income related to Predecessors of the 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 September 30, 2017 (In millions) L&S G&P Total Revenues and other income: Segment revenues $ 378 $ 669 $ 1,047 Segment other income 11 1 12 Total segment revenues and other income 389 670 1,059 Costs and expenses: Segment cost of revenues 176 276 452 Segment operating income before portion attributable to noncontrolling interests and Predecessor 213 394 607 Segment portion attributable to noncontrolling interests and Predecessor — 45 45 Segment operating income attributable to MPLX LP $ 213 $ 349 $ 562 Three Months Ended September 30, 2016 (In millions) L&S G&P Total Revenues and other income: Segment revenues $ 339 $ 567 $ 906 Segment other income 12 1 13 Total segment revenues and other income 351 568 919 Costs and expenses: Segment cost of revenues 153 239 392 Segment operating income before portion attributable to noncontrolling interests and Predecessor 198 329 527 Segment portion attributable to noncontrolling interests and Predecessor 74 36 110 Segment operating income attributable to MPLX LP $ 124 $ 293 $ 417 Nine Months Ended September 30, 2017 (In millions) L&S G&P Total Revenues and other income: Segment revenues $ 1,095 $ 1,869 $ 2,964 Segment other income 35 2 37 Total segment revenues and other income 1,130 1,871 3,001 Costs and expenses: Segment cost of revenues 500 781 1,281 Segment operating income before portion attributable to noncontrolling interests and Predecessor 630 1,090 1,720 Segment portion attributable to noncontrolling interests and Predecessor 53 119 172 Segment operating income attributable to MPLX LP $ 577 $ 971 $ 1,548 Nine Months Ended September 30, 2016 (In millions) L&S G&P Total Revenues and other income: Segment revenues $ 901 $ 1,595 $ 2,496 Segment other income 42 1 43 Total segment revenues and other income 943 1,596 2,539 Costs and expenses: Segment cost of revenues 392 662 1,054 Segment operating income before portion attributable to noncontrolling interests and Predecessor 551 934 1,485 Segment portion attributable to noncontrolling interests and Predecessor 216 113 329 Segment operating income attributable to MPLX LP $ 335 $ 821 $ 1,156 Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2017 2016 2017 2016 Reconciliation to Income from operations: L&S segment operating income attributable to MPLX LP $ 213 $ 124 $ 577 $ 335 G&P segment operating income attributable to MPLX LP 349 293 971 821 Segment operating income attributable to MPLX LP 562 417 1,548 1,156 Segment portion attributable to unconsolidated affiliates (47 ) (41 ) (125 ) (130 ) Segment portion attributable to Predecessor — 74 53 216 Income (loss) from equity method investments 23 6 29 (72 ) Other income - related parties 13 11 38 29 Unrealized derivative (losses) gains (1) (17 ) (2 ) 2 (23 ) Depreciation and amortization (164 ) (151 ) (515 ) (438 ) Impairment expense — — — (130 ) General and administrative expenses (59 ) (56 ) (174 ) (172 ) Income from operations $ 311 $ 258 $ 856 $ 436 Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2017 2016 2017 2016 Reconciliation to Total revenues and other income: Total segment revenues and other income $ 1,059 $ 919 $ 3,001 $ 2,539 Revenue adjustment from unconsolidated affiliates (107 ) (100 ) (287 ) (303 ) Income (loss) from equity method investments 23 6 29 (72 ) Other income - related parties 13 11 38 29 Unrealized derivative (losses) gains related to product sales (1) (8 ) 2 1 (12 ) Total revenues and other income $ 980 $ 838 $ 2,782 $ 2,181 (1) The Partnership makes a distinction between realized or unrealized gains and losses on derivatives. During the period when a derivative contract is outstanding, changes in the fair value of the derivative are recorded as an unrealized gain or loss. When a derivative contract matures or is settled, the previously recorded unrealized gain or loss is reversed and the realized gain or loss of the contract is recorded. Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2017 2016 2017 2016 Reconciliation to Net income attributable to noncontrolling interests and Predecessor: Segment portion attributable to noncontrolling interests and Predecessor $ 45 $ 110 $ 172 $ 329 Portion of noncontrolling interests and Predecessor related to items below segment income from operations (21 ) (39 ) (84 ) (157 ) Portion of operating income attributable to noncontrolling interests of unconsolidated affiliates (23 ) (18 ) (49 ) (20 ) Net income attributable to noncontrolling interests and Predecessor $ 1 $ 53 $ 39 $ 152 The following table reconciles segment capital expenditures to total capital expenditures: Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2017 2016 2017 2016 L&S segment capital expenditures $ 120 $ 188 $ 353 $ 369 G&P segment capital expenditures 333 183 957 668 Total segment capital expenditures 453 371 1,310 1,037 Less: Capital expenditures for Partnership-operated, non-wholly-owned subsidiaries in G&P segment 101 34 306 94 Total capital expenditures $ 352 $ 337 $ 1,004 $ 943 Total assets by reportable segment were: (In millions) September 30, 2017 December 31, 2016 Cash and cash equivalents $ 3 $ 234 L&S 4,520 2,978 G&P 14,715 14,297 Total assets $ 19,238 $ 17,509

Inventories (Notes)

Inventories (Notes)	9 Months Ended
	Sep. 30, 2017
Inventory Disclosure [Abstract]
Inventory Disclosure [Text Block]	Inventories Inventories consist of the following: (In millions) September 30, 2017 December 31, 2016 NGLs $ 3 $ 2 Line fill 9 9 Spare parts, materials and supplies 52 44 Total inventories $ 64 $ 55

Property, Plant and Equipment

Property, Plant and Equipment	9 Months Ended
	Sep. 30, 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) September 30, 2017 December 31, 2016 Natural gas gathering and NGL transportation pipelines and facilities $ 5,101 $ 4,748 Processing, fractionation and storage facilities (1) 3,753 3,547 Pipelines and related assets 2,181 1,799 Barges and towing vessels 484 479 Terminals and related assets (1) 794 759 Land, building, office equipment and other 755 757 Construction-in-progress 986 1,013 Total 14,054 13,102 Less accumulated depreciation 2,132 1,694 Property, plant and equipment, net $ 11,922 $ 11,408 (1) Certain prior period amounts have been updated to conform to current period presentation.

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 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: September 30, 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 — (5 ) — (6 ) Embedded derivatives in commodity contracts — (52 ) — (54 ) Total carrying value in Consolidated Balance Sheets $ — $ (57 ) $ — $ (60 ) The following table provides additional information about the significant unobservable inputs used in the valuation of Level 3 instruments as of September 30, 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 Liabilities Forward ethane prices (per Gal) (1) $0.27 - $0.28 Oct. 17 - Dec. 17 Forward propane prices (per Gal) (1) $0.68 - $0.91 Oct. 17 - Dec. 18 Forward isobutane prices (per Gal) (1) $0.82 - $1.06 Oct. 17 - Dec. 18 Forward normal butane prices (per Gal) (1) $0.76 - $1.03 Oct. 17 - Dec. 18 Forward natural gasoline prices (per Gal) (1) $1.18 - $1.22 Oct. 17 - Dec. 18 Embedded derivatives in commodity contracts Assets ERCOT Pricing (per MegaWatt Hour) $24.19 - $26.05 Oct. 17 - Dec. 17 Liabilities Forward propane prices (per Gal) (1) $0.61 - $0.91 Oct. 17 - Dec. 22 Forward isobutane prices (per Gal) (1) $0.75 - $1.06 Oct. 17 - Dec. 22 Forward normal butane prices (per Gal) (1) $0.69 - $1.03 Oct. 17 - Dec. 22 Forward natural gasoline prices (per Gal) (1) $1.15 - $1.22 Oct. 17 - Dec. 22 Forward natural gas prices (per MMBtu) (2) $2.30 - $3.11 Oct. 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 decrease over time. (2) Natural gas prices used in the valuations 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 two embedded derivatives in commodity contracts, as follows: • A single 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. • An embedded derivative related to utilities costs discussed further in Note 13. Increases in the forward Electric Reliability Council of Texas (“ERCOT”) prices result in a decrease in the fair value of the 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 are 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 (October 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 and nine months ended September 30, 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 September 30, 2017 Nine Months Ended September 30, 2017 (In millions) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Fair value at beginning of period $ 2 $ (43 ) $ (6 ) $ (54 ) Total losses (realized and unrealized) included in earnings (1) (10 ) (12 ) (3 ) (4 ) Settlements 3 3 4 6 Fair value at end of period $ (5 ) $ (52 ) $ (5 ) $ (52 ) The amount of total losses for the period included in earnings attributable to the change in unrealized losses relating to liabilities still held at end of period $ (7 ) $ (10 ) $ (4 ) $ (4 ) Three Months Ended September 30, 2016 Nine Months Ended September 30, 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 $ (4 ) $ (40 ) $ 7 $ (32 ) Total gains (losses) (realized and unrealized) included in earnings (1) 2 (6 ) (5 ) (17 ) Settlements (1 ) 2 (6 ) 5 Netting adjustment (2) — — 1 — Fair value at end of period $ (3 ) $ (44 ) $ (3 ) $ (44 ) The amount of total losses for the period included in earnings attributable to the change in unrealized losses relating to liabilities still held at end of period $ — $ (4 ) $ (4 ) $ (15 ) (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 and Cost of revenues . (2) Certain derivative positions are subject to master netting agreements; therefore, the Partnership has elected to offset derivative assets and liabilities where legally permissible. The Partnership may hold positions with certain counterparties, which for GAAP purposes are classified within different levels of the fair value hierarchy and may be legally permissible to offset. This adjustment represents the total impact of offsetting Level 2 positions with Level 3 positions as of September 30, 2016 . 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: September 30, 2017 December 31, 2016 (In millions) Fair Value Carrying Value Fair Value Carrying Value Long-term debt $ 7,619 $ 6,869 $ 4,953 $ 4,422 SMR liability 106 92 108 96

Derivative Financial Instrument

Derivative Financial Instruments (Notes)	9 Months Ended
	Sep. 30, 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. A portion of 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 all unrealized gains and losses from the changes in fair value of derivatives in current earnings. The Partnership makes a distinction between realized or unrealized gains and losses on derivatives. During the period when a derivative contract is outstanding, changes in the fair value of the derivative are recorded as an unrealized gain or loss. When a derivative contract matures or is settled, the previously recorded unrealized gain or loss is reversed and the realized gain or loss of the contract is recorded. Volume of Commodity Derivative Activity As of September 30, 2017 , the Partnership had the following outstanding commodity contracts that were executed to manage the cash flow risk associated with future sales of NGLs and purchases of natural gas: Derivative contracts not designated as hedging instruments Financial Position Notional Quantity (net) Crude Oil (bbl) Short 18,400 Natural Gas (MMBtu) Long 1,096,539 NGLs (gal) Short 33,387,904 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 September 30, 2017 and December 31, 2016 , the estimated fair value of this contract was a liability of $52 million and $54 million , respectively. The Partnership has a commodity contract that gives it an option to fix a component of the utilities cost to an index price on electricity at a plant location in the Southwest through the fourth quarter of 2018. The contract’s pricing is currently fixed through the fourth quarter of 2017 with the ability to fix the pricing for its remaining year. Changes in the fair value as of the derivative component of this contract were recognized as Cost of Revenues in the Consolidated Statements of Income. As of September 30, 2017 , the estimated fair value of this contract was a liability of less than $1 million . 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 , as updated by our Current Report on Form 8-K filed on May 1, 2017. The impact of the Partnership’s derivative instruments on its Consolidated Balance Sheets is summarized below: (In millions) September 30, 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 $ — $ (15 ) $ — $ (13 ) Other noncurrent assets / deferred credits and other liabilities — (42 ) — (47 ) Total $ — $ (57 ) $ — $ (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: September 30, 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 $ — $ — $ — $ (5 ) $ — $ (5 ) Embedded derivatives in commodity contracts — — — (10 ) — (10 ) Total current derivative instruments — — — (15 ) — (15 ) Non-current Commodity contracts — — — — — — Embedded derivatives in commodity contracts — — — (42 ) — (42 ) Total non-current derivative instruments — — — (42 ) — (42 ) Total derivative instruments $ — $ — $ — $ (57 ) $ — $ (57 ) 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 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 September 30, Nine Months Ended September 30, (In millions) 2017 2016 2017 2016 Product sales Realized (loss) gain $ (2 ) $ — $ (3 ) $ 6 Unrealized (loss) gain (8 ) 2 1 (12 ) Total derivative (loss) gain related to product sales (10 ) 2 (2 ) (6 ) Purchased product costs Realized loss (1) (2 ) (1 ) (6 ) (4 ) Unrealized (loss) gain (9 ) (3 ) 1 (12 ) Total derivative loss related to purchased product costs (11 ) (4 ) (5 ) (16 ) Cost of revenues Realized loss (1) — — — (2 ) Unrealized (loss) gain — (1 ) — 1 Total derivative loss related to cost of revenues — (1 ) — (1 ) Total derivative losses $ (21 ) $ (3 ) $ (7 ) $ (23 ) (1) Certain prior period amounts have been updated to conform to current period presentation.

Debt

Debt	9 Months Ended
	Sep. 30, 2017
Debt Disclosure [Abstract]
Debt	Debt The Partnership’s outstanding borrowings consisted of the following: (In millions) September 30, 2017 December 31, 2016 MPLX LP: Bank revolving credit facility due 2022 $ 420 $ — Term loan facility due 2019 — 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 7 8 Total 7,277 4,858 Unamortized debt issuance costs (27 ) (7 ) Unamortized discount (401 ) (428 ) Amounts due within one year (1 ) (1 ) Total long-term debt due after one year $ 6,848 $ 4,422 Credit Agreements On July 21, 2017, the Partnership entered into a syndicated credit agreement to replace its previously outstanding $2.0 billion five -year bank revolving credit facility with a $2.25 billion five -year bank revolving credit facility that expires in July 2022 (the “MPLX Credit Agreement 2022”). The financial covenants and the interest rate terms contained in the new credit agreement are substantially the same as those contained in the previous bank revolving credit facility. During the nine months ended September 30, 2017 , the Partnership had no borrowings under the previous bank revolving credit facility. During the nine months ended September 30, 2017 , the Partnership borrowed $420 million under the MPLX Credit Agreement 2022, at an average interest rate of 2.702 percent . At September 30, 2017 , the Partnership had $420 million outstanding borrowings and $3 million letters of credit outstanding under the new facility, resulting in total availability of $1.827 billion , or 81.2 percent of the borrowing capacity . The $250 million term loan facility was drawn in full on November 20, 2014. On July 19, 2017, MPLX LP prepaid the entire outstanding principal of this loan facility with cash on hand. The borrowings under this facility between January 1, 2017 and July 19, 2017 were at an average interest rate of 2.407 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	9 Months Ended
	Sep. 30, 2017
Supplemental Cash Flow Elements [Abstract]
Supplemental Cash Flow Information [Text Block]	Supplemental Cash Flow Information Nine Months Ended September 30, (In millions) 2017 2016 Net cash provided by operating activities included: Interest paid (net of amounts capitalized) $ 207 $ 158 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) 337 — (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: Nine Months Ended September 30, (In millions) 2017 2016 Increase in capital accruals $ 55 $ —

Equity-Based Compensation Plan

Equity-Based Compensation Plan	9 Months Ended
	Sep. 30, 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 nine months ended September 30, 2017 : Number Weighted Outstanding at December 31, 2016 1,173,411 $ 33.09 Granted 653,721 36.36 Settled (288,584 ) 33.50 Forfeited (113,107 ) 34.59 Outstanding at September 30, 2017 1,425,441 34.39 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 DCF 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 nine months ended September 30, 2017 : Number of Outstanding at December 31, 2016 1,799,249 Granted 1,407,062 Settled (464,500 ) Forfeited (35,217 ) Outstanding at September 30, 2017 2,706,594

