MPLX (MPLX) 10-Q 2 May 16 2016 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2016	Apr. 29, 2016
Document Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2016
Document Fiscal Year Focus	2,016
Document Fiscal Period Focus	Q1
Trading Symbol	MPLX
Entity Registrant Name	MPLX LP
Entity Central Index Key	1,552,000
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Common Units [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		331,281,560
Class B Units [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		7,981,756
General Partner Units [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		7,506,482

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) shares in Millions, $ in Millions		3 Months Ended
		Mar. 31, 2016		Mar. 31, 2015
Revenues and other income:
Service revenue		$ 229		$ 16	[1]
Service revenue - related parties		150		142	[1]
Rental income		70		0	[1]
Rental income - related parties		26		25	[1]
Product sales		100		0	[1]
Product sales - related parties		3		0	[1]
Income from equity method investments		5		0	[1],[2]
Other income		2		1	[1]
Other income - related parties		24		17	[1]
Total revenues and other income		609		201	[1]
Costs and expenses:
Cost of revenues (excludes items below)		89		42	[1]
Purchased product costs		79		0	[1]
Rental cost of sales		14		0	[1]
Purchases - related parties		76		40	[1]
Depreciation and amortization		132		19	[1],[2]
Impairment expense		129		0	[1],[2]
General and administrative expenses		52		22	[1]
Other taxes		11		4	[1]
Total costs and expenses		582		127	[1]
Income from operations		27		74	[1]
Related party interest and other financial costs		1		0
Interest expense (net of amounts capitalized of $7 million and $1 million, respectively)		55		5	[1]
Other financial costs		(12)		(1)	[1]
(Loss) income before income taxes		(41)	[3]	68	[1]
Benefit for income taxes		4	[3]	0	[1]
Net (loss) income		(37)		68	[1],[2],[4]
Less: Net income attributable to noncontrolling interests and Predecessor		23		22	[1]
Net (loss) income attributable to MPLX LP	[5]	(60)		46	[1]
Less: General partner’s interest in net income attributable to MPLX LP		39		4	[1]
Limited partners’ interest in net (loss) income attributable to MPLX LP		$ (99)		$ 42	[1]
Weighted average limited partner units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic		307		82
Weighted Average Limited Partnership Units Outstanding, Diluted		307		82
MPC [Member]
Revenues and other income:
Service revenue - related parties		$ 150		$ 142
Product sales - related parties		3		0
Other income - related parties		17		16
Limited Partners Common Units [Member]
Costs and expenses:
Net (loss) income attributable to MPLX LP	[5]	$ (99)		$ 20
Per Unit Data (See Note 6)
Common - basic (in USD per unit)		$ (0.33)		$ 0.46
Common - diluted (in USD per unit)		$ (0.33)		$ 0.46
Weighted average limited partner units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic		300		43
Weighted Average Limited Partnership Units Outstanding, Diluted		300		43
Cash distributions declared per limited partner common unit		$ 0.5050		$ 0.4100
Limited Partners Subordinated Units [Member] | MPC [Member]
Costs and expenses:
Net (loss) income attributable to MPLX LP	[5]			$ 17
Per Unit Data (See Note 6)
Common - basic (in USD per unit)				$ 0.46
Common - diluted (in USD per unit)				0.46
Subordinated - basic and diluted (in USD per unit)		$ 0		$ 0.46
Weighted average limited partner units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic				37
Weighted Average Limited Partnership Units Outstanding, Diluted				37
Weighted average limited partner units outstanding basic and diluted (in shares)		0		37
[1]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.
[2]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.
[3]	(1) Financial information has been retrospectively adjusted for the acquisition of HSM from MPC. See Notes 1 and 3. Prior to this acquisition, MPC paid all income taxes related to HSM.
[4]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.
[5]	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
	Mar. 31, 2016	Mar. 31, 2015
Income Statement [Abstract]
Interest Costs Capitalized	$ 7	$ 1

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions		Mar. 31, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents		$ 4	$ 43	[1]
Receivables, net		250	245	[1]
Receivables - related parties		102	187	[1]
Inventories		45	51	[1]
Other current assets		28	50	[1]
Total current assets		429	576	[1]
Equity method investments		2,598	2,458	[1]
Property, plant and equipment, net		10,195	9,997	[1]
Intangibles, net		520	466	[1]
Goodwill		2,200	2,570	[1]
Long-term receivables - related parties		25	25	[1]
Other noncurrent assets		11	12	[1]
Total assets		15,978	16,104	[1]
Current liabilities:
Accounts payable		105	91	[1]
Accrued liabilities		159	187	[1]
Payables - related parties		512	54	[1]
Deferred revenue - related parties		35	32	[1]
Accrued property, plant and equipment		140	168	[1]
Accrued taxes		24	27	[1]
Accrued interest payable		56	54	[1]
Other current liabilities		14	12	[1]
Total current liabilities		1,045	625	[1]
Long-term deferred revenue		6	4	[1]
Long-term deferred revenue - related parties		9	9	[1]
Long-term debt		4,715	5,255	[1]
Deferred income taxes		382	378	[1]
Deferred credits and other liabilities		166	166	[1]
Total liabilities		$ 6,323	$ 6,437	[1]
Commitments and contingencies (see Note 18)
Equity
Total MPLX LP partners’ capital		$ 9,641	$ 9,654	[1]
Noncontrolling interest		14	13	[1]
Total equity		9,655	9,667	[1],[2]
Total liabilities and equity		15,978	16,104	[1]
MPC [Member]
Current assets:
Receivables - related parties		96	175
Long-term receivables - related parties		25	25
Current liabilities:
Payables - related parties		491	33
Limited Partners Common Units [Member] | MPC [Member]
Equity
Total MPLX LP partners’ capital		1,086	465	[1]
Limited Partners Class B Units [Member]
Equity
Total MPLX LP partners’ capital		266	266	[1]
General Partner [Member] | MPC [Member]
Equity
Total MPLX LP partners’ capital		484	819	[1]
Public [Member] | Limited Partners Common Units [Member]
Equity
Total MPLX LP partners’ capital		7,805	7,691	[1]
Predecessor [Member] | MPC [Member]
Equity
Total MPLX LP partners’ capital		$ 0	413	[1]
Total equity	[2]		$ 413
[1]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.
[2]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - shares	Mar. 31, 2016	Dec. 31, 2015
Units outstanding	7,506,460
Limited Partners Common Units [Member] | Public [Member]
Units issued	251,000,000	240,000,000
Units outstanding	251,000,000	240,000,000
Limited Partners Class B Units [Member]
Units issued	8,000,000	8,000,000
Units outstanding	8,000,000	8,000,000
MPC [Member] | Limited Partners Common Units [Member]
Units issued	79,000,000	57,000,000
Units outstanding	79,000,000	57,000,000
MPC [Member] | General Partner [Member]
Units issued	8,000,000	7,000,000
Units outstanding	8,000,000	7,000,000

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016		Mar. 31, 2015
Operating activities:
Net (loss) income	$ (37)		$ 68	[1],[2],[3]
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization of deferred financing costs	11		0	[2]
Depreciation and amortization	132		19	[1],[2]
Impairment expense	129		0	[1],[2]
Deferred income taxes	(4)		0	[2]
Equity in earnings from unconsolidated affiliates	5		0	[1],[2]
Distributions from unconsolidated affiliates	38		0	[2]
Changes in:
Current receivables	(5)		2	[2]
Inventories	(1)		0	[2]
Change in fair value of derivatives	(12)		0	[2]
Current accounts payable and accrued liabilities	(15)		2	[2]
Receivables from / liabilities to related parties	23		(18)	[2]
All other, net	15		(1)	[2]
Net cash provided by operating activities	295		72	[2]
Investing activities:
Additions to property, plant and equipment	(291)		(35)	[2]
Investments - loans from (to) related parties	77		(8)	[2]
Investments in unconsolidated affiliates	29		0	[2]
All other, net	3		0	[2]
Net cash used in investing activities	(240)		(43)	[2]
Financing activities:
Long-term debt - borrowings	306		528	[2]
Long-term debt - repayments	(857)		(415)	[2]
Related party debt - borrowings	1,437		0	[2]
Related party debt - repayments	1,007		0	[2]
Debt issuance costs	0		4	[2]
Net proceeds from equity offerings	321		0	[2]
Distributions to unitholders and general partner	(190)		(33)	[2]
Distributions to noncontrolling interests	(1)		0	[2]
Contributions from noncontrolling interests	2		0	[2]
All other, net	(1)		0	[2]
Net cash (used in) provided by financing activities	(94)		76	[2]
Net (decrease) increase in cash and cash equivalents	(39)		105	[2]
Cash and cash equivalents at beginning of period	43	[4]	27	[2]
Cash and cash equivalents at end of period	4		132	[2]
MPC [Member]
Financing activities:
Distributions to MPC from Predecessor	104
Predecessor [Member] | MPC [Member]
Operating activities:
Net (loss) income	23		22	[3]
Financing activities:
Distributions to MPC from Predecessor	$ 104		$ 0	[2]
[1]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.
[2]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.
[3]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.
[4]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.

Consolidated Statements of Equi

Consolidated Statements of Equity - USD ($) $ in Millions		Total		MPC [Member]	Limited Partners Common Units [Member]Public [Member]	Limited Partners Common Units [Member]MPC [Member]	Limited Partners Class B Units [Member]	Limited Partners Subordinated Units [Member]MPC [Member]	General Partner [Member]MPC [Member]	Noncontrolling Interest [Member]	Noncontrolling Interest [Member]MPC [Member]
Beginning Balance (Predecessor [Member]) at Dec. 31, 2014	[1]			$ 321
Beginning Balance at Dec. 31, 2014	[1]	$ 784			$ 639	$ 261		$ 217	$ (660)		$ 6
Distributions to MPC from Predecessor | Predecessor [Member]	[2]			0
Net (loss) income | Predecessor [Member]	[1]			22
Net (loss) income	[1]	68	[2],[3]		12	11		19	4		0
Distributions to unitholders and general partner	[1]	(33)			(9)	(8)		(14)	(2)
Contributions from noncontrolling interests	[2]	0
Ending Balance (Predecessor [Member]) at Mar. 31, 2015	[1]			343
Ending Balance at Mar. 31, 2015	[1]	819			642	264		$ 222	(658)		$ 6
Beginning Balance (Predecessor [Member]) at Dec. 31, 2015	[1]			413
Beginning Balance at Dec. 31, 2015	[1]	9,667	[4]		7,691	465	$ 266		819	$ 13
Distributions to MPC from Predecessor | Predecessor [Member]				(104)
Distributions to MPC from Predecessor				(104)
Issuance of units under ATM Program		321			315				6
Net (loss) income | Predecessor [Member]				23
Net (loss) income		(37)			(80)	(19)			39
Contribution from MPC		15		15		12			3
Distribution to MPC		(15)				(12)			(3)
Allocation of MPC's net investment at acquisition | Predecessor [Member]				$ (332)
Allocation of MPC's net investment at acquisition						669			(337)
Distributions to unitholders and general partner		(190)			(120)	(29)			(41)
Distributions to noncontrolling interest		(1)								(1)
Contributions from noncontrolling interests		2								2
Equity-based compensation		2			2
Deferred income tax impact from changes in equity		(5)			(3)				(2)
Ending Balance at Mar. 31, 2016		$ 9,655			$ 7,805	$ 1,086	$ 266		$ 484	$ 14
[1]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.
[2]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.
[3]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.
[4]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.

Description of the Business and

Description of the Business and Basis of Presentation	3 Months Ended
	Mar. 31, 2016
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 MPC. 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 and storage of crude oil and refined petroleum products. On December 4, 2015, the Partnership completed a merger with MarkWest (the “MarkWest Merger”). See Note 3 for additional information. The Partnership’s business consists of two segments based on the nature of services it offers: Logistics and Storage (“L&S”) focused on crude oil and refined products and Gathering and Processing (“G&P”) focused on natural gas and NGLs. See Note 8 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, including MPC, have been recorded as Noncontrolling interest in the accompanying Consolidated Balance Sheets. Intercompany investments, accounts and transactions have been eliminated. The Partnership’s investments in which the Partnership exercises significant influence but does not control and does not have a controlling financial interest are accounted for using the equity method. The Partnership’s investments in a VIE in which the Partnership exercises significant influence but does not control and is not the primary beneficiary are also accounted for using the equity method. The accompanying consolidated financial statements of the Partnership have been prepared in accordance with GAAP. Reclassifications have been made in connection with the MarkWest Merger and HSM acquisition to conform to current classifications. These reclassifications had no effect on previously reported results of operations or retained earnings. Effective March 31, 2016, the Partnership acquired MPC’s inland marine business. This business is operated through Hardin Street Marine LLC (“HSM”). HSM’s related assets, liabilities and results of the operations are collectively referred to as the “Predecessor.” The acquisition from MPC was a transfer between entities under common control. As an entity under common control with MPC, the Partnership recorded the assets acquired 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. Accordingly, the accompanying financial statements and related notes of MPLX LP have been retrospectively adjusted to include the historical results of the assets acquired from MPC prior to the effective date of the acquisition. See Note 3 for additional information regarding the HSM acquisition. The accompanying financial statements and related notes present the combined financial position, results of operations, cash flows and equity of the Predecessor at historical cost. The financial statements of the Predecessor have been prepared from the separate records maintained by MPC and may not necessarily be indicative of the conditions or the results of operations that would have existed if the Predecessor had been operated as an unaffiliated entity. Based on the terms of certain natural gas gathering, transportation and processing agreements, the Partnership is considered to be the lessor under several implicit operating lease arrangements in accordance with GAAP. The Partnership’s primary implicit lease operations relate to a natural gas gathering agreement in the Marcellus shale for which it earns a fixed-fee for providing gathering services to a single producer customer using a dedicated gathering system. As the gathering system is expanded, the fixed-fee charged to the producer is adjusted to include the additional gathering assets in the lease. Other significant implicit leases relate to a natural gas processing agreement in the Marcellus shale and a natural gas processing agreement in the Southern Appalachia region for which the Partnership earns minimum monthly fees for providing processing services to a single producer using a dedicated processing plant. Similarly, the Partnership is considered to be the lessor under implicit operating lease arrangements with MPC in accordance with GAAP. The Partnership’s primary implicit lease operations with MPC relate to the transportation agreement between HSM and MPC. The rental expense related to the HSM implicit lease is depreciation of the HSM assets. These revenues and costs from implicit leases are recorded as Rental income and Rental expenses , respectively, on the Consolidated Statements of Income. All other services are provided to MPC on an as-needed basis and recorded as Service revenue-related parties on the Consolidated Statements of Income. 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 our Annual Report on Form 10-K for the year ended December 31, 2015 . The results of operations for the three months ended March 31, 2016 are not necessarily indicative of the results to be expected for the full year. In preparing our consolidated statements of equity, net income attributable to MPLX LP is allocated to unitholders in accordance with their respective ownership percentages. However, when distributions related to the incentive distribution rights are made, earnings equal to the amount of those distributions are first allocated to the general partner before the remaining earnings are allocated to the unitholders based on their respective ownership percentages. The allocation of net income attributable to MPLX LP for purposes of calculating net income per limited partner unit is described in Note 6 .

Accounting Standards

Accounting Standards	3 Months Ended
	Mar. 31, 2016
Accounting Standards [Abstract]
Accounting Standards	Accounting Standards Recently Adopted In September 2015, the FASB issued an accounting standards update that eliminates the requirement to restate prior period financial statements for measurement period adjustments for business combinations. This update requires that the cumulative impact of a measurement period adjustment be recognized in the reporting period in which the adjustment is identified. The standard is effective for interim and annual periods beginning after December 15, 2015 with early application permitted. The Partnership adopted this standard during the three months ended March 31, 2016 and as a result recorded its measurement period adjustments in the first quarter 2016 on a cumulative prospective basis as additional analysis was completed on the preliminary purchase price allocation for the acquisition of MarkWest. See Notes 3 and 15 for further discussion and detail related to the adjustment. In April 2015, the FASB issued an accounting standards update requiring that the earnings of transferred net assets prior to the dropdown date of the net assets to a master limited partnership be allocated entirely to the general partner when calculating earnings per unit under the two class method. Under this guidance, previously reported earnings per unit of the limited partners will not change as a result of a dropdown transaction. The change is effective for fiscal years and interim periods within those fiscal years beginning after December 15, 2015. Retrospective application is required and early adoption is permitted. The adoption of this standard did not have a material impact on the consolidated results of operations, financial position or cash flows. In April 2015, the FASB issued an accounting standards update clarifying whether a customer should account for a cloud computing arrangement as an acquisition of a software license or as a service arrangement by providing characteristics that a cloud computing arrangement must have in order to be accounted for as a software license acquisition. The change is effective for fiscal years and interim periods within those fiscal years beginning after December 15, 2015. Retrospective or prospective application is allowed and early adoption is permitted. The adoption of this standard did not have a material impact on the consolidated results of operations, financial position or cash flows. In February 2015, the FASB issued an accounting standards update making targeted changes to the current consolidation guidance. The new standard changes the considerations related to substantive rights, related parties, and decision making fees when applying the VIE consolidation model and eliminates certain guidance for limited partnerships and similar entities under the voting interest consolidation model. The update is effective for fiscal years and interim periods within those fiscal years beginning after December 15, 2015. Early adoption is permitted. The adoption of this standard did not have a material impact on the consolidated results of operations, financial position, cash flows or required disclosures. Not Yet Adopted 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 will also increase the amount an employer can withhold for tax purposes without triggering liability accounting. Lastly, the update allows employers to make a policy election to account for forfeitures as they occur. The changes are effective for fiscal years beginning after December 15, 2016 and early adoption is permitted. The Partnership is in the process of determining the impact of the new standard on the consolidated financial statements. In February 2016, the FASB issued an accounting standards update on lease accounting. This update requires lessees to put most leases on their balance sheets. The new standard also requires new disclosures to help financial statement users better understand the amount, timing and uncertainty of cash flows arising from leases. The accounting standards update will be effective on a retrospective or modified retrospective basis for annual reporting periods beginning after December 15, 2018, and interim periods within those years, with early adoption permitted. The Partnership is in the process of determining the impact of the new standard on the consolidated financial statements, and expects such impact to 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 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. Upon adoption, entities will be required to make a cumulative-effect adjustment to the consolidated results of operations as of the beginning of the first reporting period the guidance is effective. Early adoption is permitted only for the amendment regarding presentation of the liability’s credit risk. The Partnership is in the process of determining the impact of the new standard on the consolidated financial statements. In August 2014, the FASB issued an accounting standards update requiring management to assess an entity’s ability to continue as a going concern and to provide related footnote disclosures in certain circumstances. Management is required to assess if there is substantial doubt about an entity’s ability to continue as a going concern within one year after the date that the financial statements are issued. Disclosures will be required if conditions give rise to substantial doubt and the type of disclosure will be determined based on whether management’s plans will be able to alleviate the substantial doubt. This accounting standards update is effective for the first annual period ending after December 15, 2016, and for annual periods and interim periods thereafter with early application permitted. The adoption of this standard is not expected to have a material impact on the consolidated results of operations, financial position or cash flows. In May 2014, the FASB issued an initial accounting standards update for revenue recognition, which has had subsequent updates. The new standard is aligned with the International Accounting Standards Board’s revenue recognition standard. The guidance in the update states that revenue is recognized when a customer obtains control of a good or service. Recognition of the revenue will involve a multiple step approach including identifying the contract, identifying the separate performance obligations, determining the transaction price, allocating the price to the performance obligations and then recognizing the revenue as the obligations are satisfied. Additional disclosures will be required to provide adequate information to understand the nature, amount, timing and uncertainty of reported revenues and revenues expected to be recognized. The accounting standards update will be effective on a retrospective or modified retrospective basis for annual reporting periods beginning after December 15, 2017, and interim periods within those years, with early adoption permitted no earlier than January 1, 2017. The Partnership is in the process of determining the impact of the new standard on the consolidated financial statements.

