MPLX (MPLX) 10-Q 3 Aug 16 2016 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2016	Jul. 27, 2016
Document Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2016
Document Fiscal Year Focus	2,016
Document Fiscal Period Focus	Q2
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		335,635,872
Class B Units [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		3,990,878
General Partner Units [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		7,513,899

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) shares in Millions, $ in Millions		3 Months Ended			6 Months Ended
		Jun. 30, 2016	Jun. 30, 2015		Jun. 30, 2016		Jun. 30, 2015
Revenues and other income:
Service revenue		$ 233	$ 16	[1]	$ 462		$ 32	[1]
Service revenue - related parties		145	152	[1]	295		294	[1]
Rental income		71	0	[1]	141		0	[1]
Rental income - related parties		29	25	[1]	55		50	[1]
Product sales		137	0	[1]	237		0	[1]
Product sales - related parties		3	0	[1]	6		0	[1]
Loss from equity method investments		(83)	0	[1]	(78)	[2]	0	[1],[3]
Other income		1	2	[1]	3		3	[1]
Other income - related parties		28	18	[1]	52		35	[1]
Total revenues and other income		564	213	[1]	1,173		414	[1]
Costs and expenses:
Cost of revenues (excludes items below)		84	46	[1]	173		88	[1]
Purchased product costs		114	0	[1]	193		0	[1]
Rental cost of sales		14	0	[1]	28		0	[1]
Purchases - related parties		78	40	[1]	154		80	[1]
Depreciation and amortization		137	20	[1]	269		39	[1],[3]
Impairment expense		1	0	[1]	130		0	[1],[3]
General and administrative expenses		49	21	[1]	101		43	[1]
Other taxes		11	4	[1]	22		8	[1]
Total costs and expenses		488	131	[1]	1,070		258	[1]
Income from operations		76	82	[1]	103		156	[1]
Related party interest and other financial costs		0	0	[1]	1		0	[1]
Interest expense (net of amounts capitalized of $7 million, $1 million, $14 million and $1 million, respectively)		52	6	[1]	107		11	[1]
Other financial costs		12	0	[1]	24		1	[1]
Income (loss) before income taxes		12	76	[1]	(29)	[4]	144	[1]
Benefit for income taxes		8	0	[1]	12	[4]	0	[1]
Net income (loss)		20	76	[1]	(17)		144	[1],[3]
Net income attributable to noncontrolling interest		1	1	[1]	1		1	[1]
Less: Net income attributable to Predecessor		0	24	[1]	23		46	[1]
Net income (loss) attributable to MPLX LP	[5]	19	51	[1]	(41)		97	[1]
Less: Preferred unit distributions		9	0	[1]	9		0
Less: General partner’s interest in net income attributable to MPLX LP		46	7	[1]	85		11	[1]
Limited partners’ interest in net (loss) income attributable to MPLX LP		$ (36)	$ 44	[1]	$ (135)		$ 86	[1]
Weighted average limited partner units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic		355	82		331		82
Weighted Average Limited Partnership Units Outstanding, Diluted		355	82		331		82
MPC [Member]
Revenues and other income:
Service revenue - related parties		$ 145	$ 152		$ 295		$ 294
Rental income - related parties		29	25		55		50
Product sales - related parties		3	0		6		0
Other income - related parties		16	18		33		34
Limited Partners Common Units [Member]
Costs and expenses:
Net income (loss) attributable to MPLX LP	[5]	$ (35)	$ 21		$ (135)		$ 42
Per Unit Data (See Note 6)
Common - basic (in USD per unit)		$ (0.11)	$ 0.50		$ (0.43)		$ 0.96
Common - diluted (in USD per unit)		$ (0.11)	$ 0.50		$ (0.43)		$ 0.96
Weighted average limited partner units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic		331	43		316		43
Weighted Average Limited Partnership Units Outstanding, Diluted		331	43		316		43
Cash distributions declared per limited partner common unit		$ 0.5100	$ 0.4400		$ 1.0150		$ 0.8500
Limited Partners Subordinated Units [Member] | MPC [Member]
Costs and expenses:
Net income (loss) attributable to MPLX LP	[5]		$ 19				$ 36
Per Unit Data (See Note 6)
Common - basic (in USD per unit)			$ 0.50				$ 0.96
Common - diluted (in USD per unit)			0.50				0.96
Subordinated - basic and diluted (in USD per unit)		$ 0	$ 0.50		$ 0		$ 0.96
Weighted average limited partner units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic			37				37
Weighted Average Limited Partnership Units Outstanding, Diluted			37				37
Weighted average limited partner units outstanding basic and diluted (in shares)		0	37		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]	(2)Income (loss) from equity method investments includes the impact of any basis differential amortization or accretion.
[3]	(1)Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.
[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.
[5]	llocation of net income (loss) attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.

Consolidated Statements of Inc3

Consolidated Statements of Income Consolidated Statements of Income (Parenthetical) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Income Statement [Abstract]
Interest Costs Capitalized	$ 7	$ 1	$ 14	$ 1

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions		Jun. 30, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents		$ 35	$ 43
Receivables, net		265	245
Receivables - related parties		113	187
Inventories		49	51
Other current assets		24	50
Total current assets		486	576
Equity method investments		2,485	2,458
Property, plant and equipment, net		10,360	9,997
Intangibles, net		511	466
Goodwill		2,199	2,570
Long-term receivables - related parties		26	25
Other noncurrent assets		12	12
Total assets		16,079	16,104
Current liabilities:
Accounts payable		102	91
Accrued liabilities		180	187
Payables - related parties		65	54
Deferred revenue - related parties		38	32
Accrued property, plant and equipment		163	168
Accrued taxes		32	27
Accrued interest payable		53	54
Other current liabilities		17	12
Total current liabilities		650	625
Long-term deferred revenue		9	4
Long-term deferred revenue - related parties		10	9
Long-term debt		4,400	5,255
Deferred income taxes		368	378
Deferred credits and other liabilities		176	166
Total liabilities		5,613	6,437
Commitments and contingencies (see Note 19)
Temporary Equity, Carrying Amount, Including Portion Attributable to Noncontrolling Interests		993	0
Equity
Total MPLX LP partners’ capital		9,458	9,654
Noncontrolling interest		15	13
Total equity		9,473	9,667
Total liabilities, preferred units and equity		16,079	16,104
MPC [Member]
Current assets:
Receivables - related parties		104	175
Long-term receivables - related parties		26	25
Current liabilities:
Payables - related parties		51	33
Limited Partners Common Units [Member] | MPC [Member]
Equity
Total MPLX LP partners’ capital		1,049	465
Limited Partners Class B Units [Member]
Equity
Total MPLX LP partners’ capital		266	266
General Partner [Member] | MPC [Member]
Equity
Total MPLX LP partners’ capital		485	819
Public [Member] | Limited Partners Common Units [Member]
Equity
Total MPLX LP partners’ capital		7,658	7,691
Predecessor [Member] | MPC [Member]
Equity
Total MPLX LP partners’ capital		$ 0	413
Total equity	[1]		$ 413
[1]	(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	Jun. 30, 2016	Dec. 31, 2015
Units outstanding	7,506,520
Limited Partners Common Units [Member] | Public [Member]
Units issued	252,000,000	240,000,000
Units outstanding	252,000,000	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		6 Months Ended
		Jun. 30, 2016		Jun. 30, 2015
Operating activities:
Net (loss) income		$ (17)		$ 144	[1],[2]
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization of deferred financing costs		23		1	[2]
Depreciation and amortization		269		39	[1],[2]
Impairment expense		130		0	[1],[2]
Deferred income taxes		(13)		(1)	[2]
Asset retirement expenditures		2		0	[2]
Loss from equity method investments		(78)	[3]	0	[1],[2]
Distributions from unconsolidated affiliates		78		0	[2]
Changes in:
Current receivables		(20)		(2)	[2]
Inventories		3		0	[2]
Change in fair value of derivatives		(25)		0	[2]
Current accounts payable and accrued liabilities		18		12	[2]
Receivables from / liabilities to related parties		6		(19)	[2]
All other, net		21		(1)	[2]
Net cash provided by operating activities		593		173	[2]
Investing activities:
Additions to property, plant and equipment		(569)		(70)	[2]
Investments - loans from (to) related parties		77		(38)	[2]
Investments in unconsolidated affiliates		39		0	[2]
All other, net		5		(1)	[2]
Net cash used in investing activities		(526)		(109)	[2]
Financing activities:
Long-term debt - borrowings		434		528	[2]
Long-term debt - repayments		(1,311)		(415)	[2]
Related party debt - borrowings		1,853		0	[2]
Related party debt - repayments		1,861		0	[2]
Debt issuance costs		0		4	[2]
Net proceeds from equity offerings		321		1	[2]
Issuance of redeemable preferred units		984		0	[2]
Distributions to unitholders and general partner		(391)		(70)	[2]
Distributions to noncontrolling interests		(1)		(1)	[2]
Contributions from noncontrolling interests		2		0	[2]
All other, net		(1)		0	[2]
Net cash (used in) provided by financing activities		(75)		39	[2]
Net (decrease) increase in cash and cash equivalents		(8)		103	[2]
Cash and cash equivalents at beginning of period		43		27	[2]
Cash and cash equivalents at end of period		35		130	[2]
MPC [Member]
Financing activities:
Distributions to MPC from Predecessor		104		1
Predecessor [Member] | MPC [Member]
Operating activities:
Net (loss) income	[4]			46
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]	(2)Income (loss) from equity method investments includes the impact of any basis differential amortization or accretion.
[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]Public [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		$ 784			$ 639	$ 261		$ 217	$ (660)		$ 6
Distributions to MPC from Predecessor | Predecessor [Member]	[2]			0
Distributions to MPC from Predecessor				(1)
Issuance of units under ATM Program		1			1
Net (loss) income | Predecessor [Member]	[1]			46
Net (loss) income		144	[2],[3]		25	21		40	11		1
Distributions to unitholders and general partner		(70)			(19)	(16)		(29)	(6)
Distributions to noncontrolling interest		(1)									(1)
Contributions from noncontrolling interests	[2]	0
Equity-based compensation		1			1
Ending Balance (Predecessor [Member]) at Jun. 30, 2015	[1]			367
Ending Balance at Jun. 30, 2015		859			647	266		$ 228	(655)		$ 6
Beginning Balance (Predecessor [Member]) at Dec. 31, 2015	[1]			413
Beginning Balance at Dec. 31, 2015		9,667			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		(17)
Net (loss) income | Predecessor [Member]				23
Net (loss) income		(26)			(107)	(28)			85	1
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		(391)			(248)	(57)			(86)
Distributions to noncontrolling interest		(1)								(1)
Contributions from noncontrolling interests		2								2
Equity-based compensation		5			5
Deferred income tax impact from changes in equity		0			2				(2)
Ending Balance at Jun. 30, 2016		$ 9,473			$ 7,658	$ 1,049	$ 266		$ 485	$ 15
[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.

Description of the Business and

Description of the Business and Basis of Presentation	6 Months Ended
	Jun. 30, 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 petroleum products and Gathering and Processing (“G&P”) focused on natural gas and NGLs. See Note 9 for additional information regarding operations. Basis of Presentation – The Partnership’s consolidated financial statements include all majority-owned and controlled subsidiaries. For non-wholly-owned consolidated subsidiaries, the interests owned by third parties, 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 HSM. HSM’s related assets, liabilities and results of 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 consolidated financial statements and related notes of MPLX LP have been retrospectively adjusted to include the historical results of the assets acquired from MPC prior to the effective 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. Revenues and costs related to the portion of the revenue earned under these contracts considered to be implicit leases are recorded as Rental income and Rental cost of sales , respectively, on the Consolidated Statements of Income. 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 services agreement between HSM and MPC. Revenue related to this agreement is recorded as Rental income - related parties on the Consolidated Statements of Income. The rental cost of sales related to the HSM implicit lease is depreciation of the HSM assets. 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. These interim consolidated financial statements are unaudited; however, in the opinion of the Partnership’s management, these statements reflect all adjustments necessary for a fair statement of the results for the periods reported. All such adjustments are of a normal, recurring nature unless otherwise disclosed. These interim consolidated financial statements, including the notes, have been prepared in accordance with the rules and regulations of the SEC applicable to interim period financial statements and do not include all of the information and disclosures required by GAAP for complete financial statements. These interim consolidated financial statements should be read in conjunction with the audited consolidated financial statements and notes thereto included in the Annual Report on Form 10-K for the year ended December 31, 2015, as updated by our Current Report on Form 8-K/A filed on May 20, 2016. The results of operations for the three and six months ended June 30, 2016 are not necessarily indicative of the results to be expected for the full year. In preparing the Consolidated Statements of Equity, net income attributable to MPLX LP is allocated to preferred unitholders based on a fixed distribution schedule, as discussed in Note 8 , and subsequently allocated to the general partner and limited partner unitholders. Distributions, although earned, are not accrued for until declared. However, when distributions related to the incentive distribution rights are made, earnings equal to the amount of those distributions are first allocated to the general partner before the remaining earnings are allocated to the limited partner unitholders based on their respective ownership percentages. The allocation of net income attributable to MPLX LP for purposes of calculating net income per limited partner unit is described in Note 6 .