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 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 September 30, 2017 and December 31, 2016 , accrued liabilities for remediation totaled $14 million and $3 million , respectively. However, it is not presently possible to estimate the ultimate amount of all remediation costs that might be incurred or the penalties, if any, which may be imposed. At December 31, 2016 , there was less than $1 million in receivables from MPC for indemnification of environmental costs related to incidents occurring prior to the Initial Offering. There were no such receivables at September 30, 2017 . 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. In connection with these negotiations, MarkWest Liberty Midstream has received a revised settlement proposal from the EPA which proposes to lower the proposed cash penalty to approximately $1.24 million and the estimated cost of proposed supplemental environmental projects to an estimated cost of approximately $1.6 million . 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 Marathon Pipe Line LLC (“MPL”). These complaints, which have been amended since filing, assert claims of common law nuisance and contribution under the Illinois Contribution Act and other laws for environmental cleanup costs that may be imposed on Premcor and Apex by the State of Illinois. On September 6, 2016, the trial court approved a settlement between Apex and the State of Illinois whereby Apex agreed to settle all claims against it for a $10 million payment. Premcor has objected to this ruling and is seeking an appeal. There are several third-party defendants in the litigation and MPL has asserted cross-claims in contribution against the various third-party defendants. This litigation is currently pending in the Third Judicial Circuit Court, Madison County, Illinois. 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 losses) 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 September 30, 2017 , the Partnership’s contractual commitments to acquire property, plant and equipment totaled $520 million . These commitments at September 30, 2017 were primarily related to plant expansion projects for the Marcellus and Southwest Operations. In addition, from time to time and in the ordinary course of business, the Partnership and its affiliates provide guarantees of the Partnership’s subsidiaries payment and performance obligations in the G&P segment. Certain natural gas processing and gathering arrangements require the Partnership to construct new natural gas processing plants, natural gas gathering pipelines and NGL pipelines and contain certain fees and charges if specified construction milestones are not achieved for reasons other than force majeure. In certain cases, certain producers may have the right to cancel the processing arrangements if there are significant delays that are not due to force majeure. As of September 30, 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)	9 Months Ended
	Sep. 30, 2017
Accounting Policies [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 until declared. However, when distributions related to the IDRs 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 IDRs.
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.

Net Income Per Limited Partne26

Net Income Per Limited Partner Unit Earnings Per Share, Policy (Policies)	9 Months Ended
	Sep. 30, 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 until declared. However, when distributions related to the IDRs 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 IDRs.

Acquisitions (Tables)

Acquisitions (Tables)	9 Months Ended
	Sep. 30, 2017
Business Acquisition [Line Items]
Schedule of Business Acquisitions, by Acquisition [Table Text Block]	The following tables present the Partnership’s previously reported unaudited Consolidated Statements of Income for the three and nine months ended September 30, 2016 , retrospectively adjusted for the acquisition of HST, WHC and MPLXT: Three Months Ended September 30, 2016 (In millions, except per unit data) MPLX LP (Previously Reported) HST/WHC MPLXT Eliminations (1) MPLX LP (Currently Reported) Revenues and other income: Service revenue $ 250 $ — $ — $ — $ 250 Service revenue - related parties 153 28 72 — 253 Rental income 77 — — — 77 Rental income - related parties 29 13 26 — 68 Product sales 157 — — — 157 Product sales - related parties 2 — — — 2 Income from equity method investments 6 — — — 6 Gain on sale of assets 1 — — — 1 Other income 2 — — — 2 Other income - related parties 26 — — (4 ) 22 Total revenues and other income 703 41 98 (4 ) 838 Costs and expenses: Cost of revenues (excludes items below) 90 10 22 — 122 Purchased product costs 117 — — — 117 Rental cost of sales 11 2 — — 13 Rental cost of sales - related parties — 1 — (1 ) — Purchases - related parties 84 4 24 (3 ) 109 Depreciation and amortization 138 4 9 — 151 General and administrative expenses 46 2 8 — 56 Other taxes 10 — 2 — 12 Total costs and expenses 496 23 65 (4 ) 580 Income from operations 207 18 33 — 258 Interest expense (net of amounts capitalized) 51 — — — 51 Other financial costs 13 — — — 13 Income before income taxes 143 18 33 — 194 Net income 143 18 33 — 194 Less: Net income attributable to noncontrolling interests 2 — — — 2 Less: Net income attributable to Predecessor — 18 33 — 51 Net income attributable to MPLX LP 141 — — — 141 Less: Preferred unit distributions 16 — — — 16 Less: General partner’s interest in net income attributable to MPLX LP 51 — — — 51 Limited partners’ interest in net income attributable to MPLX LP $ 74 $ — $ — $ — $ 74 (1) Represents intercompany transactions eliminated during the consolidation process, in accordance with GAAP. Nine Months Ended September 30, 2016 (In millions, except per unit data) MPLX LP (Previously Reported) HST/WHC MPLXT Eliminations (1) MPLX LP (Currently Reported) Revenues and other income: Service revenue $ 712 $ — $ — $ — $ 712 Service revenue - related parties 448 82 146 — 676 Rental income 218 — — — 218 Rental income - related parties 84 36 52 — 172 Product sales 394 — — — 394 Product sales - related parties 8 — — — 8 Loss from equity method investments (72 ) — — — (72 ) Gain on sale of assets 1 — — — 1 Other income 5 — — — 5 Other income - related parties 78 — — (11 ) 67 Total revenues and other income 1,876 118 198 (11 ) 2,181 Costs and expenses: Cost of revenues (excludes items below) 263 24 42 — 329 Purchased product costs 310 — — — 310 Rental cost of sales 39 3 — — 42 Rental cost of sales - related parties — 2 — (1 ) 1 Purchases - related parties 238 13 45 (10 ) 286 Depreciation and amortization 407 12 19 — 438 Impairment expense 130 — — — 130 General and administrative expenses 147 5 20 — 172 Other taxes 32 2 3 — 37 Total costs and expenses 1,566 61 129 (11 ) 1,745 Income from operations 310 57 69 — 436 Related party interest and other financial income 1 — — — 1 Interest expense (net of amounts capitalized) 158 — — — 158 Other financial costs 37 — — — 37 Income before income taxes 114 57 69 — 240 Benefit for income taxes (12 ) — — — (12 ) Net income 126 57 69 — 252 Less: Net income attributable to noncontrolling interests 3 — — — 3 Less: Net income attributable to Predecessor 23 57 69 — 149 Net income attributable to MPLX LP 100 — — — 100 Less: Preferred unit distributions 25 — — — 25 Less: General partner’s interest in net income attributable to MPLX LP 136 — — — 136 Limited partners’ interest in net loss attributable to MPLX LP $ (61 ) $ — $ — $ — $ (61 ) (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, WHC and MPLXT: Nine Months Ended September 30, 2016 (In millions) MPLX LP (Previously Reported) HST/WHC MPLXT MPLX LP (Currently Reported) Increase (decrease) in cash and cash equivalents Operating activities: Net income $ 126 $ 57 $ 69 $ 252 Adjustments to reconcile net income to net cash provided by (used in) operating activities: Amortization of deferred financing costs 34 — — 34 Depreciation and amortization 407 12 19 438 Impairment expense 130 — — 130 Deferred income taxes (16 ) — — (16 ) Asset retirement expenditures (3 ) (1 ) — (4 ) Gain on disposal of assets (1 ) — — (1 ) Loss from equity method investments 72 — — 72 Distributions from unconsolidated affiliates 111 — — 111 Changes in: Current receivables (44 ) 1 — (43 ) Inventories (4 ) — — (4 ) Fair value of derivatives 28 — — 28 Current accounts payable and accrued liabilities 59 (1 ) 6 64 Receivables from / liabilities to related parties 15 3 (122 ) (104 ) All other, net 18 2 (2 ) 18 Net cash provided by (used in) operating activities 932 73 (30 ) 975 Investing activities: Additions to property, plant and equipment (874 ) (36 ) (33 ) (943 ) Investments - net related party loans 77 (37 ) 63 103 Investments in unconsolidated affiliates (56 ) — — (56 ) All other, net 4 — — 4 Net cash (used in) provided by investing activities (849 ) (73 ) 30 (892 ) Financing activities: Long-term debt - borrowings 434 — — 434 - repayments (1,312 ) — — (1,312 ) Related party debt - borrowings 2,215 — — 2,215 - repayments (2,223 ) — — (2,223 ) Net proceeds from equity offerings 510 — — 510 Issuance of redeemable preferred units 984 — — 984 Distributions to preferred unitholders (9 ) — — (9 ) Distributions to unitholders and general partner (612 ) — — (612 ) Distributions to noncontrolling interests (3 ) — — (3 ) Contributions from noncontrolling interests 4 — — 4 Consideration payment to Class B unitholders (25 ) — — (25 ) All other, net (2 ) — — (2 ) Contribution from MPC 225 — — 225 Distributions to MPC from Predecessor (104 ) — — (104 ) Net cash provided by financing activities 82 — — 82 Net increase in cash and cash equivalents 165 — — 165 Cash and cash equivalents at beginning of period 43 — — 43 Cash and cash equivalents at end of period $ 208 $ — $ — $ 208
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, since the March 1, 2017 acquisition date, are as follows: (In millions) Three Months Ended September 30, 2017 Seven Months Ended September 30, 2017 Revenues and other income $ 19 $ 45 Income from operations 6 17

Equity Method Investments (Tabl

Equity Method Investments (Tables)	9 Months Ended
	Sep. 30, 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 nine months ended September 30, 2017 and 2016 is as follows: Nine Months Ended September 30, 2017 (In millions) MarkWest Utica EMG Other VIEs Non-VIEs Total Revenues and other income $ 137 $ 49 $ 178 $ 364 Costs and expenses 72 29 115 216 Income from operations 65 20 63 148 Net income 65 19 28 112 Income from equity method investments (1) 6 7 16 29 Nine Months Ended September 30, 2016 (In millions) MarkWest Utica EMG Other VIEs (2) Non-VIEs Total Revenues and other income $ 165 $ 13 $ 108 $ 286 Costs and expenses 70 107 80 257 Income (loss) from operations 95 (94 ) 28 29 Net income (loss) 94 (94 ) 28 28 Income (loss) from equity method investments (1) 10 (88 ) 6 (72 ) (1) Income (loss) from equity method investments includes the impact of any basis differential amortization or accretion. (2) Includes an impairment charge of $89 million for the nine months ended September 30, 2016 related to the Partnership’s investment in Ohio Condensate Company, L.L.C., which does not appear separately in this table. Summarized balance sheet information for the Partnership’s equity method investments as of September 30, 2017 and December 31, 2016 is as follows: September 30, 2017 (In millions) MarkWest Utica EMG (1) Other VIEs Non-VIEs Total Current assets $ 72 $ 47 $ 379 $ 498 Noncurrent assets 2,092 878 4,614 7,584 Current liabilities 37 55 492 584 Noncurrent liabilities 2 12 562 576 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, L.L.C.’s (“MarkWest Utica EMG”) noncurrent assets include its investment in its subsidiary Ohio Gathering Company, L.L.C. (“Ohio Gathering”), which does not appear elsewhere in this table. The investment was $794 million as of September 30, 2017 and December 31, 2016 .

Related Party Agreements and 29

Related Party Agreements and Transactions (Tables)	9 Months Ended
	Sep. 30, 2017
Related Party Transactions [Abstract]
Schedule of Sales to Related Parties	Sales to related parties were as follows: Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2017 2016 2017 2016 Service revenues MPC $ 276 $ 253 $ 801 $ 676 Rental income MPC $ 70 $ 68 $ 207 $ 172 Product sales (1) MPC $ 2 $ 2 $ 6 $ 8 (1) There were additional product sales to MPC that net to zero within the consolidated financial statements as the transactions are recorded net due to the terms of the agreements under which such product was sold. For the three and nine months ended September 30, 2017 , these sales totaled $63 million and $173 million , respectively. For the three and nine months ended September 30, 2016 , these sales totaled $13 million and $25 million , respectively.
Summary of Fees Received from 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 September 30, Nine Months Ended September 30, (In millions) 2017 2016 2017 2016 MPC $ 9 $ 10 $ 30 $ 36 MarkWest Utica EMG 5 5 13 12 Ohio Gathering 4 5 12 12 Other 4 2 14 7 Total $ 22 $ 22 $ 69 $ 67
Summary of Charges for Services from Related Parties	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 September 30, Nine Months Ended September 30, (In millions) 2017 2016 2017 2016 Purchases - related parties $ 17 $ 11 $ 50 $ 29 General and administrative expenses 9 11 28 33 Total $ 26 $ 22 $ 78 $ 62
Summary of Related Party Costs Added to Property, Plant and Equipment	These costs added to Property, plant and equipment were as follows: Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2017 2016 2017 2016 MPC $ 11 $ 14 $ 33 $ 36
Schedule of Employee Services Expenses from Related Parties	Employee services expenses from related parties were as follows: Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2017 2016 2017 2016 Purchases - related parties $ 97 $ 98 $ 280 $ 257 General and administrative expenses 25 27 74 75 Total $ 122 $ 125 $ 354 $ 332
Schedule of Receivables from Related Parties	Receivables from related parties, which for December 31, 2016 included reimbursements from the MarkWest Merger to be provided by MPC for the conversion of Class B units, were as follows: (In millions) September 30, 2017 December 31, 2016 MPC $ 144 $ 242 MarkWest Utica EMG 2 2 Ohio Gathering 2 2 Other 4 1 Total $ 152 $ 247
Schedule of Long Term Receivables with Related Parties	Long-term receivables with related parties, which includes straight-line rental income, were as follows: (In millions) September 30, 2017 December 31, 2016 MPC $ 18 $ 11
Schedule of Payables to Related Parties	Payables to related parties were as follows: (In millions) September 30, 2017 December 31, 2016 MPC (1) $ 277 $ 63 MarkWest Utica EMG 30 24 Other 10 — Total $ 317 $ 87 (1) Balance includes approximately $202 million related to the loan with MPC Investment as discussed above.
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) September 30, 2017 December 31, 2016 Minimum volume deficiencies - MPC $ 55 $ 48 Project reimbursements - MPC 27 9 Total $ 82 $ 57

Net Income Per Limited Partne30

Net Income Per Limited Partner Unit (Tables)	9 Months Ended
	Sep. 30, 2017
Earnings Per Share [Abstract]
Schedule of Distributions By Partner By Class	For the three months ended September 30, 2017 , the Partnership had dilutive potential common units consisting of certain equity-based compensation awards. For the three months ended September 30, 2016 and the nine months ended September 30, 2017 and 2016 , the Partnership had dilutive potential common units consisting of certain equity-based compensation awards and Class B units. Potential common units omitted from the diluted earnings per unit calculation for the three and nine months ended September 30, 2017 were less than one million and for the three and nine months ended September 30, 2016 were less than one million and approximately eight million , respectively. Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2017 2016 2017 2016 Net income attributable to MPLX LP $ 216 $ 141 $ 556 $ 100 Less: Limited partners’ distributions declared on Preferred units (1) 16 16 49 25 General partner’s distributions declared (including IDRs) (1) 88 54 229 148 Limited partners’ distributions declared on common units (1) 232 179 648 507 Undistributed net loss attributable to MPLX LP $ (120 ) $ (108 ) $ (370 ) $ (580 ) (1) See Note 7 for distribution information.
Schedule of Basic and Diluted Earnings Per Unit	Three Months Ended September 30, 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) $ 88 $ 232 $ 16 $ 336 Undistributed net loss attributable to MPLX LP (2 ) (118 ) — (120 ) Net income attributable to MPLX LP (1) $ 86 $ 114 $ 16 $ 216 Weighted average units outstanding: Basic 8 394 31 433 Diluted 8 395 31 434 Net income attributable to MPLX LP per limited partner unit: Basic $ 0.29 Diluted $ 0.29 Three Months Ended September 30, 2016 (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) $ 54 $ 179 $ 16 $ 249 Undistributed net loss attributable to MPLX LP (3 ) (105 ) — (108 ) Net income attributable to MPLX LP (1) $ 51 $ 74 $ 16 $ 141 Weighted average units outstanding: Basic 7 341 31 379 Diluted 7 346 31 384 Net income attributable to MPLX LP per limited partner unit: Basic $ 0.22 Diluted $ 0.21 Nine Months Ended September 30, 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) $ 229 $ 648 $ 49 $ 926 Undistributed net loss attributable to MPLX LP (7 ) (363 ) — (370 ) Net income attributable to MPLX LP (1) $ 222 $ 285 $ 49 $ 556 Weighted average units outstanding: Basic 8 378 31 417 Diluted 8 381 31 420 Net income attributable to MPLX LP per limited partner unit: Basic $ 0.75 Diluted $ 0.75 Nine Months Ended September 30, 2016 (In millions, except per unit data) General Partner Limited Partners’ Common Units Redeemable Preferred Units Total Basic and diluted net loss attributable to MPLX LP per unit: Net income (loss) attributable to MPLX LP: Distributions declared (including IDRs) $ 148 $ 507 $ 25 $ 680 Undistributed net loss attributable to MPLX LP (12 ) (568 ) — (580 ) Net income (loss) attributable to MPLX LP (1) $ 136 $ (61 ) $ 25 $ 100 Weighted average units outstanding: Basic 7 324 16 347 Diluted 7 324 16 347 Net loss attributable to MPLX LP per limited partner unit: Basic $ (0.19 ) Diluted $ (0.19 ) (1) Allocation of net income (loss) attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.