Acquisitions

Acquisitions	3 Months Ended
	Mar. 31, 2016
Equity [Abstract]
Acquisitions	Acquisitions 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 Holdings LLC and MPC Investment LLC (“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 general partner interest in the Partnership. The acquisition closed on March 31, 2016 and the fair value of the common units and general partner units issued was $669 million and $14 million , respectively, as recorded on the Consolidated Statements of Equity. MPC agreed to waive distributions in the first quarter of 2016 on MPLX common units issued in connection with this transaction. MPC will not receive general partner distributions or incentive distribution rights that would otherwise accrue on such MPLX common units with respect to the first quarter distributions. The value of these waived distributions was $15 million . The inland marine business, comprised of 18 tow boats and 205 barges which transports light products, heavy oils, crude oil, renewable fuels, chemicals and feedstocks in the Midwest and U.S. Gulf Coast regions, accounts for nearly 60 percent of the total volumes MPC ships by inland marine vessels. The Partnership accounts for HSM as a reporting unit of the L&S segment. The Partnership retrospectively adjusted the historical financial results for all periods to include HSM as required for transactions between entities under common control. The following table presents the Partnership’s previously reported Consolidated Statement of Income, retrospectively adjusted for the acquisition of HSM: Three Months Ended March 31, 2015 (In millions, except per unit data) MPLX LP (Previously Reported) HSM MPLX LP (Currently Reported) Revenues and other income: Service revenue $ 16 $ — $ 16 Service revenue - related parties 111 31 142 Rental income - related parties 4 21 25 Other income 1 — 1 Other income - related parties 6 11 17 Total revenues and other income 138 63 201 Costs and expenses: Cost of revenues (excludes items below) 28 14 42 Purchases - related parties 24 16 40 Depreciation and amortization 12 7 19 General and administrative expenses 19 3 22 Other taxes 3 1 4 Total costs and expenses 86 41 127 Income from operations 52 22 74 Interest expense (net of amounts capitalized of $1 million) 5 — 5 Other financial costs 1 — 1 Income before income taxes 46 22 68 Net income 46 22 68 Less: Net income attributable to Predecessor — 22 22 Net income attributable to MPLX LP 46 — 46 Less: General partner’s interest in net income attributable to MPLX LP 4 — 4 Limited partners’ interest in net income attributable to MPLX LP $ 42 $ — $ 42 The following table presents the Partnership’s previously reported Consolidated Balance Sheet, retrospectively adjusted for the acquisition of HSM: December 31, 2015 (In millions) MPLX LP (Previously Reported) HSM MPLX LP (Currently Reported) Assets Current assets: Cash and cash equivalents $ 43 $ — $ 43 Receivables, net 244 1 245 Receivables - related parties 88 99 187 Inventories 49 2 51 Other current assets 50 — 50 Total current assets 474 102 576 Equity method investments 2,458 — 2,458 Property, plant and equipment, net 9,683 314 9,997 Intangibles, net 466 — 466 Goodwill 2,559 11 2,570 Long-term receivables - related parties 25 — 25 Other noncurrent assets 12 — 12 Total assets $ 15,677 $ 427 $ 16,104 Liabilities Current liabilities: Accounts payable $ 89 $ 2 $ 91 Accrued liabilities 186 1 187 Payables - related parties 47 7 54 Deferred revenue - related parties 32 — 32 Accrued property, plant and equipment 166 2 168 Accrued taxes 26 1 27 Accrued interest payable 54 — 54 Other current liabilities 12 — 12 Total current liabilities 612 13 625 Long-term deferred revenue 4 — 4 Long-term deferred revenue - related parties 9 — 9 Long-term debt 5,255 — 5,255 Deferred income taxes 377 1 378 Deferred credits and other liabilities 166 — 166 Total liabilities 6,423 14 6,437 Equity Common unitholders - public 7,691 — 7,691 Class B unitholders 266 — 266 Common unitholder - MPC 465 — 465 General partner - MPC 819 — 819 Equity of Predecessor — 413 413 Total MPLX LP partners’ capital 9,241 413 9,654 Noncontrolling interest 13 — 13 Total equity 9,254 413 9,667 Total liabilities and equity $ 15,677 $ 427 $ 16,104 The following table presents the Partnership’s previously reported Consolidated Statement of Cash Flows, retrospectively adjusted for the acquisition of HSM: Three Months Ended March 31, 2015 (In millions) MPLX LP (Previously Reported) HSM MPLX LP (Currently Reported) Increase (decrease) in cash and cash equivalents Operating activities: Net income $ 46 $ 22 $ 68 Adjustments to reconcile net income to net cash provided by operating activities: Depreciation and amortization 12 7 19 Changes in: Current receivables 2 — 2 Current accounts payable and accrued liabilities 5 (3 ) 2 Receivables from / liabilities to related parties (5 ) (13 ) (18 ) All other, net (1 ) — (1 ) Net cash provided by operating activities 59 13 72 Investing activities: Additions to property, plant and equipment (30 ) (5 ) (35 ) Investments - loans to related parties — (8 ) (8 ) Net cash used in investing activities (30 ) (13 ) (43 ) Financing activities: Long-term debt - borrowings 528 — 528 - repayments (415 ) — (415 ) Debt issuance costs (4 ) — (4 ) Distributions to unitholders and general partner (33 ) — (33 ) Net cash provided by financing activities 76 — 76 Net increase in cash and cash equivalents 105 — 105 Cash and cash equivalents at beginning of period 27 — 27 Cash and cash equivalents at end of period $ 132 $ — $ 132 Purchase of MarkWest Energy Partners, L.P. On December 4, 2015 , a wholly-owned subsidiary of the Partnership merged with MarkWest. Each common unit of MarkWest issued and outstanding immediately prior to the effective time of the MarkWest Merger was converted into a right to receive 1.09 common units representing limited partner interests in MPLX LP, plus a one-time cash payment of $6.20 per unit. Each Class B unit of MarkWest issued and outstanding immediately prior to the effective time of the MarkWest Merger was converted into the right to receive one Class B unit of MPLX LP. Each Class B unit of MPLX LP will convert into 1.09 common units of MPLX LP and the right to receive $6.20 in cash, and the conversion of the Class B units will occur in equal installments on July 1, 2016 and July 1, 2017 . MPC contributed approximately $1.3 billion of cash to the Partnership to pay the aggregate cash consideration to MarkWest unitholders, without receiving any new equity in exchange. At closing, MPC made a payment of $1.2 billion to MarkWest common unitholders and the remaining $50 million will be paid in equal amounts in July 2016 and 2017 in connection with the conversion of the Class B units to common units of MPLX LP. The Partnership’s financial results and operating statistics reflect the results of MarkWest from the date of the acquisition. The components of the fair value of consideration transferred are as follows: (In millions) Fair value of units issued $ 7,326 Cash 1,230 Payable to MarkWest Class B unitholders 50 Total fair value of consideration transferred $ 8,606 The following table summarizes the preliminary purchase price allocation. Subsequent to December 31, 2015 , additional analysis was completed and adjustments were made to the preliminary purchase price allocation as noted in the table below. The estimated fair value of assets acquired and liabilities and noncontrolling interests assumed at the acquisition date as of March 31, 2016 , are as follows: (In millions) As Originally Reported Adjustments As Adjusted Cash and cash equivalents $ 12 $ — $ 12 Receivables 164 — 164 Inventories 33 (1 ) 32 Other current assets 44 — 44 Equity method investments 2,457 143 2,600 Property, plant and equipment 8,474 43 8,517 Intangibles 468 65 533 Other noncurrent assets 5 — 5 Total assets acquired 11,657 250 11,907 Accounts payable 322 — 322 Accrued liabilities 13 6 19 Accrued taxes 21 — 21 Other current liabilities 44 — 44 Long-term debt 4,567 — 4,567 Deferred income taxes 374 3 377 Deferred credits and other liabilities 151 — 151 Noncontrolling interest 13 — 13 Total liabilities and noncontrolling interest assumed 5,505 9 5,514 Net assets acquired excluding goodwill 6,152 241 6,393 Goodwill 2,454 (241 ) 2,213 Net assets acquired $ 8,606 $ — $ 8,606 Adjustments to the preliminary purchase price stem mainly from additional information obtained by management in the first quarter about facts and circumstances that existed at the acquisition date, including updates to forecasted employee benefit costs, maintenance capital expenditures and completion of certain valuations to determine the underlying fair value of certain acquired assets. The adjustment to intangibles mainly relates to a misstatement in the original preliminary purchase price allocation. The correction of the error resulted in a $68 million reduction to the carrying value of goodwill and an offsetting increase of $64 million in intangibles and $2 million in each of equity method investments and property, plant and equipment. Management concluded that the correction of the error is immaterial to the consolidated financial statements of all periods presented. The Partnership is still completing its analysis of the final purchase price allocation. As further discussed in Note 15, the Partnership recorded a goodwill impairment charge based on the implied fair value of goodwill as of the interim impairment analysis date. Therefore, any future adjustments to the purchase price allocation may be offset by adjustments to the Impairment expense line item in the Consolidated Statements of Income. The increase to the fair value of intangibles and property, plant and equipment noted above resulted in additional amortization and depreciation expense of approximately $1 million recognized for the three months ended March 31, 2016, in Depreciation and amortization in the Consolidated Statements of Income , that would have been recorded for the year ended December 31, 2015, had the fair value adjustments been recorded as of December 4, 2015. The increase in the fair value of equity investments above would not have resulted in a material effect to the income from equity method investments had the fair value adjustment been recorded as of December 4, 2015. The purchase price allocation resulted in the recognition of $2.2 billion of goodwill in three reporting units within the Partnership’s G&P segment, substantially all of which is not deductible for tax purposes. Goodwill represents the complementary aspects of the highly diverse asset base of MarkWest and MPLX LP that will provide significant additional opportunities across multiple segments of the hydrocarbon value chain. The fair value of the common units issued was determined on the basis of the closing market price of the Partnership’s units as of the effective time of the transaction and is considered a Level 1 measurement. The fair value of the Class B units issued was determined based on reference to the value of the common units, adjusted for a lack of distributions prior to their stated conversion dates, and is considered a Level 2 measurement. The fair values of the long-term debt and SMR liabilities were determined as of the acquisition date using the methods discussed in Note 12 . The fair value of the equity method investments was determined based on applying the discounted cash flow method, which is an income approach, to the Partnership’s equity method investments on an individual basis. Key assumptions include discount rates of 9.4 percent to 11.1 percent and terminal values based on the Gordon growth method to capitalize the cash flows, using a 2.5 percent long-term growth rate. Intangibles represent customer contracts and related relationships. The fair value of the intangibles was determined based on applying the multi-period excess earnings method, which is an income approach. Key assumptions include attrition rates by reporting unit ranging from 5.0 percent to 10.0 percent and discount rates by reporting unit ranging from 11.5 percent to 12.8 percent . The fair value of property, plant and equipment was determined primarily based on the cost approach. Key assumptions include inputs to the valuation methodology such as recent purchases of similar items and published data for similar items. Components were adjusted for economic and functional obsolescence, location, normal useful lives and capacity (if applicable). The fair value measurements for equity method investments, intangibles, and property, plant and equipment are based on significant inputs that are not observable in the market and, therefore, represent Level 3 measurements. The amounts of revenue and income from operations associated with MarkWest are not included in the Consolidated Statement of Income for the period ended March 31, 2015 . Unaudited Pro Forma Financial Information The following unaudited pro forma financial information presents consolidated results assuming the MarkWest Merger occurred on January 1, 2014. The pro forma financial information does not give effect to potential synergies that could result from the acquisition and is not necessarily indicative of the results of future operations. (In millions, except per unit data) Three Months Ended March 31, 2015 Revenues and other income $ 664 Net income attributable to MPLX LP 64 Net income attributable to MPLX LP per unit - basic 0.19 Net income attributable to MPLX LP per unit - diluted 0.18 The unaudited pro forma information includes adjustments primarily to align accounting policies, to adjust depreciation expense to reflect the fair value of property, plant and equipment, increase amortization expense related to identifiable intangible assets, adjust interest expense related to the fair value of MarkWest’s long-term debt, as well as the related income tax effects. MarkWest has a 60 percent legal ownership interest in MarkWest Utica EMG. MarkWest Utica EMG’s inability to fund its planned activities without subordinated financial support qualify it as a VIE. The financing structure for MarkWest Utica EMG at its inception resulted in a de-facto agent relationship under which MarkWest was deemed to be the primary beneficiary of MarkWest Utica EMG. Therefore, MarkWest consolidated MarkWest Utica EMG in its historical financial statements. In the fourth quarter of 2015, based on economic conditions and other pertinent factors, the accounting for its investment in MarkWest Utica EMG was re-assessed. As of December 4, 2015 , the entity has been deconsolidated. For purposes of this pro forma financial information, MarkWest Utica EMG has been consolidated for the period prior to the acquisition consistent with its treatment in the historical periods presented. A summary of the amounts included in the historical financial statements of MarkWest for the period ended March 31, 2015 related to MarkWest Utica EMG are as follows: (in millions) Three Months Ended March 31, 2015 Revenues and other income $ 33 Cost of revenue excluding depreciation and amortization 7 Depreciation and amortization 16 Net income attributable to noncontrolling interest 14 Net loss (4 ) EMG Utica, LLC (“EMG Utica”), a joint venture partner in MarkWest Utica EMG, received a special non-cash allocation of income of approximately $10 million for the period ended March 31, 2015 . See Note 4 for a description of the transaction and its impact on the financial statements. Net income of MarkWest would not have changed had MarkWest Utica EMG been deconsolidated for the period ended March 31, 2015 .

Equity Method Investments (Note

Equity Method Investments (Notes)	3 Months Ended
	Mar. 31, 2016
Equity Method Investments and Joint Ventures [Abstract]
Equity Method Investments and Joint Ventures Disclosure [Text Block]	Equity Method Investments MarkWest Utica EMG Effective January 1, 2012, MarkWest Utica Operating Company, LLC (“Utica Operating”), a wholly owned and consolidated subsidiary of MarkWest, and EMG Utica (together the “Members”) executed agreements to form a joint venture, MarkWest Utica EMG, to develop significant natural gas gathering, processing and NGL fractionation, transportation and marketing infrastructure in eastern Ohio. The related limited liability company agreement has been amended from time to time (the liability company agreement as currently in effect is referred to as the “Amended LLC Agreement”). The aggregate funding commitment of EMG Utica was $950 million (the “Minimum EMG Investment”). Thereafter, Utica Operating was required to fund, as needed, 100 percent of future capital for MarkWest Utica EMG until such time as the aggregate capital that had been contributed by the Members reached $2 billion , which occurred prior to the MarkWest Merger. Until such time as the investment balances of Utica Operating and EMG Utica are in the ratio of 70 percent and 30 percent , respectively (such time being referred to as the “Second Equalization Date”), EMG Utica will have the right, but not the obligation, to fund up to 10 percent of each capital call for MarkWest Utica EMG, and Utica Operating will be required to fund all remaining capital not elected to be funded by EMG Utica. After the Second Equalization Date, Utica Operating and EMG Utica will have the right, but not the obligation, to fund their pro rata portion (based on their respective investment balances) of any additional required capital and may also fund additional capital that the other party elects not to fund. As of March 31, 2016 , EMG Utica has contributed approximately $998 million and Utica Operating has contributed approximately $1.5 billion to MarkWest Utica EMG. Under the Amended LLC Agreement, after EMG Utica has contributed more than $500 million to MarkWest Utica EMG and prior to December 31, 2016, EMG Utica’s investment balance will also be increased by a quarterly special non-cash allocation of income (“Preference Amount”) that is based upon the amount of capital contributed by EMG Utica in excess of $500 million . No Preference Amount will accrue to EMG Utica’s investment balance after December 31, 2016. EMG Utica received a special non-cash allocation of income of approximately $4 million for the period ended March 31, 2016 . Under the Amended LLC Agreement, Utica Operating will continue to receive 60 percent of cash generated by MarkWest Utica EMG that is available for distribution until the earlier of December 31, 2016 and the date on which Utica Operating’s investment balance equals 60 percent of the aggregate investment balances of the Members. After the earlier of those dates, 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 March 31, 2016 , Utica Operating’s investment balance in MarkWest Utica EMG was approximately 56 percent. MarkWest Utica EMG is deemed to be a VIE. As of the date of the MarkWest Merger, Utica Operating is not deemed to be the primary beneficiary due to EMG Utica’s voting rights on significant matters. The Partnership’s portion of MarkWest Utica EMG’s net assets, which was $2.3 billion at March 31, 2016 , is reported under the caption Equity method investments on the Consolidated Balance Sheets (see basis differential discussion below). 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 period ended March 31, 2016 . The Partnership receives management fee revenue for engineering and construction, administrative and personnel services (“Operational Service revenue”) for operating MarkWest Utica EMG. The amount of Operational Service revenue related to MarkWest Utica EMG for the period ended March 31, 2016 was $2 million and is reported as Other income-related parties in the Consolidated Statements of Income. Ohio Gathering Ohio Gathering is a subsidiary of MarkWest Utica EMG and is engaged in providing natural gas gathering services in the Utica Shale in eastern Ohio. Ohio Gathering is a joint venture between MarkWest Utica EMG and Summit Midstream Partners, LLC (“Summit”). As Ohio Gathering is a subsidiary of MarkWest Utica EMG, which is accounted for as an equity method investment, the Partnership reports its portion of Ohio Gathering’s net assets as a component of its investment in MarkWest Utica EMG. The Partnership receives Operational Service revenue for operating Ohio Gathering. The amount of Operational Service revenue related to Ohio Gathering for the period ended March 31, 2016 was approximately $4 million and is reported as Other income-related parties in the Consolidated Statements of Income. Ohio Condensate In December 2013, MarkWest formed Utica Condensate for the purpose of engaging in wellhead condensate gathering, stabilization, terminalling, storage and marketing in Ohio. As of March 31, 2016 , the Partnership owned 100 percent of Utica Condensate. Utica Condensate’s business is conducted solely through its subsidiary, Ohio Condensate Company L.L.C. (“Ohio Condensate”), which is a joint venture between Utica Condensate and Summit. As of March 31, 2016 , Utica Condensate owned 60 percent of Ohio Condensate. The Partnership accounts for Ohio Condensate, which is a VIE, as an equity method investment as MPLX LP exercises significant influence, but does not control Ohio Condensate and is not its primary beneficiary due to Summit’s voting rights on significant matters. The Partnership’s portion of Ohio Condensate’s net assets are reported under the caption Equity method investments on the Consolidated Balance Sheets. The Partnership receives Operational Service revenue for operating Ohio Condensate. The amount of Operational Service revenue related to Ohio Condensate for the period ended March 31, 2016 was $1 million and is reported as Other income-related parties in the Consolidated Statements of Income. Summarized financial information for the period ended March 31, 2016 for equity method investments is as follows: (In millions) MarkWest Utica EMG (1) Other VIEs Non-VIEs Total Income statement data: Revenue 61 5 20 86 Income from operations 39 1 7 47 Net income 23 1 5 29 Balance sheet data: Current assets 125 8 56 189 Noncurrent assets 2,215 176 373 2,764 Current liabilities 95 6 24 125 Noncurrent liabilities 2 15 — 17 (1) MarkWest Utica EMG’s noncurrent assets includes its investment in its subsidiary Ohio Gathering, which does not appear elsewhere in this table. The investment was $798 million as of March 31, 2016 . Summarized balance sheet information as of December 31, 2015 for equity method investments is as follows: (In millions) MarkWest Utica EMG (1) Other VIEs Non-VIEs Total Balance sheet data: Current assets 113 7 30 150 Noncurrent assets 2,207 169 243 2,619 Current liabilities 77 7 18 102 Noncurrent liabilities 1 12 — 13 (1) MarkWest Utica EMG’s noncurrent assets includes its investment in its subsidiary Ohio Gathering, which does not appear elsewhere in this table. The investment was $781 million as of December 31, 2015 . As of March 31, 2016 and December 31, 2015 , the carrying value of the Partnership’s equity method investments was $1.1 billion and $961 million , respectively, higher than the underlying net assets of the investees. The increase is due to the measurement period adjustments discussed in Note 3 that were recorded during the quarter ended March 31, 2016 . This basis difference is being amortized or accreted into net income over the remaining estimated useful lives of the underlying net assets, except for $469 million of excess related to goodwill.

Related Party Agreements and Tr

Related Party Agreements and Transactions	3 Months Ended
	Mar. 31, 2016
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. Centennial owns a products pipeline and storage facility. • Muskegon Pipeline LLC (“Muskegon”), in which MPC has a 60 percent interest. Muskegon owns a common carrier products pipeline. • MarkWest Utica EMG, in which MPLX LP has a 60 percent interest. MarkWest Utica EMG is engaged in significant natural gas processing and NGL fractionation, transportation and marketing in eastern Ohio. • Ohio Gathering, in which MPLX LP has a 36 percent indirect interest. Ohio Gathering is a subsidiary of MarkWest Utica EMG providing natural gas gathering service in the Utica Shale region of eastern Ohio. • Jefferson Dry Gas, in which MPLX LP has a 67 percent interest. Jefferson Dry Gas is engaged in dry natural gas gathering in the county of Jefferson County, Ohio. • Ohio Condensate, in which MPLX LP has a 60 percent interest. Ohio Condensate is engaged in wellhead condensate gathering, stabilization, terminalling, transportation and storage within certain defined areas of Ohio. Related Party Agreements The Partnership has various long-term, fee-based commercial agreements with MPC. Under these agreements, the Partnership provides pipeline transportation and storage services to MPC, and MPC has committed to provide the Partnership with minimum quarterly throughput and storage volumes of crude oil and refined products and minimum storage volumes of butane. In addition, the Partnership is party to a loan agreement with MPC Investment, a wholly owned subsidiary of MPC. Under the terms of the agreement, MPC Investment will make a loan or loans to the Partnership on a revolving basis as requested by the Partnership and as agreed to by MPC Investment, in an amount or amounts that do not result in the aggregate principal amount of all loans outstanding exceeding $500 million at any one time. The entire unpaid principal amount of the loan, together with all accrued and unpaid interest and other amounts (if any), shall become due and payable on December 4, 2020 . MPC Investment may demand payment of all or any portion of the outstanding principal amount of the loan, together with all accrued and unpaid interest and other amounts (if any), at any time prior to December 4, 2020 . Borrowings under the loan will bear interest at LIBOR plus 1.50 percent . During the three months ended March 31, 2016 , the Partnership borrowed $1.4 billion and repaid $1.0 billion , for an outstanding balance at March 31, 2016 of $438 million , which is included in Payables-related parties on the Consolidated Balance Sheets. Borrowings were at an average interest rate of 1.93 percent , per annum. For additional information regarding the Partnership’s commercial and other agreements with MPC, see Item 1. Business in our Annual Report on Form 10-K for the year ended December 31, 2015 . The Partnership believes the terms and conditions under its agreements with MPC are generally comparable to those with unrelated parties. HSM Agreements As discussed in Note 3 , the Partnership acquired HSM on March 31, 2016. HSM has various operating, management services and employee services agreements with MPC, which are discussed below. On January 1, 2015, HSM entered into a long-term, fee-based transportation services agreement with MPC with an initial term of six years and automatically renews for two additional renewal terms of five years each unless either party provides the other party with written notice of its intent to terminate at least 12 months prior to the end of the then-current term. Under the agreement, HSM provides marine transportation of crude oil, feedstocks and refined petroleum products, as well as related services. Under the agreement MPC pays HSM monthly for the following: the specified day rate for equipment and charges for services related to transportation, tankerman services and cleaning and repair charges. Fleeting services are billed monthly. HSM entered into a guaranteed supply agreement with MPC on January 1, 2015 for the supply of fuel for use by HSM's towing vessels and barges. HSM must purchase the fuel on a ratable basis throughout the initial term or any successor term or MPC may reduce the sales volume on a pro-rated basis for each successive renewal term. The guaranteed supply agreement has an initial term of one year and automatically renews for successive one year renewal terms unless either party provides the other party with written notice of non-renewal at least 60 days prior to the end of the then-current term. HSM entered into a management services agreement with MPC on January 1, 2015 with an initial term of six years and automatically renews for two additional renewal terms of five years each unless either party provides the other party with written notice of its intent to terminate at least 12 months prior to the end of the then-current term. Under this agreement, HSM provides management services to assist MPC in the oversight and management of the MPC marine business. HSM receives a fixed annual fee in monthly installments for providing the required management services. This fee is adjusted annually on the anniversary of the contract for inflation and any changes in the scope of the management services provided. On January 1, 2015, under an employee services agreement, HSM employees were transferred to Marathon Petroleum Logistics Services LLC ("MPLS"), a wholly owned subsidiary of MPC. Under the agreement HSM reimburses MPLS for employee benefit expenses along with certain operational and management services provided in support of HSM’s areas of operation. The employee services agreement has an initial term of six years and automatically renews for two additional renewal terms of five years each unless either party provides the other party with written notice of its intent to terminate at least 12 months prior to the end of the then-current term. Related Party Transactions Sales to related parties were as follows: Three Months Ended March 31, (In millions) 2016 2015 Service revenues MPC $ 150 $ 142 Rental income MPC $ 26 $ 25 Product sales MPC $ 3 $ — 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 is included in Other income-related parties on the Consolidated Statements of Income was as follows: Three Months Ended March 31, (In millions) 2016 2015 MPC $ 17 $ 16 MarkWest Utica EMG 2 — Ohio Gathering 4 — Ohio Condensate 1 — Centennial — 1 Total $ 24 $ 17 MPC provides executive management services and certain general and administrative services to the Partnership under the terms of an omnibus agreement. Expenses incurred under these agreements are shown in the table below by the income statement line where they were recorded. Charges for services included in Purchases-related parties primarily relate to services that support the Partnership’s operations and maintenance activities, as well as compensation expenses. Charges for services included in General and administrative expenses primarily relate to services that support the Partnership’s executive management, accounting and human resources activities. These charges were as follows: Three Months Ended March 31, (In millions) 2016 2015 Purchases - related parties $ 6 $ 7 General and administrative expenses 8 11 Total 14 18 Also under terms of the omnibus agreement, some service costs related to engineering services are associated with assets under construction. These costs added to Property, plant and equipment were as follows: Three Months Ended March 31, (In millions) 2016 2015 MPC $ 9 $ 2 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. Employee services expenses from related parties were as follows: Three Months Ended March 31, (In millions) 2016 2015 Purchases - related parties $ 70 $ 33 General and administrative expenses 21 7 Total $ 91 $ 40 Receivables from related parties were as follows: (In millions) March 31, 2016 December 31, 2015 MPC $ 96 $ 175 MarkWest Utica EMG 1 4 Ohio Gathering 3 5 Other 2 3 Total $ 102 $ 187 Long-term receivables related to reimbursements from the MarkWest Merger to be provided by MPC for the conversion of Class B units were as follows: (In millions) March 31, 2016 December 31, 2015 MPC $ 25 $ 25 Payables to related parties were as follows: (In millions) March 31, 2016 December 31, 2015 MPC $ 491 $ 33 MarkWest Utica EMG 21 21 Total $ 512 $ 54 During the three months ended March 31, 2016 and 2015 , MPC did not ship its minimum committed volumes on certain pipeline systems. In addition, capital projects the Partnership is undertaking at the request of MPC are reimbursed in cash and recognized in income over the remaining term of the applicable transportation services agreements. The Deferred revenue-related parties balance associated with the minimum volume deficiencies and project reimbursements were as follows: (In millions) March 31, 2016 December 31, 2015 Minimum volume deficiencies - MPC $ 38 $ 36 Project reimbursements - MPC 6 5 Total $ 44 $ 41

Net Income Per Limited Partner

Net Income Per Limited Partner Unit	3 Months Ended
	Mar. 31, 2016
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 and to subordinated 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 and subordinated 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, subordinated units, general partner units, certain equity-based compensation awards and incentive distribution rights (“IDRs”). As discussed in Note 1 , the HSM acquisition was a transfer between entities under common control. As an entity 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 HSM will be included in the net income (loss) per unit calculation prospectively as described above. As discussed further in Note 7 , the subordinated units, all of which were owned by MPC, were converted into common units during the third quarter of 2015. For purposes of calculating net income (loss) per unit, the subordinated units were treated as if they converted to common units on July 1, 2015. For the three months ended March 31, 2016 , the Partnership had dilutive potential common units consisting of certain equity-based compensation awards and Class B units. Diluted net income (loss) per limited partner unit for the three months ended March 31, 2015 is the same as basic net income (loss) per limited partner unit since the inclusion of the any potential common units would have been anti-dilutive. Potential common shares omitted from the diluted earnings per unit calculation was approximately 10 million . Three Months Ended March 31, (In millions) 2016 2015 Net (loss) income attributable to MPLX LP $ (60 ) $ 46 Less: General partner’s distributions declared (including IDRs) (1) 44 4 Limited partners’ distributions declared on common units (1) 156 18 Limited partner’s distributions declared on subordinated units (1) — 15 Undistributed net (loss) income attributable to MPLX LP $ (260 ) $ 9 (1) See Note 7 for distribution information. Three Months Ended March 31, 2016 (In millions, except per unit data) General Partner Limited Partners’ Common Units Total Basic and diluted net income (loss) attributable to MPLX LP per unit: Net income (loss) attributable to MPLX LP: Distributions declared (including IDRs) $ 44 $ 156 $ 200 Undistributed net loss attributable to MPLX LP (5 ) (255 ) (260 ) Net income (loss) attributable to MPLX LP (1) $ 39 $ (99 ) $ (60 ) Weighted average units outstanding: Basic 7 300 307 Diluted 7 300 307 Net loss attributable to MPLX LP per limited partner unit: Basic $ (0.33 ) Diluted $ (0.33 ) Three Months Ended March 31, 2015 (In millions, except per unit data) General Partner Limited Partners’ Common Units Limited Total Basic and diluted net income attributable to MPLX LP per unit: Net income attributable to MPLX LP: Distributions declared (including IDRs) $ 4 $ 18 $ 15 $ 37 Undistributed net income attributable to MPLX LP 5 2 2 9 Net income attributable to MPLX LP (1) $ 9 $ 20 $ 17 $ 46 Weighted average units outstanding: Basic 2 43 37 82 Diluted 2 43 37 82 Net income attributable to MPLX LP per limited partner unit: Basic $ 0.46 $ 0.46 Diluted $ 0.46 $ 0.46 (1) Allocation of net income (loss) attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.