Accounting Standards

Accounting Standards	6 Months Ended
	Jun. 30, 2016
Accounting Standards [Abstract]
Accounting Standards	Accounting Standards Recently Adopted In September 2015, the FASB issued an accounting standard update that eliminates the requirement to restate prior period financial statements for measurement period adjustments related to business combinations. This accounting standard update requires that the cumulative impact of a measurement period adjustment be recognized in the reporting period in which the adjustment is identified. The change was effective for interim and annual periods beginning after December 15, 2015. The Partnership recognized measurement period adjustments during the first and second quarters of 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 16 for further discussion and detail related to these measurement period adjustments. In April 2015, the FASB issued an accounting standard 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 was effective for fiscal years and interim periods within those fiscal years beginning after December 15, 2015. Retrospective application is required. The Partnership adopted this accounting standard update in the first quarter of 2016 and it 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 standard 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 was effective for fiscal years and interim periods within those fiscal years beginning after December 15, 2015. Retrospective or prospective application is allowed. The Partnership adopted this accounting standard update prospectively in the first quarter of 2016 and it 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 standard update making targeted changes to the current consolidation guidance. The accounting standard update 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 change was effective for fiscal years and interim periods within those fiscal years beginning after December 15, 2015. The Partnership adopted this accounting standard update in the first quarter of 2016 and it did not have a material impact on the consolidated results of operations, financial position or cash flows. Not Yet Adopted In June 2016, the FASB issued an accounting standard update related to the accounting for credit losses on certain financial instruments. The guidance requires that for most financial assets, losses are based on an expected loss approach which includes estimates of losses over the life of exposure that considers historical, current and forecasted information. Expanded disclosures related to the methods used to estimate the losses as well as a specific disaggregation of balances for financial assets are also required. The change is effective for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years, with early adoption permitted for fiscal years beginning after December 15, 2018, and interim periods within those fiscal years. The Partnership does not expect application of this accounting standard update to have a material impact on the consolidated financial statements. In March 2016, the FASB issued an accounting standard update on the accounting for employee share-based payments. This accounting standard update requires the recognition of income tax effects of awards through the income statement when awards vest or are settled. It will also increase the amount an employer can withhold for tax purposes without triggering liability accounting. Lastly, it allows employers to make a policy election to account for forfeitures as they occur. The changes 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 standard update on lease accounting. This accounting standard update requires lessees to record virtually all leases on their balance sheets. The accounting standard update also requires expanded disclosures to help financial statement users better understand the amount, timing and uncertainty of cash flows arising from leases. The change will be effective on a retrospective or modified retrospective basis for fiscal years 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 accounting standard update on the consolidated financial statements and expects such impact to be material. In January 2016, the FASB issued an accounting standard update requiring unconsolidated equity investments, not accounted for under the equity method, to be measured at fair value with changes in fair value recognized in net income. The accounting standard update also requires the use of the exit price notion when measuring the fair value of financial instruments for disclosure purposes and the separate presentation of financial assets and liabilities by measurement category and form on the balance sheet and accompanying notes. The accounting standard update eliminates the requirement to disclose the methods and assumptions used in estimating the fair value of financial instruments measured at amortized cost. Lastly, the accounting standard update requires separate presentation in other comprehensive income of the portion of the total change in the fair value of a liability resulting from a change in the instrument-specific credit risk when electing to measure the liability at fair value in accordance with the fair value option for financial instruments. The changes are effective for fiscal years and interim periods within those fiscal years beginning after December 15, 2017. 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 guidance regarding presentation of the liability’s credit risk. The Partnership is in the process of determining the impact of the accounting standard update on the consolidated financial statements. In August 2014, the FASB issued an accounting standard 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 issuance of the financial statements. 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. The change will be effective for the first fiscal period ending after December 15, 2016, and for fiscal periods and interim periods thereafter with early application permitted. The adoption of this accounting standard update is not expected to have a material impact on the Partnership’s financial reporting. In May 2014, the FASB issued an initial accounting standard update for revenue recognition for contracts with customers. The guidance in the accounting standard update states that revenue is recognized when a customer obtains control of a good or service. Recognition of the revenue will involve a multiple step approach including identifying the contract, identifying the separate performance obligations, determining the transaction price, allocating the price to the performance obligations and then recognizing the revenue as the obligations are satisfied. Additional disclosures will be required to provide adequate information to understand the nature, amount, timing and uncertainty of reported revenues and revenues expected to be recognized. The change will be effective on a retrospective or modified retrospective basis for fiscal years beginning after December 15, 2017, and interim periods within those years, with early adoption permitted no earlier than January 1, 2017. The Partnership is in the process of determining the impact of the accounting standard update on the consolidated financial statements.

Acquisitions

Acquisitions	6 Months Ended
	Jun. 30, 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 LLC (“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 did not receive general partner distributions or incentive distribution rights that would have otherwise accrued 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 transport light products, heavy oils, crude oil, renewable fuels, chemicals and feedstocks in the Midwest and U.S. Gulf Coast regions, accounted for nearly 60 percent of the total volumes MPC shipped by inland marine vessels as of March 31, 2016. The Partnership accounts for HSM as a reporting unit of the L&S segment. The Partnership retrospectively adjusted the historical financial results for all periods to include HSM as required for transactions between entities under common control. For the previously reported Consolidated Balance Sheets retrospectively adjusted for the acquisition of HSM, see the Annual Report on Form 10-K for the year ended December 31, 2015 , as updated by our Current Report on Form 8-K/A filed on May 20, 2016. The following table presents the Partnership’s previously reported Consolidated Statement of Income, retrospectively adjusted for the acquisition of HSM: Three Months Ended June 30, 2015 (In millions) MPLX LP (Previously Reported) HSM MPLX LP (Currently Reported) Revenues and other income: Service revenue $ 16 $ — $ 16 Service revenue - related parties 119 33 152 Rental income - related parties 4 21 25 Other income 2 — 2 Other income - related parties 6 12 18 Total revenues and other income 147 66 213 Costs and expenses: Cost of revenues (excludes items below) 31 15 46 Purchases - related parties 24 16 40 Depreciation and amortization 13 7 20 General and administrative expenses 18 3 21 Other taxes 3 1 4 Total costs and expenses 89 42 131 Income from operations 58 24 82 Interest expense (net of amounts capitalized of $1 million) 6 — 6 Other financial costs — — — Income before income taxes 52 24 76 Net income 52 24 76 Less: Net income attributable to noncontrolling interests 1 — 1 Less: Net income attributable to Predecessor — 24 24 Net income attributable to MPLX LP 51 — 51 Less: General partner’s interest in net income attributable to MPLX LP 7 — 7 Limited partners’ interest in net income attributable to MPLX LP $ 44 $ — $ 44 Six Months Ended June 30, 2015 (In millions) MPLX LP (Previously Reported) HSM MPLX LP (Currently Reported) Revenues and other income: Service revenue $ 32 $ — $ 32 Service revenue - related parties 230 64 294 Rental income - related parties 8 42 50 Other income 3 — 3 Other income - related parties 12 23 35 Total revenues and other income 285 129 414 Costs and expenses: Cost of revenues (excludes items below) 59 29 88 Purchases - related parties 48 32 80 Depreciation and amortization 25 14 39 General and administrative expenses 37 6 43 Other taxes 6 2 8 Total costs and expenses 175 83 258 Income from operations 110 46 156 Interest expense (net of amounts capitalized of $1 million) 11 — 11 Other financial costs 1 — 1 Income before income taxes 98 46 144 Net income 98 46 144 Less: Net income attributable to noncontrolling interests 1 — 1 Less: Net income attributable to Predecessor — 46 46 Net income attributable to MPLX LP 97 — 97 Less: General partner’s interest in net income attributable to MPLX LP 11 — 11 Limited partners’ interest in net income attributable to MPLX LP $ 86 $ — $ 86 The following table presents the Partnership’s previously reported Consolidated Statement of Cash Flows, retrospectively adjusted for the acquisition of HSM: Six Months Ended June 30, 2015 (In millions) MPLX LP (Previously Reported) HSM MPLX LP (Currently Reported) Increase (decrease) in cash and cash equivalents Operating activities: Net income $ 98 $ 46 $ 144 Adjustments to reconcile net income to net cash provided by operating activities: Amortization of deferred financing costs 1 — 1 Depreciation and amortization 25 14 39 Deferred income taxes — (1 ) (1 ) Changes in: Current receivables (2 ) — (2 ) Current accounts payable and accrued liabilities 14 (2 ) 12 Receivables from / liabilities to related parties (8 ) (11 ) (19 ) All other, net — (1 ) (1 ) Net cash provided by operating activities 128 45 173 Investing activities: Additions to property, plant and equipment (64 ) (6 ) (70 ) Investments - loans to related parties — (38 ) (38 ) All other, net — (1 ) (1 ) Net cash used in investing activities (64 ) (45 ) (109 ) Financing activities: Long-term debt - borrowings 528 — 528 - repayments (415 ) — (415 ) Debt issuance costs (4 ) — (4 ) Net proceeds from equity offerings 1 — 1 Distributions to unitholders and general partner (70 ) — (70 ) Distributions to MPC from Predecessor (1 ) — (1 ) Net cash provided by financing activities 39 — 39 Net increase in cash and cash equivalents 103 — 103 Cash and cash equivalents at beginning of period 27 — 27 Cash and cash equivalents at end of period $ 130 $ — $ 130 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, the first of which occurred on July 1, 2016 and the second of which will occur on 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 is payable in equal amounts, the first of which was paid in July 2016 and the second of which will be paid in July 2017, in connection with the conversion of the Class B units to common units of MPLX LP. The Partnership’s financial results 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 Paid/payable to MarkWest Class B unitholders 50 Total fair value of consideration transferred $ 8,606 The following table summarizes the final 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 fair value of assets acquired and liabilities and noncontrolling interests assumed at the acquisition date as of June 30, 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 and second quarters of 2016 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. As further discussed in Note 16 , in the first quarter of 2016 the Partnership recorded a goodwill impairment charge based on the implied fair value of goodwill as of the interim impairment analysis date. During the second quarter of 2016, the Partnership finalized its analysis of the final purchase price allocation. The completion of the purchase price allocation resulted in a refinement of the impairment expense recorded, as more fully discussed in Note 16 . 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 six months ended June 30, 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 had a material effect on 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 13 . 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 June 30, 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. (In millions, except per unit data) Three Months Ended June 30, 2015 Six Months Ended June 30, 2015 Revenues and other income $ 668 $ 1,332 Net (loss) income attributable to MPLX LP (11 ) 53 Net income attributable to MPLX LP per unit - basic (0.19 ) (0.10 ) Net income attributable to MPLX LP per unit - diluted (0.19 ) (0.10 ) The unaudited pro forma financial information includes adjustments primarily to align accounting policies, adjust depreciation expense to reflect the fair value of property, plant and equipment, increase amortization expense related to identifiable intangible assets and adjust interest expense related to the fair value of MarkWest’s long-term debt, as well as the related income tax effects. 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. 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 related to MarkWest Utica EMG are as follows: (In millions) Three Months Ended June 30, 2015 Six Months Ended June 30, 2015 Revenues and other income $ 34 $ 67 Cost of revenue excluding depreciation and amortization 7 14 Depreciation and amortization 16 32 Net income attributable to noncontrolling interest 15 29 Net loss (5 ) (9 ) EMG Utica, LLC (“EMG Utica”), a joint venture partner in MarkWest Utica EMG, received a special non-cash allocation of income of approximately $11 million and $21 million for the three and six months ended June 30, 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 June 30, 2015 .