Equity (Tables)

Equity (Tables)	9 Months Ended
	Sep. 30, 2017
Equity [Abstract]
Schedule of Stockholders Equity [Table Text Block]	The changes in the number of units outstanding during the nine months ended September 30, 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) 183,509 — 3,745 187,254 Issuance of units under the ATM Program (2) 13,846,998 — 282,591 14,129,589 Contribution of HST/WHC/MPLXT (3) 12,960,376 — 264,497 13,224,873 Contribution of the Joint-Interest Acquisition (3) 18,511,134 — 377,778 18,888,912 Class B conversion (4) 4,350,057 (3,990,878 ) 7,330 366,509 Balance at September 30, 2017 407,045,362 — 8,307,046 415,352,408 (1) As a result of the unit-based compensation awards issued during the period, MPLX GP contributed less than $1 million in exchange for 3,745 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 $10 million in exchange for 282,591 general partner units to maintain its two percent GP Interest. (3) See Note 3 for information regarding this acquisition. (4) On July 1, 2017 , 3,990,878 Class B units converted to 4,350,057 common units and were eligible to receive the second quarter 2017 distribution. As a result of the Class B conversion, MPLX GP contributed less than $1 million in exchange for 7,330 general partner units to maintain its two percent GP Interest.
Schedule Of Calculation Of Net Income Applicable to Partners [Table Text Block]	The following table presents the allocation of the general partner’s GP Interest in net income attributable to MPLX LP: Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2017 2016 2017 2016 Net income attributable to MPLX LP $ 216 $ 141 $ 556 $ 100 Less: Preferred unit distributions 16 16 49 25 General partner's IDRs and other 83 49 216 137 Net income (loss) attributable to MPLX LP available to general and limited partners $ 117 $ 76 $ 291 $ (62 ) General partner's two percent GP Interest in net income (loss) attributable to MPLX LP $ 3 $ 2 $ 6 $ (1 ) General partner's IDRs and other 83 49 216 137 General partner's GP Interest in net income attributable to MPLX LP $ 86 $ 51 $ 222 $ 136
Distributions Made to Limited Partner, by Distribution [Table Text Block]	The allocation of total quarterly cash distributions to general, limited and Preferred unitholders is as follows for the three and nine months ended September 30, 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 September 30, Nine Months Ended September 30, (In millions) 2017 2016 2017 2016 General partner's distributions: General partner's distributions on general partner units $ 7 $ 5 $ 18 $ 13 General partner's distributions on IDRs 81 49 211 135 Total distribution on general partner units and IDRs $ 88 $ 54 $ 229 $ 148 Common and preferred unit distributions: Common unitholders, includes common units of general partner $ 232 $ 179 $ 648 $ 507 Preferred unit distributions 16 16 49 25 Total cash distributions declared $ 336 $ 249 $ 926 $ 680

Redeemable Preferred Units (Tab

Redeemable Preferred Units (Tables)	9 Months Ended
	Sep. 30, 2017
Temporary Equity Disclosure [Abstract]
Temporary Equity [Table Text Block]	The changes in the redeemable preferred balance from December 31, 2016 through September 30, 2017 are summarized below: (In millions) Redeemable Preferred Units Balance at December 31, 2016 $ 1,000 Net income 49 Distributions received by Preferred unitholders (49 ) Balance at September 30, 2017 $ 1,000

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2017
Segment Reporting [Abstract]
Schedule of Segment Reporting Information, by Segment [Table Text Block]	The tables below present information about income from operations and capital expenditures for the reported segments: Three Months Ended September 30, 2017 (In millions) L&S G&P Total Revenues and other income: Segment revenues $ 378 $ 669 $ 1,047 Segment other income 11 1 12 Total segment revenues and other income 389 670 1,059 Costs and expenses: Segment cost of revenues 176 276 452 Segment operating income before portion attributable to noncontrolling interests and Predecessor 213 394 607 Segment portion attributable to noncontrolling interests and Predecessor — 45 45 Segment operating income attributable to MPLX LP $ 213 $ 349 $ 562 Three Months Ended September 30, 2016 (In millions) L&S G&P Total Revenues and other income: Segment revenues $ 339 $ 567 $ 906 Segment other income 12 1 13 Total segment revenues and other income 351 568 919 Costs and expenses: Segment cost of revenues 153 239 392 Segment operating income before portion attributable to noncontrolling interests and Predecessor 198 329 527 Segment portion attributable to noncontrolling interests and Predecessor 74 36 110 Segment operating income attributable to MPLX LP $ 124 $ 293 $ 417 Nine Months Ended September 30, 2017 (In millions) L&S G&P Total Revenues and other income: Segment revenues $ 1,095 $ 1,869 $ 2,964 Segment other income 35 2 37 Total segment revenues and other income 1,130 1,871 3,001 Costs and expenses: Segment cost of revenues 500 781 1,281 Segment operating income before portion attributable to noncontrolling interests and Predecessor 630 1,090 1,720 Segment portion attributable to noncontrolling interests and Predecessor 53 119 172 Segment operating income attributable to MPLX LP $ 577 $ 971 $ 1,548 Nine Months Ended September 30, 2016 (In millions) L&S G&P Total Revenues and other income: Segment revenues $ 901 $ 1,595 $ 2,496 Segment other income 42 1 43 Total segment revenues and other income 943 1,596 2,539 Costs and expenses: Segment cost of revenues 392 662 1,054 Segment operating income before portion attributable to noncontrolling interests and Predecessor 551 934 1,485 Segment portion attributable to noncontrolling interests and Predecessor 216 113 329 Segment operating income attributable to MPLX LP $ 335 $ 821 $ 1,156
Reconciliation of Operating Profit (Loss) from Segments to Consolidated [Table Text Block]	Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2017 2016 2017 2016 Reconciliation to Income from operations: L&S segment operating income attributable to MPLX LP $ 213 $ 124 $ 577 $ 335 G&P segment operating income attributable to MPLX LP 349 293 971 821 Segment operating income attributable to MPLX LP 562 417 1,548 1,156 Segment portion attributable to unconsolidated affiliates (47 ) (41 ) (125 ) (130 ) Segment portion attributable to Predecessor — 74 53 216 Income (loss) from equity method investments 23 6 29 (72 ) Other income - related parties 13 11 38 29 Unrealized derivative (losses) gains (1) (17 ) (2 ) 2 (23 ) Depreciation and amortization (164 ) (151 ) (515 ) (438 ) Impairment expense — — — (130 ) General and administrative expenses (59 ) (56 ) (174 ) (172 ) Income from operations $ 311 $ 258 $ 856 $ 436
Reconciliation of Revenue from Segments to Consolidated [Table Text Block]	Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2017 2016 2017 2016 Reconciliation to Total revenues and other income: Total segment revenues and other income $ 1,059 $ 919 $ 3,001 $ 2,539 Revenue adjustment from unconsolidated affiliates (107 ) (100 ) (287 ) (303 ) Income (loss) from equity method investments 23 6 29 (72 ) Other income - related parties 13 11 38 29 Unrealized derivative (losses) gains related to product sales (1) (8 ) 2 1 (12 ) Total revenues and other income $ 980 $ 838 $ 2,782 $ 2,181 (1) The Partnership makes a distinction between realized or unrealized gains and losses on derivatives. During the period when a derivative contract is outstanding, changes in the fair value of the derivative are recorded as an unrealized gain or loss. When a derivative contract matures or is settled, the previously recorded unrealized gain or loss is reversed and the realized gain or loss of the contract is recorded.
Reconciliation of Net Income Attributable to Noncontrolling Interests [Table Text Block]	Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2017 2016 2017 2016 Reconciliation to Net income attributable to noncontrolling interests and Predecessor: Segment portion attributable to noncontrolling interests and Predecessor $ 45 $ 110 $ 172 $ 329 Portion of noncontrolling interests and Predecessor related to items below segment income from operations (21 ) (39 ) (84 ) (157 ) Portion of operating income attributable to noncontrolling interests of unconsolidated affiliates (23 ) (18 ) (49 ) (20 ) Net income attributable to noncontrolling interests and Predecessor $ 1 $ 53 $ 39 $ 152
Reconciliation of Other Significant Reconciling Items from Segments to Consolidated [Table Text Block]	The following table reconciles segment capital expenditures to total capital expenditures: Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2017 2016 2017 2016 L&S segment capital expenditures $ 120 $ 188 $ 353 $ 369 G&P segment capital expenditures 333 183 957 668 Total segment capital expenditures 453 371 1,310 1,037 Less: Capital expenditures for Partnership-operated, non-wholly-owned subsidiaries in G&P segment 101 34 306 94 Total capital expenditures $ 352 $ 337 $ 1,004 $ 943
Reconciliation of Assets from Segment to Consolidated [Table Text Block]	Total assets by reportable segment were: (In millions) September 30, 2017 December 31, 2016 Cash and cash equivalents $ 3 $ 234 L&S 4,520 2,978 G&P 14,715 14,297 Total assets $ 19,238 $ 17,509

Inventories (Tables)

Inventories (Tables)	9 Months Ended
	Sep. 30, 2017
Inventory Disclosure [Abstract]
Schedule of Inventory, Current [Table Text Block]	Inventories consist of the following: (In millions) September 30, 2017 December 31, 2016 NGLs $ 3 $ 2 Line fill 9 9 Spare parts, materials and supplies 52 44 Total inventories $ 64 $ 55

Property, Plant and Equipment (

Property, Plant and Equipment (Tables)	9 Months Ended
	Sep. 30, 2017
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment	Property, plant and equipment with associated accumulated depreciation is shown below: (In millions) September 30, 2017 December 31, 2016 Natural gas gathering and NGL transportation pipelines and facilities $ 5,101 $ 4,748 Processing, fractionation and storage facilities (1) 3,753 3,547 Pipelines and related assets 2,181 1,799 Barges and towing vessels 484 479 Terminals and related assets (1) 794 759 Land, building, office equipment and other 755 757 Construction-in-progress 986 1,013 Total 14,054 13,102 Less accumulated depreciation 2,132 1,694 Property, plant and equipment, net $ 11,922 $ 11,408 (1) Certain prior period amounts have been updated to conform to current period presentation.

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block]	The following table presents the financial instruments carried at fair value classified by the valuation hierarchy: September 30, 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 — (5 ) — (6 ) Embedded derivatives in commodity contracts — (52 ) — (54 ) Total carrying value in Consolidated Balance Sheets $ — $ (57 ) $ — $ (60 )
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 September 30, 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 Liabilities Forward ethane prices (per Gal) (1) $0.27 - $0.28 Oct. 17 - Dec. 17 Forward propane prices (per Gal) (1) $0.68 - $0.91 Oct. 17 - Dec. 18 Forward isobutane prices (per Gal) (1) $0.82 - $1.06 Oct. 17 - Dec. 18 Forward normal butane prices (per Gal) (1) $0.76 - $1.03 Oct. 17 - Dec. 18 Forward natural gasoline prices (per Gal) (1) $1.18 - $1.22 Oct. 17 - Dec. 18 Embedded derivatives in commodity contracts Assets ERCOT Pricing (per MegaWatt Hour) $24.19 - $26.05 Oct. 17 - Dec. 17 Liabilities Forward propane prices (per Gal) (1) $0.61 - $0.91 Oct. 17 - Dec. 22 Forward isobutane prices (per Gal) (1) $0.75 - $1.06 Oct. 17 - Dec. 22 Forward normal butane prices (per Gal) (1) $0.69 - $1.03 Oct. 17 - Dec. 22 Forward natural gasoline prices (per Gal) (1) $1.15 - $1.22 Oct. 17 - Dec. 22 Forward natural gas prices (per MMBtu) (2) $2.30 - $3.11 Oct. 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 decrease over time. (2) Natural gas prices used in the valuations 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, 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 and nine months ended September 30, 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 September 30, 2017 Nine Months Ended September 30, 2017 (In millions) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Fair value at beginning of period $ 2 $ (43 ) $ (6 ) $ (54 ) Total losses (realized and unrealized) included in earnings (1) (10 ) (12 ) (3 ) (4 ) Settlements 3 3 4 6 Fair value at end of period $ (5 ) $ (52 ) $ (5 ) $ (52 ) The amount of total losses for the period included in earnings attributable to the change in unrealized losses relating to liabilities still held at end of period $ (7 ) $ (10 ) $ (4 ) $ (4 ) Three Months Ended September 30, 2016 Nine Months Ended September 30, 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 $ (4 ) $ (40 ) $ 7 $ (32 ) Total gains (losses) (realized and unrealized) included in earnings (1) 2 (6 ) (5 ) (17 ) Settlements (1 ) 2 (6 ) 5 Netting adjustment (2) — — 1 — Fair value at end of period $ (3 ) $ (44 ) $ (3 ) $ (44 ) The amount of total losses for the period included in earnings attributable to the change in unrealized losses relating to liabilities still held at end of period $ — $ (4 ) $ (4 ) $ (15 ) (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 and Cost of revenues . (2) Certain derivative positions are subject to master netting agreements; therefore, the Partnership has elected to offset derivative assets and liabilities where legally permissible. The Partnership may hold positions with certain counterparties, which for GAAP purposes are classified within different levels of the fair value hierarchy and may be legally permissible to offset. This adjustment represents the total impact of offsetting Level 2 positions with Level 3 positions as of September 30, 2016 .
Fair Value Carrying Value by Balance Sheet Grouping [Table Text Block]	The following table summarizes the fair value and carrying value of the long-term debt, excluding capital leases, and SMR liability: September 30, 2017 December 31, 2016 (In millions) Fair Value Carrying Value Fair Value Carrying Value Long-term debt $ 7,619 $ 6,869 $ 4,953 $ 4,422 SMR liability 106 92 108 96