Equity

Equity	3 Months Ended
	Mar. 31, 2016
Equity [Abstract]
Equity	Equity Units Outstanding – After giving effect to the common units and general partner units issued in connection with the HSM acquisition described in Note 3 , the Partnership had 331,280,516 common units outstanding as of March 31, 2016 . Of that number, 79,466,136 were owned by MPC, which also owned the two percent general partner interest, represented by 7,506,460 general partner units. Following payment of the cash distribution for the second quarter of 2015, the requirements for the conversion of all subordinated units were satisfied under the partnership agreement. As a result, effective August 17, 2015 , the 36,951,515 subordinated units owned by MPC were converted into common units on a one-for-one basis and thereafter participate on terms equal with all other common units in distributions of available cash. The conversion did not impact the amount of the cash distributions paid by the Partnership or the total units outstanding. ATM Program – On March 4, 2016 , the Partnership entered into an amended and restated distribution agreement providing for the continuous issuance of up to an aggregate of $500 million of common units, in amounts, at prices and on terms to be determined by market conditions and other factors at the time of any offerings (such continuous offering program, or at-the-market program, referred to as the “ATM Program”). The Partnership expects the net proceeds from sales under the ATM Program will be used for general partnership purposes. During the three months ended March 31, 2016 , the sale of common units under the ATM Program generated net proceeds of approximately $315 million . The changes in the number of units outstanding from December 31, 2015 through March 31, 2016 are summarized below: (In units) Common Class B General Partner Total Balance at December 31, 2015 296,687,176 7,981,756 6,800,475 311,469,407 Unit-based compensation awards (1) 34,338 — 701 35,039 Issuance of units under the ATM Program (2) 12,025,000 — 245,406 12,270,406 Contribution of HSM (3) 22,534,002 — 459,878 22,993,880 Balance at March 31, 2016 331,280,516 7,981,756 7,506,460 346,768,732 (1) As a result of the unit-based compensation awards issued during the period, MPLX GP contributed less than $1 million in exchange for 701 general partner units to maintain its two percent general partner interest. (2) As a result of common units issued under the ATM Program during the period, MPLX GP contributed $6 million in exchange for 245,406 general partner units to maintain its two percent general partner interest. (3) See Note 3 for information regarding the HSM acquisition. Issuance of Additional Securities – The partnership agreement authorizes the issuance of an unlimited number of additional partnership securities for the consideration and on the terms and conditions determined by the general partner without the approval of the unitholders. See Note 19 for information on the issuance of Series A Convertible Preferred Units. Net (Loss) Income Allocation – In preparing the Consolidated Statements of Equity, net (loss) income attributable to MPLX LP is allocated to the unitholders in accordance with their respective ownership percentages. However, when distributions related to the incentive distribution rights are made, earnings equal to the amount of those distributions are first allocated to the general partner before the remaining earnings are allocated to the unitholders based on their respective ownership percentages. The following table presents the allocation of the general partner’s interest in net income attributable to MPLX LP: Three Months Ended March 31, (In millions) 2016 2015 Net (loss) income attributable to MPLX LP $ (60 ) $ 46 Less: General partner's incentive distribution rights and other 41 3 Net (loss) income attributable to MPLX LP available to general and limited partners $ (101 ) $ 43 General partner's two percent interest in net (loss) income attributable to MPLX LP $ (2 ) $ 1 General partner's incentive distribution rights and other 41 3 General partner's interest in net income attributable to MPLX LP $ 39 $ 4 Cash distributions – The partnership agreement sets forth the calculation to be used to determine the amount and priority of cash distributions that the common and subordinated unitholders and general partner will receive. In accordance with the partnership agreement, on April 19, 2016 , the Partnership declared a quarterly cash distribution of $0.5050 per unit, resulting in total distributions of $200 million . These distributions will be paid on May 13, 2016 to unitholders of record on May 3, 2016 . The allocation of total quarterly cash distributions to general and limited partners is as follows for the three months ended March 31, 2016 and 2015 . The Partnership’s distributions are declared subsequent to quarter end; therefore, the following table represents total cash distributions applicable to the period in which the distributions were earned. Three Months Ended March 31, (In millions) 2016 2015 General partner's distributions: General partner's distributions $ 4 $ 1 General partner's incentive distribution rights distributions 40 3 Total general partner's distributions $ 44 $ 4 Limited partners' distributions: Common unitholders $ 156 $ 18 Subordinated unitholders — 15 Total limited partners' distributions 156 33 Total cash distributions declared $ 200 $ 37

Segment Information (Notes)

Segment Information (Notes)	3 Months Ended
	Mar. 31, 2016
Segment Reporting [Abstract]
Segment Reporting Disclosure [Text Block]	Segment Information The Partnership’s chief operating decision maker is the chief executive officer (“CEO”) of its general partner. The CEO reviews the Partnership’s discrete financial information, makes operating decisions, assesses financial performance and allocates resources on a type of service basis. The Partnership has two reportable segments: L&S and G&P. Each of these segments is organized and managed based upon the nature of the products and services it offers. • L&S - transports and stores crude oil and refined petroleum products. Segment information for prior periods includes HSM as it is an entity under common control. • G&P - gathers, processes and transports natural gas; gathers, transports, fractionates, stores and markets NGLs. This segment is the result of the MarkWest Merger on December 4, 2015 discussed in more detail in Note 3 . Segment information for periods prior to the MarkWest Merger does not include amounts for these operations. 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 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. The tables below present information about income from operations and capital expenditures for the reported segments: Three months ended March 31, 2016 (In millions) L&S G&P Total Revenues and other income: Segment revenues $ 192 $ 498 $ 690 Segment other income 19 — 19 Total segment revenues and other income 211 498 709 Costs and expenses: Segment cost of revenues 89 200 289 Segment operating income before portion attributable to noncontrolling interest 122 298 420 Segment portion attributable to noncontrolling interest and Predecessor 34 41 75 Segment operating income attributable to MPLX LP $ 88 $ 257 $ 345 Three months ended March 31, 2015 (In millions) L&S Revenues and other income: Segment revenues $ 183 Segment other income 18 Total segment revenues and other income 201 Costs and expenses: Segment cost of revenues 86 Segment operating income before portion attributable to noncontrolling interest and Predecessor 115 Segment portion attributable to noncontrolling interest and Predecessor 33 Segment operating income attributable to MPLX LP $ 82 Three months ended March 31, (In millions) 2016 2015 Reconciliation to Income from operations: Segment operating income attributable to MPLX LP $ 345 $ 82 Segment portion attributable to unconsolidated affiliates (83 ) — Segment portion attributable to noncontrolling interest and Predecessor 75 33 Income from equity method investments 5 — Other income - related parties 7 — Unrealized derivative losses (9 ) — Impairment expense (129 ) — Depreciation and amortization (132 ) (19 ) General and administrative expenses (52 ) (22 ) Income from operations $ 27 $ 74 Three months ended March 31, (In millions) 2016 2015 Reconciliation to Total revenues and other income: Total segment revenues and other income $ 709 $ 201 Revenue adjustment from unconsolidated affiliates (104 ) — Income from equity method investments 5 — Other income - related parties 7 — Unrealized derivative loss (8 ) — Total revenues and other income $ 609 $ 201 Three months ended March 31, (In millions) 2016 2015 Reconciliation to Net income attributable to noncontrolling interests and Predecessor Segment portion attributable to noncontrolling interest and Predecessor $ 75 $ 33 Portion of noncontrolling interests and Predecessor related to items below segment income from operations (29 ) (11 ) Portion of operating income attributable to noncontrolling interest of unconsolidated affiliates (23 ) — Net income attributable to noncontrolling interests and Predecessor $ 23 $ 22 The following reconciles segment capital expenditures to total capital expenditures: Three months ended March 31, (In millions) 2016 2015 L&S segment capital expenditures $ 62 $ 35 G&P segment capital expenditures (1) 273 — Total segment capital expenditures 335 35 Less: Capital expenditures for Partnership operated, non-wholly owned subsidiaries (44 ) — Total capital expenditures $ 291 $ 35 (1) The G&P segment includes $44 million of capital expenditures related to Partnership operated, non-wholly owned subsidiaries. Total assets by reportable segment were: March 31, December 31, (In millions) 2016 2015 L&S $ 1,792 $ 1,858 G&P 14,186 14,246 Total assets $ 15,978 $ 16,104

Income Taxes (Notes)

Income Taxes (Notes)	3 Months Ended
	Mar. 31, 2016
Income Tax Disclosure [Abstract]
Income Tax Disclosure [Text Block]	Income Tax The Partnership is not a taxable entity for United States federal income tax purposes or for the majority of states that impose an income tax. Taxes on the Partnership’s net income generally are borne by its partners through the allocation of taxable income. The Partnership’s income tax provision results from partnership activity in the states of Texas and Tennessee. MarkWest Hydrocarbon is a tax paying entity for both federal and state tax purposes. The Partnership’s income tax activity was less than $1 million for the three months ended March 31, 2015 . A reconciliation of the provision for income tax and the amount computed by applying the federal statutory rate of 35 percent to the income before income taxes for the three months ended March 31, 2016 is as follows: (In millions) MarkWest Hydrocarbon Partnership Eliminations Consolidated (1) Income before provision for income tax $ (12 ) $ (30 ) $ 1 $ (41 ) Federal statutory rate 35 % — % — % Federal income tax at statutory rate (2) (4 ) — — (4 ) State income taxes net of federal benefit - MarkWest Hydrocarbon (1 ) — — (1 ) Provision on income from Class A units (2) (1 ) — — (1 ) Other 1 1 — 2 Provision for income tax $ (5 ) $ 1 $ — $ (4 ) (1) Financial information has been retrospectively adjusted for the acquisition of HSM from MPC. See Notes 1 and 3 . Prior to this acquisition, MPC paid all income taxes related to HSM. (2) MarkWest Hydrocarbon pays tax on its share of the Partnership’s income or loss as a result of its ownership of Class A units.

Inventories (Notes)

Inventories (Notes)	3 Months Ended
	Mar. 31, 2016
Inventory Disclosure [Abstract]
Inventory Disclosure [Text Block]	Inventories Inventories consist of the following: (In millions) March 31, 2016 December 31, 2015 NGLs $ 1 $ 3 Line fill 4 5 Spare parts, materials and supplies 40 43 Total inventories $ 45 $ 51

Property, Plant and Equipment

Property, Plant and Equipment	3 Months Ended
	Mar. 31, 2016
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment	Property, Plant and Equipment Property, plant and equipment with associated accumulated depreciation was: (In millions) March 31, 2016 December 31, 2015 Natural gas gathering and NGL transportation pipelines and facilities $ 4,443 $ 4,307 Processing, fractionation and storage facilities 3,197 3,185 Pipelines and related assets 1,143 1,128 Barges and towing vessels 478 475 Land, building, office equipment and other 625 606 Construction in progress 1,078 946 Total 10,964 10,647 Less accumulated depreciation 769 650 Property, plant and equipment, net $ 10,195 $ 9,997

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2016
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. (In millions) March 31, 2016 December 31, 2015 Assets Liabilities Assets Liabilities Significant other observable inputs (Level 2) Commodity contracts 1 — $ 2 $ — Significant unobservable inputs (Level 3) Commodity contracts 1 (1 ) 7 — Embedded derivatives in commodity contracts — (34 ) — (32 ) Total carrying value in Consolidated Balance Sheets 2 (35 ) $ 9 $ (32 ) The following table provides additional information about the significant unobservable inputs used in the valuation of Level 3 instruments as of March 31, 2016 . The market approach is used for valuation of all instruments. Level 3 Instrument Balance Sheet Classification Unobservable Inputs Value Range Time Period Commodity contracts Assets Forward ethane prices (per gallon) (1) $0.17 - $0.21 Apr. 2016 - Dec. 2016 Forward propane prices (per gallon) (1) $0.44 - $0.49 Apr. 2016 - Dec. 2016 Forward isobutane prices (per gallon) (1) $0.54 - $0.58 Apr. 2016 - Dec. 2016 Forward normal butane prices (per gallon) (1) $0.50 - $0.56 Apr. 2016 - Dec. 2016 Forward natural gasoline prices (per gallon) (1) $0.90 - $0.91 Apr. 2016 - Dec. 2016 Liabilities Forward propane prices (per gallon) (1) $0.44 - $0.49 Apr. 2016 - Dec. 2016 Forward isobutane prices (per gallon) (1) $0.54 - $0.58 Apr. 2016 - Dec. 2016 Forward normal butane prices (per gallon) (1) $0.50 - $0.56 Apr. 2016 - Dec. 2016 Forward natural gasoline prices (per gallon) (1) $0.90 - $0.91 Apr. 2016 - Dec. 2016 Embedded derivatives in commodity contracts Liabilities Forward propane prices (per gallon) (1) $0.44 - $0.51 Apr. 2016 - Dec. 2022 Forward isobutane prices (per gallon) (1) $0.54 - $0.63 Apr. 2016 - Dec. 2022 Forward normal butane prices (per gallon) (1) $0.49 - $0.61 Apr. 2016 - Dec. 2022 Forward natural gasoline prices (per gallon) (1) $0.90 - $1.01 Apr. 2016 - Dec. 2022 Forward natural gas prices (per mmbtu) (2) $1.79 - $3.37 Apr. 2016 - Dec. 2022 ERCOT Pricing (per MegaWatt Hour) $21.34 - $44.27 Apr. 2016 - Dec. 2016 Probability of renewal (3) 50.0% Probability of renewal for second 5-yr term (3) 75.0% (1) NGL prices used in the valuation are generally at the lower end of the range in the early years and increase over time. (2) Natural gas prices used in the valuations are generally at the lower end of the range in the early years and increase 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 forward NGL prices result in an increase (decrease) in the fair value of the embedded derivative. 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 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 reviewed and validated on a quarterly basis by the Risk Department by comparing the pricing and option volatilities to actual market data and/or data provided by at least one other independent third-party pricing service. Management is responsible for the valuation of the Natural Gas Embedded Derivative discussed in Note 13 . Included in the valuation of the Natural Gas Embedded Derivative are assumptions about the forward price curves for NGLs and natural gas for periods in which price curves are not available from third-party pricing services due to insufficient market data. The Risk Department must develop forward price curves for NGLs and natural gas through the initial contract term (April 2016 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 • The producer customer’s potential business strategy decision points that may exist at the time the counterparty would elect whether to renew the contracts Changes in Level 3 Fair Value Measurements The tables below include a rollforward of the balance sheet amounts for the three months ended March 31, 2016 (including the change in fair value) for assets and liabilities classified by the Partnership within Level 3 of the valuation hierarchy, except for the changes in goodwill. See Note 15 for a rollforward of goodwill, which included a Level 3 valuation adjustment during the three months ended March 31, 2016 . Three months ended March 31, 2016 (In millions) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Fair value at beginning of period $ 7 $ (32 ) Total loss (realized and unrealized) included in earnings (1) (1 ) (4 ) Settlements (6 ) 2 Fair value at end of period $ — $ (34 ) The amount of total loss for the period included in earnings attributable to the change in unrealized loss relating to assets still held at end of period $ (2 ) $ (3 ) (1) Gains and losses on Commodity Derivative Contracts classified as Level 3 are recorded in Product sales in the accompanying Consolidated Statements of Income. Gains and losses on Embedded Derivatives in Commodity Contracts are recorded in Costs of revenue and Purchased product costs . Fair Values – Reported The Partnership’s primary financial instruments are cash and cash equivalents, receivables, receivables from related parties, accounts payable, payables to related parties, and long-term debt. The Partnership’s fair value assessment incorporates a variety of considerations, including (1) the short-term duration of the instruments, (2) MPC’s investment-grade credit rating and (3) the historical incurrence of and expected future insignificance of bad debt expense, which includes an evaluation of counterparty credit risk. The Partnership believes the carrying values of its current assets and liabilities approximate fair value. The recorded value of the amounts outstanding under the bank revolving credit facility, if any, approximates fair value due to the variable interest rate that approximates current market rates. Derivative instruments are recorded at fair value, based on available market information (see Note 13 ). The SMR liability and $4.1 billion aggregate principal of the Partnership’s long-term debt were recorded at fair value in connection with the MarkWest Merger as of December 4, 2015, which established a new cost basis for each of those liabilities. The fair value of the 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 fair values are considered Level 3 measurements and SMR liability fair values are considered Level 2 measurements. The following table summarizes the fair value and carrying value of the Partnership’s long-term debt, excluding capital leases, and SMR liability. March 31, 2016 December 31, 2015 (In millions) Fair Value Carrying Value Fair Value Carrying Value Long-term debt $ 4,804 $ 4,715 $ 5,212 $ 5,255 SMR liability $ 102 $ 99 $ 99 $ 100

Derivative Financial Instrument

Derivative Financial Instruments (Notes)	3 Months Ended
	Mar. 31, 2016
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Instruments and Hedging Activities Disclosure [Text Block]	Derivative Financial Instruments Commodity Derivatives NGL and natural gas prices are volatile and are impacted by changes in fundamental supply and demand, as well as market uncertainty, availability of NGL transportation and fractionation capacity and a variety of additional factors that are beyond the Partnership’s control. The Partnership’s profitability is directly affected by prevailing commodity prices primarily as a result of processing or conditioning at its own or third-party processing plants, purchasing and selling or gathering and transporting volumes of natural gas at index-related prices and the cost of third-party transportation and fractionation services. To the extent that commodity prices influence the level of natural gas drilling by the Partnership’s producer customers, such prices also affect profitability. To protect itself financially against adverse price movements and to maintain more stable and predictable cash flows so that the Partnership can meet its cash distribution objectives, debt service and capital plans, the Partnership executes a strategy governed by its risk management policy. The Partnership has a committee comprised of senior management that oversees risk management activities, continually monitors the risk management program and adjusts its strategy as conditions warrant. The Partnership enters into certain derivative contracts to reduce the risks associated with unfavorable changes in the prices of natural gas, NGLs and crude oil. Derivative contracts utilized are swaps and options 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 2016. 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 & Resources L.L.C. (“MarkWest Liberty Midstream”) to enter into derivative positions without posting cash collateral. The Partnership uses standardized agreements that allow for offset of certain positive and negative exposures (“master netting arrangements”) in the event of default or other terminating events, including bankruptcy. The Partnership records derivative contracts at fair value in the Consolidated Balance Sheets and has not elected hedge accounting or the normal purchases and normal sales designation (except for electricity and certain other qualifying contracts, for which the normal purchases and normal sales designation has been elected). The Partnership’s accounting may cause volatility in the Consolidated Statements of Income as the Partnership recognizes in current earnings all unrealized gains and losses from the changes in fair value of derivatives. Volume of Commodity Derivative Activity As of March 31, 2016 , the Partnership had the following outstanding commodity contracts that were executed to manage the cash flow risk associated with future sales of NGLs: Derivative contracts not designated as hedging instruments Financial Position Notional Quantity (net) Crude Oil (bbl) Short 82,500 Natural Gas (MMBtu) Long 745,826 NGLs (gal) Short 55,759,065 Embedded Derivatives in Commodity Contracts The Partnership has a commodity contract with a producer customer in the Southern Appalachian region that creates a floor on the frac spread for gas purchases of 9,000 Dth/d. The commodity contract is a component of a broader regional arrangement that also includes a keep-whole processing agreement. For accounting purposes, these contracts have been aggregated into a single contract and are evaluated together. In February 2011, the Partnership executed agreements with the producer customer to extend the commodity contract and the related processing agreement from March 31, 2015 to December 31, 2022, with the producer customer’s option to extend the agreement for two successive five year terms through December 31, 2032. The purchase of gas at prices based on the frac spread and the option to extend the agreements have been identified as a single embedded derivative, which is recorded at fair value. The probability of renewal is determined based on extrapolated pricing curves, a review of the overall expected favorability of the contracts based on such pricing curves, and assumptions about the counterparty’s potential business strategy decision points that may exist at the time the counterparty would elect whether to renew the contract. The changes in fair value of this embedded derivative are based on the difference between the contractual and index pricing, the probability of the producer customer exercising its option to extend and the estimated favorability of these contracts compared to current market conditions. The changes in fair value are recorded in earnings through Purchased product costs in the Consolidated Statements of Income. As of March 31, 2016 , the estimated fair value of this contract was a liability of $33 million . 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 its plant location in the Southwest operations through the fourth quarter of 2017. The contract is currently fixed through the fourth quarter of 2016 with the ability to fix the commodity contract for its remaining year. Changes in the fair value of the derivative component of this contract were recognized as Cost of revenues in the Consolidated Statements of Income. As of March 31, 2016 , the estimated fair value of this contract was a liability of $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, 2015. The impact of the Partnership’s derivative instruments on its Consolidated Balance Sheets is summarized below: (In millions) March 31, 2016 December 31, 2015 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 $ 2 $ (7 ) $ 9 $ (5 ) Other noncurrent assets / deferred credits and other liabilities — (28 ) — (27 ) Total $ 2 $ (35 ) $ 9 $ (32 ) (1) Includes embedded derivatives in commodity contracts as discussed above. Certain derivative positions are subject to master netting agreements, therefore the Partnership has elected to offset derivative assets and liabilities that are legally permissible to be offset. The net amounts in the table below equal the balances presented in the Consolidated Balance Sheets: March 31, 2016 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 $ 3 $ (1 ) $ 2 $ (2 ) $ 1 $ (1 ) Embedded derivatives in commodity contracts — — — (6 ) — (6 ) Total current derivative instruments 3 (1 ) 2 (8 ) 1 (7 ) Non-current Commodity contracts — — — — — — Embedded derivatives in commodity contracts — — — (28 ) — (28 ) Total non-current derivative instruments — — — (28 ) — (28 ) Total derivative instruments $ 3 $ (1 ) $ 2 $ (36 ) $ 1 $ (35 ) In the table above, the Partnership does not offset a counterparty’s current derivative contracts with the counterparty’s non-current derivative contracts, although the Partnership’s master netting arrangements would allow current and non-current positions to be offset in the event of default. Additionally, in the event of a default, the Partnership’s master netting arrangements would allow for the offsetting of all transactions executed under the master netting arrangement. These types of transactions may include non-derivative instruments, derivatives qualifying for scope exceptions, receivables and payables arising from settled positions and other forms of non-cash collateral (such as letters of credit). The impact of the Partnership’s derivative contracts not designated as hedging instruments and the location of gain or (loss) recognized in the Consolidated Statements of Income is summarized below: (In millions) March 31, 2016 Product sales Realized gain 7 Unrealized loss (8 ) Total revenue: derivative loss from product sales (1 ) Purchased product costs Unrealized loss (1 ) Total loss (2 )