Equity Method Investments (Note

Equity Method Investments (Notes)	6 Months Ended
	Jun. 30, 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 limited 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 June 30, 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 and approximately $8 million for the three and six months ended June 30, 2016 , respectively. 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 June 30, 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 June 30, 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 June 30, 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 three and six months ended June 30, 2016 was $5 million and $7 million , respectively, and is reported as Other income-related parties in the Consolidated Statements of Income. Ohio Gathering Ohio Gathering is a subsidiary of MarkWest Utica EMG and is engaged in providing natural gas gathering services in the Utica Shale in eastern Ohio. Ohio Gathering is a joint venture between MarkWest Utica EMG and Summit Midstream Partners, LLC (“Summit”). As Ohio Gathering is a subsidiary of MarkWest Utica EMG, which is accounted for as an equity method investment, the Partnership reports its portion of Ohio Gathering’s net assets as a component of its investment in MarkWest Utica EMG. The Partnership receives Operational Service revenue for operating Ohio Gathering. The amount of Operational Service revenue related to Ohio Gathering for the three and six months ended June 30, 2016 was approximately $3 million and $7 million , respectively, and is reported as Other income-related parties in the Consolidated Statements of Income. Ohio Condensate In December 2013, MarkWest formed MarkWest Utica EMG Condensate L.L.C. (“Utica Condensate”) for the purpose of engaging in wellhead condensate gathering, stabilization, terminalling, storage and marketing in Ohio. As of June 30, 2016 , the Partnership owned 100 percent of Utica Condensate. Utica Condensate’s business is conducted solely through its subsidiary, Ohio Condensate, which is a joint venture between Utica Condensate and Summit. As of June 30, 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 three and six months ended June 30, 2016 was $1 million and $2 million , respectively, and is reported as Other income-related parties in the Consolidated Statements of Income. Summarized financial information for the six months ended June 30, 2016 for equity method investments is as follows: Six Months Ended June 30, 2016 (In millions) MarkWest Utica EMG (1) Ohio Condensate Other VIEs Non-VIEs Total Revenue $ 113 $ 10 $ — $ 68 $ 191 Gross margin 113 10 — 32 155 Income (loss) from operations 68 (94 ) — 18 (8 ) Net income (loss) 68 (94 ) — 18 (8 ) Income (loss) from equity method investments (2) 7 (88 ) — 3 (78 ) Summarized balance sheet information as of June 30, 2016 and December 31, 2015 for equity method investments is as follows: June 30, 2016 (In millions) MarkWest Utica EMG (1) Ohio Condensate Other VIEs Non-VIEs Total Current assets $ 138 $ 7 $ — $ 38 $ 183 Noncurrent assets 2,193 31 55 385 2,664 Current liabilities 108 6 — 26 140 Noncurrent liabilities 2 14 — — 16 December 31, 2015 (In millions) MarkWest Utica EMG (1) Ohio Condensate Other VIEs Non-VIEs Total Current assets $ 113 $ 7 $ — $ 30 $ 150 Noncurrent assets 2,207 127 42 243 2,619 Current liabilities 77 6 1 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 $788 million and $781 million as of June 30, 2016 and December 31, 2015 , respectively. (2) Income (loss) from equity method investments includes the impact of any basis differential amortization or accretion. As of June 30, 2016 , the carrying value of the Partnership’s equity method investments was $1.1 billion higher than the underlying net assets of the investees. This basis difference is being amortized or accreted into net income over the remaining estimated useful lives of the underlying net assets, except for $459 million of excess related to goodwill. During the second quarter of 2016, the Partnership completed its purchase price allocation related to the MarkWest Merger. As a result, a portion of the basis differential related to goodwill for Utica EMG was reclassified to fixed assets and will be amortized prospectively. During the second quarter of 2016, forecasts for Ohio Condensate were reduced to align with updated forecasts for customer requirements. As the operator of that entity responsible for maintaining its financial records, the Partnership completed a fixed asset impairment analysis as of June 30, 2016, in accordance with ASC Topic 360, to determine the potential fixed asset impairment charge. The resulting fixed asset impairment charge recorded within Ohio Condensate’s financial statements was $96 million . Based on the Partnership’s 60 percent ownership of Ohio Condensate, approximately $58 million was recorded in the second quarter of 2016 in Loss from equity method investments on the accompanying Consolidated Statements of Income. The Partnership’s investment in Ohio Condensate, which was established at fair value in connection with the MarkWest Merger, exceeded its proportionate share of the underlying net assets. Therefore, in conjunction with the ASC Topic 360 impairment analysis, the Partnership completed an equity method impairment analysis in accordance with ASC Topic 323 to determine the potential additional equity method impairment charge to be recorded on the Partnership’s consolidated financial statements resulting from an other-than-temporary impairment. As a result, an additional impairment charge of approximately $31 million was recorded in the second quarter of 2016 in Loss from equity method investments on the accompanying Consolidated Statements of Income, which eliminated the basis differential established in connection with the MarkWest Merger. The fair value of Ohio Condensate and its underlying fixed assets was determined based upon 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 estimate of the fair value under the discounted cash flow method include management’s best estimates of the expected future results using a probability weighted average set of cash flow forecasts and a discount rate of 11.2 percent . An increase to the discount rate of 50 basis points would have resulted in an additional charge of $1 million on the Consolidated Statements of Income. Fair value determinations require considerable judgment and are sensitive to changes in underlying assumptions and factors. As such, the fair value of the Ohio Condensate equity method investment and its underlying fixed assets represents a Level 3 measurement. As a result, there can be no assurance that the estimates and assumptions made for purposes of the interim impairment test will prove to be an accurate prediction of the future.

Related Party Agreements and Tr

Related Party Agreements and Transactions	6 Months Ended
	Jun. 30, 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 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 six months ended June 30, 2016 , the Partnership borrowed $1.9 billion and repaid $1.9 billion , resulting in no outstanding balance at June 30, 2016 . 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 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, HSM employees were transferred to Marathon Petroleum Logistics Services LLC ("MPLS"), a wholly-owned subsidiary of MPC, and HSM and MPLS entered into an employee services agreement. 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 June 30, Six Months Ended June 30, (In millions) 2016 2015 2016 2015 Service revenues MPC $ 145 $ 152 $ 295 $ 294 Rental income MPC $ 29 $ 25 $ 55 $ 50 Product sales (1) MPC $ 3 $ — $ 6 $ — (1) For the three and six months ended June 30, 2016 , there were $7 million and $12 million , respectively, of additional product sales to MPC that net to zero within our consolidated financial statements, as the transactions are recorded net due to the terms of the agreements under which such product was sold. Related party sales to MPC consist of crude oil and refined products pipeline transportation services based on regulated tariff rates, storage services based on contracted rates and transportation services provided by HSM. Under the Partnership’s pipeline transportation services agreements, if MPC fails to transport its minimum throughput volumes during any quarter, then MPC will pay the Partnership a deficiency payment equal to the volume of the deficiency multiplied by the tariff rate then in effect. The deficiency amounts are recorded as Deferred revenue-related parties . MPC may then apply the amount of any such deficiency payments as a credit for volumes transported on the applicable pipeline system in excess of its minimum volume commitment during the following four or eight quarters under the terms of the applicable transportation services agreement. The Partnership recognizes revenues for the deficiency payments when credits are used for volumes transported in excess of minimum quarterly volume commitments, when it becomes impossible to physically transport volumes necessary to utilize the credits or upon the expiration of the credits. The use or expiration of the credits is a decrease in Deferred revenue-related parties . The revenue received from related parties, included in Other income-related parties on the Consolidated Statements of Income, was as follows: Three Months Ended June 30, Six Months Ended June 30, (In millions) 2016 2015 2016 2015 MPC $ 16 $ 18 $ 33 $ 34 MarkWest Utica EMG 5 — 7 — Ohio Gathering 3 — 7 — Ohio Condensate 1 — 2 — Other 3 — 3 1 Total $ 28 $ 18 $ 52 $ 35 MPC provides executive management services and certain general and administrative services to the Partnership under the terms of an omnibus agreement. Expenses incurred under this agreement are shown in the table below by the income statement line where they were recorded. Charges for services included in Purchases-related parties primarily relate to services that support the Partnership’s operations and maintenance activities, as well as compensation expenses. Charges for services included in General and administrative expenses primarily relate to services that support the Partnership’s executive management, accounting and human resources activities. These charges were as follows: Three Months Ended June 30, Six Months Ended June 30, (In millions) 2016 2015 2016 2015 Purchases - related parties $ 5 $ 7 $ 11 $ 14 General and administrative expenses 7 11 15 22 Total $ 12 $ 18 $ 26 $ 36 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 June 30, Six Months Ended June 30, (In millions) 2016 2015 2016 2015 MPC $ 9 $ 4 $ 18 $ 6 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 June 30, Six Months Ended June 30, (In millions) 2016 2015 2016 2015 Purchases - related parties $ 73 $ 33 $ 143 $ 66 General and administrative expenses 19 8 40 15 Total $ 92 $ 41 $ 183 $ 81 Receivables from related parties were as follows: (In millions) June 30, 2016 December 31, 2015 MPC $ 104 $ 175 MarkWest Utica EMG 5 4 Ohio Gathering 3 5 Other 1 3 Total $ 113 $ 187 Long-term receivables with related parties, which include reimbursements from the MarkWest Merger to be provided by MPC for the conversion of Class B units and straight-line rental income, were as follows: (In millions) June 30, 2016 December 31, 2015 MPC $ 26 $ 25 Payables to related parties were as follows: (In millions) June 30, 2016 December 31, 2015 MPC $ 51 $ 33 MarkWest Utica EMG 14 21 Total $ 65 $ 54 During the six months ended June 30, 2016 and the year ended December 31, 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) June 30, 2016 December 31, 2015 Minimum volume deficiencies - MPC $ 43 $ 36 Project reimbursements - MPC 5 5 Total $ 48 $ 41