Derivative Financial Instrume37

Derivative Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Schedule of Notional Amounts of Outstanding Derivative Positions [Table Text Block]	As of September 30, 2017 , the Partnership had the following outstanding commodity contracts that were executed to manage the cash flow risk associated with future sales of NGLs and purchases of natural gas: Derivative contracts not designated as hedging instruments Financial Position Notional Quantity (net) Crude Oil (bbl) Short 18,400 Natural Gas (MMBtu) Long 1,096,539 NGLs (gal) Short 33,387,904
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) September 30, 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 $ — $ (15 ) $ — $ (13 ) Other noncurrent assets / deferred credits and other liabilities — (42 ) — (47 ) Total $ — $ (57 ) $ — $ (60 ) (1) Includes embedded derivatives in commodity contracts as discussed above.
Offsetting Assets and Liabilities [Table Text Block]	The net amounts in the table below equal the balances presented in the Consolidated Balance Sheets: September 30, 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 $ — $ — $ — $ (5 ) $ — $ (5 ) Embedded derivatives in commodity contracts — — — (10 ) — (10 ) Total current derivative instruments — — — (15 ) — (15 ) Non-current Commodity contracts — — — — — — Embedded derivatives in commodity contracts — — — (42 ) — (42 ) Total non-current derivative instruments — — — (42 ) — (42 ) Total derivative instruments $ — $ — $ — $ (57 ) $ — $ (57 )
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 September 30, Nine Months Ended September 30, (In millions) 2017 2016 2017 2016 Product sales Realized (loss) gain $ (2 ) $ — $ (3 ) $ 6 Unrealized (loss) gain (8 ) 2 1 (12 ) Total derivative (loss) gain related to product sales (10 ) 2 (2 ) (6 ) Purchased product costs Realized loss (1) (2 ) (1 ) (6 ) (4 ) Unrealized (loss) gain (9 ) (3 ) 1 (12 ) Total derivative loss related to purchased product costs (11 ) (4 ) (5 ) (16 ) Cost of revenues Realized loss (1) — — — (2 ) Unrealized (loss) gain — (1 ) — 1 Total derivative loss related to cost of revenues — (1 ) — (1 ) Total derivative losses $ (21 ) $ (3 ) $ (7 ) $ (23 ) (1) Certain prior period amounts have been updated to conform to current period presentation.

Debt (Tables)

Debt (Tables)	9 Months Ended
	Sep. 30, 2017
Debt Disclosure [Abstract]
Summary of Outstanding Borrowings	The Partnership’s outstanding borrowings consisted of the following: (In millions) September 30, 2017 December 31, 2016 MPLX LP: Bank revolving credit facility due 2022 $ 420 $ — Term loan facility due 2019 — 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 7 8 Total 7,277 4,858 Unamortized debt issuance costs (27 ) (7 ) Unamortized discount (401 ) (428 ) Amounts due within one year (1 ) (1 ) Total long-term debt due after one year $ 6,848 $ 4,422

Supplemental Cash Flow Inform39

Supplemental Cash Flow Information (Tables)	9 Months Ended
	Sep. 30, 2017
Supplemental Cash Flow Elements [Abstract]
Summary of Supplemental Cash Flow Information [Table Text Block]	Nine Months Ended September 30, (In millions) 2017 2016 Net cash provided by operating activities included: Interest paid (net of amounts capitalized) $ 207 $ 158 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) 337 — (1) Contribution of assets to Sherwood Midstream and Sherwood Midstream Holdings. See Note 4 .
Summary of Reconciliation of Additions to Property, Plant and Equipment to Total Capital Expenditures [Table Text Block]	The following is the change of additions to property, plant and equipment related to capital accruals: Nine Months Ended September 30, (In millions) 2017 2016 Increase in capital accruals $ 55 $ —

Equity-Based Compensation Plan

Equity-Based Compensation Plan (Tables)	9 Months Ended
	Sep. 30, 2017
Phantom Units [Member]
Equity Transactions And Share Based Compensation [Line Items]
Summary of Share-based Compensation, Restricted Stock Units Award Activity	The following is a summary of phantom unit award activity of MPLX LP common units for the nine months ended September 30, 2017 : Number Weighted Outstanding at December 31, 2016 1,173,411 $ 33.09 Granted 653,721 36.36 Settled (288,584 ) 33.50 Forfeited (113,107 ) 34.59 Outstanding at September 30, 2017 1,425,441 34.39
Performance Shares [Member]
Equity Transactions And Share Based Compensation [Line Items]
Summary of Share-based Compensation, Restricted Stock Units Award Activity	The following is a summary of the equity-classified performance unit award activity for the nine months ended September 30, 2017 : Number of Outstanding at December 31, 2016 1,799,249 Granted 1,407,062 Settled (464,500 ) Forfeited (35,217 ) Outstanding at September 30, 2017 2,706,594

Description of Business and Bas

Description of Business and Basis of Presentation - Additional Information (Detail)	Mar. 01, 2017	Sep. 30, 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

Accounting Standards Adoption o

Accounting Standards Adoption of ASC 606 (Details) - Subsequent Event [Member] - Difference between Revenue Guidance in Effect before and after Topic 606 [Member] $ in Millions	Jan. 01, 2018USD ($)
Minimum [Member]
Item Effected [Line Items]
Effect on Future Earnings, Amount	$ 300
Maximum [Member]
Item Effected [Line Items]
Effect on Future Earnings, Amount	$ 350

Acquisitions Joint-Interest Acq

Acquisitions Joint-Interest Acquisition (Details)		Sep. 01, 2017USD ($)miinshares	Mar. 01, 2017USD ($)	Mar. 31, 2016USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2016USD ($)	Sep. 30, 2017USD ($)		Sep. 30, 2016USD ($)		Dec. 31, 2016USD ($)
Schedule of Equity Method Investments [Line Items]
Limited Liability Company (LLC) or Limited Partnership (LP), Managing Member or General Partner, Ownership Interest		2.00%	2.00%	2.00%				2.00%
Income (loss) from equity method investments						$ 23,000,000	$ 6,000,000	$ 29,000,000	[1]	$ (72,000,000)	[1]
Equity method investments					$ 3,997,000,000	3,997,000,000		3,997,000,000				$ 2,471,000,000
Contribution from MPC								675,000,000		225,000,000
LOOP LOCAP SAX and Explorer Llc [Member]
Schedule of Equity Method Investments [Line Items]
Payments to Acquire Interest in Joint Venture		$ 420,000,000
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned		630,000,000
Business Acquisition, Total Consideration, Value Assigned		1,050,000,000
Business Acquisition, Equity Interest Issued or Issuable, Fair Value Assigned		653,000,000
Business Acquisition, Total Consideration, Fair Value Assigned		$ 1,070,000,000
Income (loss) from equity method investments					0
Equity method investments					$ 645,000,000	$ 645,000,000		645,000,000
Loop Llc and Explorer Pipeline [Member]
Schedule of Equity Method Investments [Line Items]
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Net Prior Service Cost (Credit) Adjustment, Tax								$ 0
Illinois Extension Pipeline Company LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage		35.00%			35.00%	35.00%		35.00%
Pipeline length | mi		168
Pipeline diameter | in		24
Number of pump stations		2
Loop Llc [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage		40.70%			40.70%	40.70%		40.70%
Locap Llc [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage		58.52%			58.52%	58.52%		58.52%
Explorer Pipeline [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage		24.51%			24.51%	24.51%		24.51%
Pipeline length | mi		1,830
General Partner Units | LOOP LOCAP SAX and Explorer Llc [Member]
Schedule of Equity Method Investments [Line Items]
Partners' Capital Account, Units, Acquisitions | shares	[2]	377,778
General Partner Common Units | LOOP LOCAP SAX and Explorer Llc [Member]
Schedule of Equity Method Investments [Line Items]
Partners' Capital Account, Units, Acquisitions | shares	[2]	13,719,017
MPLX Logistics LLC [Member] | Limited Partners Common Units | LOOP LOCAP SAX and Explorer Llc [Member]
Schedule of Equity Method Investments [Line Items]
Partners' Capital Account, Units, Acquisitions | shares	[2]	3,350,893
MPLX Holdings Inc [Member] | Limited Partners Common Units | LOOP LOCAP SAX and Explorer Llc [Member]
Schedule of Equity Method Investments [Line Items]
Partners' Capital Account, Units, Acquisitions | shares	[2]	1,441,224
MPC
Schedule of Equity Method Investments [Line Items]
Distributions to MPC								$ 1,931,000,000		0
Contribution from MPC			$ 6,000,000	$ 15,000,000		$ 10,000,000
MPC | LOOP LOCAP SAX and Explorer Llc [Member]
Schedule of Equity Method Investments [Line Items]
Distributions to MPC								13,000,000
General Partner | MPC
Schedule of Equity Method Investments [Line Items]
Distributions to MPC								1,394,000,000
Contribution from MPC										$ 141,000,000
General Partner | MPC | LOOP LOCAP SAX and Explorer Llc [Member]
Schedule of Equity Method Investments [Line Items]
Distributions to MPC								$ 13,000,000
[1]	Income (loss) from equity method investments includes the impact of any basis differential amortization or accretion.
[2]	See Note 3 for information regarding this acquisition.

Acquisitions Acquisition of HST

Acquisitions Acquisition of HST, WHC & MPLXT (Details) bbl in Millions, $ in Millions		Sep. 01, 2017	Mar. 01, 2017USD ($)misharesbbl	Mar. 31, 2016USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2017USD ($)shares		Sep. 30, 2016USD ($)
Business Acquisition [Line Items]
Limited Liability Company (LLC) or Limited Partnership (LP), Managing Member or General Partner, Ownership Interest		2.00%	2.00%	2.00%		2.00%
Contribution from MPC | $						$ 675		$ 225
Number of natural gas liquids storage caverns			9
Number of light product terminals			62
HST & WHC & MPLXT [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
Partners' Capital Account, Units, Acquisitions	[1]					13,224,873
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
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions			264,497			264,497	[1]
HST & WHC & MPLXT [Member] | Limited Partners Common Units
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions	[1]					12,960,376
HST & WHC & MPLXT [Member] | General Partner Common Units
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions			9,197,900
MPLX Logistics LLC [Member] | HST & WHC & MPLXT [Member] | Limited Partners Common Units
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions			2,630,427
MPLX Holdings Inc [Member] | HST & WHC & MPLXT [Member] | Limited Partners Common Units
Business Acquisition [Line Items]
Partners' Capital Account, Units, Acquisitions			1,132,049
Natural gas liquids storage caverns [Member]
Business Acquisition [Line Items]
Storage Capacity | bbl			1.8
Light-product terminal [Member]
Business Acquisition [Line Items]
Storage Capacity | bbl			23.6
Refined Products [Member]
Business Acquisition [Line Items]
Miles of pipeline | mi			430
Crude Oil [Member]
Business Acquisition [Line Items]
Miles of pipeline | mi			174
Wholly Owned Properties [Member]
Business Acquisition [Line Items]
Number of light product terminals			59
Property Subject to Operating Lease [Member]
Business Acquisition [Line Items]
Number of light product terminals			1
Partially Owned Properties [Member]
Business Acquisition [Line Items]
Number of light product terminals			2
MPC
Business Acquisition [Line Items]
Contribution from MPC | $			$ 6	$ 15	$ 10
[1]	See Note 3 for information regarding this acquisition.