Debt

Debt	3 Months Ended
	Mar. 31, 2016
Debt Disclosure [Abstract]
Debt	Debt The Partnership’s outstanding borrowings at March 31, 2016 and December 31, 2015 consisted of the following: (In millions) March 31, 2016 December 31, 2015 MPLX LP: Bank revolving credit facility due 2020 326 $ 877 Term loan facility due 2019 250 250 5.500% senior notes due 2023 710 710 4.500% senior notes due 2023 989 989 4.875% senior notes due 2024 1,149 1,149 4.000% senior notes due 2025 500 500 4.875% senior notes due 2025 1,189 1,189 Consolidated subsidiaries: MarkWest - 4.500% - 5.500% senior notes, due 2023 - 2025 63 63 MPL - capital lease obligations due 2020 9 9 Total 5,185 5,736 Unamortized debt issuance costs (8 ) (8 ) Unamortized discount (1) (461 ) (472 ) Amounts due within one year (1 ) (1 ) Total long-term debt due after one year $ 4,715 $ 5,255 (1) Includes $453 million and $465 million discount as of March 31, 2016 and December 31, 2015 , respectively, related to the difference between the fair value and the principal amount of the assumed MarkWest debt. Credit Agreements During the three months ended March 31, 2016 , the Partnership borrowed $306 million under the bank revolving credit facility, at an average interest rate of 1.896 percent , per annum, and repaid $857 million under the bank revolving credit facility. At March 31, 2016 , the Partnership had $326 million in borrowings and $8 million letters of credit outstanding under this facility, resulting in total availability of $1.7 billion , or 85 percent of the borrowing capacity. The $250 million term loan facility was drawn in full on November 20, 2014. The borrowings under this facility during the three months ended March 31, 2016 were at an average interest rate of 1.928 percent .

Goodwill (Notes)

Goodwill (Notes)	3 Months Ended
	Mar. 31, 2016
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets Disclosure [Text Block]	Goodwill The Partnership annually evaluates goodwill for impairment as of November 30, as well as whenever events or changes in circumstances indicate it is more likely than not that the fair value of a reporting unit with goodwill is less than its carrying amount. During the first quarter of 2016, the Partnership determined that an interim impairment analysis of the goodwill recorded in connection with the MarkWest Merger was necessary based on consideration of a number of first quarter events and circumstances, including i) continued deterioration of near term commodity prices as well as longer term pricing trends, ii) recent guidance on reductions to forecasted capital spending, the slowing of drilling activity and the resulting reduced production growth forecasts released or communicated by the Partnership’s producer customers and iii) increases in cost of capital. The combination of these factors was considered to be a triggering event requiring an interim impairment test. Based on the first step of the interim goodwill impairment analysis, the fair value for the three reporting units to which goodwill was assigned in connection with the MarkWest Merger was less than the respective carrying value. In step two of the impairment analysis, the implied fair values of the goodwill were compared to the carrying values within those reporting units. Based on this assessment, it was determined that goodwill was impaired in two of the three reporting units. Accordingly, the Partnership recorded an impairment charge of approximately $129 million in the first quarter of 2016. The fair value of the reporting units for the interim goodwill impairment analysis was determined based on applying the discounted cash flow method, which is an income approach, and the guideline public company method, which is a market approach. The discounted cash flow fair value estimate is based on known or knowable information at the interim measurement date. The significant assumptions that were used to develop the estimates of the fair values under the discounted cash flow method include management’s best estimates of the expected future results and discount rates, which range from 10.5 percent to 11.5 percent . The fair value of the intangibles was determined based on applying the multi-period excess earnings method, which is an income approach. Key assumptions include attrition rates by reporting unit ranging from 5.0 percent to 10.0 percent and discount rates by reporting unit ranging from 11.5 percent to 12.8 percent . Fair value determinations require considerable judgment and are sensitive to changes in underlying assumptions and factors. As a result, there can be no assurance that the estimates and assumptions made for purposes of the interim goodwill impairment test will prove to be an accurate prediction of the future. The fair value measurements for the individual reporting units’ overall fair values, and the fair values of the goodwill assigned thereto, represent Level 3 measurements. The changes in carrying amount of goodwill for 2016 were as follows: (In millions) L&S G&P Total Balance as of December 31, 2015 $ 116 $ 2,454 $ 2,570 Purchase price allocation adjustments (1) — (241 ) (241 ) Impairment expense — (129 ) (129 ) Balance as of March 31, 2016 $ 116 $ 2,084 $ 2,200 (1) See Note 3 for further discussion on purchase price allocation adjustments.

Supplemental Cash Flow Informat

Supplemental Cash Flow Information	3 Months Ended
	Mar. 31, 2016
Supplemental Cash Flow Elements [Abstract]
Supplemental Cash Flow Information	Supplemental Cash Flow Information Three Months Ended March 31, (In millions) 2016 2015 Net cash provided by operating activities included: Interest paid (net of amounts capitalized) $ 53 $ 2 Non-cash investing and financing activities: Net transfers of property, plant and equipment from materials and supplies inventories $ (5 ) $ — The Consolidated Statements of Cash Flows exclude changes to the Consolidated Balance Sheets that did not affect cash. The following is the change of additions to property, plant and equipment related to capital accruals: Three Months Ended March 31, (In millions) 2016 2015 (Decrease) increase in capital accruals (20 ) 5 In connection with the acquisition of HSM described in Note 3 , MPC agreed to waive first quarter 2016 distributions on the MPLX common units issued in connection with the transaction. MPC will not receive general partner distributions or incentive distribution rights that would otherwise accrue on such MPLX common units with respect to the first quarter distributions. The value of these waived distributions was $15 million .

Equity-Based Compensation Plan

Equity-Based Compensation Plan	3 Months Ended
	Mar. 31, 2016
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 limited partner units for the three months ended March 31, 2016 : Number Weighted Outstanding at December 31, 2015 1,031,219 $ 35.49 Granted 437,978 29.32 Settled (41,463 ) 51.13 Forfeited (4,592 ) 30.27 Outstanding at March 31, 2016 1,423,142 33.15 Performance Units – The Partnership grants performance units under the MPLX LP 2012 Incentive Compensation Plan to certain officers of our 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 and had a weighted-average grant date fair value per unit of $0.63 for 2016 , as calculated using a Monte Carlo valuation model. The following is a summary of the equity-classified performance unit award activity for the three months ended March 31, 2016 : Number of Outstanding at December 31, 2015 1,521,392 Granted 781,875 Settled (433,011 ) Forfeited (53,507 ) Outstanding at March 31, 2016 1,816,749

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2016
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies The Partnership is the subject of, or a party to, a number of pending or threatened legal actions, contingencies and commitments involving a variety of matters, including laws and regulations relating to the environment. Some of these matters are discussed below. For matters for which the Partnership has not recorded an accrued liability, the Partnership is unable to estimate a range of possible losses for the reasons discussed in more detail below. However, the ultimate resolution of some of these contingencies could, individually or in the aggregate, be material. Environmental Matters – The Partnership is subject to federal, state and local laws and regulations relating to the environment. These laws generally provide for control of pollutants released into the environment and require responsible parties to undertake remediation of hazardous waste disposal sites. Penalties may be imposed for non-compliance. At March 31, 2016 and December 31, 2015 , accrued liabilities for remediation totaled $5 million and $1 million , respectively. However, it is not presently possible to estimate the ultimate amount of all remediation costs that might be incurred or the penalties, if any, which may be imposed. At March 31, 2016 , there was less than $1 million in receivables from MPC for indemnification of environmental costs related to incidents occurring prior to the initial public offering. There were $1 million in receivables from MPC for indemnification at December 31, 2015 . On July 6, 2015, representatives from the EPA and the United States Department of Justice entered 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. The government has presented MarkWest Liberty Midstream with subpoenas to provide documents related to its pipeline and compressor facilities located in Pennsylvania. MarkWest Liberty Midstream is providing information in response to the subpoenas and related requests for information from EPA, state and other agencies, and is in discussions with the agencies regarding issues associated with its operations of, and any permit related obligations for, its gathering facilities in the region. Immediately following the July 6, 2015 search, MarkWest Liberty Midstream commenced its own assessment of its operations of launcher/receiver facilities. MarkWest Liberty Midstream’s review to date has determined that MarkWest Liberty Midstream’s operations have been conducted in a manner fully protective of its employees and the public, and that other than potentially having to obtain certain permits at a relatively small number of individual sites, MarkWest Liberty has operated in substantial compliance with applicable laws and regulations. It is possible that, in connection with any potential or asserted civil or criminal enforcement action associated with this matter, MarkWest Liberty Midstream will incur material assessments, penalties or fines, incur material defense costs and expenses, be required to modify operations or construction activities which could increase operating costs and capital expenditures, or be subject to other obligations or restrictions that could restrict or prohibit our activities, any or all of which could adversely affect our results of operations, financial position or cash flows. The amount of any potential assessments, penalties, fines, restrictions, requirements, modifications, costs or expenses that may be incurred in connection with any potential enforcement action cannot be reasonably estimated or determined at this time. We are involved in a number of other environmental enforcement matters arising in the ordinary course of business. While the outcome and impact on us cannot be predicted with certainty, management believes the resolution of these environmental matters will not, individually or collectively, have a material adverse effect on our consolidated results of operations, financial position or cash flows. Litigation Relating to the MarkWest Merger – In July 2015, a purported class action lawsuit asserting claims challenging the MarkWest Merger was filed in the Court of Chancery of the State of Delaware by a purported unitholder of MarkWest. In August 2015, two similar putative class action lawsuits were filed in the Court of Chancery of the State of Delaware by plaintiffs who purport to be unitholders of MarkWest. On September 9, 2015, these lawsuits were consolidated into one action pending in the Court of Chancery of the State of Delaware, now captioned In re MarkWest Energy Partners, L.P. Unitholder Litigation . On October 1, 2015, the plaintiffs filed a consolidated complaint against the individual members of the board of directors of MarkWest Energy GP, LLC (the “MarkWest GP Board”), MPLX LP, MPLX GP, MPC and Sapphire Holdco LLC, a wholly owned subsidiary of MPLX LP, asserting in connection with the MarkWest Merger and related disclosures that, among other things, (i) the MarkWest GP Board breached its duties in approving the MarkWest Merger with MPLX LP and (ii) MPC, MPLX LP, MPLX GP and Sapphire Holdco LLC aided and abetted such breaches. On February 4, 2016, the Court approved a stipulation and proposed order to dismiss all claims with prejudice as to the named plaintiffs, but the Court retained jurisdiction to adjudicate an application for a mootness fee by plaintiffs' counsel for an award of attorneys’ fees and reimbursement of expenses. On March 28, 2016, the plaintiffs filed an application for reimbursement of approximately $2 million of fees and expenses. We intend to vigorously defend against any application for a mootness fee and do not expect the resolution of such matter to have a material adverse effect. Other Lawsuits – In 2003, the State of Illinois brought an action against the Premcor Refining Group, Inc. (“Premcor”) and Apex Refining Company (“Apex”) asserting claims for environmental cleanup related to the refinery owned by these entities in the Hartford/Wood River, Illinois area. In 2006, Premcor and Apex filed third-party complaints against numerous owners and operators of petroleum products facilities in the Hartford/Wood River, Illinois area, including MPL. These complaints, which have been amended since filing, assert claims of common law nuisance and contribution under the Illinois Contribution Act and other laws for environmental cleanup costs that may be imposed on Premcor and Apex by the State of Illinois. There are several third-party defendants in the litigation and MPL has asserted cross-claims in contribution against the various third-party defendants. This litigation is currently pending in the Third Judicial Circuit Court, Madison County, Illinois. While the ultimate outcome of these litigated matters remains uncertain, neither the likelihood of an unfavorable outcome nor the ultimate liability, if any, with respect to this matter can be determined at this time and the Partnership is unable to estimate a reasonably possible loss (or range of loss) for this litigation. Under the omnibus agreement, MPC will indemnify the Partnership for the full cost of any losses should MPL be deemed responsible for any damages in this lawsuit. The Partnership is also a party to a number of other lawsuits and other proceedings arising in the ordinary course of business. While the ultimate outcome and impact to us cannot be predicted with certainty, the Partnership believes the resolution of these other lawsuits and proceedings will not have a material adverse effect on its consolidated financial position, results of operations or cash flows. Guarantees – Over the years, the Partnership has sold various assets in the normal course of its business. Certain of the related agreements contain performance and general guarantees, including guarantees regarding inaccuracies in representations, warranties, covenants and agreements, and environmental and general indemnifications that require the Partnership to perform upon the occurrence of a triggering event or condition. These guarantees and indemnifications are part of the normal course of selling assets. The Partnership is typically not able to calculate the maximum potential amount of future payments that could be made under such contractual provisions because of the variability inherent in the guarantees and indemnities. Most often, the nature of the guarantees and indemnities is such that there is no appropriate method for quantifying the exposure because the underlying triggering event has little or no past experience upon which a reasonable prediction of the outcome can be based. Contractual Commitments and Contingencies – At March 31, 2016 , the Partnership’s contractual commitments to acquire property, plant and equipment totaled $247 million . 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. These commitments at March 31, 2016 were primarily related to plant expansion projects for the Marcellus and Southwest operations and the Cornerstone Pipeline project. Certain natural gas processing and gathering arrangements require the Partnership to construct new natural gas processing plants, natural gas gathering pipelines and NGL pipelines and contain certain fees and charges if specified construction milestones are not achieved for reasons other than force majeure. In certain cases, certain producers may have the right to cancel the processing arrangements if there are significant delays that are not due to force majeure. As of March 31, 2016 , management does not believe there are any indications that the Partnership will not be able to meet the construction milestones, that force majeure does not apply, or that such fees and charges will otherwise be triggered.

Subsequent Event

Subsequent Event	3 Months Ended
	Mar. 31, 2016
Subsequent Event [Abstract]
Subsequent Events [Text Block]	Subsequent Event On April 27, 2016 , the Partnership entered into an agreement for the private placement of $1.0 billion of newly authorized 6.5 percent Series A Convertible Preferred Units (the “Preferred Units”) at a price of $32.50 per unit or approximately 30.8 million units. The net proceeds, after deducting offering and transaction expenses, are expected to be approximately $984 million . The Partnership expects to use the proceeds for capital expenditures, repayment of debt, and general partnership purposes. The Preferred Units will be entitled to receive an annual distribution, which will initially equal $2.1125 per unit. The Preferred Units are convertible into MPLX common units on a one for one basis after three years, at the purchasers’ option, and after four years at the Partnership’s option, subject to certain conditions. Closing of the transaction is scheduled to occur in May 2016.

Description of the Business a27

Description of the Business and Basis of Presentation Description of Business and Basis of Presentation (Policies)	3 Months Ended
	Mar. 31, 2016
Accounting Policies [Abstract]
Revenue Recognition, Policy [Policy Text Block]	Based on the terms of certain natural gas gathering, transportation and processing agreements, the Partnership is considered to be the lessor under several implicit operating lease arrangements in accordance with GAAP. The Partnership’s primary implicit lease operations relate to a natural gas gathering agreement in the Marcellus shale for which it earns a fixed-fee for providing gathering services to a single producer customer using a dedicated gathering system. As the gathering system is expanded, the fixed-fee charged to the producer is adjusted to include the additional gathering assets in the lease. Other significant implicit leases relate to a natural gas processing agreement in the Marcellus shale and a natural gas processing agreement in the Southern Appalachia region for which the Partnership earns minimum monthly fees for providing processing services to a single producer using a dedicated processing plant. Similarly, the Partnership is considered to be the lessor under implicit operating lease arrangements with MPC in accordance with GAAP. The Partnership’s primary implicit lease operations with MPC relate to the transportation agreement between HSM and MPC. The rental expense related to the HSM implicit lease is depreciation of the HSM assets. These revenues and costs from implicit leases are recorded as Rental income and Rental expenses , respectively, on the Consolidated Statements of Income. All other services are provided to MPC on an as-needed basis and recorded as Service revenue-related parties on the Consolidated Statements of Income.
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 Unit	Net income (loss) per unit applicable to common limited partner units and to subordinated 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 and subordinated 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, subordinated units, general partner units, certain equity-based compensation awards and incentive distribution rights (“IDRs”).

Acquisitions (Tables)

Acquisitions (Tables)	3 Months Ended
	Mar. 31, 2016
Business Acquisition [Line Items]
Business Acquisition, Pro Forma Information [Table Text Block]	The following unaudited pro forma financial information presents consolidated results assuming the MarkWest Merger occurred on January 1, 2014. The pro forma financial information does not give effect to potential synergies that could result from the acquisition and is not necessarily indicative of the results of future operations. (In millions, except per unit data) Three Months Ended March 31, 2015 Revenues and other income $ 664 Net income attributable to MPLX LP 64 Net income attributable to MPLX LP per unit - basic 0.19 Net income attributable to MPLX LP per unit - diluted 0.18
Hardin Street Marine [Member]
Business Acquisition [Line Items]
Schedule of Business Acquisitions, by Acquisition [Table Text Block]	The Partnership retrospectively adjusted the historical financial results for all periods to include HSM as required for transactions between entities under common control. The following table presents the Partnership’s previously reported Consolidated Statement of Income, retrospectively adjusted for the acquisition of HSM: Three Months Ended March 31, 2015 (In millions, except per unit data) MPLX LP (Previously Reported) HSM MPLX LP (Currently Reported) Revenues and other income: Service revenue $ 16 $ — $ 16 Service revenue - related parties 111 31 142 Rental income - related parties 4 21 25 Other income 1 — 1 Other income - related parties 6 11 17 Total revenues and other income 138 63 201 Costs and expenses: Cost of revenues (excludes items below) 28 14 42 Purchases - related parties 24 16 40 Depreciation and amortization 12 7 19 General and administrative expenses 19 3 22 Other taxes 3 1 4 Total costs and expenses 86 41 127 Income from operations 52 22 74 Interest expense (net of amounts capitalized of $1 million) 5 — 5 Other financial costs 1 — 1 Income before income taxes 46 22 68 Net income 46 22 68 Less: Net income attributable to Predecessor — 22 22 Net income attributable to MPLX LP 46 — 46 Less: General partner’s interest in net income attributable to MPLX LP 4 — 4 Limited partners’ interest in net income attributable to MPLX LP $ 42 $ — $ 42 The following table presents the Partnership’s previously reported Consolidated Balance Sheet, retrospectively adjusted for the acquisition of HSM: December 31, 2015 (In millions) MPLX LP (Previously Reported) HSM MPLX LP (Currently Reported) Assets Current assets: Cash and cash equivalents $ 43 $ — $ 43 Receivables, net 244 1 245 Receivables - related parties 88 99 187 Inventories 49 2 51 Other current assets 50 — 50 Total current assets 474 102 576 Equity method investments 2,458 — 2,458 Property, plant and equipment, net 9,683 314 9,997 Intangibles, net 466 — 466 Goodwill 2,559 11 2,570 Long-term receivables - related parties 25 — 25 Other noncurrent assets 12 — 12 Total assets $ 15,677 $ 427 $ 16,104 Liabilities Current liabilities: Accounts payable $ 89 $ 2 $ 91 Accrued liabilities 186 1 187 Payables - related parties 47 7 54 Deferred revenue - related parties 32 — 32 Accrued property, plant and equipment 166 2 168 Accrued taxes 26 1 27 Accrued interest payable 54 — 54 Other current liabilities 12 — 12 Total current liabilities 612 13 625 Long-term deferred revenue 4 — 4 Long-term deferred revenue - related parties 9 — 9 Long-term debt 5,255 — 5,255 Deferred income taxes 377 1 378 Deferred credits and other liabilities 166 — 166 Total liabilities 6,423 14 6,437 Equity Common unitholders - public 7,691 — 7,691 Class B unitholders 266 — 266 Common unitholder - MPC 465 — 465 General partner - MPC 819 — 819 Equity of Predecessor — 413 413 Total MPLX LP partners’ capital 9,241 413 9,654 Noncontrolling interest 13 — 13 Total equity 9,254 413 9,667 Total liabilities and equity $ 15,677 $ 427 $ 16,104 The following table presents the Partnership’s previously reported Consolidated Statement of Cash Flows, retrospectively adjusted for the acquisition of HSM: Three Months Ended March 31, 2015 (In millions) MPLX LP (Previously Reported) HSM MPLX LP (Currently Reported) Increase (decrease) in cash and cash equivalents Operating activities: Net income $ 46 $ 22 $ 68 Adjustments to reconcile net income to net cash provided by operating activities: Depreciation and amortization 12 7 19 Changes in: Current receivables 2 — 2 Current accounts payable and accrued liabilities 5 (3 ) 2 Receivables from / liabilities to related parties (5 ) (13 ) (18 ) All other, net (1 ) — (1 ) Net cash provided by operating activities 59 13 72 Investing activities: Additions to property, plant and equipment (30 ) (5 ) (35 ) Investments - loans to related parties — (8 ) (8 ) Net cash used in investing activities (30 ) (13 ) (43 ) Financing activities: Long-term debt - borrowings 528 — 528 - repayments (415 ) — (415 ) Debt issuance costs (4 ) — (4 ) Distributions to unitholders and general partner (33 ) — (33 ) Net cash provided by financing activities 76 — 76 Net increase in cash and cash equivalents 105 — 105 Cash and cash equivalents at beginning of period 27 — 27 Cash and cash equivalents at end of period $ 132 $ — $ 132
MarkWest [Member]
Business Acquisition [Line Items]
Components Of The Fair Value Of Consideration Transferred [Table Text Block]	The components of the fair value of consideration transferred are as follows: (In millions) Fair value of units issued $ 7,326 Cash 1,230 Payable to MarkWest Class B unitholders 50 Total fair value of consideration transferred $ 8,606
Schedule of Recognized Identified Assets Acquired and Liabilities Assumed [Table Text Block]	The following table summarizes the preliminary purchase price allocation. Subsequent to December 31, 2015 , additional analysis was completed and adjustments were made to the preliminary purchase price allocation as noted in the table below. The estimated fair value of assets acquired and liabilities and noncontrolling interests assumed at the acquisition date as of March 31, 2016 , are as follows: (In millions) As Originally Reported Adjustments As Adjusted Cash and cash equivalents $ 12 $ — $ 12 Receivables 164 — 164 Inventories 33 (1 ) 32 Other current assets 44 — 44 Equity method investments 2,457 143 2,600 Property, plant and equipment 8,474 43 8,517 Intangibles 468 65 533 Other noncurrent assets 5 — 5 Total assets acquired 11,657 250 11,907 Accounts payable 322 — 322 Accrued liabilities 13 6 19 Accrued taxes 21 — 21 Other current liabilities 44 — 44 Long-term debt 4,567 — 4,567 Deferred income taxes 374 3 377 Deferred credits and other liabilities 151 — 151 Noncontrolling interest 13 — 13 Total liabilities and noncontrolling interest assumed 5,505 9 5,514 Net assets acquired excluding goodwill 6,152 241 6,393 Goodwill 2,454 (241 ) 2,213 Net assets acquired $ 8,606 $ — $ 8,606
MarkWest Utica EMG [Member]
Business Acquisition [Line Items]
Historical Information for Deconsolidated Entity [Table Text Block]	A summary of the amounts included in the historical financial statements of MarkWest for the period ended March 31, 2015 related to MarkWest Utica EMG are as follows: (in millions) Three Months Ended March 31, 2015 Revenues and other income $ 33 Cost of revenue excluding depreciation and amortization 7 Depreciation and amortization 16 Net income attributable to noncontrolling interest 14 Net loss (4 )