Net Income Per Limited Partner

Net Income Per Limited Partner Unit	6 Months Ended
	Jun. 30, 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, preferred 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 and six months ended June 30, 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 and six months ended June 30, 2016 is the same as basic net income (loss) per limited partner unit since the inclusion of any potential common units would have been anti-dilutive. Potential common units omitted from the diluted earnings per unit calculation was approximately 10 million . Three Months Ended June 30, Six Months Ended June 30, (In millions) 2016 2015 2016 2015 Net (loss) income attributable to MPLX LP $ 19 $ 51 $ (41 ) $ 97 Less: Limited partners’ distributions declared on preferred units (1) 9 — 9 — General partner’s distributions declared (including IDRs) (1) 50 6 94 10 Limited partners’ distributions declared on common units (1) 172 19 328 37 Limited partner’s distributions declared (1) — 17 — 32 Undistributed net (loss) income attributable to MPLX LP $ (212 ) $ 9 $ (472 ) $ 18 (1) See Note 7 for distribution information. Three Months Ended June 30, 2016 (In millions, except per unit data) General Partner Limited Partners’ Common Units Preferred 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) $ 50 $ 172 $ 9 $ 231 Undistributed net loss attributable to MPLX LP (5 ) (207 ) — (212 ) Net income (loss) attributable to MPLX LP (1) $ 45 $ (35 ) $ 9 $ 19 Weighted average units outstanding: Basic 7 331 17 355 Diluted 7 331 17 355 Net loss attributable to MPLX LP per limited partner unit: Basic $ (0.11 ) Diluted $ (0.11 ) Three Months Ended June 30, 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) $ 6 $ 19 $ 17 $ 42 Undistributed net income attributable to MPLX LP 5 2 2 9 Net income attributable to MPLX LP (1) $ 11 $ 21 $ 19 $ 51 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 $ 0.50 $ 0.50 Diluted $ 0.50 $ 0.50 Six Months Ended June 30, 2016 (In millions, except per unit data) General Partner Limited Partners’ Common Units Preferred 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) $ 94 $ 328 $ 9 $ 431 Undistributed net loss attributable to MPLX LP (9 ) (463 ) — (472 ) Net income (loss) attributable to MPLX LP (1) $ 85 $ (135 ) $ 9 $ (41 ) Weighted average units outstanding: Basic 7 316 8 331 Diluted 7 316 8 331 Net loss attributable to MPLX LP per limited partner unit: Basic $ (0.43 ) Diluted $ (0.43 ) Six Months Ended June 30, 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) $ 10 $ 37 $ 32 $ 79 Undistributed net income attributable to MPLX LP 9 5 4 18 Net income attributable to MPLX LP (1) $ 19 $ 42 $ 36 $ 97 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 $ 0.96 $ 0.96 Diluted $ 0.96 $ 0.96 (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	6 Months Ended
	Jun. 30, 2016
Equity [Abstract]
Equity	Equity Units Outstanding – The Partnership had 331,283,429 common units outstanding as of June 30, 2016 . Of that number, 79,466,136 were owned by MPC, which also owned the two percent general partner interest, represented by 7,506,520 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 six months ended June 30, 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 June 30, 2016 are summarized below: (In units) Common Class B (1) General Partner Total Balance at December 31, 2015 296,687,176 7,981,756 6,800,475 311,469,407 Unit-based compensation awards (2) 37,251 — 761 38,012 Issuance of units under the ATM Program (3) 12,025,000 — 245,406 12,270,406 Contribution of HSM (4) 22,534,002 — 459,878 22,993,880 Balance at June 30, 2016 331,283,429 7,981,756 7,506,520 346,771,705 (1) On July 1, 2016, 3,990,878 Class B units converted to 4,350,057 common units and will be eligible to receive the second quarter distribution. (2) As a result of the unit-based compensation awards issued during the period, MPLX GP contributed less than $1 million in exchange for 761 general partner units to maintain its two percent general partner interest. (3) 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. (4) 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. Net (Loss) Income Allocation – In preparing the Consolidated Statements of Equity, net (loss) income attributable to MPLX LP is allocated to preferred unitholders based on a fixed distribution schedule, as discussed in Note 8 , and subsequently allocated to remaining 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 June 30, Six Months Ended June 30, (In millions) 2016 2015 2016 2015 Net (loss) income attributable to MPLX LP $ 19 $ 51 $ (41 ) $ 97 Less: Preferred unit distributions 9 — 9 — General partner's incentive distribution rights and other 47 6 88 9 Net (loss) income attributable to MPLX LP available to general and limited partners $ (37 ) $ 45 $ (138 ) $ 88 General partner's two percent interest in net (loss) income attributable to MPLX LP $ (1 ) $ 1 $ (3 ) $ 2 General partner's incentive distribution rights and other 47 6 88 9 General partner's interest in net income attributable to MPLX LP $ 46 $ 7 $ 85 $ 11 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 July 22, 2016 , the Partnership declared a quarterly cash distribution of $0.5100 per unit, resulting in total distributions of $222 million . These distributions will be paid on August 12, 2016 to unitholders of record on August 2, 2016 . The allocation of total quarterly cash distributions to preferred, general and limited partners is as follows for the three and six months ended June 30, 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 June 30, Six Months Ended June 30, (In millions) 2016 2015 2016 2015 General partner's distributions: General partner's distributions $ 4 $ 1 $ 8 $ 2 General partner's incentive distribution rights distributions 46 6 86 9 Total general partner's distributions $ 50 $ 7 $ 94 $ 11 Limited partners' distributions: Common unitholders $ 172 $ 19 $ 328 $ 37 Subordinated unitholders — 16 — 31 Total limited partners' distributions 172 35 328 68 Preferred unit distributions 9 — 9 — Total cash distributions declared $ 231 $ 42 $ 431 $ 79

Redeemable Preferred Units (Not

Redeemable Preferred Units (Notes)	6 Months Ended
	Jun. 30, 2016
Temporary Equity Disclosure [Abstract]
Temporary Equity Disclosure [Text Block]	8 . Redeemable Preferred Units Private Placement of Preferred Units – On May 13, 2016, MPLX completed the private placement of approximately 30.8 million 6.5 percent Series A Convertible Preferred Units (the "Preferred Units") for a cash purchase price of $32.50 per unit. The aggregate net proceeds of approximately $984 million from the sale of the Preferred Units was used for capital expenditures, repayment of debt and general partnership purposes. The Preferred Units rank senior to all common units with respect to distributions and rights upon liquidation. The holders of the Preferred Units are entitled to receive cumulative quarterly distributions equal to $0.528125 per unit commencing for the quarter ended June 30, 2016, with a prorated amount from the date of issuance. Following the second anniversary of the issuance of the Preferred Units, the holders of the Preferred Units will receive as a distribution the greater of $0.528125 per unit or the amount of per unit distributions paid to common units. Since the Preferred Unit distribution was declared subsequent to the end of the second quarter of 2016, the distribution was not accrued to the Preferred Unit holders’ capital account. For the quarter ended June 30, 2016, the Preferred Units will receive an earned aggregate cash distribution of $9 million , based on the quarterly per unit distribution prorated for the 49-day period the Preferred Units were outstanding during the second quarter of 2016. The changes in the redeemable preferred balance for 2016 were as follows: (In millions) Redeemable Preferred Units Issuance of MPLX LP redeemable preferred units on May 13, 2016 $ 984 Net income allocated for May 13, 2016 through June 30, 2016 9 Balance at June 30, 2016 $ 993 The purchasers may convert their Preferred Units into common units, at any time after the third anniversary of the issuance date or prior to liquidation, dissolution or winding up of the Partnership, in full or in part, subject to minimum conversion amounts and conditions. After the fourth anniversary of the issuance date, the Partnership may convert the Preferred Units into common units at any time, in whole or in part, subject to certain minimum conversion amounts and conditions, if the closing price of MPLX common units is greater than $48.75 for the 20 day trading period immediately preceding the conversion notice date. The conversion rate for the Preferred Units shall be the quotient of (a) the sum of (i) $32.50, plus (ii) any unpaid cash distributions on the applicable Preferred Unit, divided by (b) $32.50. The holders of the Preferred Units are entitled to vote on an as-converted basis with the common unitholders and will have certain other class voting rights with respect to any amendment to the partnership agreement that would adversely affect any rights, preferences or privileges of the Preferred Units. In addition, upon certain events involving a change in control the holders of Preferred Units may elect, among other potential elections, to convert their Preferred Units to common units at the then change of control conversion rate. The Preferred Units are considered redeemable securities under GAAP due to the existence of redemption provisions upon a deemed liquidation event which is outside the Partnership’s control. Therefore they are presented as temporary equity in the mezzanine section of the Consolidated Balance Sheets. The Preferred Units have been recorded at their issuance date fair value, net of issuance costs. Income allocations increase the carrying value, and declared distributions decreased the carrying value of the Preferred Units. Because the Preferred Units are not currently redeemable and not probable of becoming redeemable, adjustment to the initial carrying amount is not necessary and would only be required if it becomes probable that the Preferred Units would become redeemable.

Segment Information (Notes)

Segment Information (Notes)	6 Months Ended
	Jun. 30, 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), property, plant and equipment impairment, 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 June 30, 2016 (In millions) L&S G&P Total Revenues and other income: Segment revenues $ 193 $ 530 $ 723 Segment other income 18 — 18 Total segment revenues and other income 211 530 741 Costs and expenses: Segment cost of revenues 88 223 311 Segment operating income before portion attributable to noncontrolling interest 123 307 430 Segment portion attributable to noncontrolling interest and Predecessor — 36 36 Segment operating income attributable to MPLX LP $ 123 $ 271 $ 394 Three Months Ended June 30, 2015 (In millions) L&S Revenues and other income: Segment revenues $ 193 Segment other income 20 Total segment revenues and other income 213 Costs and expenses: Segment cost of revenues 90 Segment operating income before portion attributable to noncontrolling interest and Predecessor 123 Segment portion attributable to noncontrolling interest and Predecessor 35 Segment operating income attributable to MPLX LP $ 88 Six Months Ended June 30, 2016 (In millions) L&S G&P Total Revenues and other income: Segment revenues $ 385 $ 1,028 $ 1,413 Segment other income 37 — 37 Total segment revenues and other income 422 1,028 1,450 Costs and expenses: Segment cost of revenues 177 423 600 Segment operating income before portion attributable to noncontrolling interest 245 605 850 Segment portion attributable to noncontrolling interest and Predecessor 34 77 111 Segment operating income attributable to MPLX LP $ 211 $ 528 $ 739 Six Months Ended June 30, 2015 (In millions) L&S Revenues and other income: Segment revenues $ 376 Segment other income 38 Total segment revenues and other income 414 Costs and expenses: Segment cost of revenues 176 Segment operating income before portion attributable to noncontrolling interest and Predecessor 238 Segment portion attributable to noncontrolling interest and Predecessor 68 Segment operating income attributable to MPLX LP $ 170 Three Months Ended June 30, Six Months Ended June 30, (In millions) 2016 2015 2016 2015 Reconciliation to Income from operations: Segment operating income attributable to MPLX LP $ 394 $ 88 $ 739 $ 170 Segment portion attributable to unconsolidated affiliates (83 ) — (166 ) — Segment portion attributable to noncontrolling interest and Predecessor 36 35 111 68 Loss from equity method investments (83 ) — (78 ) — Other income - related parties 11 — 18 — Unrealized derivative losses (12 ) — (21 ) — Impairment expense (1 ) — (130 ) — Depreciation and amortization (137 ) (20 ) (269 ) (39 ) General and administrative expenses (49 ) (21 ) (101 ) (43 ) Income from operations $ 76 $ 82 $ 103 $ 156 Three Months Ended June 30, Six Months Ended June 30, (In millions) 2016 2015 2016 2015 Reconciliation to Total revenues and other income: Total segment revenues and other income $ 741 $ 213 $ 1,450 $ 414 Revenue adjustment from unconsolidated affiliates (99 ) — (203 ) — Loss from equity method investments (83 ) — (78 ) — Other income - related parties 11 — 18 — Unrealized derivative loss (6 ) — (14 ) — Total revenues and other income $ 564 $ 213 $ 1,173 $ 414 Three Months Ended June 30, Six Months Ended June 30, (In millions) 2016 2015 2016 2015 Reconciliation to Net income attributable to noncontrolling interests and Predecessor Segment portion attributable to noncontrolling interest and Predecessor $ 36 $ 35 $ 111 $ 68 Portion of noncontrolling interests and Predecessor related to items below segment income from operations (56 ) (10 ) (85 ) (21 ) Portion of operating income attributable to noncontrolling interest of unconsolidated affiliates 21 — (2 ) — Net income attributable to noncontrolling interests and Predecessor $ 1 $ 25 $ 24 $ 47 The following reconciles segment capital expenditures to total capital expenditures: Three Months Ended June 30, Six Months Ended June 30, (In millions) 2016 2015 2016 2015 L&S segment capital expenditures $ 82 $ 35 $ 144 $ 70 G&P segment capital expenditures (1) 212 — 485 — Total segment capital expenditures 294 35 629 70 Less: Capital expenditures for Partnership operated, non-wholly-owned subsidiaries 16 — 60 — Total capital expenditures $ 278 $ 35 $ 569 $ 70 (1) The G&P segment includes $16 million and $60 million of capital expenditures related to Partnership operated, non-wholly-owned subsidiaries for the three and six months ended June 30, 2016 . Total assets by reportable segment were: (In millions) June 30, 2016 December 31, 2015 L&S $ 1,952 $ 1,858 G&P 14,127 14,246 Total assets $ 16,079 $ 16,104

Income Taxes (Notes)

Income Taxes (Notes)	6 Months Ended
	Jun. 30, 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 (benefit) 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 and six months ended June 30, 2015 . A reconciliation of the benefit for income tax and the amount computed by applying the federal statutory rate of 35 percent to the income before income taxes for the six months ended June 30, 2016 is as follows: (In millions) MarkWest Hydrocarbon Partnership Eliminations Consolidated (1) Income before (benefit) provision for income tax $ (35 ) $ 4 $ 2 $ (29 ) Federal statutory rate 35 % — % — % Federal income tax at statutory rate (2) (12 ) — — (12 ) Change in state statutory rate (1 ) — — (1 ) State income taxes net of federal benefit - MarkWest Hydrocarbon (1 ) — — (1 ) Provision on income from Class A units (2) — — — — Other 1 1 — 2 (Benefit) provision for income tax $ (13 ) $ 1 $ — $ (12 ) (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)	6 Months Ended
	Jun. 30, 2016
Inventory Disclosure [Abstract]
Inventory Disclosure [Text Block]	Inventories Inventories consist of the following: (In millions) June 30, 2016 December 31, 2015 NGLs $ 2 $ 3 Line fill 6 5 Spare parts, materials and supplies 41 43 Total inventories $ 49 $ 51