Acquisitions Acquisition of H45

Acquisitions Acquisition of HST, WHC & MPLXT - Income Statement (Details) - USD ($) $ in Millions		3 Months Ended			9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016		Sep. 30, 2017		Sep. 30, 2016
Business Acquisition [Line Items]
Service revenue		$ 299	$ 250		$ 845		$ 712
Service revenue - related parties		276	253	[1]	801		676	[1]
Rental income		69	77		208		218
Rental income - related parties		70	68	[1]	207		172	[1]
Product sales		217	157		611		394
Product sales - related parties		2	2		6		8
Income (loss) from equity method investments		23	6		29	[2]	(72)	[2]
Gain on sale of assets		0	1		1		1
Other income		2	2		5		5
Other income - related parties		22	22	[1]	69		67	[1]
Total revenues and other income		980	838	[1]	2,782		2,181	[1]
Cost of revenues (excludes items below)		129	122	[1]	381		329	[1]
Purchased product costs		170	117		441		310
Rental cost of sales		19	13	[1]	44		42	[1]
Rental cost of sales - related parties		0	0	[1]	1		1	[1]
Purchases - related parties		114	109	[1]	330		286	[1]
Depreciation and amortization		164	151	[1]	515		438	[1],[3]
Impairment expense		0	0		0		130
General and administrative expenses		59	56	[1]	174		172	[1]
Other taxes		14	12		40		37
Total costs and expenses		669	580	[1]	1,926		1,745	[1]
Income from operations		311	258	[1]	856		436	[1]
Related party interest and other financial costs		1	0		1		1
Interest expense (net of amounts capitalized)		77	51		217		158
Other financial costs		15	13		40		37
Income (loss) before income taxes		218	194	[1]	598		240	[1]
Provision (benefit) for income taxes		(1)	0		(3)		12
Net income (loss)		217	194	[1]	595		252	[1],[3]
Less: Net income (loss) attributable to noncontrolling interest		1	2		3		3
Less: Net income attributable to Predecessor		0	51	[1]	36		149	[1]
Net income (loss) attributable to MPLX LP	[4]	216	141		556		100
Less: Preferred unit distributions		16	16		49		25
Less: General partner’s interest in net income attributable to MPLX LP		86	51		222		136
Limited partners' interest in net income (loss) attributable to MPLX LP		$ 114	74		$ 285		(61)
Scenario, Previously Reported [Member]
Business Acquisition [Line Items]
Service revenue			250				712
Service revenue - related parties			153				448
Rental income			77				218
Rental income - related parties			29				84
Product sales			157				394
Product sales - related parties			2				8
Income (loss) from equity method investments			6				(72)
Gain on sale of assets			1				1
Other income			2				5
Other income - related parties			26				78
Total revenues and other income			703				1,876
Cost of revenues (excludes items below)			90				263
Purchased product costs			117				310
Rental cost of sales			11				39
Rental cost of sales - related parties			0				0
Purchases - related parties			84				238
Depreciation and amortization			138				407
Impairment expense							130
General and administrative expenses			46				147
Other taxes			10				32
Total costs and expenses			496				1,566
Income from operations			207				310
Related party interest and other financial costs							1
Interest expense (net of amounts capitalized)			51				158
Other financial costs			13				37
Income (loss) before income taxes			143				114
Provision (benefit) for income taxes							12
Net income (loss)			143				126
Less: Net income (loss) attributable to noncontrolling interest			2				3
Less: Net income attributable to Predecessor			0				23
Net income (loss) attributable to MPLX LP			141				100
Less: Preferred unit distributions			16				25
Less: General partner’s interest in net income attributable to MPLX LP			51				136
Limited partners' interest in net income (loss) attributable to MPLX LP			74				(61)
Scenario, Adjustment [Member]
Business Acquisition [Line Items]
Service revenue			0				0
Service revenue - related parties			0				0
Rental income			0				0
Rental income - related parties			0				0
Product sales			0				0
Product sales - related parties			0				0
Income (loss) from equity method investments			0				0
Other income			0				0
Other income - related parties	[5]		(4)				(11)
Total revenues and other income			(4)				(11)
Cost of revenues (excludes items below)			0				0
Purchased product costs			0				0
Rental cost of sales			0				0
Rental cost of sales - related parties	[5]		(1)				(1)
Purchases - related parties	[5]		(3)				(10)
Depreciation and amortization			0				0
Impairment expense							0
General and administrative expenses			0				0
Other taxes			0				0
Total costs and expenses			(4)				(11)
Income from operations			0				0
Related party interest and other financial costs							0
Interest expense (net of amounts capitalized)			0				0
Other financial costs			0				0
Income (loss) before income taxes			0				0
Provision (benefit) for income taxes							0
Net income (loss)			0				0
Less: Net income (loss) attributable to noncontrolling interest			0				0
Less: Net income attributable to Predecessor			0				0
Net income (loss) attributable to MPLX LP			0				0
Less: Preferred unit distributions			0				0
Less: General partner’s interest in net income attributable to MPLX LP			0				0
Limited partners' interest in net income (loss) attributable to MPLX LP			0				0
HST & WHC [Member] | Scenario, Adjustment [Member]
Business Acquisition [Line Items]
Service revenue			0				0
Service revenue - related parties			28				82
Rental income			0				0
Rental income - related parties			13				36
Product sales			0				0
Product sales - related parties			0				0
Income (loss) from equity method investments			0				0
Other income			0				0
Other income - related parties			0				0
Total revenues and other income			41				118
Cost of revenues (excludes items below)			10				24
Purchased product costs			0				0
Rental cost of sales			2				3
Rental cost of sales - related parties			1				2
Purchases - related parties			4				13
Depreciation and amortization			4				12
Impairment expense							0
General and administrative expenses			2				5
Other taxes			0				2
Total costs and expenses			23				61
Income from operations			18				57
Related party interest and other financial costs							0
Interest expense (net of amounts capitalized)			0				0
Other financial costs			0				0
Income (loss) before income taxes			18				57
Provision (benefit) for income taxes							0
Net income (loss)			18				57
Less: Net income (loss) attributable to noncontrolling interest			0				0
Less: Net income attributable to Predecessor			18				57
Net income (loss) attributable to MPLX LP			0				0
Less: Preferred unit distributions			0				0
Less: General partner’s interest in net income attributable to MPLX LP			0				0
Limited partners' interest in net income (loss) attributable to MPLX LP			0				0
MPLX Terminals [Member] | Scenario, Adjustment [Member]
Business Acquisition [Line Items]
Service revenue			0				0
Service revenue - related parties			72				146
Rental income			0				0
Rental income - related parties			26				52
Product sales			0				0
Product sales - related parties			0				0
Income (loss) from equity method investments			0				0
Other income			0				0
Other income - related parties			0				0
Total revenues and other income			98				198
Cost of revenues (excludes items below)			22				42
Purchased product costs			0				0
Rental cost of sales			0				0
Rental cost of sales - related parties			0				0
Purchases - related parties			24				45
Depreciation and amortization			9				19
Impairment expense							0
General and administrative expenses			8				20
Other taxes			2				3
Total costs and expenses			65				129
Income from operations			33				69
Related party interest and other financial costs							0
Interest expense (net of amounts capitalized)			0				0
Other financial costs			0				0
Income (loss) before income taxes			33				69
Provision (benefit) for income taxes							0
Net income (loss)			33				69
Less: Net income (loss) attributable to noncontrolling interest			0				0
Less: Net income attributable to Predecessor			33				69
Net income (loss) attributable to MPLX LP			0				0
Less: Preferred unit distributions			0				0
Less: General partner’s interest in net income attributable to MPLX LP			0				0
Limited partners' interest in net income (loss) attributable to MPLX LP			$ 0				$ 0
[1]	Financial information has been retrospectively adjusted for the acquisition of HST, WHC and MPLXT from MPC. See Notes 1 and 3.
[2]	Income (loss) from equity method investments includes the impact of any basis differential amortization or accretion.
[3]	Financial information has been retrospectively adjusted for the acquisition of HST, WHC and MPLXT from MPC. See Notes 1 and 3.
[4]	Allocation of net income (loss) attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.
[5]	Represents intercompany transactions eliminated during the consolidation process, in accordance with GAAP.

Acquisitions Acquisition of H46

Acquisitions Acquisition of HST, WHC & MPLXT - Cash Flow (Details) - USD ($) $ in Millions		3 Months Ended			9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016		Sep. 30, 2017		Sep. 30, 2016		Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Net income (loss)		$ 217	$ 194	[1]	$ 595		$ 252	[1],[2]
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Amortization of deferred financing costs					38		34
Depreciation and amortization		164	151	[1]	515		438	[1],[2]
Impairment expense		0	0		0		130
Deferred income taxes					2		(16)
Asset retirement expenditures					2		4	[2]
Gain on disposal of assets					1		1
(Income) loss from equity method investments		23	6		29	[3]	(72)	[3]
Distributions from unconsolidated affiliates					136		111
Changes in:
Current receivables					(20)		(43)	[2]
Inventories					3		4
Fair value of derivatives					3		(28)
Current accounts payable and accrued liabilities					6		64	[2]
Receivables from / liabilities to related parties					61		(104)	[2]
All other, net					43		18	[2]
Net cash provided by (used in) operating activities					1,338		975	[2]
Additions to property, plant and equipment		(352)	(337)		(1,004)		(943)	[2]
Investments - net related party loans					80		103	[2]
Investments in unconsolidated affiliates					690		56
All other, net					(2)		4
Net cash used in investing activities					(1,837)		(892)	[2]
Long-term debt - borrowings					2,661		434
Long-term debt - repayments					(251)		(1,312)
Related party debt - borrowings					829		2,215
Related party debt - repayments					627		2,223
Net proceeds from equity offerings					483		510
Issuance of redeemable preferred units					0		984
Distributions to preferred unitholders					(49)		(9)
Distributions to unitholders and general partner					(800)		(612)
Distributions to noncontrolling interests					(4)		(3)
Contributions from noncontrolling interests					128		4
Consideration payment to Class B unitholders					(25)		(25)
All other, net					(8)		(2)
Contribution from MPC					0		225
Net cash provided by financing activities					268		82
Net increase (decrease) in cash and cash equivalents					(231)		165
Cash and cash equivalents		$ 3	208		3		208		$ 234	$ 43
Scenario, Previously Reported [Member]
Business Acquisition [Line Items]
Net income (loss)			143				126
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Amortization of deferred financing costs							34
Depreciation and amortization			138				407
Impairment expense							130
Deferred income taxes							(16)
Asset retirement expenditures							3
Gain on disposal of assets							1
(Income) loss from equity method investments			6				(72)
Distributions from unconsolidated affiliates							111
Changes in:
Current receivables							(44)
Inventories							4
Fair value of derivatives							(28)
Current accounts payable and accrued liabilities							59
Receivables from / liabilities to related parties							15
All other, net							18
Net cash provided by (used in) operating activities							932
Additions to property, plant and equipment							(874)
Investments - net related party loans							77
Investments in unconsolidated affiliates							56
All other, net							4
Net cash used in investing activities							(849)
Long-term debt - borrowings							434
Long-term debt - repayments							(1,312)
Related party debt - borrowings							2,215
Related party debt - repayments							2,223
Net proceeds from equity offerings							510
Issuance of redeemable preferred units							984
Distributions to preferred unitholders							(9)
Distributions to unitholders and general partner							(612)
Distributions to noncontrolling interests							(3)
Contributions from noncontrolling interests							4
Consideration payment to Class B unitholders	[2]						(25)
All other, net							(2)
Contribution from MPC	[2]						225
Net cash provided by financing activities							82
Net increase (decrease) in cash and cash equivalents							165
Cash and cash equivalents			208				208			43
Scenario, Adjustment [Member]
Business Acquisition [Line Items]
Net income (loss)			0				0
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Depreciation and amortization			0				0
Impairment expense							0
(Income) loss from equity method investments			0				0
HST & WHC [Member] | Scenario, Adjustment [Member]
Business Acquisition [Line Items]
Net income (loss)			18				57
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Amortization of deferred financing costs							0
Depreciation and amortization			4				12
Impairment expense							0
Deferred income taxes							0
Asset retirement expenditures							1
Gain on disposal of assets							0
(Income) loss from equity method investments			0				0
Distributions from unconsolidated affiliates							0
Changes in:
Current receivables							1
Inventories							0
Fair value of derivatives							0
Current accounts payable and accrued liabilities							(1)
Receivables from / liabilities to related parties							3
All other, net							2
Net cash provided by (used in) operating activities							73
Additions to property, plant and equipment							(36)
Investments - net related party loans							(37)
Investments in unconsolidated affiliates							0
All other, net							0
Net cash used in investing activities							(73)
Long-term debt - borrowings							0
Long-term debt - repayments							0
Related party debt - borrowings							0
Related party debt - repayments							0
Net proceeds from equity offerings							0
Issuance of redeemable preferred units							0
Distributions to unitholders and general partner							0
Distributions to noncontrolling interests							0
Contributions from noncontrolling interests							0
All other, net							0
Net cash provided by financing activities							0
Net increase (decrease) in cash and cash equivalents							0
Cash and cash equivalents			0				0			0
MPLX Terminals [Member] | Scenario, Adjustment [Member]
Business Acquisition [Line Items]
Net income (loss)			33				69
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Amortization of deferred financing costs							0
Depreciation and amortization			9				19
Impairment expense							0
Deferred income taxes							0
Asset retirement expenditures							0
Gain on disposal of assets							0
(Income) loss from equity method investments			0				0
Distributions from unconsolidated affiliates							0
Changes in:
Current receivables							0
Inventories							0
Fair value of derivatives							0
Current accounts payable and accrued liabilities							6
Receivables from / liabilities to related parties							(122)
All other, net							(2)
Net cash provided by (used in) operating activities							(30)
Additions to property, plant and equipment							(33)
Investments - net related party loans							63
Investments in unconsolidated affiliates							0
All other, net							0
Net cash used in investing activities							30
Long-term debt - borrowings							0
Long-term debt - repayments							0
Related party debt - borrowings							0
Related party debt - repayments							0
Net proceeds from equity offerings							0
Issuance of redeemable preferred units							0
Distributions to unitholders and general partner							0
Distributions to noncontrolling interests							0
Contributions from noncontrolling interests							0
All other, net							0
Net cash provided by financing activities							0
Net increase (decrease) in cash and cash equivalents							0
Cash and cash equivalents			$ 0				0			$ 0
MPC
Changes in:
Distributions to MPC from Predecessor					(1,931)		0
MPC | Scenario, Previously Reported [Member]
Changes in:
Distributions to MPC from Predecessor							(104)
MPC | Predecessor
Changes in:
Distributions to MPC from Predecessor					$ (113)		(104)
MPC | HST & WHC [Member] | Scenario, Adjustment [Member]
Changes in:
Distributions to MPC from Predecessor							0
MPC | MPLX Terminals [Member] | Scenario, Adjustment [Member]
Changes in:
Distributions to MPC from Predecessor							$ 0
[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.
[3]	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 ($)bbl / dmiin	Sep. 30, 2017USD ($)	Sep. 30, 2016USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2016USD ($)	[1]
Business Acquisition [Line Items]
Additions to property, plant and equipment		$ (352)	$ (337)		$ (1,004)	$ (943)
Ozark Pipeline [Member]
Business Acquisition [Line Items]
Additions to property, plant and equipment	$ (219)
Pipeline length | mi	433
Pipeline diameter | in	22
Pipeline barrels capable of transportation | bbl / d	230
Revenues and other income		19		$ 45
Income from operations		$ 6		$ 17
[1]	Financial information has been retrospectively adjusted for the acquisition of HST, WHC and MPLXT from MPC. See Notes 1 and 3.

Acquisitions MarEn Bakken (Deta

Acquisitions MarEn Bakken (Details) $ in Millions	Feb. 15, 2017USD ($)Quarterly_reporting_period	Sep. 30, 2017USD ($)	Sep. 30, 2016USD ($)	Dec. 31, 2016USD ($)
Schedule of Equity Method Investments [Line Items]
Investments in unconsolidated affiliates		$ 690	$ 56
Equity method investments		3,997		$ 2,471
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		$ 520
Bakken Pipeline System [Member]
Schedule of Equity Method Investments [Line Items]
Percentage of Ownership Interest in Joint Venture Acquired	36.75%
Equity Method Investment, Ownership Percentage	9.1875%
MarEn Bakken Company LLC [Member]
Schedule of Equity Method Investments [Line Items]
Investments in unconsolidated affiliates	$ 2,000
MPC
Schedule of Equity Method Investments [Line Items]
Incentive Distribution Rights Forfeited per Quarter	$ 1.6
Number of Quarters IDRs Forfeited | Quarterly_reporting_period	12
Prorated Incentive Distribution Rights Forfeited	$ 0.8

Acquisition of Hardin Street Ma

Acquisition of Hardin Street Marine LLC (Details) $ in Millions	Sep. 01, 2017	Mar. 01, 2017USD ($)	Mar. 31, 2016USD ($)shares	Sep. 30, 2017USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2016USD ($)
Business Acquisition [Line Items]
Limited Liability Company (LLC) or Limited Partnership (LP), Managing Member or General Partner, Ownership Interest	2.00%	2.00%	2.00%		2.00%
Contribution from MPC					$ 675	$ 225
Number of tow boats			18
Number of barges			219
Hardin Street Marine [Member]
Business Acquisition [Line Items]
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned			$ 600
Business Acquisition, Effective Date of Acquisition			Mar. 31, 2016
Limited Partner | 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
General Partner | 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
MPC
Business Acquisition [Line Items]
Contribution from MPC		$ 6	$ 15	$ 10
Percentage of MPC 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		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017		Sep. 30, 2016		Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Revenues and other income				$ 364		$ 286
Costs and expenses				216		257
Income (loss) from operations				148		29
Net income (loss)				112		28
Income (loss) from equity method investments		$ 23	$ 6	29	[1]	(72)	[1]
Current assets		498		498				$ 87
Noncurrent assets		7,584		7,584				2,695
Current liabilities		584		584				60
Noncurrent liabilities		576		576				15
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity		1,100		1,100				1,100
MarkWest Utica EMG [Member]
Schedule of Equity Method Investments [Line Items]
Revenues and other income				137		165
Costs and expenses				72		70
Income (loss) from operations				65		95
Net income (loss)				65		94
Income (loss) from equity method investments	[1]			6		10
Current assets		72		72				45
Noncurrent assets	[2]	2,092		2,092				2,173
Current liabilities		37		37				30
Noncurrent liabilities		2		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]
Revenues and other income				49		13
Costs and expenses				29		107	[3]
Income (loss) from operations				20		(94)
Net income (loss)				19		(94)
Income (loss) from equity method investments	[1]			7		(88)
Current assets		47		47				2
Noncurrent assets		878		878				132
Current liabilities		55		55				4
Noncurrent liabilities		12		12				13
Non-VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Revenues and other income				178		108
Costs and expenses				115		80
Income (loss) from operations				63		28
Net income (loss)				28		28
Income (loss) from equity method investments	[1]			16		6
Current assets		379		379				40
Noncurrent assets		4,614		4,614				390
Current liabilities		492		492				26
Noncurrent liabilities		562		562				0
Ohio Condensate Company, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Impairment related to equity method investment from asset impairment and elimination of basis differential						$ 89
Ohio Gathering Company, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Investments in subsidiary		$ 794		$ 794				$ 794
[1]	Income (loss) from equity method investments includes the impact of any basis differential amortization or accretion.
[2]	MarkWest Utica EMG, L.L.C.’s (“MarkWest Utica EMG”) noncurrent assets include its investment in its subsidiary Ohio Gathering Company, L.L.C. (“Ohio Gathering”), which does not appear elsewhere in this table. The investment was $794 million as of September 30, 2017 and December 31, 2016.
[3]	Includes an impairment charge of $89 million for the nine months ended September 30, 2016 related to the Partnership’s investment in Ohio Condensate Company, L.L.C., which does not appear separately in this table.