Equity Method Investments (Tabl

Equity Method Investments (Tables)	3 Months Ended
	Mar. 31, 2016
Equity Method Investments and Joint Ventures [Abstract]
Summarized Financial Information For Equity Method Investees Table [Text Block]	Summarized financial information for the period ended March 31, 2016 for equity method investments is as follows: (In millions) MarkWest Utica EMG (1) Other VIEs Non-VIEs Total Income statement data: Revenue 61 5 20 86 Income from operations 39 1 7 47 Net income 23 1 5 29 Balance sheet data: Current assets 125 8 56 189 Noncurrent assets 2,215 176 373 2,764 Current liabilities 95 6 24 125 Noncurrent liabilities 2 15 — 17 (1) MarkWest Utica EMG’s noncurrent assets includes its investment in its subsidiary Ohio Gathering, which does not appear elsewhere in this table. The investment was $798 million as of March 31, 2016 . Summarized balance sheet information as of December 31, 2015 for equity method investments is as follows: (In millions) MarkWest Utica EMG (1) Other VIEs Non-VIEs Total Balance sheet data: Current assets 113 7 30 150 Noncurrent assets 2,207 169 243 2,619 Current liabilities 77 7 18 102 Noncurrent liabilities 1 12 — 13 (1) MarkWest Utica EMG’s noncurrent assets includes its investment in its subsidiary Ohio Gathering, which does not appear elsewhere in this table. The investment was $781 million as of December 31, 2015 .

Related Party Agreements and 30

Related Party Agreements and Transactions (Tables)	3 Months Ended
	Mar. 31, 2016
Related Party Transactions [Abstract]
Sales to Related Parties	Sales to related parties were as follows: Three Months Ended March 31, (In millions) 2016 2015 Service revenues MPC $ 150 $ 142 Rental income MPC $ 26 $ 25 Product sales MPC $ 3 $ —
Summary of Fees Received for Operating Pipelines for Related Parties Included in Other Income - Related Parties	The revenue received from related parties is included in Other income-related parties on the Consolidated Statements of Income was as follows: Three Months Ended March 31, (In millions) 2016 2015 MPC $ 17 $ 16 MarkWest Utica EMG 2 — Ohio Gathering 4 — Ohio Condensate 1 — Centennial — 1 Total $ 24 $ 17
Summary of Charges for Services Included in Purchases from Related Parties	Three Months Ended March 31, (In millions) 2016 2015 Purchases - related parties $ 6 $ 7 General and administrative expenses 8 11 Total 14 18
Summary of Related Party Costs Added to Property, Plant and Equipment	Three Months Ended March 31, (In millions) 2016 2015 MPC $ 9 $ 2
Employee Services Expenses from Related Parties	Employee services expenses from related parties were as follows: Three Months Ended March 31, (In millions) 2016 2015 Purchases - related parties $ 70 $ 33 General and administrative expenses 21 7 Total $ 91 $ 40
Receivables from Related Parties	Receivables from related parties were as follows: (In millions) March 31, 2016 December 31, 2015 MPC $ 96 $ 175 MarkWest Utica EMG 1 4 Ohio Gathering 3 5 Other 2 3 Total $ 102 $ 187
Payables to Related Parties	Payables to related parties were as follows: (In millions) March 31, 2016 December 31, 2015 MPC $ 491 $ 33 MarkWest Utica EMG 21 21 Total $ 512 $ 54
Summary of Deferred Revenue - Related Parties	The Deferred revenue-related parties balance associated with the minimum volume deficiencies and project reimbursements were as follows: (In millions) March 31, 2016 December 31, 2015 Minimum volume deficiencies - MPC $ 38 $ 36 Project reimbursements - MPC 6 5 Total $ 44 $ 41

Net Income Per Limited Partne31

Net Income Per Limited Partner Unit (Tables)	3 Months Ended
	Mar. 31, 2016
Earnings Per Share [Abstract]
Schedule of Distributions By Partner By Class	For the three months ended March 31, 2016 , the Partnership had dilutive potential common units consisting of certain equity-based compensation awards and Class B units. Diluted net income (loss) per limited partner unit for the three months ended March 31, 2015 is the same as basic net income (loss) per limited partner unit since the inclusion of the any potential common units would have been anti-dilutive. Potential common shares omitted from the diluted earnings per unit calculation was approximately 10 million . Three Months Ended March 31, (In millions) 2016 2015 Net (loss) income attributable to MPLX LP $ (60 ) $ 46 Less: General partner’s distributions declared (including IDRs) (1) 44 4 Limited partners’ distributions declared on common units (1) 156 18 Limited partner’s distributions declared on subordinated units (1) — 15 Undistributed net (loss) income attributable to MPLX LP $ (260 ) $ 9 (1) See Note 7 for distribution information.
Schedule of Basic and Diluted Earnings Per Unit	Three Months Ended March 31, 2016 (In millions, except per unit data) General Partner Limited Partners’ Common Units Total Basic and diluted net income (loss) attributable to MPLX LP per unit: Net income (loss) attributable to MPLX LP: Distributions declared (including IDRs) $ 44 $ 156 $ 200 Undistributed net loss attributable to MPLX LP (5 ) (255 ) (260 ) Net income (loss) attributable to MPLX LP (1) $ 39 $ (99 ) $ (60 ) Weighted average units outstanding: Basic 7 300 307 Diluted 7 300 307 Net loss attributable to MPLX LP per limited partner unit: Basic $ (0.33 ) Diluted $ (0.33 ) Three Months Ended March 31, 2015 (In millions, except per unit data) General Partner Limited Partners’ Common Units Limited Total Basic and diluted net income attributable to MPLX LP per unit: Net income attributable to MPLX LP: Distributions declared (including IDRs) $ 4 $ 18 $ 15 $ 37 Undistributed net income attributable to MPLX LP 5 2 2 9 Net income attributable to MPLX LP (1) $ 9 $ 20 $ 17 $ 46 Weighted average units outstanding: Basic 2 43 37 82 Diluted 2 43 37 82 Net income attributable to MPLX LP per limited partner unit: Basic $ 0.46 $ 0.46 Diluted $ 0.46 $ 0.46 (1) Allocation of net income (loss) attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.

Equity (Tables)

Equity (Tables)	3 Months Ended
	Mar. 31, 2016
Equity [Abstract]
Schedule of Stockholders Equity [Table Text Block]	The changes in the number of units outstanding from December 31, 2015 through March 31, 2016 are summarized below: (In units) Common Class B General Partner Total Balance at December 31, 2015 296,687,176 7,981,756 6,800,475 311,469,407 Unit-based compensation awards (1) 34,338 — 701 35,039 Issuance of units under the ATM Program (2) 12,025,000 — 245,406 12,270,406 Contribution of HSM (3) 22,534,002 — 459,878 22,993,880 Balance at March 31, 2016 331,280,516 7,981,756 7,506,460 346,768,732 (1) As a result of the unit-based compensation awards issued during the period, MPLX GP contributed less than $1 million in exchange for 701 general partner units to maintain its two percent general partner interest. (2) As a result of common units issued under the ATM Program during the period, MPLX GP contributed $6 million in exchange for 245,406 general partner units to maintain its two percent general partner interest.
Schedule Of Calculation Of Net Income Applicable to Partners [Table Text Block]	Net (Loss) Income Allocation – In preparing the Consolidated Statements of Equity, net (loss) income attributable to MPLX LP is allocated to the unitholders in accordance with their respective ownership percentages. However, when distributions related to the incentive distribution rights are made, earnings equal to the amount of those distributions are first allocated to the general partner before the remaining earnings are allocated to the unitholders based on their respective ownership percentages. The following table presents the allocation of the general partner’s interest in net income attributable to MPLX LP: Three Months Ended March 31, (In millions) 2016 2015 Net (loss) income attributable to MPLX LP $ (60 ) $ 46 Less: General partner's incentive distribution rights and other 41 3 Net (loss) income attributable to MPLX LP available to general and limited partners $ (101 ) $ 43 General partner's two percent interest in net (loss) income attributable to MPLX LP $ (2 ) $ 1 General partner's incentive distribution rights and other 41 3 General partner's interest in net income attributable to MPLX LP $ 39 $ 4
Distributions Made to Limited Partner, by Distribution [Table Text Block]	distributions are declared subsequent to quarter end; therefore, the following table represents total cash distributions applicable to the period in which the distributions were earned. Three Months Ended March 31, (In millions) 2016 2015 General partner's distributions: General partner's distributions $ 4 $ 1 General partner's incentive distribution rights distributions 40 3 Total general partner's distributions $ 44 $ 4 Limited partners' distributions: Common unitholders $ 156 $ 18 Subordinated unitholders — 15 Total limited partners' distributions 156 33 Total cash distributions declared $ 200 $ 37

Segment Information (Tables)

Segment Information (Tables)	3 Months Ended
	Mar. 31, 2016
Segment Reporting [Abstract]
Schedule of Segment Reporting Information, by Segment [Table Text Block]	Three months ended March 31, 2016 (In millions) L&S G&P Total Revenues and other income: Segment revenues $ 192 $ 498 $ 690 Segment other income 19 — 19 Total segment revenues and other income 211 498 709 Costs and expenses: Segment cost of revenues 89 200 289 Segment operating income before portion attributable to noncontrolling interest 122 298 420 Segment portion attributable to noncontrolling interest and Predecessor 34 41 75 Segment operating income attributable to MPLX LP $ 88 $ 257 $ 345 Three months ended March 31, 2015 (In millions) L&S Revenues and other income: Segment revenues $ 183 Segment other income 18 Total segment revenues and other income 201 Costs and expenses: Segment cost of revenues 86 Segment operating income before portion attributable to noncontrolling interest and Predecessor 115 Segment portion attributable to noncontrolling interest and Predecessor 33 Segment operating income attributable to MPLX LP $ 82
Reconciliation of Operating Profit (Loss) from Segments to Consolidated [Table Text Block]	Three months ended March 31, (In millions) 2016 2015 Reconciliation to Income from operations: Segment operating income attributable to MPLX LP $ 345 $ 82 Segment portion attributable to unconsolidated affiliates (83 ) — Segment portion attributable to noncontrolling interest and Predecessor 75 33 Income from equity method investments 5 — Other income - related parties 7 — Unrealized derivative losses (9 ) — Impairment expense (129 ) — Depreciation and amortization (132 ) (19 ) General and administrative expenses (52 ) (22 ) Income from operations $ 27 $ 74
Reconciliation of Revenue from Segments to Consolidated [Table Text Block]	Three months ended March 31, (In millions) 2016 2015 Reconciliation to Total revenues and other income: Total segment revenues and other income $ 709 $ 201 Revenue adjustment from unconsolidated affiliates (104 ) — Income from equity method investments 5 — Other income - related parties 7 — Unrealized derivative loss (8 ) — Total revenues and other income $ 609 $ 201
Reconciliation of Net Income Attributable to Noncontrolling Interests [Table Text Block]	Three months ended March 31, (In millions) 2016 2015 Reconciliation to Net income attributable to noncontrolling interests and Predecessor Segment portion attributable to noncontrolling interest and Predecessor $ 75 $ 33 Portion of noncontrolling interests and Predecessor related to items below segment income from operations (29 ) (11 ) Portion of operating income attributable to noncontrolling interest of unconsolidated affiliates (23 ) — Net income attributable to noncontrolling interests and Predecessor $ 23 $ 22
Reconciliation of Other Significant Reconciling Items from Segments to Consolidated [Table Text Block]	The following reconciles segment capital expenditures to total capital expenditures: Three months ended March 31, (In millions) 2016 2015 L&S segment capital expenditures $ 62 $ 35 G&P segment capital expenditures (1) 273 — Total segment capital expenditures 335 35 Less: Capital expenditures for Partnership operated, non-wholly owned subsidiaries (44 ) — Total capital expenditures $ 291 $ 35 (1) The G&P segment includes $44 million of capital expenditures related to Partnership operated, non-wholly owned subsidiaries.
Reconciliation of Assets from Segment to Consolidated [Table Text Block]	Total assets by reportable segment were: March 31, December 31, (In millions) 2016 2015 L&S $ 1,792 $ 1,858 G&P 14,186 14,246 Total assets $ 15,978 $ 16,104

Income Taxes (Tables)

Income Taxes (Tables)	3 Months Ended
	Mar. 31, 2016
Income Tax Disclosure [Abstract]
Schedule of Effective Income Tax Rate Reconciliation [Table Text Block]	A reconciliation of the provision for income tax and the amount computed by applying the federal statutory rate of 35 percent to the income before income taxes for the three months ended March 31, 2016 is as follows: (In millions) MarkWest Hydrocarbon Partnership Eliminations Consolidated (1) Income before provision for income tax $ (12 ) $ (30 ) $ 1 $ (41 ) Federal statutory rate 35 % — % — % Federal income tax at statutory rate (2) (4 ) — — (4 ) State income taxes net of federal benefit - MarkWest Hydrocarbon (1 ) — — (1 ) Provision on income from Class A units (2) (1 ) — — (1 ) Other 1 1 — 2 Provision for income tax $ (5 ) $ 1 $ — $ (4 ) (1) Financial information has been retrospectively adjusted for the acquisition of HSM from MPC. See Notes 1 and 3 . Prior to this acquisition, MPC paid all income taxes related to HSM. (2) MarkWest Hydrocarbon pays tax on its share of the Partnership’s income or loss as a result of its ownership of Class A units.

Inventories (Tables)

Inventories (Tables)	3 Months Ended
	Mar. 31, 2016
Inventory Disclosure [Abstract]
Schedule of Inventory, Current [Table Text Block]	Inventories consist of the following: (In millions) March 31, 2016 December 31, 2015 NGLs $ 1 $ 3 Line fill 4 5 Spare parts, materials and supplies 40 43 Total inventories $ 45 $ 51

Property, Plant and Equipment (

Property, Plant and Equipment (Tables)	3 Months Ended
	Mar. 31, 2016
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment	roperty, plant and equipment with associated accumulated depreciation was: (In millions) March 31, 2016 December 31, 2015 Natural gas gathering and NGL transportation pipelines and facilities $ 4,443 $ 4,307 Processing, fractionation and storage facilities 3,197 3,185 Pipelines and related assets 1,143 1,128 Barges and towing vessels 478 475 Land, building, office equipment and other 625 606 Construction in progress 1,078 946 Total 10,964 10,647 Less accumulated depreciation 769 650 Property, plant and equipment, net $ 10,195 $ 9,997

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2016
Fair Value Disclosures [Abstract]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block]	The tables below include a rollforward of the balance sheet amounts for the three months ended March 31, 2016 (including the change in fair value) for assets and liabilities classified by the Partnership within Level 3 of the valuation hierarchy, except for the changes in goodwill. See Note 15 for a rollforward of goodwill, which included a Level 3 valuation adjustment during the three months ended March 31, 2016 . Three months ended March 31, 2016 (In millions) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Fair value at beginning of period $ 7 $ (32 ) Total loss (realized and unrealized) included in earnings (1) (1 ) (4 ) Settlements (6 ) 2 Fair value at end of period $ — $ (34 ) The amount of total loss for the period included in earnings attributable to the change in unrealized loss relating to assets still held at end of period $ (2 ) $ (3 ) (1) Gains and losses on Commodity Derivative Contracts classified as Level 3 are recorded in Product sales in the accompanying Consolidated Statements of Income. Gains and losses on Embedded Derivatives in Commodity Contracts are recorded in Costs of revenue and Purchased product costs .
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block]	The following table presents the financial instruments carried at fair value classified by the valuation hierarchy. (In millions) March 31, 2016 December 31, 2015 Assets Liabilities Assets Liabilities Significant other observable inputs (Level 2) Commodity contracts 1 — $ 2 $ — Significant unobservable inputs (Level 3) Commodity contracts 1 (1 ) 7 — Embedded derivatives in commodity contracts — (34 ) — (32 ) Total carrying value in Consolidated Balance Sheets 2 (35 ) $ 9 $ (32 )
Fair Value, by Balance Sheet Grouping [Table Text Block]	The following table summarizes the fair value and carrying value of the Partnership’s long-term debt, excluding capital leases, and SMR liability. March 31, 2016 December 31, 2015 (In millions) Fair Value Carrying Value Fair Value Carrying Value Long-term debt $ 4,804 $ 4,715 $ 5,212 $ 5,255 SMR liability $ 102 $ 99 $ 99 $ 100
Fair Value Inputs Assets and Liabilities Quantitative Information [Table Text Block]	The following table provides additional information about the significant unobservable inputs used in the valuation of Level 3 instruments as of March 31, 2016 . The market approach is used for valuation of all instruments. Level 3 Instrument Balance Sheet Classification Unobservable Inputs Value Range Time Period Commodity contracts Assets Forward ethane prices (per gallon) (1) $0.17 - $0.21 Apr. 2016 - Dec. 2016 Forward propane prices (per gallon) (1) $0.44 - $0.49 Apr. 2016 - Dec. 2016 Forward isobutane prices (per gallon) (1) $0.54 - $0.58 Apr. 2016 - Dec. 2016 Forward normal butane prices (per gallon) (1) $0.50 - $0.56 Apr. 2016 - Dec. 2016 Forward natural gasoline prices (per gallon) (1) $0.90 - $0.91 Apr. 2016 - Dec. 2016 Liabilities Forward propane prices (per gallon) (1) $0.44 - $0.49 Apr. 2016 - Dec. 2016 Forward isobutane prices (per gallon) (1) $0.54 - $0.58 Apr. 2016 - Dec. 2016 Forward normal butane prices (per gallon) (1) $0.50 - $0.56 Apr. 2016 - Dec. 2016 Forward natural gasoline prices (per gallon) (1) $0.90 - $0.91 Apr. 2016 - Dec. 2016 Embedded derivatives in commodity contracts Liabilities Forward propane prices (per gallon) (1) $0.44 - $0.51 Apr. 2016 - Dec. 2022 Forward isobutane prices (per gallon) (1) $0.54 - $0.63 Apr. 2016 - Dec. 2022 Forward normal butane prices (per gallon) (1) $0.49 - $0.61 Apr. 2016 - Dec. 2022 Forward natural gasoline prices (per gallon) (1) $0.90 - $1.01 Apr. 2016 - Dec. 2022 Forward natural gas prices (per mmbtu) (2) $1.79 - $3.37 Apr. 2016 - Dec. 2022 ERCOT Pricing (per MegaWatt Hour) $21.34 - $44.27 Apr. 2016 - Dec. 2016 Probability of renewal (3) 50.0% Probability of renewal for second 5-yr term (3) 75.0% (1) NGL prices used in the valuation are generally at the lower end of the range in the early years and increase over time. (2) Natural gas prices used in the valuations are generally at the lower end of the range in the early years and increase over time. (3) The producer counterparty to the embedded derivative has the option to renew the gas purchase agreement and the related keep-whole processing agreement for two successive five -year terms after 2022. The embedded gas purchase agreement cannot be renewed without the renewal of the related keep-whole processing agreement. Due to the significant number of years until the renewal options are exercisable and the high level of uncertainty regarding the counterparty’s future business strategy, the future commodity price environment, and the future competitive environment for midstream services in the Southern Appalachian region, management determined that a 50 percent probability of renewal for the first five-year term and 75 percent for the second five-year term are appropriate assumptions. Included in this assumption is a further extension of management’s estimates of future frac spreads through 2032.

Derivative Financial Instrume38

Derivative Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 2016
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Schedule of Derivative Instruments in Statement of Financial Position, Fair Value [Table Text Block]	The impact of the Partnership’s derivative instruments on its Consolidated Balance Sheets is summarized below: (In millions) March 31, 2016 December 31, 2015 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 $ 2 $ (7 ) $ 9 $ (5 ) Other noncurrent assets / deferred credits and other liabilities — (28 ) — (27 ) Total $ 2 $ (35 ) $ 9 $ (32 ) (1) Includes embedded derivatives in commodity contracts as discussed above.
Schedule of Notional Amounts of Outstanding Derivative Positions [Table Text Block]	As of March 31, 2016 , the Partnership had the following outstanding commodity contracts that were executed to manage the cash flow risk associated with future sales of NGLs: Derivative contracts not designated as hedging instruments Financial Position Notional Quantity (net) Crude Oil (bbl) Short 82,500 Natural Gas (MMBtu) Long 745,826 NGLs (gal) Short 55,759,065
Offsetting Assets and Liabilities [Table Text Block]	The net amounts in the table below equal the balances presented in the Consolidated Balance Sheets: March 31, 2016 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 $ 3 $ (1 ) $ 2 $ (2 ) $ 1 $ (1 ) Embedded derivatives in commodity contracts — — — (6 ) — (6 ) Total current derivative instruments 3 (1 ) 2 (8 ) 1 (7 ) Non-current Commodity contracts — — — — — — Embedded derivatives in commodity contracts — — — (28 ) — (28 ) Total non-current derivative instruments — — — (28 ) — (28 ) Total derivative instruments $ 3 $ (1 ) $ 2 $ (36 ) $ 1 $ (35 )
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: (In millions) March 31, 2016 Product sales Realized gain 7 Unrealized loss (8 ) Total revenue: derivative loss from product sales (1 ) Purchased product costs Unrealized loss (1 ) Total loss (2 )

Debt (Tables)

Debt (Tables)	3 Months Ended
	Mar. 31, 2016
Debt Disclosure [Abstract]
Summary of Outstanding Borrowings	The Partnership’s outstanding borrowings at March 31, 2016 and December 31, 2015 consisted of the following: (In millions) March 31, 2016 December 31, 2015 MPLX LP: Bank revolving credit facility due 2020 326 $ 877 Term loan facility due 2019 250 250 5.500% senior notes due 2023 710 710 4.500% senior notes due 2023 989 989 4.875% senior notes due 2024 1,149 1,149 4.000% senior notes due 2025 500 500 4.875% senior notes due 2025 1,189 1,189 Consolidated subsidiaries: MarkWest - 4.500% - 5.500% senior notes, due 2023 - 2025 63 63 MPL - capital lease obligations due 2020 9 9 Total 5,185 5,736 Unamortized debt issuance costs (8 ) (8 ) Unamortized discount (1) (461 ) (472 ) Amounts due within one year (1 ) (1 ) Total long-term debt due after one year $ 4,715 $ 5,255 (1) Includes $453 million and $465 million discount as of March 31, 2016 and December 31, 2015 , respectively, related to the difference between the fair value and the principal amount of the assumed MarkWest debt.