Property, Plant and Equipment

Property, Plant and Equipment	6 Months Ended
	Jun. 30, 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) June 30, 2016 December 31, 2015 Natural gas gathering and NGL transportation pipelines and facilities $ 4,573 $ 4,307 Processing, fractionation and storage facilities 3,456 3,185 Pipelines and related assets 1,186 1,128 Barges and towing vessels 478 475 Land, building, office equipment and other 662 606 Construction in progress 898 946 Total 11,253 10,647 Less accumulated depreciation 893 650 Property, plant and equipment, net $ 10,360 $ 9,997

Fair Value Measurements

Fair Value Measurements	6 Months Ended
	Jun. 30, 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 14 . 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) June 30, 2016 December 31, 2015 Assets Liabilities Assets Liabilities Significant other observable inputs (Level 2) Commodity contracts $ — $ — $ 2 $ — Significant unobservable inputs (Level 3) Commodity contracts — (4 ) 7 — Embedded derivatives in commodity contracts 1 (41 ) — (32 ) Total carrying value in Consolidated Balance Sheets $ 1 $ (45 ) $ 9 $ (32 ) The following table provides additional information about the significant unobservable inputs used in the valuation of Level 3 instruments as of June 30, 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 Liabilities Forward ethane prices (per gallon) (1) $0.24 - $0.28 July 2016 - Dec. 2016 Forward propane prices (per gallon) (1) $0.52 - $0.57 July 2016 - Dec. 2016 Forward isobutane prices (per gallon) (1) $0.69 - $0.72 July 2016 - Dec. 2016 Forward normal butane prices (per gallon) (1) $0.63 - $0.69 July 2016 - Dec. 2016 Forward natural gasoline prices (per gallon) (1) $0.99 - $1.03 July 2016 - Dec. 2016 Embedded derivatives in commodity contracts Assets ERCOT Pricing (per MegaWatt Hour) $26.47 - $57.95 July 2016 - Dec. 2016 Liabilities Forward propane prices (per gallon) (1) $0.52 - $0.58 July 2016 - Dec. 2022 Forward isobutane prices (per gallon) (1) $0.67 - $0.73 July 2016 - Dec. 2022 Forward normal butane prices (per gallon) (1) $0.62 - $0.71 July 2016 - Dec. 2022 Forward natural gasoline prices (per gallon) (1) $0.99 - $1.10 July 2016 - Dec. 2022 Forward natural gas prices (per mmbtu) (2) $2.61 - $3.35 July 2016 - Dec. 2022 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 14 . Increases (decreases) in the frac spread result in an increase (decrease) in the fair value of the embedded derivative liability. An increase in the probability of renewal would result in an increase in the fair value of the related embedded derivative liability. • An embedded derivative related to utilities costs discussed further in Note 14 . Increases in the forward ERCOT prices result in an increase in the fair value of the embedded derivative asset. 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 14 . 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 (July 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 and six months ended June 30, 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 4 for detail of the Ohio Condensate equity method impairment charge, which included a Level 3 valuation adjustment during the three and six months ended June 30, 2016 . See Note 16 for a rollforward of goodwill, which included a Level 3 valuation adjustment during the three and six months ended June 30, 2016 . Three Months Ended June 30, 2016 Six Months Ended June 30, 2016 (In millions) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Fair value at beginning of period $ — $ (34 ) $ 7 $ (32 ) Total loss (realized and unrealized) included in earnings (1) (6 ) (7 ) (7 ) (11 ) Settlements 1 1 (5 ) 3 Netting adjustment (2) 1 — 1 — Fair value at end of period $ (4 ) $ (40 ) $ (4 ) $ (40 ) 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 $ (5 ) $ (8 ) $ (6 ) $ (11 ) (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 . (2) Certain derivative positions are subject to master netting agreements; therefore, the Partnership has elected to offset derivative assets and liabilities where legally permissible. The Partnership may hold positions with certain counterparties, which for GAAP purposes are classified within different levels of the fair value hierarchy and may be legally permissible to offset. This adjustment represents the total impact of offsetting Level 2 positions with Level 3 positions as of June 30, 2016. Fair Values – Reported The Partnership’s primary financial instruments are cash and cash equivalents, receivables, receivables from related parties, accounts payable, payables to related parties, and long-term debt. The Partnership’s fair value assessment incorporates a variety of considerations, including (1) the short-term duration of the instruments, (2) MPC’s investment-grade credit rating and (3) the historical incurrence of and expected future insignificance of bad debt expense, which includes an evaluation of counterparty credit risk. The Partnership believes the carrying values of its current assets and liabilities approximate fair value. The recorded value of the amounts outstanding under the bank revolving credit facility, if any, approximates fair value due to the variable interest rate that approximates current market rates. Derivative instruments are recorded at fair value, based on available market information (see Note 14 ). 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 and SMR liability fair values are considered Level 3 measurements. The following table summarizes the fair value and carrying value of the Partnership’s long-term debt, excluding capital leases, and SMR liability. June 30, 2016 December 31, 2015 (In millions) Fair Value Carrying Value Fair Value Carrying Value Long-term debt $ 4,748 $ 4,400 $ 5,212 $ 5,255 SMR liability $ 105 $ 98 $ 99 $ 100

Derivative Financial Instrument

Derivative Financial Instruments (Notes)	6 Months Ended
	Jun. 30, 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 June 30, 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 184,000 Natural Gas (MMBtu) Long 1,088,484 NGLs (gal) Short 64,810,176 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 June 30, 2016 , the estimated fair value of this contract was a liability of $41 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 a 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 June 30, 2016 , the estimated fair value of this contract was an asset 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, as updated by our Current Report on Form 8-K/A filed on May 20, 2016. The impact of the Partnership’s derivative instruments on its Consolidated Balance Sheets is summarized below: (In millions) June 30, 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 $ 1 $ (9 ) $ 9 $ (5 ) Other noncurrent assets / deferred credits and other liabilities — (36 ) — (27 ) Total $ 1 $ (45 ) $ 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: June 30, 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 $ 2 $ (2 ) $ — $ (6 ) $ 2 $ (4 ) Embedded derivatives in commodity contracts 1 — 1 (5 ) — (5 ) Total current derivative instruments 3 (2 ) 1 (11 ) 2 (9 ) Non-current Commodity contracts — — — — — — Embedded derivatives in commodity contracts — — — (36 ) — (36 ) Total non-current derivative instruments — — — (36 ) — (36 ) Total derivative instruments $ 3 $ (2 ) $ 1 $ (47 ) $ 2 $ (45 ) In the table above, the Partnership does not offset a counterparty’s current derivative contracts with the counterparty’s non-current derivative contracts, although the Partnership’s master netting arrangements would allow current and non-current positions to be offset in the event of default. Additionally, in the event of a default, the Partnership’s master netting arrangements would allow for the offsetting of all transactions executed under the master netting arrangement. These types of transactions may include non-derivative instruments, derivatives qualifying for scope exceptions, receivables and payables arising from settled positions and other forms of non-cash collateral (such as letters of credit). The impact of the Partnership’s derivative contracts not designated as hedging instruments and the location of gain or (loss) recognized in the Consolidated Statements of Income is summarized below: (In millions) Three Months Ended June 30, 2016 Six Months Ended June 30, 2016 Product sales Realized (loss) gain $ (1 ) $ 6 Unrealized loss (6 ) (14 ) Total revenue: derivative loss from product sales (7 ) (8 ) Purchased product costs Unrealized loss (8 ) (9 ) Cost of Revenues Unrealized gain 2 2 Total loss $ (13 ) $ (15 )

Debt

Debt	6 Months Ended
	Jun. 30, 2016
Debt Disclosure [Abstract]
Debt	Debt The Partnership’s outstanding borrowings at June 30, 2016 and December 31, 2015 consisted of the following: (In millions) June 30, 2016 December 31, 2015 MPLX LP: Bank revolving credit facility due 2020 $ — $ 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 4,859 5,736 Unamortized debt issuance costs (8 ) (8 ) Unamortized discount (1) (450 ) (472 ) Amounts due within one year (1 ) (1 ) Total long-term debt due after one year $ 4,400 $ 5,255 (1) Includes $442 million and $465 million discount as of June 30, 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 six months ended June 30, 2016 , the Partnership borrowed $434 million under the bank revolving credit facility, at an average interest rate of 1.899 percent , per annum, and repaid $1.3 billion under the bank revolving credit facility. At June 30, 2016 , the Partnership had no outstanding borrowings and $8 million letters of credit outstanding under this facility, resulting in total availability of $1.99 billion , or 99.6 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 six months ended June 30, 2016 were at an average interest rate of 1.931 percent .

Goodwill (Notes)

Goodwill (Notes)	6 Months Ended
	Jun. 30, 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. In the second quarter of 2016, the Partnership completed its purchase price allocation, which resulted in an additional $1 million of impairment expense that would have been recorded in the first quarter of 2016 had the purchase price allocation been completed as of that date. This adjustment to the impairment expense was the result of completing an evaluation of the deferred tax liabilities associated with the MarkWest Merger and their impact on the resulting goodwill that was recognized. 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 Gross goodwill as of December 31, 2015 $ 116 $ 2,454 $ 2,570 Accumulated impairment losses — — — Balance as of December 31, 2015 116 2,454 2,570 Purchase price allocation adjustments (1) — (241 ) (241 ) Impairment losses — (130 ) (130 ) Balance as of June 30, 2016 $ 116 $ 2,083 $ 2,199 Gross goodwill as of June 30, 2016 $ 116 $ 2,213 $ 2,329 Accumulated impairment losses — (130 ) (130 ) Balance as of June 30, 2016 $ 116 $ 2,083 $ 2,199 (1) See Note 3 for further discussion on purchase price allocation adjustments.

Supplemental Cash Flow Informat

Supplemental Cash Flow Information	6 Months Ended
	Jun. 30, 2016
Supplemental Cash Flow Elements [Abstract]
Supplemental Cash Flow Information	Supplemental Cash Flow Information Six Months Ended June 30, (In millions) 2016 2015 Net cash provided by operating activities included: Interest paid (net of amounts capitalized) $ 109 $ 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: Six Months Ended June 30, (In millions) 2016 2015 (Decrease) increase in capital accruals $ (6 ) $ 13 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 did not receive general partner distributions or incentive distribution rights that would have otherwise accrued 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	6 Months Ended
	Jun. 30, 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 six months ended June 30, 2016 : Number Weighted Outstanding at December 31, 2015 1,031,219 $ 35.49 Granted 445,555 29.31 Settled (43,660 ) 51.21 Forfeited (19,490 ) 31.36 Outstanding at June 30, 2016 1,413,624 33.11 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 six months ended June 30, 2016 : Number of Outstanding at December 31, 2015 1,521,392 Granted 789,375 Settled (458,011 ) Forfeited (53,507 ) Outstanding at June 30, 2016 1,799,249

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
	Jun. 30, 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 June 30, 2016 and December 31, 2015 , accrued liabilities for remediation totaled $7 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 June 30, 2016 , there was less than $1 million in receivables from MPC for indemnification of environmental costs related to incidents occurring prior to the Initial Offering. There were $1 million in receivables from MPC for indemnification at December 31, 2015 . In July 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. MarkWest Liberty Midstream has provided information in response to subpoenas presented by the government and similar requests for information from the EPA, state and other agencies related to MarkWest's pipeline and compressor stations located in Pennsylvania. The Partnership is engaged in ongoing discussions with EPA and the U.S. Attorney’s office regarding alleged omissions associated with permits or related regulatory obligations for its launcher/receiver facilities in the region. MarkWest Liberty Midstream’s internal review has determined that its operations have been conducted consistent with industry practices and in a manner protective of its employees and the public. It is possible however, 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. The Partnership is involved in a number of other environmental enforcement matters arising in the ordinary course of business. While the outcome and impact on MPLX cannot be predicted with certainty, management believes the resolution of these environmental matters will not, individually or collectively, have a material adverse effect on its consolidated results of operations, financial position or cash flows. 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 purported 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, 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. On May 17, 2016, the plaintiffs withdrew the fee application and the case is now dismissed. 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 the Partnership cannot be predicted with certainty, the Partnership believes the resolution of these other lawsuits and proceedings will not have a material adverse effect on its consolidated financial position, results of operations or cash flows. Guarantees – Over the years, the Partnership has sold various assets in the normal course of its business. Certain of the related agreements contain performance and general guarantees, including guarantees regarding inaccuracies in representations, warranties, covenants and agreements, and environmental and general indemnifications that require the Partnership to perform upon the occurrence of a triggering event or condition. These guarantees and indemnifications are part of the normal course of selling assets. The Partnership is typically not able to calculate the maximum potential amount of future payments that could be made under such contractual provisions because of the variability inherent in the guarantees and indemnities. Most often, the nature of the guarantees and indemnities is such that there is no appropriate method for quantifying the exposure because the underlying triggering event has little or no past experience upon which a reasonable prediction of the outcome can be based. Contractual Commitments and Contingencies – At June 30, 2016 , the Partnership’s contractual commitments to acquire property, plant and equipment totaled $190 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 June 30, 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 June 30, 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.