Equity Method Investments Utica

Equity Method Investments Utica EMG (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016		Sep. 30, 2017	Sep. 30, 2016		Dec. 31, 2016	Feb. 28, 2013
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 3,997			$ 3,997			$ 2,471
Operational service revenue	$ 22	$ 22	[1]	$ 69	$ 67	[1]
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%			70.00%
Threshold Percentage of Noncontrolling Ownership Interest in Joint Venture	30.00%			30.00%
Maximum Percentage of Additional Noncontrolling Capital Contribution before Threshold Ownership				10.00%
Actual Capital Contributed by Noncontrolling Owners	$ 1,200			$ 1,200
Actual Capital Contribution to Date	$ 1,500			$ 1,500
Noncontrolling Owners Preference Threshold								$ 500
Noncontrolling Owners Accrual of Preference Amount		4			12
Percentage of aggregate investment balance	56.00%			56.00%
Equity method investments	$ 2,200			$ 2,200			$ 2,200
Operational service revenue	$ 5	$ 5		$ 13	$ 12
[1]	Financial information has been retrospectively adjusted for the acquisition of HST, WHC and MPLXT from MPC. See Notes 1 and 3.

Investments & NCI Ohio Gatherin

Investments & NCI Ohio Gathering (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016		Sep. 30, 2017	Sep. 30, 2016
Schedule of Equity Method Investments [Line Items]
Operational service revenue	$ 22	$ 22	[1]	$ 69	$ 67	[1]
Ohio Gathering Company, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	34.00%			34.00%
Operational service revenue	$ 4	$ 5		$ 12	$ 12
[1]	Financial information has been retrospectively adjusted for the acquisition of HST, WHC and MPLXT from MPC. See Notes 1 and 3.

Investments & NCI Sherwood Mids

Investments & NCI Sherwood Midstream (Details) bbl / d in Thousands, $ in Millions	Jan. 01, 2017USD ($)bbl / d	Sep. 30, 2017USD ($)	Sep. 30, 2016USD ($)	[2]	Sep. 30, 2017USD ($)		Sep. 30, 2016USD ($)		Dec. 31, 2016USD ($)
Schedule of Equity Method Investments [Line Items]
Contribution of fixed assets to joint venture					$ 337	[1]	$ 0
Assets, Current		$ 571			571				$ 868
Liabilities, Current		1,048			1,048				763
Equity method investments		3,997			3,997				$ 2,471
Operational service revenue		$ 22	$ 22		$ 69		$ 67	[2]
Sherwood Midstream LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage		50.00%			50.00%
Contribution of fixed assets to joint venture	$ 134
Payments to Acquire Interest in Joint Venture	$ 20
Equity method investments		$ 220			$ 220
Operational service revenue		2			6
MarkWest Ohio Fractionation Company, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Capacity | bbl / d	20
Assets, Current		51			51
Assets, Noncurrent		406			406
Liabilities, Current		$ 26			$ 26
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
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
[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.

Investments & NCI Sherwood Mi54

Investments & NCI Sherwood Midstream Holdings (Details) - USD ($) $ in Millions	Jan. 01, 2017	Sep. 30, 2017	Sep. 30, 2017		Sep. 30, 2016	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Contribution of fixed assets to joint venture			$ 337	[1]	$ 0
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity		$ 1,100	1,100			$ 1,100
Distributions from unconsolidated affiliates - return of capital			24		$ 0
Equity method investments		3,997	$ 3,997			$ 2,471
Sherwood Midstream Holdings [Member]
Schedule of Equity Method Investments [Line Items]
Fair Value Of Assets Contributed	$ 209	$ 10
Equity Method Investment, Ownership Percentage		86.00%	86.00%
Contribution of fixed assets to joint venture	203
Gain on sale of assets	2
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity		$ 14	$ 14
Distributions from unconsolidated affiliates - return of capital	$ 45
Equity method investments		$ 163	$ 163
Direct Ownership Interest [Member] | Sherwood Midstream Holdings [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	79.00%
Indirect Ownership Interest [Member] | Sherwood Midstream Holdings [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage		14.70%	14.70%
Sherwood Midstream LLC [Member] | Sherwood Midstream Holdings [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	21.00%
Payments to Acquire Interest in Joint Venture	$ 44	$ 4
[1]	Contribution of assets to Sherwood Midstream and Sherwood Midstream Holdings. See Note 4.

Related Party Agreements and 55

Related Party Agreements and Transactions - Additional Information (Detail) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017USD ($)	Dec. 31, 2016USD ($)	Sep. 01, 2017
Centennial [Member] | MPC
Related Party Transaction [Line Items]
Equity Method Investment, Ownership Percentage	50.00%
Muskegon [Member] | MPC
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	86.00%
Illinois Extension Pipeline Company LLC [Member]
Related Party Transaction [Line Items]
Equity Method Investment, Ownership Percentage	35.00%		35.00%
Number of pump stations			2
Loop Llc [Member]
Related Party Transaction [Line Items]
Equity Method Investment, Ownership Percentage	40.70%		40.70%
Locap Llc [Member]
Related Party Transaction [Line Items]
Equity Method Investment, Ownership Percentage	58.52%		58.52%
Explorer Pipeline [Member]
Related Party Transaction [Line Items]
Equity Method Investment, Ownership Percentage	24.51%		24.51%
Transportation Services Agreements [Member] | Transportation of crude and refined product services [Member] | Commercial Agreements [Member] | MPC
Related Party Transaction [Line Items]
Number of Renewals	2
Term Of Agreements	4 years
Three Year Storage Services Agreement [Member] | Storage Services Tank Farm [Member] | Commercial Agreements [Member] | MPC
Related Party Transaction [Line Items]
Term Of Agreements	3 years
Ten Year Storage Services Agreement [Member] | Storage Services Butane and Propane Caverns [Member] | Commercial Agreements [Member] | MPC
Related Party Transaction [Line Items]
Term Of Agreements	10 years
Terminal Services Agreements [Member] | Terminal storage and related services [Member] | Commercial Agreements [Member] | MPC
Related Party Transaction [Line Items]
Number of Renewals	2
Term Of Agreements	5 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 Short-term Debt	$ 829	$ 2,500
Repayments of Short-term Debt	627	2,500
Intercompany loan outstanding	$ 202	$ 0
Line of Credit Facility, Interest Rate During Period	2.721%	1.939%

Related Party Agreements and 56

Related Party Agreements and Transactions - Sales to Related Parties (Detail) - USD ($) $ in Millions		3 Months Ended			9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016		Sep. 30, 2017	Sep. 30, 2016
Related Party Transaction [Line Items]
Service revenue - related parties		$ 276	$ 253	[1]	$ 801	$ 676	[1]
Rental income - related parties		70	68	[1]	207	172	[1]
Product sales - related parties		2	2		6	8
MPC
Related Party Transaction [Line Items]
Service revenue - related parties		276	253		801	676
Rental income - related parties		70	68		207	172
Product sales - related parties	[2]	2	2		6	8
Product sales to MPC that net to zero		$ 63	$ 13		$ 173	$ 25
[1]	Financial information has been retrospectively adjusted for the acquisition of HST, WHC and MPLXT from MPC. See Notes 1 and 3.
[2]	There were additional product sales to MPC that net to zero within the consolidated financial statements as the transactions are recorded net due to the terms of the agreements under which such product was sold. For the three and nine months ended September 30, 2017, these sales totaled $63 million and $173 million, respectively. For the three and nine months ended September 30, 2016, these sales totaled $13 million and $25 million, respectively.

Related Party Agreements and 57

Related Party Agreements and Transactions - Other Income - Related Parties (Detail) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016		Sep. 30, 2017	Sep. 30, 2016
Related Party Transaction [Line Items]
Other income - related parties	$ 22	$ 22	[1]	$ 69	$ 67	[1]
MPC
Related Party Transaction [Line Items]
Other income - related parties	9	10		30	36
MarkWest Utica EMG [Member]
Related Party Transaction [Line Items]
Other income - related parties	5	5		13	12
Ohio Gathering Company, LLC [Member]
Related Party Transaction [Line Items]
Other income - related parties	4	5		12	12
Other [Member]
Related Party Transaction [Line Items]
Other income - related parties	$ 4	$ 2		$ 14	$ 7
[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 58

Related Party Agreements and Transaction - Summary of Charges for Omnibus Agreement (Detail) - MPC - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Related Party Transaction [Line Items]
Charges for services under an omnibus agreement	$ 26	$ 22	$ 78	$ 62
Purchases - related parties
Related Party Transaction [Line Items]
Charges for services under an omnibus agreement	17	11	50	29
General and administrative expenses
Related Party Transaction [Line Items]
Charges for services under an omnibus agreement	$ 9	$ 11	$ 28	$ 33

Related Party Agreements and 59

Related Party Agreements and Transactions - Summary of Related Party Costs Added to Property, Plant and Equipment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
MPC | Construction-in-progress
Property, Plant and Equipment [Line Items]
Costs added to property, plant and equipment	$ 11	$ 14	$ 33	$ 36

Related Party Agreements and 60

Related Party Agreements and Transactions - Employee Services Agreements (Detail) - MPC - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	$ 122	$ 125	$ 354	$ 332
Purchases - related parties
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	97	98	280	257
General and administrative expenses
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	$ 25	$ 27	$ 74	$ 75

Related Party Agreements and 61

Related Party Agreements and Transactions - Receivables from Related Parties (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Related Party Transaction [Line Items]
Receivables - related parties	$ 152	$ 247
MPC
Related Party Transaction [Line Items]
Receivables - related parties	144	242
MarkWest Utica EMG [Member]
Related Party Transaction [Line Items]
Receivables - related parties	2	2
Ohio Gathering Company, LLC [Member]
Related Party Transaction [Line Items]
Receivables - related parties	2	2
Other [Member]
Related Party Transaction [Line Items]
Receivables - related parties	$ 4	$ 1

Related Party Agreements and 62

Related Party Agreements and Transactions Related Party Agreements and Transactions - Long-term Receivables from Related Parties (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Related Party Transaction [Line Items]
Long-term receivables - related parties	$ 18	$ 11
MPC
Related Party Transaction [Line Items]
Long-term receivables - related parties	$ 18	$ 11

Related Party Agreements and 63

Related Party Agreements and Transactions - Payables to Related Parties (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Related Party Transaction [Line Items]
Payables - related parties	$ 317	$ 87
MPC
Related Party Transaction [Line Items]
Payables - related parties	277	63
MarkWest Utica EMG [Member]
Related Party Transaction [Line Items]
Payables - related parties	30	24
Other [Member]
Related Party Transaction [Line Items]
Payables - related parties	10	0
Related Party Revolving Credit Agreement [Member] | MPC Investment [Member]
Related Party Transaction [Line Items]
Intercompany loan outstanding	$ 202	$ 0

Related Party Agreements and 64

Related Party Agreements and Transactions - Summary of Deferred Revenue - Related Parties (Detail) - MPC - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Related Party Transaction [Line Items]
Deferred revenue related parties	$ 82	$ 57
Minimum volume deficiencies [Member]
Related Party Transaction [Line Items]
Deferred revenue related parties	55	48
Project reimbursements [Member]
Related Party Transaction [Line Items]
Deferred revenue related parties	$ 27	$ 9

Net Income Per Limited Partne65

Net Income Per Limited Partner Unit - Schedule of Distributions by Partner by Class (Detail) - USD ($) shares in Millions, $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Net Income Per Share [Line Items]
Potentially Dilutive Units			1	1	8
Net income (loss) attributable to MPLX LP	[1]	$ 216	$ 141	$ 556	$ 100
Less: Distribution declared		336	249	926	680
Undistributed net income (loss) attributable to MPLX LP		(120)	(108)	(370)	(580)
Preferred Units
Net Income Per Share [Line Items]
Net income (loss) attributable to MPLX LP	[1]	16	16	49	25
Less: Distribution declared	[2]	16	16	49	25
Undistributed net income (loss) attributable to MPLX LP		0	0	0	0
General Partner | MPC
Net Income Per Share [Line Items]
Net income (loss) attributable to MPLX LP	[1]	86	51	222	136
Less: Distribution declared	[2]	88	54	229	148
Undistributed net income (loss) attributable to MPLX LP		(2)	(3)	(7)	(12)
Limited Partners Common Units
Net Income Per Share [Line Items]
Net income (loss) attributable to MPLX LP	[1]	114	74	285	(61)
Less: Distribution declared	[2]	232	179	648	507
Undistributed net income (loss) attributable to MPLX LP		$ (118)	$ (105)	$ (363)	$ (568)
[1]	Allocation of net income (loss) attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.
[2]	See Note 7 for distribution information.

Net Income Per Limited Partne66

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		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Net income (loss) attributable to MPLX LP:
Distributions declared (including IDRs)		$ 336	$ 249	$ 926	$ 680
Undistributed net income (loss) attributable to MPLX LP		(120)	(108)	(370)	(580)
Net income (loss) attributable to MPLX LP	[1]	$ 216	$ 141	$ 556	$ 100
Weighted average units outstanding:
Common - basic (in shares)		433	379	417	347
Common - diluted (in shares)		434	384	420	347
Limited Partners Common Units
Net income (loss) attributable to MPLX LP:
Distributions declared (including IDRs)	[2]	$ 232	$ 179	$ 648	$ 507
Undistributed net income (loss) attributable to MPLX LP		(118)	(105)	(363)	(568)
Net income (loss) attributable to MPLX LP	[1]	$ 114	$ 74	$ 285	$ (61)
Weighted average units outstanding:
Common - basic (in shares)		394	341	378	324
Common - diluted (in shares)		395	346	381	324
Net income attributable to MPLX LP per limited partner unit:
Basic (in USD per unit)		$ 0.29	$ 0.22	$ 0.75	$ (0.19)
Diluted (in USD per unit)		$ 0.29	$ 0.21	$ 0.75	$ (0.19)
Preferred Units
Net income (loss) attributable to MPLX LP:
Distributions declared (including IDRs)	[2]	$ 16	$ 16	$ 49	$ 25
Undistributed net income (loss) attributable to MPLX LP		0	0	0	0
Net income (loss) attributable to MPLX LP	[1]	$ 16	$ 16	$ 49	$ 25
Weighted average units outstanding:
Common - basic (in shares)		31	31	31	16
Common - diluted (in shares)		31	31	31	16
MPC | General Partner
Net income (loss) attributable to MPLX LP:
Distributions declared (including IDRs)	[2]	$ 88	$ 54	$ 229	$ 148
Undistributed net income (loss) attributable to MPLX LP		(2)	(3)	(7)	(12)
Net income (loss) attributable to MPLX LP	[1]	$ 86	$ 51	$ 222	$ 136
Weighted average units outstanding:
Common - basic (in shares)		8	7	8	7
Common - diluted (in shares)		8	7	8	7
[1]	Allocation of net income (loss) attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.
[2]	See Note 7 for distribution information.