Goodwill (Tables)

Goodwill (Tables)	3 Months Ended
	Mar. 31, 2016
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Goodwill [Table Text Block]	The changes in carrying amount of goodwill for 2016 were as follows: (In millions) L&S G&P Total Balance as of December 31, 2015 $ 116 $ 2,454 $ 2,570 Purchase price allocation adjustments (1) — (241 ) (241 ) Impairment expense — (129 ) (129 ) Balance as of March 31, 2016 $ 116 $ 2,084 $ 2,200 (1) See Note 3 for further discussion on purchase price allocation adjustments.

Supplemental Cash Flow Inform41

Supplemental Cash Flow Information (Tables)	3 Months Ended
	Mar. 31, 2016
Supplemental Cash Flow Elements [Abstract]
Summary of Supplemental Cash Flow Information	Three Months Ended March 31, (In millions) 2016 2015 Net cash provided by operating activities included: Interest paid (net of amounts capitalized) $ 53 $ 2 Non-cash investing and financing activities: Net transfers of property, plant and equipment from materials and supplies inventories $ (5 ) $ —
Summary of Reconciliation of Additions to Property, Plant and Equipment to Total Capital Expenditures	Three Months Ended March 31, (In millions) 2016 2015 (Decrease) increase in capital accruals (20 ) 5

Equity-Based Compensation Plan

Equity-Based Compensation Plan (Tables)	3 Months Ended
	Mar. 31, 2016
Phantom Units [Member]
Equity Transactions And Share Based Compensation [Line Items]
Schedule of Share-based Compensation, Restricted Stock Units Award Activity [Table Text Block]	The following is a summary of phantom unit award activity of MPLX LP common limited partner units for the three months ended March 31, 2016 : Number Weighted Outstanding at December 31, 2015 1,031,219 $ 35.49 Granted 437,978 29.32 Settled (41,463 ) 51.13 Forfeited (4,592 ) 30.27 Outstanding at March 31, 2016 1,423,142 33.15
Performance Shares [Member]
Equity Transactions And Share Based Compensation [Line Items]
Schedule of Share-based Compensation, Restricted Stock Units Award Activity [Table Text Block]	The following is a summary of the equity-classified performance unit award activity for the three months ended March 31, 2016 : Number of Outstanding at December 31, 2015 1,521,392 Granted 781,875 Settled (433,011 ) Forfeited (53,507 ) Outstanding at March 31, 2016 1,816,749

Description of Business and Bas

Description of Business and Basis of Presentation - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2016
Accounting Policies [Abstract]
Number of Reportable Segments	2

Acquisitions - Acquisition of H

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

Acquisitions - Purchase of HSM

Acquisitions - Purchase of HSM retrospective adjustments (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016		Mar. 31, 2015		Dec. 31, 2015		Dec. 31, 2014
Revenues and other income:
Service revenue		$ 229		$ 16	[1]
Service revenue - related parties		150		142	[1]
Rental income - related parties		26		25	[1]
Other income		2		1	[1]
Other income - related parties		24		17	[1]
Total revenues and other income		609		201	[1]
Costs and expenses:
Cost of revenues (excludes items below)		89		42	[1]
Purchases - related parties		76		40	[1]
Depreciation and amortization		132		19	[1],[2]
General and administrative expenses		52		22	[1]
Other taxes		11		4	[1]
Total costs and expenses		582		127	[1]
Income from operations		27		74	[1]
Interest expense (net of amounts capitalized of $7 million and $1 million, respectively)		55		5	[1]
Other financial costs		12		1	[1]
Income before income taxes		(41)	[3]	68	[1]
Net (loss) income		(37)		68	[1],[2],[4]
Less: Net income attributable to noncontrolling interests and Predecessor		23		22	[1]
Net income attributable to MPLX LP	[5]	(60)		46	[1]
Less: General partner’s interest in net income attributable to MPLX LP		39		4	[1]
Limited partners’ interest in net (loss) income attributable to MPLX LP		(99)		42	[1]
Current assets:
Cash and cash equivalents		4		132	[2]	$ 43	[6]	$ 27	[2]
Receivables, net		250				245	[6]
Receivables - related parties		102				187	[6]
Inventories		45				51	[6]
Other current assets		28				50	[6]
Total current assets		429				576	[6]
Equity method investments		2,598				2,458	[6]
Property, plant and equipment, net		10,195				9,997	[6]
Intangibles, net		520				466	[6]
Goodwill		2,200				2,570	[6]
Long-term receivables - related parties		25				25	[6]
Other noncurrent assets		11				12	[6]
Assets		15,978				16,104	[6]
Current liabilities:
Accounts payable		105				91	[6]
Accrued liabilities		159				187	[6]
Payables - related parties		512				54	[6]
Deferred revenue - related parties		35				32	[6]
Accrued property, plant and equipment		140				168	[6]
Accrued taxes		24				27	[6]
Accrued interest payable		56				54	[6]
Other current liabilities		14				12	[6]
Total current liabilities		1,045				625	[6]
Long-term deferred revenue		6				4	[6]
Long-term deferred revenue - related parties		9				9	[6]
Long-term debt		4,715				5,255	[6]
Deferred income taxes		382				378	[6]
Deferred credits and other liabilities		166				166	[6]
Total liabilities		6,323				6,437	[6]
Equity
Total MPLX LP partners’ capital		9,641				9,654	[6]
Noncontrolling interest		14				13	[6]
Total equity		9,655		819	[4]	9,667	[4],[6]	784	[4]
Total liabilities and equity		15,978				16,104	[6]
Changes in:
Current receivables		5		(2)	[2]
Current accounts payable and accrued liabilities		(15)		2	[2]
Receivables from / liabilities to related parties		(23)		18	[2]
All other, net		15		(1)	[2]
Net Cash Provided by (Used in) Operating Activities		295		72	[2]
Investing activities:
Additions to property, plant and equipment		291		35	[2]
Investments - loans to related parties		(77)		8	[2]
Net cash used in investing activities		(240)		(43)	[2]
Financing activities:
Long-term debt - borrowings		306		528	[2]
Repayments of Long-term Debt		857		415	[2]
Debt issuance costs		0		4	[2]
Distributions to unitholders and general partner		190		33	[2]
Net cash (used in) provided by financing activities		(94)		76	[2]
Cash and Cash Equivalents, Period Increase (Decrease)		(39)		105	[2]
MPLX LP (Previously Reported)
Revenues and other income:
Service revenue				16
Service revenue - related parties				111
Rental income - related parties				4
Other income				1
Other income - related parties				6
Total revenues and other income				138
Costs and expenses:
Cost of revenues (excludes items below)				28
Purchases - related parties				24
Depreciation and amortization				12
General and administrative expenses				19
Other taxes				3
Total costs and expenses				86
Income from operations				52
Interest expense (net of amounts capitalized of $7 million and $1 million, respectively)				5
Other financial costs				1
Income before income taxes				46
Net (loss) income				46
Less: Net income attributable to noncontrolling interests and Predecessor				0
Net income attributable to MPLX LP				46
Less: General partner’s interest in net income attributable to MPLX LP				4
Limited partners’ interest in net (loss) income attributable to MPLX LP				42
Current assets:
Cash and cash equivalents				132		43		27
Receivables, net						244
Receivables - related parties						88
Inventories						49
Other current assets						50
Total current assets						474
Equity method investments						2,458
Property, plant and equipment, net						9,683
Intangibles, net						466
Goodwill						2,559
Long-term receivables - related parties						25
Other noncurrent assets						12
Assets						15,677
Current liabilities:
Accounts payable						89
Accrued liabilities						186
Payables - related parties						47
Deferred revenue - related parties						32
Accrued property, plant and equipment						166
Accrued taxes						26
Accrued interest payable						54
Other current liabilities						12
Total current liabilities						612
Long-term deferred revenue						4
Long-term deferred revenue - related parties						9
Long-term debt						5,255
Deferred income taxes						377
Deferred credits and other liabilities						166
Total liabilities						6,423
Equity
Total MPLX LP partners’ capital						9,241
Noncontrolling interest						13
Total equity						9,254
Total liabilities and equity						15,677
Changes in:
Current receivables				(2)
Current accounts payable and accrued liabilities				5
Receivables from / liabilities to related parties				5
All other, net				(1)
Net Cash Provided by (Used in) Operating Activities				59
Investing activities:
Additions to property, plant and equipment				30
Investments - loans to related parties				0
Net cash used in investing activities				(30)
Financing activities:
Long-term debt - borrowings				528
Repayments of Long-term Debt				415
Debt issuance costs				4
Distributions to unitholders and general partner				33
Net cash (used in) provided by financing activities				76
Cash and Cash Equivalents, Period Increase (Decrease)				105
Hardin Street Marine Business [Member] | HSM
Revenues and other income:
Service revenue				0
Service revenue - related parties				31
Rental income - related parties				21
Other income				0
Other income - related parties				11
Total revenues and other income				63
Costs and expenses:
Cost of revenues (excludes items below)				14
Purchases - related parties				16
Depreciation and amortization				7
General and administrative expenses				3
Other taxes				1
Total costs and expenses				41
Income from operations				22
Interest expense (net of amounts capitalized of $7 million and $1 million, respectively)				0
Other financial costs				0
Income before income taxes				22
Net (loss) income				22
Less: Net income attributable to noncontrolling interests and Predecessor				22
Net income attributable to MPLX LP				0
Less: General partner’s interest in net income attributable to MPLX LP				0
Limited partners’ interest in net (loss) income attributable to MPLX LP				0
Current assets:
Cash and cash equivalents				0		0		0
Receivables, net						1
Receivables - related parties						99
Inventories						2
Other current assets						0
Total current assets						102
Equity method investments						0
Property, plant and equipment, net						314
Intangibles, net						0
Goodwill						11
Long-term receivables - related parties						0
Other noncurrent assets						0
Assets						427
Current liabilities:
Accounts payable						2
Accrued liabilities						1
Payables - related parties						7
Deferred revenue - related parties						0
Accrued property, plant and equipment						2
Accrued taxes						1
Accrued interest payable						0
Other current liabilities						0
Total current liabilities						13
Long-term deferred revenue						0
Long-term deferred revenue - related parties						0
Long-term debt						0
Deferred income taxes						1
Deferred credits and other liabilities						0
Total liabilities						14
Equity
Total MPLX LP partners’ capital						413
Noncontrolling interest						0
Total equity						413
Total liabilities and equity						427
Changes in:
Current receivables				0
Current accounts payable and accrued liabilities				(3)
Receivables from / liabilities to related parties				13
All other, net				0
Net Cash Provided by (Used in) Operating Activities				13
Investing activities:
Additions to property, plant and equipment				5
Investments - loans to related parties				8
Net cash used in investing activities				(13)
Financing activities:
Long-term debt - borrowings				0
Repayments of Long-term Debt				0
Debt issuance costs				0
Distributions to unitholders and general partner				0
Net cash (used in) provided by financing activities				0
Cash and Cash Equivalents, Period Increase (Decrease)				0
Limited Partners Common Units [Member]
Costs and expenses:
Net income attributable to MPLX LP	[5]	(99)		20
Limited Partners Common Units [Member] | Public [Member]
Equity
Total MPLX LP partners’ capital		7,805				7,691	[6]
Limited Partners Common Units [Member] | Public [Member] | MPLX LP (Previously Reported)
Equity
Total MPLX LP partners’ capital						7,691
Limited Partners Common Units [Member] | Public [Member] | Hardin Street Marine Business [Member] | HSM
Equity
Total MPLX LP partners’ capital						0
Limited Partners Class B Units [Member]
Equity
Total MPLX LP partners’ capital		266				266	[6]
Limited Partners Class B Units [Member] | MPLX LP (Previously Reported)
Equity
Total MPLX LP partners’ capital						266
Limited Partners Class B Units [Member] | Hardin Street Marine Business [Member] | HSM
Equity
Total MPLX LP partners’ capital						0
MPC [Member]
Revenues and other income:
Service revenue - related parties		150		142
Other income - related parties		17		16
Current assets:
Receivables - related parties		96				175
Long-term receivables - related parties		25				25
Current liabilities:
Payables - related parties		491				33
MPC [Member] | MPLX LP (Previously Reported)
Equity
Total MPLX LP partners’ capital						0
MPC [Member] | Predecessor [Member]
Costs and expenses:
Net (loss) income		23		22	[4]
Equity
Total MPLX LP partners’ capital		0				413	[6]
Total equity	[4]			343		413		$ 321
MPC [Member] | Hardin Street Marine Business [Member] | Predecessor [Member]
Equity
Total MPLX LP partners’ capital						413
MPC [Member] | Limited Partners Common Units [Member]
Equity
Total MPLX LP partners’ capital		1,086				465	[6]
MPC [Member] | Limited Partners Common Units [Member] | MPLX LP (Previously Reported)
Equity
Total MPLX LP partners’ capital						465
MPC [Member] | Limited Partners Common Units [Member] | Hardin Street Marine Business [Member] | HSM
Equity
Total MPLX LP partners’ capital						0
MPC [Member] | General Partner [Member]
Costs and expenses:
Net income attributable to MPLX LP	[5]	39		$ 9
Equity
Total MPLX LP partners’ capital		$ 484				819	[6]
MPC [Member] | General Partner [Member] | MPLX LP (Previously Reported)
Equity
Total MPLX LP partners’ capital						819
MPC [Member] | General Partner [Member] | Hardin Street Marine Business [Member] | HSM
Equity
Total MPLX LP partners’ capital						$ 0
[1]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.
[2]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.
[3]	(1) Financial information has been retrospectively adjusted for the acquisition of HSM from MPC. See Notes 1 and 3. Prior to this acquisition, MPC paid all income taxes related to HSM.
[4]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.
[5]	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.
[6]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.

Acquisitions - Purchase of HS46

Acquisitions - Purchase of HSM retrospective adjustments (Parenthetical) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Business Combinations [Abstract]
Interest Costs Capitalized	$ 7	$ 1

Acquisitions - Purchase of Mark

Acquisitions - Purchase of MarkWest Energy Partners LP (Details) $ / shares in Units, $ in Millions	Jul. 01, 2017	Jul. 01, 2016	Dec. 04, 2015USD ($)$ / shares	Dec. 31, 2015USD ($)		Mar. 31, 2016USD ($)	Mar. 31, 2015USD ($)	[2],[3]
Business Acquisition [Line Items]
Goodwill				$ 2,570	[1]	$ 2,200
Depreciation and amortization						132	$ 19
MarkWest [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition			Dec. 4, 2015
Common Units Conversion Ratio			1.09
Cash consideration to unitholders | $ / shares			$ 6.20
Cash			$ 1,230
Payable to MarkWest Class B unitholders			50
Goodwill			2,213
Equity method investments			2,600
Misstatement of Original Purchase Price Allocation [Member] | MarkWest [Member]
Business Acquisition [Line Items]
Goodwill			68
Equity method investments			2
Customer Relationships [Member] | Misstatement of Original Purchase Price Allocation [Member] | MarkWest [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill			64
Depreciation and amortization						$ 1
MPC [Member] | MarkWest [Member]
Business Acquisition [Line Items]
Cash			1,200	$ 1,300
Payable to MarkWest Class B unitholders			$ 50
Income Approach Valuation Technique [Member] | Equity Method Investments [Member]
Business Acquisition [Line Items]
Fair Value Inputs, Long-term Revenue Growth Rate			2.50%
Income Approach Valuation Technique [Member] | Minimum [Member] | Equity Method Investments [Member]
Business Acquisition [Line Items]
Fair Value Inputs, Discount Rate			9.40%			10.50%
Income Approach Valuation Technique [Member] | Minimum [Member] | Finite-Lived Intangible Assets [Member]
Business Acquisition [Line Items]
Fair Value Inputs, Discount Rate			11.50%			11.50%
Fair Value Inputs, Attrition Rate			5.00%			5.00%
Income Approach Valuation Technique [Member] | Maximum [Member] | Equity Method Investments [Member]
Business Acquisition [Line Items]
Fair Value Inputs, Discount Rate			11.10%			11.50%
Income Approach Valuation Technique [Member] | Maximum [Member] | Finite-Lived Intangible Assets [Member]
Business Acquisition [Line Items]
Fair Value Inputs, Discount Rate			12.80%			12.80%
Fair Value Inputs, Attrition Rate			10.00%			10.00%
Subsequent Event [Member] | Limited Partners Subordinated Units [Member]
Business Acquisition [Line Items]
Equity Instruments Conversion Date	Jul. 1, 2017	Jul. 1, 2016
[1]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.
[2]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.
[3]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.

Acquisitions - Fair Value Consi

Acquisitions - Fair Value Consideration Transferred - MarkWest) (Details) - MarkWest [Member] $ in Millions	Dec. 04, 2015USD ($)
Business Acquisition [Line Items]
Fair value of units issued	$ 7,326
Cash	1,230
Payable to MarkWest Class B unitholders	50
Total fair value of consideration transferred	$ 8,606

Acquisitions - Schedule of Asse

Acquisitions - Schedule of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015		Dec. 04, 2015
Business Acquisition [Line Items]
Goodwill	$ 2,200	$ 2,570	[1]
MarkWest [Member]
Business Acquisition [Line Items]
Cash and cash equivalents				$ 12
Receivables				164
Inventories				32
Other current assets				44
Equity method investments				2,600
Property, plant and equipment				8,517
Intangibles				533
Other noncurrent assets				5
Total assets acquired				11,907
Accounts payable				322
Accrued liabilities				19
Accrued taxes				21
Other current liabilities				44
Long-term debt				4,567
Deferred income taxes				377
Deferred credits and other liabilities				151
Noncontrolling interest				13
Total liabilities and noncontrolling interest assumed				5,514
Net assets acquired excluding goodwill				6,393
Goodwill				2,213
Net assets acquired				8,606
As Originally Reported
Business Acquisition [Line Items]
Goodwill		$ 2,559
As Originally Reported | MarkWest [Member]
Business Acquisition [Line Items]
Cash and cash equivalents				12
Receivables				164
Inventories				33
Other current assets				44
Equity method investments				2,457
Property, plant and equipment				8,474
Intangibles				468
Other noncurrent assets				5
Total assets acquired				11,657
Accounts payable				322
Accrued liabilities				13
Accrued taxes				21
Other current liabilities				44
Long-term debt				4,567
Deferred income taxes				374
Deferred credits and other liabilities				151
Noncontrolling interest				13
Total liabilities and noncontrolling interest assumed				5,505
Net assets acquired excluding goodwill				6,152
Goodwill				2,454
Net assets acquired				8,606
Adjustments | MarkWest [Member]
Business Acquisition [Line Items]
Cash and cash equivalents				0
Receivables				0
Inventories				(1)
Other current assets				0
Equity method investments				143
Property, plant and equipment				43
Intangibles				65
Other noncurrent assets				0
Total assets acquired				250
Accounts payable				0
Accrued liabilities				6
Accrued taxes				0
Other current liabilities				0
Long-term debt				0
Deferred income taxes				3
Deferred credits and other liabilities				0
Noncontrolling interest				0
Total liabilities and noncontrolling interest assumed				9
Net assets acquired excluding goodwill				241
Goodwill				(241)
Net assets acquired				$ 0
[1]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.

Acquisitions - Unaudited Pro Fo

Acquisitions - Unaudited Pro Forma Financial Information (Details) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2015USD ($)$ / shares
Business Combinations [Abstract]
Revenues and other income | $	$ 664
Net income attributable to MPLX LP | $	$ 64
Net income attributable to MPLX LP per unit - basic | $ / shares	$ 0.19
Net income attributable to MPLX LP per unit - diluted | $ / shares	$ 0.18

Acquisitions - Pro Forma Narrat

Acquisitions - Pro Forma Narrative (Details)	Dec. 04, 2015	Mar. 31, 2016
Business Acquisition [Line Items]
Business Acquisition, Pro Forma Information, Description		The unaudited pro forma information includes adjustments primarily to align accounting policies, to adjust depreciation expense to reflect the fair value of property, plant and equipment, increase amortization expense related to identifiable intangible assets, adjust interest expense related to the fair value of MarkWest’s long-term debt, as well as the related income tax effects.
MarkWest Utica EMG [Member]
Business Acquisition [Line Items]
Equity Method Investment, Ownership Percentage		60.00%
Deconsolidation, Date	Dec. 4, 2015

Acquisitions - Pro Forma Inform

Acquisitions - Pro Forma Information - Historical MarkWest Utica EMG Information (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015
Business Acquisition [Line Items]
Total revenues and other income		$ 609	$ 201	[1]
Net income attributable to MPLX LP	[2]	$ (60)	46	[1]
EMG Utica, LLC [Member]
Business Acquisition [Line Items]
Noncontrolling Owners Accrual of Preference Amount			10
MarkWest Utica EMG [Member]
Business Acquisition [Line Items]
Total revenues and other income			33
Operating Expenses			7
Depreciation and amortization			16
Net income attributable to noncontrolling interest			14
Net income attributable to MPLX LP			$ (4)
[1]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.
[2]	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 Method Investments (Deta

Equity Method Investments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	[2]	Dec. 31, 2015		Feb. 28, 2013
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 2,598			$ 2,458	[1]
Other income - related parties	24	$ 17
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity	$ 1,100			$ 961
Markwest Utica Emg [Member]
Schedule of Equity Method Investments [Line Items]
Minimum Capital Contributed by Noncontrolling Owners						$ 950
Percentage of Required Capital Contribution by Reporting Entity after NCI						100.00%
Aggregate Contributions to VIE Threshold						$ 2,000
Percentage of Ownership Interest in Joint Venture, Maximum	70.00%
Threshold Percentage of Noncontrolling Ownership Interest in Joint Venture	30.00%
Maximum Percentage of Additional Noncontrolling Capital Contribution before Threshold Ownership	10.00%
Actual Capital Contributed by Noncontrolling Owners	$ 998
Actual Capital Contribution to Date	1,500
Noncontrolling Owners Preference Threshold						$ 500
Noncontrolling Owners Accrual of Preference Amount	$ 4
Percentage of Cash Generation Received	60.00%
Percentage Investment Balance Target for Distribution of Available Cash	60.00%
Percentage of aggregate investment balance	56.00%
Equity method investments	$ 2,300
Other income - related parties	2
Equity Method Investment Difference Between Carrying Amount And Underlying Equity Portion Related To Goodwill Not Amortized	469
Ohio Gathering Company, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Other income - related parties	$ 4
Equity Method Investment, Ownership Percentage	36.00%
Utica Condensate [Member]
Schedule of Equity Method Investments [Line Items]
Subsidiary of Limited Liability Company or Limited Partnership, Ownership Interest	100.00%
Ohio Condensate Company, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Other income - related parties	$ 1
Equity Method Investment, Ownership Percentage	60.00%
[1]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.
[2]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.