Description of the Business a27

Description of the Business and Basis of Presentation Description of Business and Basis of Presentation (Policies)	6 Months Ended
	Jun. 30, 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. Revenues and costs related to the portion of the revenue earned under these contracts considered to be implicit leases are recorded as Rental income and Rental cost of sales , respectively, on the Consolidated Statements of Income. 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 services agreement between HSM and MPC. Revenue related to this agreement is recorded as Rental income - related parties on the Consolidated Statements of Income. The rental cost of sales related to the HSM implicit lease is depreciation of the HSM assets. 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	These 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, preferred units, certain equity-based compensation awards and incentive distribution rights (“IDRs”).

Acquisitions (Tables)

Acquisitions (Tables)	6 Months Ended
	Jun. 30, 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. (In millions, except per unit data) Three Months Ended June 30, 2015 Six Months Ended June 30, 2015 Revenues and other income $ 668 $ 1,332 Net (loss) income attributable to MPLX LP (11 ) 53 Net income attributable to MPLX LP per unit - basic (0.19 ) (0.10 ) Net income attributable to MPLX LP per unit - diluted (0.19 ) (0.10 )
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. For the previously reported Consolidated Balance Sheets retrospectively adjusted for the acquisition of HSM, see the Annual Report on Form 10-K for the year ended December 31, 2015 , as updated by our Current Report on Form 8-K/A filed on May 20, 2016. The following table presents the Partnership’s previously reported Consolidated Statement of Income, retrospectively adjusted for the acquisition of HSM: Three Months Ended June 30, 2015 (In millions) MPLX LP (Previously Reported) HSM MPLX LP (Currently Reported) Revenues and other income: Service revenue $ 16 $ — $ 16 Service revenue - related parties 119 33 152 Rental income - related parties 4 21 25 Other income 2 — 2 Other income - related parties 6 12 18 Total revenues and other income 147 66 213 Costs and expenses: Cost of revenues (excludes items below) 31 15 46 Purchases - related parties 24 16 40 Depreciation and amortization 13 7 20 General and administrative expenses 18 3 21 Other taxes 3 1 4 Total costs and expenses 89 42 131 Income from operations 58 24 82 Interest expense (net of amounts capitalized of $1 million) 6 — 6 Other financial costs — — — Income before income taxes 52 24 76 Net income 52 24 76 Less: Net income attributable to noncontrolling interests 1 — 1 Less: Net income attributable to Predecessor — 24 24 Net income attributable to MPLX LP 51 — 51 Less: General partner’s interest in net income attributable to MPLX LP 7 — 7 Limited partners’ interest in net income attributable to MPLX LP $ 44 $ — $ 44 Six Months Ended June 30, 2015 (In millions) MPLX LP (Previously Reported) HSM MPLX LP (Currently Reported) Revenues and other income: Service revenue $ 32 $ — $ 32 Service revenue - related parties 230 64 294 Rental income - related parties 8 42 50 Other income 3 — 3 Other income - related parties 12 23 35 Total revenues and other income 285 129 414 Costs and expenses: Cost of revenues (excludes items below) 59 29 88 Purchases - related parties 48 32 80 Depreciation and amortization 25 14 39 General and administrative expenses 37 6 43 Other taxes 6 2 8 Total costs and expenses 175 83 258 Income from operations 110 46 156 Interest expense (net of amounts capitalized of $1 million) 11 — 11 Other financial costs 1 — 1 Income before income taxes 98 46 144 Net income 98 46 144 Less: Net income attributable to noncontrolling interests 1 — 1 Less: Net income attributable to Predecessor — 46 46 Net income attributable to MPLX LP 97 — 97 Less: General partner’s interest in net income attributable to MPLX LP 11 — 11 Limited partners’ interest in net income attributable to MPLX LP $ 86 $ — $ 86 The following table presents the Partnership’s previously reported Consolidated Statement of Cash Flows, retrospectively adjusted for the acquisition of HSM: Six Months Ended June 30, 2015 (In millions) MPLX LP (Previously Reported) HSM MPLX LP (Currently Reported) Increase (decrease) in cash and cash equivalents Operating activities: Net income $ 98 $ 46 $ 144 Adjustments to reconcile net income to net cash provided by operating activities: Amortization of deferred financing costs 1 — 1 Depreciation and amortization 25 14 39 Deferred income taxes — (1 ) (1 ) Changes in: Current receivables (2 ) — (2 ) Current accounts payable and accrued liabilities 14 (2 ) 12 Receivables from / liabilities to related parties (8 ) (11 ) (19 ) All other, net — (1 ) (1 ) Net cash provided by operating activities 128 45 173 Investing activities: Additions to property, plant and equipment (64 ) (6 ) (70 ) Investments - loans to related parties — (38 ) (38 ) All other, net — (1 ) (1 ) Net cash used in investing activities (64 ) (45 ) (109 ) Financing activities: Long-term debt - borrowings 528 — 528 - repayments (415 ) — (415 ) Debt issuance costs (4 ) — (4 ) Net proceeds from equity offerings 1 — 1 Distributions to unitholders and general partner (70 ) — (70 ) Distributions to MPC from Predecessor (1 ) — (1 ) Net cash provided by financing activities 39 — 39 Net increase in cash and cash equivalents 103 — 103 Cash and cash equivalents at beginning of period 27 — 27 Cash and cash equivalents at end of period $ 130 $ — $ 130
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 Paid/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 final 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 fair value of assets acquired and liabilities and noncontrolling interests assumed at the acquisition date as of June 30, 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 related to MarkWest Utica EMG are as follows: (In millions) Three Months Ended June 30, 2015 Six Months Ended June 30, 2015 Revenues and other income $ 34 $ 67 Cost of revenue excluding depreciation and amortization 7 14 Depreciation and amortization 16 32 Net income attributable to noncontrolling interest 15 29 Net loss (5 ) (9 )

Equity Method Investments (Tabl

Equity Method Investments (Tables)	6 Months Ended
	Jun. 30, 2016
Equity Method Investments and Joint Ventures [Abstract]
Summarized Financial Information For Equity Method Investees Table [Text Block]	Summarized financial information for the six months ended June 30, 2016 for equity method investments is as follows: Six Months Ended June 30, 2016 (In millions) MarkWest Utica EMG (1) Ohio Condensate Other VIEs Non-VIEs Total Revenue $ 113 $ 10 $ — $ 68 $ 191 Gross margin 113 10 — 32 155 Income (loss) from operations 68 (94 ) — 18 (8 ) Net income (loss) 68 (94 ) — 18 (8 ) Income (loss) from equity method investments (2) 7 (88 ) — 3 (78 ) Summarized balance sheet information as of June 30, 2016 and December 31, 2015 for equity method investments is as follows: June 30, 2016 (In millions) MarkWest Utica EMG (1) Ohio Condensate Other VIEs Non-VIEs Total Current assets $ 138 $ 7 $ — $ 38 $ 183 Noncurrent assets 2,193 31 55 385 2,664 Current liabilities 108 6 — 26 140 Noncurrent liabilities 2 14 — — 16 December 31, 2015 (In millions) MarkWest Utica EMG (1) Ohio Condensate Other VIEs Non-VIEs Total Current assets $ 113 $ 7 $ — $ 30 $ 150 Noncurrent assets 2,207 127 42 243 2,619 Current liabilities 77 6 1 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 $788 million and $781 million as of June 30, 2016 and December 31, 2015 , respectively.

Related Party Agreements and 30

Related Party Agreements and Transactions (Tables)	6 Months Ended
	Jun. 30, 2016
Related Party Transactions [Abstract]
Sales to Related Parties	Sales to related parties were as follows: Three Months Ended June 30, Six Months Ended June 30, (In millions) 2016 2015 2016 2015 Service revenues MPC $ 145 $ 152 $ 295 $ 294 Rental income MPC $ 29 $ 25 $ 55 $ 50 Product sales (1) MPC $ 3 $ — $ 6 $ —
Summary of Fees Received for Operating Pipelines for Related Parties Included in Other Income - Related Parties	The revenue received from related parties, included in Other income-related parties on the Consolidated Statements of Income, was as follows: Three Months Ended June 30, Six Months Ended June 30, (In millions) 2016 2015 2016 2015 MPC $ 16 $ 18 $ 33 $ 34 MarkWest Utica EMG 5 — 7 — Ohio Gathering 3 — 7 — Ohio Condensate 1 — 2 — Other 3 — 3 1 Total $ 28 $ 18 $ 52 $ 35
Summary of Charges for Services Included in Purchases from Related Parties	Three Months Ended June 30, Six Months Ended June 30, (In millions) 2016 2015 2016 2015 Purchases - related parties $ 5 $ 7 $ 11 $ 14 General and administrative expenses 7 11 15 22 Total $ 12 $ 18 $ 26 $ 36
Summary of Related Party Costs Added to Property, Plant and Equipment	Three Months Ended June 30, Six Months Ended June 30, (In millions) 2016 2015 2016 2015 MPC $ 9 $ 4 $ 18 $ 6
Employee Services Expenses from Related Parties	Employee services expenses from related parties were as follows: Three Months Ended June 30, Six Months Ended June 30, (In millions) 2016 2015 2016 2015 Purchases - related parties $ 73 $ 33 $ 143 $ 66 General and administrative expenses 19 8 40 15 Total $ 92 $ 41 $ 183 $ 81
Receivables from Related Parties	Receivables from related parties were as follows: (In millions) June 30, 2016 December 31, 2015 MPC $ 104 $ 175 MarkWest Utica EMG 5 4 Ohio Gathering 3 5 Other 1 3 Total $ 113 $ 187
Payables to Related Parties	Payables to related parties were as follows: (In millions) June 30, 2016 December 31, 2015 MPC $ 51 $ 33 MarkWest Utica EMG 14 21 Total $ 65 $ 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) June 30, 2016 December 31, 2015 Minimum volume deficiencies - MPC $ 43 $ 36 Project reimbursements - MPC 5 5 Total $ 48 $ 41