Equity - Changes in Partners Ca

Equity - Changes in Partners Capital, Unit Rollforward (Details) - USD ($) $ in Millions		Sep. 01, 2017	Jul. 01, 2017		Mar. 01, 2017	Mar. 31, 2016	Sep. 30, 2017		Sep. 30, 2016
Stockholders Equity [Line Items]
Balance at December 31, 2016							368,555,271
Units converted	[1]						187,254
Balance at September 30, 2017							415,352,408
Sale of units for GP Interest							$ 483		$ 510
Limited Liability Company (LLC) or Limited Partnership (LP), Managing Member or General Partner, Ownership Interest		2.00%			2.00%	2.00%	2.00%
Equity Instruments Conversion Date			Jul. 1, 2017
Limited Partners Common Units
Stockholders Equity [Line Items]
Balance at December 31, 2016							357,193,288
Units converted			4,350,057	[2]			183,509	[1]
Contribution of the Joint-Interest Acquisition									21,401,137
Balance at September 30, 2017							407,045,362
Class B Units [Member]
Stockholders Equity [Line Items]
Balance at December 31, 2016							3,990,878
Units converted	[2]		3,990,878
Balance at September 30, 2017							0
General Partner Units
Stockholders Equity [Line Items]
Balance at December 31, 2016							7,371,105
Units converted	[1]						3,745
Contribution of the Joint-Interest Acquisition									436,758
Balance at September 30, 2017							8,307,046
Sale of units for GP Interest							$ 1
ATM Program [Member]
Stockholders Equity [Line Items]
Issuance of units under the ATM Program	[3]						14,129,589
ATM Program [Member] | Limited Partners Common Units
Stockholders Equity [Line Items]
Issuance of units under the ATM Program	[3]						13,846,998
ATM Program [Member] | General Partner Units
Stockholders Equity [Line Items]
Issuance of units under the ATM Program	[3]						282,591
Sale of units for GP Interest							$ 10
Class B Units [Member]
Stockholders Equity [Line Items]
Units converted	[2]						366,509
Class B Units [Member] | General Partner Units
Stockholders Equity [Line Items]
Units converted	[2]		7,330
Sale of units for GP Interest							$ 1
HST & WHC & MPLXT [Member]
Stockholders Equity [Line Items]
Contribution of HST/WHC/MPLXT	[4]						13,224,873
HST & WHC & MPLXT [Member] | Limited Partners Common Units
Stockholders Equity [Line Items]
Contribution of HST/WHC/MPLXT	[4]						12,960,376
HST & WHC & MPLXT [Member] | General Partner Units
Stockholders Equity [Line Items]
Contribution of HST/WHC/MPLXT					264,497		264,497	[4]
LOOP LOCAP SAX and Explorer Llc [Member]
Stockholders Equity [Line Items]
Contribution of the Joint-Interest Acquisition	[4]						18,888,912
LOOP LOCAP SAX and Explorer Llc [Member] | Limited Partners Common Units
Stockholders Equity [Line Items]
Contribution of the Joint-Interest Acquisition	[4]						18,511,134
LOOP LOCAP SAX and Explorer Llc [Member] | General Partner Units
Stockholders Equity [Line Items]
Contribution of HST/WHC/MPLXT	[4]	377,778
Contribution of the Joint-Interest Acquisition	[4]						(377,778)
[1]	As a result of the unit-based compensation awards issued during the period, MPLX GP contributed less than $1 million in exchange for 3,745 general partner units to maintain its two percent GP Interest.
[2]	On July 1, 2017, 3,990,878 Class B units converted to 4,350,057 common units and were eligible to receive the second quarter 2017 distribution. As a result of the Class B conversion, MPLX GP contributed less than $1 million in exchange for 7,330 general partner units to maintain its two percent GP Interest.
[3]	As a result of common units issued under the ATM Program during the period, MPLX GP contributed $10 million in exchange for 282,591 general partner units to maintain its two percent GP Interest.
[4]	See Note 3 for information regarding this acquisition.

Equity - Reorganization Transac

Equity - Reorganization Transaction (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Stockholders Equity Note [Line Items]
Contribution from MPC	$ 0	$ 225
General Partner Units
Stockholders Equity Note [Line Items]
Partners' Capital Account, Units, Contributed		436,758
Limited Partners Common Units
Stockholders Equity Note [Line Items]
Partners' Capital Account, Units, Contributed		21,401,137
MPC | Limited Partners Common Units
Stockholders Equity Note [Line Items]
Contribution from MPC		$ 84
MPC | General Partner
Stockholders Equity Note [Line Items]
Contribution from MPC		$ 141

Equity - Net Income Allocation

Equity - Net Income Allocation (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Stockholders Equity [Line Items]
Net income (loss) attributable to MPLX LP	[1]	$ 216	$ 141	$ 556	$ 100
Preferred unit distributions		(336)	(249)	(926)	(680)
General partner's IDRs and other		83	49	216	137
Net income (loss) attributable to MPLX LP available to general and limited partners		117	76	291	(62)
General partner's two percent GP Interest in net income (loss) attributable to MPLX LP		3	2	6	(1)
Less: General partner’s interest in net income attributable to MPLX LP		86	51	222	136
Preferred Units
Stockholders Equity [Line Items]
Net income (loss) attributable to MPLX LP	[1]	16	16	49	25
Preferred unit distributions	[2]	$ (16)	$ (16)	$ (49)	$ (25)
[1]	Allocation of net income (loss) attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.
[2]	See Note 7 for distribution information.

Equity - Cash Distributions (De

Equity - Cash Distributions (Details) - USD ($) $ / shares in Units, $ in Millions	Nov. 14, 2017	Nov. 06, 2017	Oct. 25, 2017	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distributions declared (including IDRs)				$ 336	$ 249	$ 926	$ 680
General partner's distributions				7	5	18	13
General partner's incentive distribution rights				81	49	211	135
Total general partner's distributions				88	54	229	148
Partners' distributions				336	249	926	680
Limited Partners Common Units
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Partners' distributions				232	179	648	507
Preferred Units [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Partners' distributions				$ 16	$ 16	$ 49	$ 25
Subsequent Event [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Distribution Made to Limited Partner, Declaration Date			Oct. 25, 2017
Distributions declared (including IDRs)			$ 320
Cash distributions declared per limited partner common unit			$ 0.5875
Distribution Made to Limited Partner, Distribution Date	Nov. 14, 2017
Distribution Made to Limited Partner, Date of Record		Nov. 6, 2017

Redeemable Preferred Units (Det

Redeemable Preferred Units (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	May 13, 2016	Sep. 30, 2017	Sep. 30, 2016
Redeemable Noncontrolling Interest [Line Items]
Issuance of redeemable preferred units		$ 0	$ 984
Series A Convertible Preferred Units [Member]
Redeemable Noncontrolling Interest [Line Items]
Issuance of preferred units	30.8
Preferred Stock, Dividend Rate, Percentage	6.50%
Shares Issued, Price Per Share	$ 32.50
Issuance of redeemable preferred units	$ 984
Preferred Stock, Dividend Rate, Per-Dollar-Amount	$ 0.528125
Preferred Units, Description	The holders may convert their Preferred units into common units at any time after the third anniversary of the issuance date or prior to liquidation, dissolution or winding up of the Partnership, in full or in part, subject to minimum conversion amounts and conditions. After the fourth anniversary of the issuance date, the Partnership may convert the Preferred units into common units at any time, in whole or in part, subject to certain minimum conversion amounts and conditions, if the closing price of MPLX LP common units is greater than $48.75 for the 20 day trading period immediately preceding the conversion notice date. The conversion rate for the Preferred units shall be the quotient of (a) the sum of (i) $32.50, plus (ii) any unpaid cash distributions on the applicable Preferred unit, divided by (b) $32.50. The holders of the Preferred units are entitled to vote on an as-converted basis with the common unitholders and will have certain other class voting rights with respect to any amendment to the Partnership Agreement that would adversely affect any rights, preferences or privileges of the Preferred units. In addition, upon certain events involving a change of control the holders of Preferred units may elect, among other potential elections, to convert their Preferred units to common units at the then-change of control conversion rate.

Redeemable Preferred Units Rede

Redeemable Preferred Units Redeemable Preferred Units Rollforward (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Redeemable Noncontrolling Interest [Line Items]
Balance at December 31, 2016	$ 1,000
Net income	49
Distributions to unitholders and general partner	(800)	$ (612)
Balance at September 30, 2017	1,000
Preferred Units
Redeemable Noncontrolling Interest [Line Items]
Distributions to unitholders and general partner	$ 49

Segment Information (Details)

Segment Information (Details)	9 Months Ended
	Sep. 30, 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			9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016		Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Segment revenues	$ 217	$ 157		$ 611	$ 394
Segment other income	2	2		5	5
Total segment revenues and other income	980	838	[1]	2,782	2,181	[1]
Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment revenues	1,047	906		2,964	2,496
Segment other income	12	13		37	43
Total segment revenues and other income	1,059	919		3,001	2,539
Segment cost of revenues	452	392		1,281	1,054
Segment operating income before portion attributable to noncontrolling interests and Predecessor	607	527		1,720	1,485
Segment portion attributable to noncontrolling interests and Predecessor	45	110		172	329
Segment operating income attributable to MPLX LP	562	417		1,548	1,156
Operating Segments [Member] | L&S [Member]
Segment Reporting Information [Line Items]
Segment revenues	378	339		1,095	901
Segment other income	11	12		35	42
Total segment revenues and other income	389	351		1,130	943
Segment cost of revenues	176	153		500	392
Segment operating income before portion attributable to noncontrolling interests and Predecessor	213	198		630	551
Segment portion attributable to noncontrolling interests and Predecessor	0	74		53	216
Segment operating income attributable to MPLX LP	213	124		577	335
Operating Segments [Member] | G&P [Member]
Segment Reporting Information [Line Items]
Segment revenues	669	567		1,869	1,595
Segment other income	1	1		2	1
Total segment revenues and other income	670	568		1,871	1,596
Segment cost of revenues	276	239		781	662
Segment operating income before portion attributable to noncontrolling interests and Predecessor	394	329		1,090	934
Segment portion attributable to noncontrolling interests and Predecessor	45	36		119	113
Segment operating income attributable to MPLX LP	$ 349	$ 293		$ 971	$ 821
[1]	Financial information has been retrospectively adjusted for the acquisition of HST, WHC and MPLXT from MPC. See Notes 1 and 3.

Segment Information - Reconcili

Segment Information - Reconciliation to Income from Operations (Details) - USD ($) $ in Millions		3 Months Ended			9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016		Sep. 30, 2017		Sep. 30, 2016
Segment Reporting, Revenue Reconciling Item [Line Items]
Segment portion attributable to Predecessor		$ 0	$ 51	[1]	$ 36		$ 149	[1]
Income (loss) from equity method investments		23	6		29	[2]	(72)	[2]
Other income - related parties		22	22	[1]	69		67	[1]
Depreciation and amortization		(164)	(151)	[1]	(515)		(438)	[1],[3]
Impairment expense		0	0		0		(130)
General and administrative expenses		(59)	(56)	[1]	(174)		(172)	[1]
Income from operations		311	258	[1]	856		436	[1]
Operating Segments [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Segment operating income attributable to MPLX LP		562	417		1,548		1,156
Segment Reconciling Items [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Segment portion attributable to unconsolidated affiliates		(47)	(41)		(125)		(130)
Segment portion attributable to Predecessor		0	74		53		216
Other income - related parties		13	11		38		29
Unrealized derivative (losses) gains(1)	[4]	(17)	(2)		2		(23)
L&S [Member] | Operating Segments [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Segment operating income attributable to MPLX LP		213	124		577		335
G&P [Member] | Operating Segments [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Segment operating income attributable to MPLX LP		$ 349	$ 293		$ 971		$ 821
[1]	Financial information has been retrospectively adjusted for the acquisition of HST, WHC and MPLXT from MPC. See Notes 1 and 3.
[2]	Income (loss) from equity method investments includes the impact of any basis differential amortization or accretion.
[3]	Financial information has been retrospectively adjusted for the acquisition of HST, WHC and MPLXT from MPC. See Notes 1 and 3.
[4]	The Partnership makes a distinction between realized or unrealized gains and losses on derivatives. During the period when a derivative contract is outstanding, changes in the fair value of the derivative are recorded as an unrealized gain or loss. When a derivative contract matures or is settled, the previously recorded unrealized gain or loss is reversed and the realized gain or loss of the contract is recorded

Segment Information - Reconci76

Segment Information - Reconciliation to Total Revenues and Other Income (Details) - USD ($) $ in Millions		3 Months Ended			9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016		Sep. 30, 2017		Sep. 30, 2016
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues and other income		$ 980	$ 838	[1]	$ 2,782		$ 2,181	[1]
Income (loss) from equity method investments		23	6		29	[2]	(72)	[2]
Other income - related parties		22	22	[1]	69		67	[1]
Operating Segments [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues and other income		1,059	919		3,001		2,539
Segment Reconciling Items [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue adjustment from unconsolidated affiliates		(107)	(100)		(287)		(303)
Other income - related parties		13	11		38		29
Unrealized derivative (losses) gains related to product sales(1)	[3]	$ (8)	$ 2		$ 1		$ (12)
[1]	Financial information has been retrospectively adjusted for the acquisition of HST, WHC and MPLXT from MPC. See Notes 1 and 3.
[2]	Income (loss) from equity method investments includes the impact of any basis differential amortization or accretion.
[3]	The Partnership makes a distinction between realized or unrealized gains and losses on derivatives. During the period when a derivative contract is outstanding, changes in the fair value of the derivative are recorded as an unrealized gain or loss. When a derivative contract matures or is settled, the previously recorded unrealized gain or loss is reversed and the realized gain or loss of the contract is recorded

Segment Information - Reconci77

Segment Information - Reconciliation to Net Income Attributable to Noncontrolling Interests (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reconciliation to Net Income Attributable to Noncontrolling Interests [Line Items]
Net income attributable to noncontrolling interests and Predecessor	$ 1	$ 53	$ 39	$ 152
Operating Segments [Member]
Reconciliation to Net Income Attributable to Noncontrolling Interests [Line Items]
Segment portion attributable to noncontrolling interests and Predecessor	45	110	172	329
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	(21)	(39)	(84)	(157)
Portion of operating income attributable to noncontrolling interests of unconsolidated affiliates	$ (23)	$ (18)	$ (49)	$ (20)

Segment Information - Reconci78

Segment Information - Reconciliation of Capital Expenditures (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting, Other Significant Reconciling Item [Line Items]
Additions to property, plant and equipment	$ (352)	$ (337)	$ (1,004)	$ (943)	[1]
Operating Segments [Member]
Segment Reporting, Other Significant Reconciling Item [Line Items]
Additions to property, plant and equipment	(453)	(371)	(1,310)	(1,037)
Operating Segments [Member] | L&S [Member]
Segment Reporting, Other Significant Reconciling Item [Line Items]
Additions to property, plant and equipment	(120)	(188)	(353)	(369)
Operating Segments [Member] | G&P [Member]
Segment Reporting, Other Significant Reconciling Item [Line Items]
Additions to property, plant and equipment	(333)	(183)	(957)	(668)
Segment Reconciling Items [Member]
Segment Reporting, Other Significant Reconciling Item [Line Items]
Additions to property, plant and equipment	$ 101	$ 34	$ 306	$ 94
[1]	Financial information has been retrospectively adjusted for the acquisition of HST, WHC and MPLXT from MPC. See Notes 1 and 3.