Equity Method Investments - Sum

Equity Method Investments - Summary of Equity Method Investment Financial Information (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016		Dec. 31, 2015
Schedule of Equity Method Investments [Line Items]
Revenue		$ 86
Income from operations		47
Net income		29
Current assets		189		$ 150
Noncurrent assets		2,764		2,619
Current liabilities		125		102
Noncurrent liabilities		17		13
Markwest Utica Emg [Member]
Schedule of Equity Method Investments [Line Items]
Revenue	[1]	61
Income from operations	[1]	39
Net income	[1]	23
Current assets		125	[1]	113	[2]
Noncurrent assets		2,215	[1]	2,207	[2]
Current liabilities		95	[1]	77	[2]
Noncurrent liabilities		2	[1]	1	[2]
Other VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Revenue		5
Income from operations		1
Net income		1
Current assets		8		7
Noncurrent assets		176		169
Current liabilities		6		7
Noncurrent liabilities		15		12
Non-VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Revenue		20
Income from operations		7
Net income		5
Current assets		56		30
Noncurrent assets		373		243
Current liabilities		24		18
Noncurrent liabilities		0		0
Utica Condensate [Member]
Schedule of Equity Method Investments [Line Items]
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures		$ 798		$ 781
[1]	(1) MarkWest Utica EMG’s noncurrent assets includes its investment in its subsidiary Ohio Gathering, which does not appear elsewhere in this table. The investment was $798 million as of March 31, 2016.
[2]	(1) MarkWest Utica EMG’s noncurrent assets includes its investment in its subsidiary Ohio Gathering, which does not appear elsewhere in this table. The investment was $781 million as of December 31, 2015.

Related Party Agreements and 55

Related Party Agreements and Transactions - Additional Information (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2016USD ($)
Centennial [Member] | MPC [Member]
Related Party Transaction [Line Items]
Equity Method Investment, Ownership Percentage	50.00%
Muskegon [Member] | MPC [Member]
Related Party Transaction [Line Items]
Equity Method Investment, Ownership Percentage	60.00%
MarkWest Utica EMG [Member]
Related Party Transaction [Line Items]
Equity Method Investment, Ownership Percentage	60.00%
Ohio Gathering Company, LLC [Member]
Related Party Transaction [Line Items]
Equity Method Investment, Ownership Percentage	36.00%
MarkWest EMG Jefferson Dry Gas Gathering Company, L.L.C. [Member]
Related Party Transaction [Line Items]
Equity Method Investment, Ownership Percentage	67.00%
Ohio Condensate Company, LLC [Member]
Related Party Transaction [Line Items]
Equity Method Investment, Ownership Percentage	60.00%
Management Services Agreements [Member] | Hardin Street Marine [Member]
Related Party Transaction [Line Items]
Term Of Agreements	6 years
Number of Renewals	2
Renewal Term Agreement	5 years
Employee Services Agreements [Member] | MPC [Member]
Related Party Transaction [Line Items]
Term Of Agreements	6 years
Number of Renewals	2
Renewal Term Agreement	5 years
Transportation Services Agreements [Member] | Marine Transportation Services [Member] | Commercial Agreements [Member] | MPC [Member]
Related Party Transaction [Line Items]
Term Of Agreements	6 years
Number of Renewals	2
Renewal Term Agreement	5 years
Guaranteed Supply Agreement [Member] | Marine Transportation Services [Member] | Commercial Agreements [Member] | MPC [Member]
Related Party Transaction [Line Items]
Term Of Agreements	1 year
Renewal Term Agreement	1 year
Related Party Revolving Credit Agreement [Member] | MPC Investment [Member]
Related Party Transaction [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 500
Line of Credit Facility, Expiration Date	Dec. 4, 2020
Debt Instrument, Description of Variable Rate Basis	LIBOR plus 1.50 percent
Proceeds from Long-term Lines of Credit	$ 1,400
Repayments of Long-term Lines of Credit	1,000
Long-term Debt, Gross	$ 438
Debt Instrument, Interest Rate, Effective Percentage	1.93%

Related Party Agreements and 56

Related Party Agreements and Transactions - Sales to Related Parties (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Related Party Transaction [Line Items]
Service revenue - related parties	$ 150	$ 142	[1]
Rental income - related parties	26	25	[1]
Product sales - related parties	3	0	[1]
MPC [Member]
Related Party Transaction [Line Items]
Service revenue - related parties	150	142
Product sales - related parties	$ 3	$ 0
[1]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.

Related Party Agreements and 57

Related Party Agreements and Transactions - Other Income - Related Parties (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Related Party Transaction [Line Items]
Other income - related parties	$ 24	$ 17	[1]
MPC [Member]
Related Party Transaction [Line Items]
Other income - related parties	17	16
MarkWest Utica EMG [Member]
Related Party Transaction [Line Items]
Other income - related parties	2	0
Ohio Gathering Company, LLC [Member]
Related Party Transaction [Line Items]
Other income - related parties	4	0
Ohio Condensate Company, LLC [Member]
Related Party Transaction [Line Items]
Other income - related parties	1	0
Centennial [Member]
Related Party Transaction [Line Items]
Other income - related parties	$ 0	$ 1
[1]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC 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 [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Related Party Transaction [Line Items]
Charges for services included in purchases from related party	$ 14	$ 18
Purchases From Related Parties [Member]
Related Party Transaction [Line Items]
Charges for services included in purchases from related party	6	7
General and Administrative Expense [Member]
Related Party Transaction [Line Items]
Charges for services included in purchases from related party	$ 8	$ 11

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
	Mar. 31, 2016	Mar. 31, 2015
MPC [Member] | Assets Under Construction [Member]
Property, Plant and Equipment [Line Items]
Costs added to property, plant and equipment	$ 9	$ 2

Related Party Agreements and 60

Related Party Agreements and Transactions - Employee Services Agreements (Detail) - MPC [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	$ 91	$ 40
Purchases From Related Parties [Member]
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	70	33
General and Administrative Expense [Member]
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	$ 21	$ 7

Related Party Agreements and 61

Related Party Agreements and Transactions - Receivables from Related Parties (Detail) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Receivables - related parties	$ 102	$ 187	[1]
MPC [Member]
Related Party Transaction [Line Items]
Receivables - related parties	96	175
MarkWest Utica EMG [Member]
Related Party Transaction [Line Items]
Receivables - related parties	1	4
Ohio Gathering Company, LLC [Member]
Related Party Transaction [Line Items]
Receivables - related parties	3	5
Other [Member]
Related Party Transaction [Line Items]
Receivables - related parties	$ 2	$ 3
[1]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.

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	Mar. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Long-term receivables - related parties	$ 25	$ 25	[1]
MPC [Member]
Related Party Transaction [Line Items]
Long-term receivables - related parties	$ 25	$ 25
[1]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.

Related Party Agreements and 63

Related Party Agreements and Transactions - Payables to Related Parties (Detail) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Payables - related parties	$ 512	$ 54	[1]
MPC [Member]
Related Party Transaction [Line Items]
Payables - related parties	491	33
MarkWest Utica EMG [Member]
Related Party Transaction [Line Items]
Payables - related parties	$ 21	$ 21
[1]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.

Related Party Agreements and 64

Related Party Agreements and Transactions - Summary of Deferred Revenue - Related Parties (Detail) - MPC [Member] - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Deferred Revenue Related Parties	$ 44	$ 41
Minimum volume deficiencies [Member]
Related Party Transaction [Line Items]
Deferred Revenue Related Parties	38	36
Project Reimbursements [Member]
Related Party Transaction [Line Items]
Deferred Revenue Related Parties	$ 6	$ 5

Net Income Per Limited Partne65

Net Income Per Limited Partner Unit - Schedule of Distributions by Partner by Class (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015
Net Income Per Share [Line Items]
Earnings Per Unit, Potentially Dilutive Units		10,000,000
Net income attributable to MPLX LP	[1]	$ (60)	$ 46	[2]
Less: Distribution declared		(200)	(37)
Undistributed net income attributable to MPLX LP		(260)	9
General Partner [Member] | MPC [Member]
Net Income Per Share [Line Items]
Net income attributable to MPLX LP	[1]	39	9
Less: Distribution declared	[3]	(44)	(4)
Undistributed net income attributable to MPLX LP		(5)	5
Limited Partners Common Units [Member]
Net Income Per Share [Line Items]
Net income attributable to MPLX LP	[1]	(99)	20
Less: Distribution declared	[3]	(156)	(18)
Undistributed net income attributable to MPLX LP		(255)	2
Limited Partners Subordinated Units [Member] | MPC [Member]
Net Income Per Share [Line Items]
Net income attributable to MPLX LP	[1]		17
Less: Distribution declared	[3]	$ 0	(15)
Undistributed net income attributable to MPLX LP			$ 2
[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]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.
[3]	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
		Mar. 31, 2016	Mar. 31, 2015
Net income attributable to MPLX LP:
Distributions declared (including IDRs)		$ 200	$ 37
Undistributed net income attributable to MPLX LP		(260)	9
Net income attributable to MPLX LP	[1]	$ (60)	$ 46	[2]
Weighted average units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic		307	82
Weighted Average Limited Partnership Units Outstanding, Diluted		307	82
Limited Partners Common Units [Member]
Net income attributable to MPLX LP:
Distributions declared (including IDRs)	[3]	$ 156	$ 18
Undistributed net income attributable to MPLX LP		(255)	2
Net income attributable to MPLX LP	[1]	$ (99)	$ 20
Weighted average units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic		300	43
Weighted Average Limited Partnership Units Outstanding, Diluted		300	43
Net income attributable to MPLX LP per limited partner unit:
Basic (in USD per unit)		$ (0.33)	$ 0.46
Diluted (in USD per unit)		$ (0.33)	$ 0.46
MPC [Member] | General Partner [Member]
Net income attributable to MPLX LP:
Distributions declared (including IDRs)	[3]	$ 44	$ 4
Undistributed net income attributable to MPLX LP		(5)	5
Net income attributable to MPLX LP	[1]	$ 39	$ 9
Weighted average units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic		7	2
Weighted Average Limited Partnership Units Outstanding, Diluted		7	2
MPC [Member] | Limited Partners Subordinated Units [Member]
Net income attributable to MPLX LP:
Distributions declared (including IDRs)	[3]	$ 0	$ 15
Undistributed net income attributable to MPLX LP			2
Net income attributable to MPLX LP	[1]		$ 17
Weighted average units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic			37
Weighted Average Limited Partnership Units Outstanding, Diluted			37
Net income attributable to MPLX LP per limited partner unit:
Basic (in USD per unit)			$ 0.46
Diluted (in USD per unit)			$ 0.46
[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]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.
[3]	See Note 7 for distribution information.

Equity (Details)

Equity (Details) - USD ($) $ / shares in Units, $ in Millions		May. 13, 2016	May. 03, 2016	Apr. 19, 2016	Mar. 04, 2016	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Stockholders Equity Note [Line Items]
Limited Liability Company (LLC) or Limited Partnership (LP), Managing Member or General Partner, Ownership Interest						2.00%
Units outstanding						7,506,460
Unit-based compensation awards(1)	[1]					35,039
Issuance of units under ATM Program						$ 321
Distributions declared (including IDRs)						$ 200	$ 37
Limited Partners Common Units [Member]
Stockholders Equity Note [Line Items]
Units outstanding						331,280,516
Subordinated Units [Member]
Stockholders Equity Note [Line Items]
Equity Instruments Conversion Date						Aug. 17, 2015
Unit-based compensation awards(1)						36,951,515
MPC [Member] | Limited Partners Common Units [Member]
Stockholders Equity Note [Line Items]
Units outstanding						79,466,136
Subsequent Event [Member]
Stockholders Equity Note [Line Items]
Distribution Made to Limited Partner, Declaration Date				Apr. 19, 2016
Cash distributions declared per limited partner common unit				$ 0.5050
Distributions declared (including IDRs)				$ 200
Distribution Made to Limited Partner, Distribution Date		May 13, 2016
Distribution Made to Limited Partner, Date of Record			May 3, 2016
ATM Program [Member]
Stockholders Equity Note [Line Items]
Equity Instrument Regulatory Filing Date					Mar. 4, 2016
Common Units Aggregate Value						$ 500
Partners' Capital Account, Public Sale of Units Net of Offering Costs						$ 315
Limited Partners Common Units [Member]
Stockholders Equity Note [Line Items]
Unit-based compensation awards(1)	[1]					34,338
Cash distributions declared per limited partner common unit						$ 0.5050	$ 0.4100
Distributions declared (including IDRs)	[2]					$ 156	$ 18
Limited Partners Common Units [Member] | MPC [Member]
Stockholders Equity Note [Line Items]
Units outstanding						79,000,000		57,000,000
General Partner Units [Member]
Stockholders Equity Note [Line Items]
Unit-based compensation awards(1)	[1]					701
Issuance of units under ATM Program						$ 1
General Partner Units [Member] | ATM Program [Member]
Stockholders Equity Note [Line Items]
Issuance of units under ATM Program						$ 6
[1]	(1) As a result of the unit-based compensation awards issued during the period, MPLX GP contributed less than $1 million in exchange for 701 general partner units to maintain its two percent general partner interest.
[2]	See Note 7 for distribution information.

Equity - Changes in Partners Ca

Equity - Changes in Partners Capital, Unit Rollforward (Details)	3 Months Ended
	Mar. 31, 2016shares
Stockholders Equity [Line Items]
Balance at December 31, 2015	311,469,407
Units converted	35,039	[1]
Balance at March 31, 2016	346,768,732
Limited Partners Common Units [Member]
Stockholders Equity [Line Items]
Balance at December 31, 2015	296,687,176
Units converted	34,338	[1]
Balance at March 31, 2016	331,280,516
Class B Units [Member]
Stockholders Equity [Line Items]
Balance at December 31, 2015	7,981,756
Balance at March 31, 2016	7,981,756
General Partner Units [Member]
Stockholders Equity [Line Items]
Balance at December 31, 2015	6,800,475
Units converted	701	[1]
Balance at March 31, 2016	7,506,460
ATM Program [Member]
Stockholders Equity [Line Items]
Issuance of units under the ATM Program(2)	12,270,406	[2]
ATM Program [Member] | Limited Partners Common Units [Member]
Stockholders Equity [Line Items]
Issuance of units under the ATM Program(2)	12,025,000	[2]
ATM Program [Member] | General Partner Units [Member]
Stockholders Equity [Line Items]
Issuance of units under the ATM Program(2)	245,406	[2]
Hardin Street Marine [Member]
Stockholders Equity [Line Items]
Partners' Capital Account, Units, Acquisitions	22,993,880	[3]
Hardin Street Marine [Member] | Limited Partners Common Units [Member]
Stockholders Equity [Line Items]
Partners' Capital Account, Units, Acquisitions	22,534,002	[3]
Hardin Street Marine [Member] | General Partner Units [Member]
Stockholders Equity [Line Items]
Partners' Capital Account, Units, Acquisitions	459,878	[3]
[1]	(1) As a result of the unit-based compensation awards issued during the period, MPLX GP contributed less than $1 million in exchange for 701 general partner units to maintain its two percent general partner interest.
[2]	(2) As a result of common units issued under the ATM Program during the period, MPLX GP contributed $6 million in exchange for 245,406 general partner units to maintain its two percent general partner interest.
[3]	(3) See Note 3 for information regarding the HSM acquisition.

Equity - Net Income Allocation

Equity - Net Income Allocation (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015
Equity [Abstract]
Net income attributable to MPLX LP	[1]	$ (60)	$ 46	[2]
General partner's incentive distribution rights and other		41	3
Net (loss) income attributable to MPLX LP available to general and limited partners		(101)	43
General partner's two percent interest in net (loss) income attributable to MPLX LP		(2)	1
Less: General partner’s interest in net income attributable to MPLX LP		$ 39	$ 4	[2]
[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]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.

Equity - Cash Distributions (De

Equity - Cash Distributions (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Incentive Distribution Made to Managing Member or General Partner [Line Items]
General partner's distributions	$ 4	$ 1
General partner's incentive distribution rights	40	3
Total general partner's distributions	44	4
Limited partners' distributions	156	33
Total cash distributions declared	200	37
Limited Partners Common Units [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Limited partners' distributions	156	18
Subordinated Units [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Limited partners' distributions	$ 0	$ 15

Segment Information (Details)

Segment Information (Details)	Dec. 04, 2015	Mar. 31, 2016
Segment Reporting Information [Line Items]
Number of Reportable Segments		2
MarkWest [Member]
Segment Reporting Information [Line Items]
Business Acquisition, Effective Date of Acquisition	Dec. 4, 2015

Segment Information - Segment O

Segment Information - Segment Operating Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Segment Reporting Information [Line Items]
Segment revenues	$ 100	$ 0	[1]
Segment other income	2	1	[1]
Total segment revenues and other income	609	201	[1]
Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment revenues	690
Segment other income	19
Total segment revenues and other income	709	201
Segment cost of revenues	289
Segment operating income	420
Segment portion attributable to noncontrolling interest and Predecessor	75	33
Segment operating income attributable to MPLX LP	345	82
Operating Segments [Member] | L&S [Member]
Segment Reporting Information [Line Items]
Segment revenues	192	183
Segment other income	19	18
Total segment revenues and other income	211	201
Segment cost of revenues	89	86
Segment operating income	122	115
Segment portion attributable to noncontrolling interest and Predecessor	34	33
Segment operating income attributable to MPLX LP	88	$ 82
Operating Segments [Member] | G&P [Member]
Segment Reporting Information [Line Items]
Segment revenues	498
Segment other income	0
Total segment revenues and other income	498
Segment cost of revenues	200
Segment operating income	298
Segment portion attributable to noncontrolling interest and Predecessor	41
Segment operating income attributable to MPLX LP	$ 257
[1]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.

Segment Information - Reconcili

Segment Information - Reconciliation to Income from Operations (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Segment Reporting, Revenue Reconciling Item [Line Items]
Income from equity method investments	$ 5	$ 0	[1],[2]
Other income - related parties	24	17	[1]
Impairment expense	(129)	0	[1],[2]
Depreciation and amortization	(132)	(19)	[1],[2]
General and administrative expenses	(52)	(22)	[1]
Income from operations	27	74	[1]
Operating Segments [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Segment operating income attributable to MPLX LP	345	82
Segment portion attributable to noncontrolling interest and Predecessor	75	33
Segment Reconciling Items [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Segment portion attributable to unconsolidated affiliates	(83)	0
Income from equity method investments	5	0
Other income - related parties	7	0
Unrealized derivative gains	$ (9)	$ 0
[1]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.
[2]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.

Segment Information - Reconci74

Segment Information - Reconciliation to Total Revenues and Other Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues and other income	$ 609	$ 201	[1]
Income from equity method investments	5	0	[1],[2]
Other income - related parties	24	17	[1]
Operating Segments [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues and other income	709	201
Segment Reconciling Items [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue adjustment from unconsolidated affiliates	(104)	0
Income from equity method investments	5	0
Other income - related parties	7	0
Unrealized derivative loss	$ (8)	$ 0
[1]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.
[2]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.

Segment Information - Reconci75

Segment Information - Reconciliation to Net Income Attributable to Noncontrolling Interests (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Reconciliation to Net Income Attributable to Noncontrolling Interests [Line Items]
Net income attributable to noncontrolling interests and Predecessor	$ 23	$ 22
Operating Segments [Member]
Reconciliation to Net Income Attributable to Noncontrolling Interests [Line Items]
Segment portion attributable to noncontrolling interest and Predecessor	75	33
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	(29)	(11)
Portion of operating income attributable to noncontrolling interest of unconsolidated affiliates	$ (23)	$ 0

Segment Information - Reconci76

Segment Information - Reconciliation of Capital Expenditures (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015
Segment Reporting, Other Significant Reconciling Item [Line Items]
Payments to Acquire Property, Plant, and Equipment		$ (291)	$ (35)	[1]
Operating Segments [Member]
Segment Reporting, Other Significant Reconciling Item [Line Items]
Payments to Acquire Property, Plant, and Equipment		(335)	(35)
Operating Segments [Member] | L&S [Member]
Segment Reporting, Other Significant Reconciling Item [Line Items]
Payments to Acquire Property, Plant, and Equipment		(62)	(35)
Operating Segments [Member] | G&P [Member]
Segment Reporting, Other Significant Reconciling Item [Line Items]
Payments to Acquire Property, Plant, and Equipment	[2]	(273)	0
Segment Reconciling Items [Member]
Segment Reporting, Other Significant Reconciling Item [Line Items]
Payments to Acquire Property, Plant, and Equipment		$ (44)	$ 0
[1]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.
[2]	(1) The G&P segment includes $44 million of capital expenditures related to Partnership operated, non-wholly owned subsidiaries.

Segment Information - Assets by

Segment Information - Assets by Segment (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Segment Reporting, Other Significant Reconciling Item [Line Items]
Assets	$ 15,978	$ 16,104	[1]
L&S [Member] | Operating Segments [Member]
Segment Reporting, Other Significant Reconciling Item [Line Items]
Assets	1,792	1,858
G&P [Member] | Operating Segments [Member]
Segment Reporting, Other Significant Reconciling Item [Line Items]
Assets	$ 14,186	$ 14,246
[1]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.

Income Taxes (Details)

Income Taxes (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016	[1]	Mar. 31, 2015
Income Taxes [Line Items]
Benefit for income taxes		$ 4		$ 0	[2]
Maximum [Member]
Income Taxes [Line Items]
Benefit for income taxes	[2]			$ (1)
[1]	(1) Financial information has been retrospectively adjusted for the acquisition of HSM from MPC. See Notes 1 and 3. Prior to this acquisition, MPC paid all income taxes related to HSM.
[2]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.

Income Taxes Income Taxes - Rec

Income Taxes Income Taxes - Reconciliation of Federal Statutory Income Tax Rate (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016		Mar. 31, 2015	[2]
Entity Information [Line Items]
Income before income taxes		$ (41)	[1]	$ 68
Federal statutory rate		35.00%
Federal income tax at statutory rate(2)	[1],[3]	$ (4)
State income taxes net of federal benefit - MarkWest Hydrocarbon	[1]	(1)
Provision on income from Class A units(2)	[1],[3]	(1)
Other	[1]	2
Provision for income tax		(4)	[1]	$ 0
Reportable Legal Entities [Member] | Mark West Hydrocarbon [Member]
Entity Information [Line Items]
Income before income taxes		(12)
Federal income tax at statutory rate(2)	[3]	(4)
State income taxes net of federal benefit - MarkWest Hydrocarbon		(1)
Provision on income from Class A units(2)	[3]	(1)
Other		1
Provision for income tax		(5)
Reportable Legal Entities [Member] | Partnership [Member]
Entity Information [Line Items]
Income before income taxes		(30)
Federal income tax at statutory rate(2)	[3]	0
State income taxes net of federal benefit - MarkWest Hydrocarbon		0
Provision on income from Class A units(2)	[3]	0
Other		1
Provision for income tax		1
Eliminations [Member]
Entity Information [Line Items]
Income before income taxes		1
Federal income tax at statutory rate(2)	[3]	0
State income taxes net of federal benefit - MarkWest Hydrocarbon		0
Provision on income from Class A units(2)	[3]	0
Other		0
Provision for income tax		$ 0
[1]	(1) Financial information has been retrospectively adjusted for the acquisition of HSM from MPC. See Notes 1 and 3. Prior to this acquisition, MPC paid all income taxes related to HSM.
[2]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.
[3]	(2) MarkWest Hydrocarbon pays tax on its share of the Partnership’s income or loss as a result of its ownership of Class A units.