Net Income Per Limited Partne31

Net Income Per Limited Partner Unit (Tables)	6 Months Ended
	Jun. 30, 2016
Earnings Per Share [Abstract]
Schedule of Distributions By Partner By Class	For the three and six months ended June 30, 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 and six months ended June 30, 2016 is the same as basic net income (loss) per limited partner unit since the inclusion of any potential common units would have been anti-dilutive. Potential common units omitted from the diluted earnings per unit calculation was approximately 10 million . Three Months Ended June 30, Six Months Ended June 30, (In millions) 2016 2015 2016 2015 Net (loss) income attributable to MPLX LP $ 19 $ 51 $ (41 ) $ 97 Less: Limited partners’ distributions declared on preferred units (1) 9 — 9 — General partner’s distributions declared (including IDRs) (1) 50 6 94 10 Limited partners’ distributions declared on common units (1) 172 19 328 37 Limited partner’s distributions declared (1) — 17 — 32 Undistributed net (loss) income attributable to MPLX LP $ (212 ) $ 9 $ (472 ) $ 18 (1) See Note 7 for distribution information.
Schedule of Basic and Diluted Earnings Per Unit	Three Months Ended June 30, 2016 (In millions, except per unit data) General Partner Limited Partners’ Common Units Preferred 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) $ 50 $ 172 $ 9 $ 231 Undistributed net loss attributable to MPLX LP (5 ) (207 ) — (212 ) Net income (loss) attributable to MPLX LP (1) $ 45 $ (35 ) $ 9 $ 19 Weighted average units outstanding: Basic 7 331 17 355 Diluted 7 331 17 355 Net loss attributable to MPLX LP per limited partner unit: Basic $ (0.11 ) Diluted $ (0.11 ) Three Months Ended June 30, 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) $ 6 $ 19 $ 17 $ 42 Undistributed net income attributable to MPLX LP 5 2 2 9 Net income attributable to MPLX LP (1) $ 11 $ 21 $ 19 $ 51 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 $ 0.50 $ 0.50 Diluted $ 0.50 $ 0.50 Six Months Ended June 30, 2016 (In millions, except per unit data) General Partner Limited Partners’ Common Units Preferred 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) $ 94 $ 328 $ 9 $ 431 Undistributed net loss attributable to MPLX LP (9 ) (463 ) — (472 ) Net income (loss) attributable to MPLX LP (1) $ 85 $ (135 ) $ 9 $ (41 ) Weighted average units outstanding: Basic 7 316 8 331 Diluted 7 316 8 331 Net loss attributable to MPLX LP per limited partner unit: Basic $ (0.43 ) Diluted $ (0.43 ) Six Months Ended June 30, 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) $ 10 $ 37 $ 32 $ 79 Undistributed net income attributable to MPLX LP 9 5 4 18 Net income attributable to MPLX LP (1) $ 19 $ 42 $ 36 $ 97 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 $ 0.96 $ 0.96 Diluted $ 0.96 $ 0.96 (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)	6 Months Ended
	Jun. 30, 2016
Equity [Abstract]
Schedule of Stockholders Equity [Table Text Block]	The changes in the number of units outstanding from December 31, 2015 through June 30, 2016 are summarized below: (In units) Common Class B (1) General Partner Total Balance at December 31, 2015 296,687,176 7,981,756 6,800,475 311,469,407 Unit-based compensation awards (2) 37,251 — 761 38,012 Issuance of units under the ATM Program (3) 12,025,000 — 245,406 12,270,406 Contribution of HSM (4) 22,534,002 — 459,878 22,993,880 Balance at June 30, 2016 331,283,429 7,981,756 7,506,520 346,771,705 (1) On July 1, 2016, 3,990,878 Class B units converted to 4,350,057 common units and will be eligible to receive the second quarter distribution. (2) As a result of the unit-based compensation awards issued during the period, MPLX GP contributed less than $1 million in exchange for 761 general partner units to maintain its two percent general partner interest. (3) 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. (4) See Note 3 for information regarding the HSM acquisition.
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 preferred unitholders based on a fixed distribution schedule, as discussed in Note 8 , and subsequently allocated to remaining 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 June 30, Six Months Ended June 30, (In millions) 2016 2015 2016 2015 Net (loss) income attributable to MPLX LP $ 19 $ 51 $ (41 ) $ 97 Less: Preferred unit distributions 9 — 9 — General partner's incentive distribution rights and other 47 6 88 9 Net (loss) income attributable to MPLX LP available to general and limited partners $ (37 ) $ 45 $ (138 ) $ 88 General partner's two percent interest in net (loss) income attributable to MPLX LP $ (1 ) $ 1 $ (3 ) $ 2 General partner's incentive distribution rights and other 47 6 88 9 General partner's interest in net income attributable to MPLX LP $ 46 $ 7 $ 85 $ 11
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 June 30, Six Months Ended June 30, (In millions) 2016 2015 2016 2015 General partner's distributions: General partner's distributions $ 4 $ 1 $ 8 $ 2 General partner's incentive distribution rights distributions 46 6 86 9 Total general partner's distributions $ 50 $ 7 $ 94 $ 11 Limited partners' distributions: Common unitholders $ 172 $ 19 $ 328 $ 37 Subordinated unitholders — 16 — 31 Total limited partners' distributions 172 35 328 68 Preferred unit distributions 9 — 9 — Total cash distributions declared $ 231 $ 42 $ 431 $ 79

Redeemable Preferred Units (Tab

Redeemable Preferred Units (Tables)	6 Months Ended
	Jun. 30, 2016
Temporary Equity Disclosure [Abstract]
Temporary Equity [Table Text Block]	The changes in the redeemable preferred balance for 2016 were as follows: (In millions) Redeemable Preferred Units Issuance of MPLX LP redeemable preferred units on May 13, 2016 $ 984 Net income allocated for May 13, 2016 through June 30, 2016 9 Balance at June 30, 2016 $ 993

Segment Information (Tables)

Segment Information (Tables)	6 Months Ended
	Jun. 30, 2016
Segment Reporting [Abstract]
Temporary Equity [Table Text Block]	The changes in the redeemable preferred balance for 2016 were as follows: (In millions) Redeemable Preferred Units Issuance of MPLX LP redeemable preferred units on May 13, 2016 $ 984 Net income allocated for May 13, 2016 through June 30, 2016 9 Balance at June 30, 2016 $ 993
Schedule of Segment Reporting Information, by Segment [Table Text Block]	Three Months Ended June 30, 2016 (In millions) L&S G&P Total Revenues and other income: Segment revenues $ 193 $ 530 $ 723 Segment other income 18 — 18 Total segment revenues and other income 211 530 741 Costs and expenses: Segment cost of revenues 88 223 311 Segment operating income before portion attributable to noncontrolling interest 123 307 430 Segment portion attributable to noncontrolling interest and Predecessor — 36 36 Segment operating income attributable to MPLX LP $ 123 $ 271 $ 394 Three Months Ended June 30, 2015 (In millions) L&S Revenues and other income: Segment revenues $ 193 Segment other income 20 Total segment revenues and other income 213 Costs and expenses: Segment cost of revenues 90 Segment operating income before portion attributable to noncontrolling interest and Predecessor 123 Segment portion attributable to noncontrolling interest and Predecessor 35 Segment operating income attributable to MPLX LP $ 88 Six Months Ended June 30, 2016 (In millions) L&S G&P Total Revenues and other income: Segment revenues $ 385 $ 1,028 $ 1,413 Segment other income 37 — 37 Total segment revenues and other income 422 1,028 1,450 Costs and expenses: Segment cost of revenues 177 423 600 Segment operating income before portion attributable to noncontrolling interest 245 605 850 Segment portion attributable to noncontrolling interest and Predecessor 34 77 111 Segment operating income attributable to MPLX LP $ 211 $ 528 $ 739 Six Months Ended June 30, 2015 (In millions) L&S Revenues and other income: Segment revenues $ 376 Segment other income 38 Total segment revenues and other income 414 Costs and expenses: Segment cost of revenues 176 Segment operating income before portion attributable to noncontrolling interest and Predecessor 238 Segment portion attributable to noncontrolling interest and Predecessor 68 Segment operating income attributable to MPLX LP $ 170
Reconciliation of Operating Profit (Loss) from Segments to Consolidated [Table Text Block]	Three Months Ended June 30, Six Months Ended June 30, (In millions) 2016 2015 2016 2015 Reconciliation to Income from operations: Segment operating income attributable to MPLX LP $ 394 $ 88 $ 739 $ 170 Segment portion attributable to unconsolidated affiliates (83 ) — (166 ) — Segment portion attributable to noncontrolling interest and Predecessor 36 35 111 68 Loss from equity method investments (83 ) — (78 ) — Other income - related parties 11 — 18 — Unrealized derivative losses (12 ) — (21 ) — Impairment expense (1 ) — (130 ) — Depreciation and amortization (137 ) (20 ) (269 ) (39 ) General and administrative expenses (49 ) (21 ) (101 ) (43 ) Income from operations $ 76 $ 82 $ 103 $ 156
Reconciliation of Revenue from Segments to Consolidated [Table Text Block]	Three Months Ended June 30, Six Months Ended June 30, (In millions) 2016 2015 2016 2015 Reconciliation to Total revenues and other income: Total segment revenues and other income $ 741 $ 213 $ 1,450 $ 414 Revenue adjustment from unconsolidated affiliates (99 ) — (203 ) — Loss from equity method investments (83 ) — (78 ) — Other income - related parties 11 — 18 — Unrealized derivative loss (6 ) — (14 ) — Total revenues and other income $ 564 $ 213 $ 1,173 $ 414
Reconciliation of Net Income Attributable to Noncontrolling Interests [Table Text Block]	Three Months Ended June 30, Six Months Ended June 30, (In millions) 2016 2015 2016 2015 Reconciliation to Net income attributable to noncontrolling interests and Predecessor Segment portion attributable to noncontrolling interest and Predecessor $ 36 $ 35 $ 111 $ 68 Portion of noncontrolling interests and Predecessor related to items below segment income from operations (56 ) (10 ) (85 ) (21 ) Portion of operating income attributable to noncontrolling interest of unconsolidated affiliates 21 — (2 ) — Net income attributable to noncontrolling interests and Predecessor $ 1 $ 25 $ 24 $ 47
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 June 30, Six Months Ended June 30, (In millions) 2016 2015 2016 2015 L&S segment capital expenditures $ 82 $ 35 $ 144 $ 70 G&P segment capital expenditures (1) 212 — 485 — Total segment capital expenditures 294 35 629 70 Less: Capital expenditures for Partnership operated, non-wholly-owned subsidiaries 16 — 60 — Total capital expenditures $ 278 $ 35 $ 569 $ 70 (1) The G&P segment includes $16 million and $60 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: (In millions) June 30, 2016 December 31, 2015 L&S $ 1,952 $ 1,858 G&P 14,127 14,246 Total assets $ 16,079 $ 16,104

Income Taxes (Tables)

Income Taxes (Tables)	6 Months Ended
	Jun. 30, 2016
Income Tax Disclosure [Abstract]
Schedule of Effective Income Tax Rate Reconciliation [Table Text Block]	A reconciliation of the benefit for income tax and the amount computed by applying the federal statutory rate of 35 percent to the income before income taxes for the six months ended June 30, 2016 is as follows: (In millions) MarkWest Hydrocarbon Partnership Eliminations Consolidated (1) Income before (benefit) provision for income tax $ (35 ) $ 4 $ 2 $ (29 ) Federal statutory rate 35 % — % — % Federal income tax at statutory rate (2) (12 ) — — (12 ) Change in state statutory rate (1 ) — — (1 ) State income taxes net of federal benefit - MarkWest Hydrocarbon (1 ) — — (1 ) Provision on income from Class A units (2) — — — — Other 1 1 — 2 (Benefit) provision for income tax $ (13 ) $ 1 $ — $ (12 ) (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)	6 Months Ended
	Jun. 30, 2016
Inventory Disclosure [Abstract]
Schedule of Inventory, Current [Table Text Block]	Inventories consist of the following: (In millions) June 30, 2016 December 31, 2015 NGLs $ 2 $ 3 Line fill 6 5 Spare parts, materials and supplies 41 43 Total inventories $ 49 $ 51

Property, Plant and Equipment (

Property, Plant and Equipment (Tables)	6 Months Ended
	Jun. 30, 2016
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment	Property, plant and equipment with associated accumulated depreciation was: (In millions) June 30, 2016 December 31, 2015 Natural gas gathering and NGL transportation pipelines and facilities $ 4,573 $ 4,307 Processing, fractionation and storage facilities 3,456 3,185 Pipelines and related assets 1,186 1,128 Barges and towing vessels 478 475 Land, building, office equipment and other 662 606 Construction in progress 898 946 Total 11,253 10,647 Less accumulated depreciation 893 650 Property, plant and equipment, net $ 10,360 $ 9,997