Segment Information - Assets by

Segment Information - Assets by Segment (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Segment Reporting, Other Significant Reconciling Item [Line Items]
Cash and cash equivalents	$ 3	$ 234	$ 208	$ 43
Assets	19,238	17,509
L&S [Member] | Operating Segments [Member]
Segment Reporting, Other Significant Reconciling Item [Line Items]
Assets	4,520	2,978
G&P [Member] | Operating Segments [Member]
Segment Reporting, Other Significant Reconciling Item [Line Items]
Assets	$ 14,715	$ 14,297

Inventories (Details)

Inventories (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Inventory Disclosure [Abstract]
NGLs	$ 3	$ 2
Line fill	9	9
Spare parts, materials and supplies	52	44
Inventories	$ 64	$ 55

Property, Plant and Equipment -

Property, Plant and Equipment - Property, Plant and Equipment (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 14,054	$ 13,102
Less accumulated depreciation	2,132	1,694
Property, plant and equipment, net	11,922	11,408
Natural gas gathering and NGL transportation pipelines and facilities
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	5,101	4,748
Processing, fractionation and storage facilities
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	3,753	3,547	[1]
Pipelines and related assets
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	2,181	1,799
Barges and towing vessels
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	484	479
Terminals and related assets
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	794	759	[1]
Land, building, office equipment and other
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	755	757
Construction-in-progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 986	$ 1,013
[1]	Certain prior period amounts have been updated to conform to current period presentation.

Fair Value Measurements - Recur

Fair Value Measurements - Recurring - Financial Instruments by Valuation Hierarchy (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	$ 0	$ 0
Derivative liability	57	60
Commodity contracts | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	0	0
Derivative liability	0	0
Commodity contracts | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	0	0
Derivative liability	5	6
Embedded derivatives in commodity contracts | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	0	0
Derivative liability	$ 52	$ 54

Fair Value Measurments - Recurr

Fair Value Measurments - Recurring - Significant Unobservable Inputs in Level 3 Valuation (Details) - Fair Value, Inputs, Level 3 [Member]	9 Months Ended
	Sep. 30, 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]	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 Fair Va

Fair Value Measurements Fair Value Measurements - Recurring (Details)	Sep. 30, 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	2

Fair Value Measurements - Rec85

Fair Value Measurements - Recurring - Changes in Level 3 Measurements (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 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		$ 2	$ (4)	$ (6)	$ 7
Total gains (losses) (realized and unrealized) included in earnings	[1]	(10)	2	(3)	(5)
Settlements		3	(1)	4	(6)
Netting adjustment			0		1	[2]
Fair value at end of period		(5)	(3)	(5)	(3)
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		(7)	0	(4)	(4)
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		(43)	(40)	(54)	(32)
Total gains (losses) (realized and unrealized) included in earnings	[1]	(12)	(6)	(4)	(17)
Settlements		3	2	6	5
Netting adjustment			0		0
Fair value at end of period		(52)	(44)	(52)	(44)
The amount of total gains (losses) for the period included in earnings attributable to the change in unrealized losses relating to liabilities still held at end of period		$ (10)	$ (4)	$ (4)	$ (15)
[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 and Cost of revenues.
[2]	Certain derivative positions are subject to master netting agreements; therefore, the Partnership has elected to offset derivative assets and liabilities where legally permissible. The Partnership may hold positions with certain counterparties, which for GAAP purposes are classified within different levels of the fair value hierarchy and may be legally permissible to offset. This adjustment represents the total impact of offsetting Level 2 positions with Level 3 positions as of September 30, 2016.

Fair Value Measurements - Repor

Fair Value Measurements - Reported (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	$ 6,869	$ 4,422
SMR liability	92	96
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	7,619	4,953
SMR liability	$ 106	$ 108

Derivative Financial Instrume87

Derivative Financial Instruments - Volume of Commodity Derivative Activity (Details) - Not Designated as Hedging Instrument [Member] - Commodity contracts	Sep. 30, 2017MMBTUbblgal
Short [Member]
Derivative [Line Items]
Nonmonetary Notional Amount of Price Risk Derivatives of Crude Oil | bbl	18,400
Nonmonetary Notional Amount of Price Risk Derivatives of Natural Gas Liquids | gal	33,387,904
Long [Member]
Derivative [Line Items]
Nonmonetary Notional Amount of Price Risk Derivatives Of Natural Gas | MMBTU	1,096,539

Derivative Financial Instrume88

Derivative Financial Instruments - Embedded Derivatives in Commodity Contracts (Details) - Embedded derivatives in commodity contracts $ in Millions	9 Months Ended
	Sep. 30, 2017USD ($)	Dec. 31, 2016USD ($)
Natural Gas [Member]
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	$ 52	$ 54
Electricity [Member]
Derivative [Line Items]
Embedded Derivative, Fair Value of Embedded Derivative Liability	$ 1

Derivative Financial Instrume89

Derivative Financial Instruments - Derivatives Balance Sheet Location (Details) - Not Designated as Hedging Instrument [Member] - Commodity contracts - USD ($) $ in Millions		Sep. 30, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	$ 0	$ 0
Derivative Liability, Fair Value, Gross Liability	[1]	57	60
Other current assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	0	0
Other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	[1]	15	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]	$ 42	$ 47
[1]	Includes embedded derivatives in commodity contracts as discussed above.

Derivative Financial Instrume90

Derivative Financial Instruments - Net Derivatives (Details) - Not Designated as Hedging Instrument [Member] $ in Millions	Sep. 30, 2017USD ($)
Assets
Gross Amount	$ 0
Gross Amounts Offset in the Consolidated Balance Sheets	0
Net Amount of Assets in the Consolidated Balance Sheets	0
Liabilities
Gross Amount	(57)
Gross Amounts Offset in the Consolidated Balance Sheets	0
Net Amount of Liabilities in the Consolidated Balance Sheets	(57)
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
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
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
Current Liabilities [Member]
Liabilities
Gross Amount	(15)
Gross Amounts Offset in the Consolidated Balance Sheets	0
Net Amount of Liabilities in the Consolidated Balance Sheets	(15)
Current Liabilities [Member] | Commodity contracts
Liabilities
Gross Amount	(5)
Gross Amounts Offset in the Consolidated Balance Sheets	0
Net Amount of Liabilities in the Consolidated Balance Sheets	(5)
Current Liabilities [Member] | Embedded derivatives in commodity contracts
Liabilities
Gross Amount	(10)
Gross Amounts Offset in the Consolidated Balance Sheets	0
Net Amount of Liabilities in the Consolidated Balance Sheets	(10)
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
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 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
Non Current Liabilities [Member]
Liabilities
Gross Amount	(42)
Gross Amounts Offset in the Consolidated Balance Sheets	0
Net Amount of Liabilities in the Consolidated Balance Sheets	(42)
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
Non Current Liabilities [Member] | Embedded derivatives in commodity contracts
Liabilities
Gross Amount	(42)
Gross Amounts Offset in the Consolidated Balance Sheets	0
Net Amount of Liabilities in the Consolidated Balance Sheets	$ (42)

Derivatives Financial Instrumen

Derivatives Financial Instruments - Derivative Income Statement Location (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016		Sep. 30, 2017	Sep. 30, 2016
Derivative [Line Items]
Total gain (loss)	$ (21)	$ (3)		$ (7)	$ (23)
Product sales
Derivative [Line Items]
Realized gain (loss)	(2)	0		(3)	6
Unrealized gain (loss)	(8)	2		1	(12)
Total gain (loss)	(10)	2		(2)	(6)
Purchased product costs
Derivative [Line Items]
Realized gain (loss)	(2)	(1)	[1]	(6)	(4)	[1]
Unrealized gain (loss)	(9)	(3)		1	(12)
Total gain (loss)	(11)	(4)		(5)	(16)
Cost of revenues
Derivative [Line Items]
Realized gain (loss)	0	0	[1]	0	(2)	[1]
Unrealized gain (loss)	0	(1)		0	1
Total gain (loss)	$ 0	$ (1)		$ 0	$ (1)
[1]	Certain prior period amounts have been updated to conform to current period presentation.

Debt - Summary of Outstanding B

Debt - Summary of Outstanding Borrowings (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Feb. 10, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Total	$ 7,277		$ 4,858
Unamortized debt issuance costs	(27)		(7)
Unamortized discount	(401)		(428)
Amounts due within one year	(1)		(1)
Total long-term debt due after one year	6,848		4,422
Capital Lease Obligations [Member] | Marathon Pipe Line LLC [Member]
Debt Instrument [Line Items]
MPL - capital lease obligations due 2020	7		8
Bank revolving credit facility due 2022 | Line of Credit [Member] | MPLX LP [Member]
Debt Instrument [Line Items]
Long-term debt, gross	420		0
Term loan facility due 2019 | Unsecured Debt [Member] | MPLX LP [Member]
Debt Instrument [Line Items]
Long-term debt, gross	0		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 March 2027 | Senior Notes [Member] | MPLX LP [Member]
Debt Instrument [Line Items]
Long-term debt, gross	1,250	$ 1,250	0
5.200% senior notes due March 2047 | Senior Notes [Member] | MPLX LP [Member]
Debt Instrument [Line Items]
Long-term debt, gross	1,000	$ 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

Debt Debt - Summary of Outstand

Debt Debt - Summary of Outstanding Borrowings - Interest Rates and Table Due Dates (Details)	9 Months Ended
	Sep. 30, 2017	Feb. 10, 2017
MarkWest [Member] | Senior Notes [Member] | 5.500% senior notes due February 2023
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.50%
Debt Instrument, Maturity Date	Feb. 15, 2023
MarkWest [Member] | Senior Notes [Member] | 4.500% senior notes due July 2023
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.50%
Debt Instrument, Maturity Date	Jul. 15, 2023
MarkWest [Member] | Senior Notes [Member] | 4.875% senior notes due December 2024
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.875%
Debt Instrument, Maturity Date	Dec. 1, 2024
MarkWest [Member] | Senior Notes [Member] | 4.875% senior notes due June 2025
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.875%
Debt Instrument, Maturity Date	Jun. 1, 2025
MPLX LP [Member] | Line of Credit [Member] | Bank revolving credit facility due 2022
Debt Instrument [Line Items]
Line of Credit Facility, Expiration Date	Jul. 21, 2022
MPLX LP [Member] | Line of Credit [Member] | Bank revolving credit facility due 2020
Debt Instrument [Line Items]
Line of Credit Facility, Expiration Date	Dec. 4, 2020
MPLX LP [Member] | Unsecured Debt [Member] | Term loan facility due 2019
Debt Instrument [Line Items]
Line of Credit Facility, Expiration Date	Nov. 20, 2019
MPLX LP [Member] | Senior Notes [Member] | 5.500% senior notes due February 2023
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.50%
Debt Instrument, Maturity Date	Feb. 15, 2023
MPLX LP [Member] | Senior Notes [Member] | 4.500% senior notes due July 2023
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.50%
Debt Instrument, Maturity Date	Jul. 15, 2023
MPLX LP [Member] | Senior Notes [Member] | 4.875% senior notes due December 2024
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.875%
Debt Instrument, Maturity Date	Dec. 1, 2024
MPLX LP [Member] | Senior Notes [Member] | 4.000% senior notes due February 2025
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.00%
Debt Instrument, Maturity Date	Feb. 15, 2025
MPLX LP [Member] | Senior Notes [Member] | 4.875% senior notes due June 2025
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.875%
Debt Instrument, Maturity Date	Jun. 1, 2025
MPLX LP [Member] | Senior Notes [Member] | 4.125% senior notes due March 2027
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.125%	4.125%
Debt Instrument, Maturity Date	Mar. 1, 2027
MPLX LP [Member] | Senior Notes [Member] | 5.200% senior notes due March 2047
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.20%	5.20%
Debt Instrument, Maturity Date	Mar. 1, 2047
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	Jul. 21, 2017	Oct. 27, 2015	Mar. 31, 2017	Sep. 30, 2017	Feb. 10, 2017	Dec. 31, 2016	Nov. 20, 2014
Bank revolving credit facility due 2020
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 2,000
Debt Instrument, Term		5 years
Proceeds from Lines of Credit				$ 0
Bank revolving credit facility due 2022
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 2,250
Debt Instrument, Term	5 years
Proceeds from Lines of Credit				$ 420
Debt Instrument, Interest Rate, Effective Percentage				2.702%
Letters of Credit Outstanding				$ 3
Line of Credit Facility, Remaining Borrowing Capacity				$ 1,827
Line of Credit Facility, Remaining Borrowing Capacity, Percentage				81.20%
Term loan facility due 2019
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage				2.407%
Line of Credit Facility, Current Borrowing Capacity							$ 250
Line of Credit [Member] | Bank revolving credit facility due 2022
Debt Instrument [Line Items]
Long-term debt, gross				$ 420		$ 0
Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount					$ 2,250
Proceeds from new debt			$ 2,220
Senior Notes [Member] | 4.125% senior notes due March 2027
Debt Instrument [Line Items]
Long-term debt, gross				$ 1,250	$ 1,250	0
Debt Instrument, Interest Rate, Stated Percentage				4.125%	4.125%
Senior Notes [Member] | 5.200% senior notes due March 2047
Debt Instrument [Line Items]
Long-term debt, gross				$ 1,000	$ 1,000	$ 0
Debt Instrument, Interest Rate, Stated Percentage				5.20%	5.20%

Supplemental Cash Flow Inform95

Supplemental Cash Flow Information - Summary of Supplemental Cash Flow Information (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017		Sep. 30, 2016
Net cash provided by operating activities included:
Interest paid (net of amounts capitalized)	$ 207		$ 158
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	$ 337	[1]	$ 0
[1]	Contribution of assets to Sherwood Midstream and Sherwood Midstream Holdings. See Note 4.

Supplemental Cash Flow Inform96

Supplemental Cash Flow Information - Summary of Reconciliation of Additions to Property, Plant and Equipment to Total Capital Expenditures (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Supplemental Cash Flow Elements [Abstract]
Increase in capital accruals	$ 55	$ 0

Equity-Based Compensation Pla97

Equity-Based Compensation Plan - Summary of Phantom Unit Award Activity (Details) - Phantom Units [Member]	9 Months Ended
	Sep. 30, 2017$ / sharesshares
Number of Units
Outstanding at December 31, 2016 | shares	1,173,411
Granted | shares	653,721
Settled | shares	288,584
Forfeited | shares	113,107
Outstanding at September 30, 2017 | shares	1,425,441
Weighted Average Fair Value
Outstanding at December 31, 2016 | $ / shares	$ 33.09
Granted | $ / shares	36.36
Settled | $ / shares	33.50
Forfeited | $ / shares	34.59
Outstanding at September 30, 2017 | $ / shares	$ 34.39

Equity-Based Compensation Pla98

Equity-Based Compensation Plan - Additional Information (Detail) - Performance Shares [Member] - MPLX LP 2012 Incentive Compensation Plan [Member]	9 Months Ended
	Sep. 30, 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 Pla99

Equity-Based Compensation Plan - Summary of Performance Unit Award Activity (Detail) - Performance Shares [Member]	9 Months Ended
	Sep. 30, 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	1,407,062
Settled	464,500
Forfeited	(35,217)
Outstanding at September 30, 2017	2,706,594

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Thousands	Sep. 06, 2016	Sep. 30, 2017	Dec. 31, 2016
Commitments And Contingencies [Line Items]
Accrued liabilities for environmental remediation		$ 14,000	$ 3,000
Contractual commitments to acquire property, plant and equipment		520,000
Environmental Loss Contingency [Member] | MPC
Commitments And Contingencies [Line Items]
Receivables from MPC for indemnification of environmental costs		0	$ 1,000
Apex [Member]
Commitments And Contingencies [Line Items]
Apex litigation settlement amount	$ 10,000
Markwest Liberty Midstream [Member]
Commitments And Contingencies [Line Items]
EPA proposed penalty		2,400
Estimated cost of proposed supplemental environmental projects		3,600
Revised EPA proposed penalty		1,240
Revised estimated cost of proposed supplemental environmental projects		$ 1,600