Inventories (Details)

Inventories (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Inventory Disclosure [Abstract]
NGLs	$ 1	$ 3
Line fill	4	5
Spare parts, materials and supplies	40	43
Inventories	$ 45	$ 51	[1]
[1]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.

Property, Plant and Equipment -

Property, Plant and Equipment - Property, Plant and Equipment (Detail) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	$ 10,964	$ 10,647
Less accumulated depreciation	769	650
Net property, plant and equipment	10,195	9,997	[1]
Gas Gathering And Transmission Equipment And Facilities [Member]
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	4,443	4,307
Processing, Fractionation And Storage Facilities [Member]
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	3,197	3,185
Pipelines and Related Assets [Member]
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	1,143	1,128
Barges and towing vessels [Member]
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	478	475
Land Building Office Equipment And Other [Member]
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	625	606
Assets Under Construction [Member]
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	$ 1,078	$ 946
[1]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.

Fair Value Measurements - Recur

Fair Value Measurements - Recurring - Financial Instruments by Valuation Hierarchy (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross Amount	$ 3
Gross Amount	(36)
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross Amount	2	$ 9
Gross Amount	(35)	(32)
Embedded derivatives in commodity contracts | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross Amount	0	0
Gross Amount	(34)	(32)
Commodity contracts | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross Amount	1	2
Gross Amount	0	0
Commodity contracts | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross Amount	1	7
Gross Amount	$ (1)	$ 0

Fair Value Measurments - Recurr

Fair Value Measurments - Recurring - Significant Unobservable Inputs in Level 3 Valuation (Details) - Fair Value, Inputs, Level 3 [Member]	3 Months Ended
	Mar. 31, 2016USD ($)
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]
Minimum [Member] | Assets [Member] | Ethanol prices [Member] | Commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price	$ 0.17	[2]
Minimum [Member] | Assets [Member] | Propane Contract [Member] | Commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price	0.44	[2]
Minimum [Member] | Assets [Member] | Isobutane Contract [Member] | Commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price	0.54	[2]
Minimum [Member] | Assets [Member] | Normal Butane Contract [Member] | Commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price	0.50	[2]
Minimum [Member] | Assets [Member] | Natural Gasoline Contract [Member] | Commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price	0.90	[2]
Minimum [Member] | Liability [Member] | Propane Contract [Member] | Commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price	0.44	[2]
Minimum [Member] | Liability [Member] | Propane Contract [Member] | Embedded derivatives in commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price	0.39	[2]
Minimum [Member] | Liability [Member] | Isobutane Contract [Member] | Commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price	0.54	[2]
Minimum [Member] | Liability [Member] | Isobutane Contract [Member] | Embedded derivatives in commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price	0.53	[2]
Minimum [Member] | Liability [Member] | Normal Butane Contract [Member] | Commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price	0.50	[2]
Minimum [Member] | Liability [Member] | Normal Butane Contract [Member] | Embedded derivatives in commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price	0.48	[2]
Minimum [Member] | Liability [Member] | Natural Gasoline Contract [Member] | Commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price	0.90	[2]
Minimum [Member] | Liability [Member] | Natural Gasoline Contract [Member] | Embedded derivatives in commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price	0.89	[2]
Minimum [Member] | Liability [Member] | Natural Gas Contract [Member] | Embedded derivatives in commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price	2.18	[3]
Minimum [Member] | Liability [Member] | ERCOT Pricing [Member] | Embedded derivatives in commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Commodity Price	23.08
Maximum [Member] | Assets [Member] | Ethanol prices [Member] | Commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price	0.21	[2]
Maximum [Member] | Assets [Member] | Propane Contract [Member] | Commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price	0.49	[2]
Maximum [Member] | Assets [Member] | Isobutane Contract [Member] | Commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price	0.58	[2]
Maximum [Member] | Assets [Member] | Normal Butane Contract [Member] | Commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price	0.56	[2]
Maximum [Member] | Assets [Member] | Natural Gasoline Contract [Member] | Commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price	0.91	[2]
Maximum [Member] | Liability [Member] | Propane Contract [Member] | Commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price	0.49	[2]
Maximum [Member] | Liability [Member] | Propane Contract [Member] | Embedded derivatives in commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price	0.49	[2]
Maximum [Member] | Liability [Member] | Isobutane Contract [Member] | Commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price	0.58	[2]
Maximum [Member] | Liability [Member] | Isobutane Contract [Member] | Embedded derivatives in commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price	0.64	[2]
Maximum [Member] | Liability [Member] | Normal Butane Contract [Member] | Commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price	0.56	[2]
Maximum [Member] | Liability [Member] | Normal Butane Contract [Member] | Embedded derivatives in commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price	0.60	[2]
Maximum [Member] | Liability [Member] | Natural Gasoline Contract [Member] | Commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price	0.91	[2]
Maximum [Member] | Liability [Member] | Natural Gasoline Contract [Member] | Embedded derivatives in commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price	1.04	[2]
Maximum [Member] | Liability [Member] | Natural Gas Contract [Member] | Embedded derivatives in commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price	3.39	[3]
Maximum [Member] | Liability [Member] | ERCOT Pricing [Member] | Embedded derivatives in commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Commodity Price	$ 44.58
[1]	(3) The producer counterparty to the embedded derivative has the option to renew the gas purchase agreement and the related keep-whole processing agreement for two successive five-year terms after 2022. The embedded gas purchase agreement cannot be renewed without the renewal of the related keep-whole processing agreement. Due to the significant number of years until the renewal options are exercisable and the high level of uncertainty regarding the counterparty’s future business strategy, the future commodity price environment, and the future competitive environment for midstream services in the Southern Appalachian region, management determined that a 50 percent probability of renewal for the first five-year term and 75 percent for the second five-year term are appropriate assumptions. Included in this assumption is a further extension of management’s estimates of future frac spreads through 2032.
[2]	(1) NGL prices used in the valuation are generally at the lower end of the range in the early years and increase over time.
[3]	(2) Natural gas prices used in the valuations are generally at the lower end of the range in the early years and increase over time.

Fair Value Measurements - Rec84

Fair Value Measurements - Recurring - Changes in Level 3 Measurements (Details) $ in Millions	3 Months Ended
	Mar. 31, 2016USD ($)
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	$ 7
Total loss (realized and unrealized) included in earnings(1)	(1)	[1]
Settlements	(6)
Fair value at end of period	0
Fair Value Measurement with Unobservable Inputs Reconciliation, Recurring Basis Asset and Liabilities Net, Change in Unrealized Gain (Loss) Included in Earnings Contract Held	(2)
Embedded Derivatives in Commodity Contracts (net)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value at beginning of period	(32)
Total loss (realized and unrealized) included in earnings(1)	(4)	[1]
Settlements	2
Fair value at end of period	(34)
Fair Value Measurement with Unobservable Inputs Reconciliation, Recurring Basis Asset and Liabilities Net, Change in Unrealized Gain (Loss) Included in Earnings Contract Held	$ (3)
[1]	(1) Gains and losses on Commodity Derivative Contracts classified as Level 3 are recorded in Product sales in the accompanying Consolidated Statements of Income. Gains and losses on Embedded Derivatives in Commodity Contracts are recorded in Costs of revenue and Purchased product costs.

Fair Value Measurements Fair Va

Fair Value Measurements Fair Value Measurements - Recurring (Details)	Mar. 31, 2016
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 - Repor

Fair Value Measurements - Reported (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015	Dec. 04, 2015
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	$ 4,715	$ 5,255
Other Liabilities, Fair Value Disclosure	99	100
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	4,804	5,212
Other Liabilities, Fair Value Disclosure	$ 102	$ 99
MarkWest [Member] | Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Gross			$ 4,100

Derivative Financial Instrume87

Derivative Financial Instruments - Volume of Commodity Derivative Activity (Details) - Not Designated as Hedging Instrument [Member] - Commodity contracts	Mar. 31, 2016MMBTUbblgal
Short [Member]
Derivative [Line Items]
Nonmonetary Notional Amount of Price Risk Derivatives of Crude Oil | bbl	82,500
Nonmonetary Notional Amount of Price Risk Derivatives of Natural Gas Liquids | gal	55,759,065
Long [Member]
Derivative [Line Items]
Nonmonetary Notional Amount Of Price Risk Derivatives Of Natural Gas | MMBTU	745,826

Derivative Financial Instrume88

Derivative Financial Instruments - Embedded Derivatives in Commodity Contracts (Details) - Embedded derivatives in commodity contracts $ in Millions	3 Months Ended
	Mar. 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	$ 33
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) - Commodity contracts - USD ($) $ in Millions		Mar. 31, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	$ 2	$ 9
Derivative Liability, Fair Value, Gross Liability	[1]	35	32
Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	2	9
Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	[1]	7	5
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]	$ 28	$ 27
[1]	(1) Includes embedded derivatives in commodity contracts as discussed above.

Derivative Financial Instrume90

Derivative Financial Instruments - Net Derivatives (Details) $ in Millions	Mar. 31, 2016USD ($)
Assets
Gross Amount	$ 3
Gross Amounts Offset in the Consolidated Balance Sheets	(1)
Net Amount of Assets in the Consolidated Balance Sheets	2
Liabilities
Gross Amount	(36)
Gross Amounts Offset in the Consolidated Balance Sheets	1
Net Amount of Liabilities in the Consolidated Balance Sheets	(35)
Current Assets [Member] | Commodity contracts
Assets
Gross Amount	3
Gross Amounts Offset in the Consolidated Balance Sheets	(1)
Net Amount of Assets in the Consolidated Balance Sheets	2
Current Liabilities [Member] | Commodity contracts
Liabilities
Gross Amount	(2)
Gross Amounts Offset in the Consolidated Balance Sheets	1
Net Amount of Liabilities in the Consolidated Balance Sheets	(1)
Non Current Assets [Member] | Commodity contracts
Assets
Gross Amount	0
Gross Amounts Offset in the Consolidated Balance Sheets	0
Net Amount of Assets in the Consolidated Balance Sheets	0
Non Current Liabilities [Member] | Commodity contracts
Liabilities
Gross Amount	0
Gross Amounts Offset in the Consolidated Balance Sheets	0
Net Amount of Liabilities in the Consolidated Balance Sheets	0
Not Designated as Hedging Instrument [Member] | Current Assets [Member]
Assets
Gross Amount	3
Gross Amounts Offset in the Consolidated Balance Sheets	(1)
Net Amount of Assets in the Consolidated Balance Sheets	2
Not Designated as Hedging Instrument [Member] | Current Assets [Member] | Embedded derivatives in commodity contracts
Assets
Gross Amount	0
Gross Amounts Offset in the Consolidated Balance Sheets	0
Net Amount of Assets in the Consolidated Balance Sheets	0
Not Designated as Hedging Instrument [Member] | Current Liabilities [Member]
Liabilities
Gross Amount	(8)
Gross Amounts Offset in the Consolidated Balance Sheets	1
Net Amount of Liabilities in the Consolidated Balance Sheets	(7)
Not Designated as Hedging Instrument [Member] | Current Liabilities [Member] | Embedded derivatives in commodity contracts
Liabilities
Gross Amount	(6)
Gross Amounts Offset in the Consolidated Balance Sheets	0
Net Amount of Liabilities in the Consolidated Balance Sheets	(6)
Not Designated as Hedging Instrument [Member] | Non Current Assets [Member]
Assets
Gross Amount	0
Gross Amounts Offset in the Consolidated Balance Sheets	0
Net Amount of Assets in the Consolidated Balance Sheets	0
Not Designated as Hedging Instrument [Member] | Non Current Assets [Member] | Embedded derivatives in commodity contracts
Assets
Gross Amount	0
Gross Amounts Offset in the Consolidated Balance Sheets	0
Net Amount of Assets in the Consolidated Balance Sheets	0
Not Designated as Hedging Instrument [Member] | Non Current Liabilities [Member]
Liabilities
Gross Amount	(28)
Gross Amounts Offset in the Consolidated Balance Sheets	0
Net Amount of Liabilities in the Consolidated Balance Sheets	(28)
Not Designated as Hedging Instrument [Member] | Non Current Liabilities [Member] | Embedded derivatives in commodity contracts
Liabilities
Gross Amount	(28)
Gross Amounts Offset in the Consolidated Balance Sheets	0
Net Amount of Liabilities in the Consolidated Balance Sheets	$ (28)

Derivatives Financial Instrumen

Derivatives Financial Instruments - Derivative Income Statement Location (Details) $ in Millions	3 Months Ended
	Mar. 31, 2016USD ($)
Product Sales [Member]
Product sales [Abstract]
Realized gain	$ 7
Unrealized loss	(8)
Total revenue: derivative loss from product sales	(1)
Purchased product costs [Abstract]
Total loss	(1)
Purchased product costs [Member]
Product sales [Abstract]
Unrealized loss	(1)
Total revenue: derivative loss from product sales	(2)
Purchased product costs [Abstract]
Total loss	$ (2)

Debt - Summary of Outstanding B

Debt - Summary of Outstanding Borrowings (Detail) - USD ($) $ in Millions		Mar. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Total		$ 5,185	$ 5,736
Unamortized debt issuance costs		(8)	(8)
Unamortized discount(1)	[1]	(461)	(472)
Amounts due within one year		(1)	(1)
Total long-term debt due after one year		4,715	5,255	[2]
Senior Notes [Member] | MarkWest [Member]
Debt Instrument [Line Items]
Unamortized discount(1)		(453)	(465)
Capital Lease Obligations [Member] | Marathon Pipe Line Llc [Member]
Debt Instrument [Line Items]
MPL - capital lease obligations due 2020		9	9
Bank revolving credit facility due 2020 | Line of Credit [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		326	877
MPLX Term Loan [Member] | Unsecured Debt [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		250	250
5.500% senior notes due 2023 | Senior Notes [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		710	710
4.500% senior notes due 2023 | Senior Notes [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		989	989
4.875% senior notes due 2024 | Senior Notes [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		1,149	1,149
4.000% senior notes due 2025 | Senior Notes [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		500	500
4.875% senior notes due 2025 | Senior Notes [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		1,189	1,189
MarkWest - 4.500% - 5.500% senior notes, due 2023 - 2025 | Senior Notes [Member] | MarkWest [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 63	$ 63
[1]	(1) Includes $453 million and $465 million discount as of March 31, 2016 and December 31, 2015, respectively, related to the difference between the fair value and the principal amount of the assumed MarkWest debt.
[2]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.

Debt - Summary of Outstanding93

Debt - Summary of Outstanding Borrowings Interest Rates and Table Due Dates (Details)	3 Months Ended
	Mar. 31, 2016
Mplx Lp [Member] | Line of Credit [Member] | MPLX Revolver [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Expiration Date	Dec. 4, 2020
Mplx Lp [Member] | Unsecured Debt [Member] | MPLX Term Loan [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Expiration Date	Nov. 20, 2019
Mplx Lp [Member] | Senior Notes [Member] | Senior Notes Due February 2023 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.50%
Debt Instrument, Maturity Date	Feb. 15, 2023
Mplx Lp [Member] | Senior Notes [Member] | Senior Notes Due July 2023 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.50%
Debt Instrument, Maturity Date	Jul. 15, 2023
Mplx Lp [Member] | Senior Notes [Member] | Senior Notes Due December 2024 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.875%
Debt Instrument, Maturity Date	Dec. 1, 2024
Mplx Lp [Member] | Senior Notes [Member] | Senior Notes Due February 2025 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.00%
Debt Instrument, Maturity Date	Feb. 15, 2025
Mplx Lp [Member] | Senior Notes [Member] | Senior Notes Due June 2025 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.875%
Debt Instrument, Maturity Date	Jun. 1, 2025
MarkWest [Member] | Senior Notes [Member] | Senior Notes Due February 2023 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.50%
Debt Instrument, Maturity Date	Feb. 15, 2023
MarkWest [Member] | Senior Notes [Member] | Senior Notes Due July 2023 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.50%
Debt Instrument, Maturity Date	Jul. 15, 2023
MarkWest [Member] | Senior Notes [Member] | Senior Notes Due December 2024 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.875%
Debt Instrument, Maturity Date	Dec. 1, 2024
MarkWest [Member] | Senior Notes [Member] | Senior Notes Due June 2025 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.875%
Debt Instrument, Maturity Date	Jun. 1, 2025
Marathon Pipe Line Llc [Member] | Capital Lease Obligations [Member]
Debt Instrument [Line Items]
Capital Lease Due Date Year	2,020

Debt - Additional Information (

Debt - Additional Information (Detail) - Mplx Lp [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
MPLX Revolver [Member]
Debt Instrument [Line Items]
Proceeds from Lines of Credit	$ 306
Debt Instrument, Interest Rate, Effective Percentage	1.896%
Repayments of Long-term Lines of Credit	$ 857
Letters of Credit Outstanding, Amount	8
Line of Credit Facility, Remaining Borrowing Capacity	$ 1,700
Line of Credit Facility, Remaining Borrowing Capacity, Percentage	85.00%
MPLX Term Loan [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage	1.928%
Line of Credit [Member] | MPLX Revolver [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 326	$ 877
Maximum [Member] | MPLX Term Loan [Member] | Letter of Credit [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 250

Goodwill (Details)

Goodwill (Details) - USD ($) $ in Millions	Dec. 04, 2015	Mar. 31, 2016	Mar. 31, 2015	[1],[2]
Goodwill [Line Items]
Number of Reporting Units		3
Number of Reporting Units Impaired		2
Impairment expense		$ 129	$ 0
Equity Method Investments [Member] | Income Approach Valuation Technique [Member] | Minimum [Member]
Goodwill [Line Items]
Fair Value Inputs, Discount Rate	9.40%	10.50%
Equity Method Investments [Member] | Income Approach Valuation Technique [Member] | Maximum [Member]
Goodwill [Line Items]
Fair Value Inputs, Discount Rate	11.10%	11.50%
Finite-Lived Intangible Assets [Member] | Income Approach Valuation Technique [Member] | Minimum [Member]
Goodwill [Line Items]
Fair Value Inputs, Discount Rate	11.50%	11.50%
Fair Value Inputs, Attrition Rate	5.00%	5.00%
Finite-Lived Intangible Assets [Member] | Income Approach Valuation Technique [Member] | Maximum [Member]
Goodwill [Line Items]
Fair Value Inputs, Discount Rate	12.80%	12.80%
Fair Value Inputs, Attrition Rate	10.00%	10.00%
[1]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.
[2]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.

Goodwill - Changes in Goodwill

Goodwill - Changes in Goodwill (Details) $ in Millions	3 Months Ended
	Mar. 31, 2016USD ($)
Goodwill [Line Items]
Balance as of December 31, 2015	$ 2,570	[1]
Purchase price allocation adjustments(1)	(241)	[2]
Impairment expense	(129)
Balance as of March 31, 2016	2,200
L&S [Member]
Goodwill [Line Items]
Balance as of December 31, 2015	116
Purchase price allocation adjustments(1)	0	[2]
Impairment expense	0
Balance as of March 31, 2016	116
G&P [Member]
Goodwill [Line Items]
Balance as of December 31, 2015	2,454
Purchase price allocation adjustments(1)	(241)	[2]
Impairment expense	(129)
Balance as of March 31, 2016	$ 2,084
[1]	(1) Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.
[2]	(1) See Note 3 for further discussion on purchase price allocation adjustments.

Supplemental Cash Flow Inform97

Supplemental Cash Flow Information - Summary of Supplemental Cash Flow Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Net cash provided by operating activities included:
Interest paid (net of amounts capitalized)	$ 53	$ 2
Non-cash investing and financing activities:
Net transfers of property, plant and equipment from materials and supplies inventories	$ (5)	$ 0

Supplemental Cash Flow Inform98

Supplemental Cash Flow Information - Summary of Reconciliation of Additions to Property, Plant and Equipment to Total Capital Expenditures (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Supplemental Cash Flow Elements [Abstract]
(Decrease) increase in capital accruals	$ (20)	$ 5

Supplemental Cash Flow Inform99

Supplemental Cash Flow Information Supplemental Cash Flow Information - Narrative (Details) $ in Millions	3 Months Ended
	Mar. 31, 2016USD ($)
Supplemental Cash Flow Elements [Abstract]
Contribution from MPC	$ 15

Equity-Based Compensation Pl100

Equity-Based Compensation Plan - Summary of Phantom Unit Award Activity (Details) - Phantom Units [Member]	3 Months Ended
	Mar. 31, 2016$ / sharesshares
Number of Units
Outstanding at December 31, 2015 | shares	1,031,219
Granted | shares	437,978
Settled | shares	41,463
Forfeited | shares	4,592
Outstanding at March 31, 2016 | shares	1,423,142
Weighted Average Fair Value
Outstanding at December 31, 2015 | $ / shares	$ 35.49
Granted (in USD per unit) | $ / shares	29.32
Settled (in USD per unit) | $ / shares	51.13
Forfeited (in USD per unit) | $ / shares	30.27
Outstanding at March 31, 2016 | $ / shares	$ 33.15

Equity-Based Compensation Pl101

Equity-Based Compensation Plan - Additional Information (Detail) - Performance Shares [Member] - MPLX LP 2012 Incentive Compensation Plan [Member]	3 Months Ended
	Mar. 31, 2016$ / shares
Equity Transactions And Share Based Compensation [Line Items]
Performance units grant date fair value (in USD per unit)	$ 0.63
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 Pl102

Equity-Based Compensation Plan - Summary of Performance Unit Award Activity (Detail) - Performance Shares [Member]	3 Months Ended
	Mar. 31, 2016shares
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Outstanding at December 31, 2015	1,521,392
Granted	781,875
Settled	433,011
Forfeited	(53,507)
Outstanding at March 31, 2016	1,816,749

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Commitments And Contingencies [Line Items]
Accrued liabilities for remediation	$ 5	$ 1
Contractual commitments to acquire property, plant and equipment	247
Environmental Loss Contingency [Member] | MPC [Member]
Commitments And Contingencies [Line Items]
Receivables from MPC for indemnification of environmental costs	1	$ 1
MarkWest Merger [Member]
Commitments And Contingencies [Line Items]
Legal Fees	$ 2

Subsequent Event - Additional I

Subsequent Event - Additional Information (Details) - Subsequent Event [Member] $ / shares in Units, shares in Millions, number in Millions, $ in Millions	Apr. 27, 2016USD ($)$ / sharesshares
Subsequent Event [Line Items]
Subsequent Event, Date	Apr. 27, 2016
Series A Convertible Preferred Units [Member]
Subsequent Event [Line Items]
Preferred Stock, Value, Issued	$ 1,000
Preferred Stock, Dividend Rate, Percentage	0.00%
Preferred Units, Price Per Unit | $ / shares	$ 32.50
Preferred Stock, Shares Issued | shares	30.8
Preferred Units, Description	The Preferred Units will be entitled to receive an annual distribution, which will initially equal $2.1125 per unit. The Preferred Units are convertible into MPLX common units on a one for one basis after three years, at the purchasers’ option, and after four years at the Partnership’s option, subject to certain conditions. Closing of the transaction is scheduled to occur in May 2016.
Series A Convertible Preferred Units [Member] | Scenario, Forecast [Member]
Subsequent Event [Line Items]
Proceeds from Issuance of Preferred Limited Partners Units	$ 984