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	6 Months Ended
	Jun. 30, 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 and six months ended June 30, 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 4 for detail of the Ohio Condensate equity method impairment charge, which included a Level 3 valuation adjustment during the three and six months ended June 30, 2016 . See Note 16 for a rollforward of goodwill, which included a Level 3 valuation adjustment during the three and six months ended June 30, 2016 . Three Months Ended June 30, 2016 Six Months Ended June 30, 2016 (In millions) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Commodity Derivative Contracts (net) Embedded Derivatives in Commodity Contracts (net) Fair value at beginning of period $ — $ (34 ) $ 7 $ (32 ) Total loss (realized and unrealized) included in earnings (1) (6 ) (7 ) (7 ) (11 ) Settlements 1 1 (5 ) 3 Netting adjustment (2) 1 — 1 — Fair value at end of period $ (4 ) $ (40 ) $ (4 ) $ (40 ) 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 $ (5 ) $ (8 ) $ (6 ) $ (11 ) (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 . (2) Certain derivative positions are subject to master netting agreements; therefore, the Partnership has elected to offset derivative assets and liabilities where legally permissible. The Partnership may hold positions with certain counterparties, which for GAAP purposes are classified within different levels of the fair value hierarchy and may be legally permissible to offset. This adjustment represents the total impact of offsetting Level 2 positions with Level 3 positions as of June 30, 2016.
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) June 30, 2016 December 31, 2015 Assets Liabilities Assets Liabilities Significant other observable inputs (Level 2) Commodity contracts $ — $ — $ 2 $ — Significant unobservable inputs (Level 3) Commodity contracts — (4 ) 7 — Embedded derivatives in commodity contracts 1 (41 ) — (32 ) Total carrying value in Consolidated Balance Sheets $ 1 $ (45 ) $ 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. June 30, 2016 December 31, 2015 (In millions) Fair Value Carrying Value Fair Value Carrying Value Long-term debt $ 4,748 $ 4,400 $ 5,212 $ 5,255 SMR liability $ 105 $ 98 $ 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 June 30, 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 Liabilities Forward ethane prices (per gallon) (1) $0.24 - $0.28 July 2016 - Dec. 2016 Forward propane prices (per gallon) (1) $0.52 - $0.57 July 2016 - Dec. 2016 Forward isobutane prices (per gallon) (1) $0.69 - $0.72 July 2016 - Dec. 2016 Forward normal butane prices (per gallon) (1) $0.63 - $0.69 July 2016 - Dec. 2016 Forward natural gasoline prices (per gallon) (1) $0.99 - $1.03 July 2016 - Dec. 2016 Embedded derivatives in commodity contracts Assets ERCOT Pricing (per MegaWatt Hour) $26.47 - $57.95 July 2016 - Dec. 2016 Liabilities Forward propane prices (per gallon) (1) $0.52 - $0.58 July 2016 - Dec. 2022 Forward isobutane prices (per gallon) (1) $0.67 - $0.73 July 2016 - Dec. 2022 Forward normal butane prices (per gallon) (1) $0.62 - $0.71 July 2016 - Dec. 2022 Forward natural gasoline prices (per gallon) (1) $0.99 - $1.10 July 2016 - Dec. 2022 Forward natural gas prices (per mmbtu) (2) $2.61 - $3.35 July 2016 - Dec. 2022 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 Instrume39

Derivative Financial Instruments (Tables)	6 Months Ended
	Jun. 30, 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) June 30, 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 $ 1 $ (9 ) $ 9 $ (5 ) Other noncurrent assets / deferred credits and other liabilities — (36 ) — (27 ) Total $ 1 $ (45 ) $ 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 June 30, 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 184,000 Natural Gas (MMBtu) Long 1,088,484 NGLs (gal) Short 64,810,176
Offsetting Assets and Liabilities [Table Text Block]	The net amounts in the table below equal the balances presented in the Consolidated Balance Sheets: June 30, 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 $ 2 $ (2 ) $ — $ (6 ) $ 2 $ (4 ) Embedded derivatives in commodity contracts 1 — 1 (5 ) — (5 ) Total current derivative instruments 3 (2 ) 1 (11 ) 2 (9 ) Non-current Commodity contracts — — — — — — Embedded derivatives in commodity contracts — — — (36 ) — (36 ) Total non-current derivative instruments — — — (36 ) — (36 ) Total derivative instruments $ 3 $ (2 ) $ 1 $ (47 ) $ 2 $ (45 )
Derivative Instruments, Gain (Loss) [Table Text Block]	The impact of the Partnership’s derivative contracts not designated as hedging instruments and the location of gain or (loss) recognized in the Consolidated Statements of Income is summarized below: (In millions) Three Months Ended June 30, 2016 Six Months Ended June 30, 2016 Product sales Realized (loss) gain $ (1 ) $ 6 Unrealized loss (6 ) (14 ) Total revenue: derivative loss from product sales (7 ) (8 ) Purchased product costs Unrealized loss (8 ) (9 ) Cost of Revenues Unrealized gain 2 2 Total loss $ (13 ) $ (15 )

Debt (Tables)

Debt (Tables)	6 Months Ended
	Jun. 30, 2016
Debt Disclosure [Abstract]
Summary of Outstanding Borrowings	The Partnership’s outstanding borrowings at June 30, 2016 and December 31, 2015 consisted of the following: (In millions) June 30, 2016 December 31, 2015 MPLX LP: Bank revolving credit facility due 2020 $ — $ 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 4,859 5,736 Unamortized debt issuance costs (8 ) (8 ) Unamortized discount (1) (450 ) (472 ) Amounts due within one year (1 ) (1 ) Total long-term debt due after one year $ 4,400 $ 5,255 (1) Includes $442 million and $465 million discount as of June 30, 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)	6 Months Ended
	Jun. 30, 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 Gross goodwill as of December 31, 2015 $ 116 $ 2,454 $ 2,570 Accumulated impairment losses — — — Balance as of December 31, 2015 116 2,454 2,570 Purchase price allocation adjustments (1) — (241 ) (241 ) Impairment losses — (130 ) (130 ) Balance as of June 30, 2016 $ 116 $ 2,083 $ 2,199 Gross goodwill as of June 30, 2016 $ 116 $ 2,213 $ 2,329 Accumulated impairment losses — (130 ) (130 ) Balance as of June 30, 2016 $ 116 $ 2,083 $ 2,199 (1) See Note 3 for further discussion on purchase price allocation adjustments.

Supplemental Cash Flow Inform42

Supplemental Cash Flow Information (Tables)	6 Months Ended
	Jun. 30, 2016
Supplemental Cash Flow Elements [Abstract]
Summary of Supplemental Cash Flow Information	Six Months Ended June 30, (In millions) 2016 2015 Net cash provided by operating activities included: Interest paid (net of amounts capitalized) $ 109 $ 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	Six Months Ended June 30, (In millions) 2016 2015 (Decrease) increase in capital accruals $ (6 ) $ 13

Equity-Based Compensation Plan

Equity-Based Compensation Plan (Tables)	6 Months Ended
	Jun. 30, 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 six months ended June 30, 2016 : Number Weighted Outstanding at December 31, 2015 1,031,219 $ 35.49 Granted 445,555 29.31 Settled (43,660 ) 51.21 Forfeited (19,490 ) 31.36 Outstanding at June 30, 2016 1,413,624 33.11
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 six months ended June 30, 2016 : Number of Outstanding at December 31, 2015 1,521,392 Granted 789,375 Settled (458,011 ) Forfeited (53,507 ) Outstanding at June 30, 2016 1,799,249

Description of Business and Bas

Description of Business and Basis of Presentation - Additional Information (Detail)	6 Months Ended
	Jun. 30, 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, 2016shares	Jun. 30, 2016USD ($)	Jun. 30, 2015USD ($)
Business Acquisition [Line Items]
Investments - loans from (to) related parties		$ 77	$ (38)	[1]
All other, net		(5)	1	[1]
Proceeds from Issuance or Sale of Equity		(321)	(1)	[1]
Deferred income taxes		(13)	(1)	[1]
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
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
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
MPC [Member]
Business Acquisition [Line Items]
Distributions to MPC from Predecessor		(104)	(1)
Contribution from MPC		$ 15
Percent of Volumes Shipped		60.00%
As Originally Reported
Business Acquisition [Line Items]
Investments - loans from (to) related parties			0
All other, net			0
Proceeds from Issuance or Sale of Equity			(1)
Deferred income taxes			0
As Originally Reported | MPC [Member]
Business Acquisition [Line Items]
Distributions to MPC from Predecessor			(1)
Hardin Street Marine Business [Member] | HSM
Business Acquisition [Line Items]
Investments - loans from (to) related parties			(38)
All other, net			1
Proceeds from Issuance or Sale of Equity			0
Deferred income taxes			(1)
Hardin Street Marine Business [Member] | HSM | MPC [Member]
Business Acquisition [Line Items]
Distributions to MPC from Predecessor			$ 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 - Purchase of HSM

Acquisitions - Purchase of HSM retrospective adjustments (Details) - USD ($) $ in Millions		3 Months Ended			6 Months Ended
		Jun. 30, 2016	Jun. 30, 2015		Jun. 30, 2016		Jun. 30, 2015		Dec. 31, 2015	Dec. 31, 2014
Revenues and other income:
Service revenue		$ 233	$ 16	[1]	$ 462		$ 32	[1]
Service revenue - related parties		145	152	[1]	295		294	[1]
Rental income - related parties		29	25	[1]	55		50	[1]
Other income		1	2	[1]	3		3	[1]
Other income - related parties		28	18	[1]	52		35	[1]
Total revenues and other income		564	213	[1]	1,173		414	[1]
Costs and expenses:
Cost of revenues (excludes items below)		84	46	[1]	173		88	[1]
Purchases - related parties		78	40	[1]	154		80	[1]
Depreciation and amortization		137	20	[1]	269		39	[1],[2]
General and administrative expenses		49	21	[1]	101		43	[1]
Other taxes		11	4	[1]	22		8	[1]
Total costs and expenses		488	131	[1]	1,070		258	[1]
Income from operations		76	82	[1]	103		156	[1]
Interest expense (net of amounts capitalized of $7 million, $1 million, $14 million and $1 million, respectively)		52	6	[1]	107		11	[1]
Other financial costs		12	0	[1]	24		1	[1]
Income before income taxes		12	76	[1]	(29)	[3]	144	[1]
Net (loss) income		20	76	[1]	(17)		144	[1],[2]
Net income attributable to noncontrolling interest		1	1	[1]	1		1	[1]
Less: Net income attributable to Predecessor		0	24	[1]	23		46	[1]
Net income attributable to MPLX LP	[4]	19	51	[1]	(41)		97	[1]
Less: General partner’s interest in net income attributable to MPLX LP		46	7	[1]	85		11	[1]
Limited partners’ interest in net (loss) income attributable to MPLX LP		(36)	44	[1]	(135)		86	[1]
Changes in:
Amortization of deferred financing costs					23		1	[2]
Deferred income taxes					(13)		(1)	[2]
Current receivables					20		2	[2]
Current accounts payable and accrued liabilities					18		12	[2]
Receivables from / liabilities to related parties					(6)		19	[2]
All other, net					21		(1)	[2]
Net Cash Provided by (Used in) Operating Activities					593		173	[2]
Investing activities:
Additions to property, plant and equipment		278	35		569		70	[2]
Investments - loans from (to) related parties					77		(38)	[2]
All other, net					(5)		1	[2]
Net cash used in investing activities					(526)		(109)	[2]
Financing activities:
Long-term debt - borrowings					434		528	[2]
Repayments of Long-term Debt					1,311		415	[2]
Debt issuance costs					0		4	[2]
Net proceeds from equity offerings					321		1	[2]
Distributions to unitholders and general partner					391		70	[2]
Net cash (used in) provided by financing activities					(75)		39	[2]
Cash and Cash Equivalents, Period Increase (Decrease)					(8)		103	[2]
Cash and cash equivalents		35	130	[2]	35		130	[2]	$ 43	$ 27	[2]
MPLX LP (Previously Reported)
Revenues and other income:
Service revenue			16				32
Service revenue - related parties			119				230
Rental income - related parties			4				8
Other income			2				3
Other income - related parties			6				12
Total revenues and other income			147				285
Costs and expenses:
Cost of revenues (excludes items below)			31				59
Purchases - related parties			24				48
Depreciation and amortization			13				25
General and administrative expenses			18				37
Other taxes			3				6
Total costs and expenses			89				175
Income from operations			58				110
Interest expense (net of amounts capitalized of $7 million, $1 million, $14 million and $1 million, respectively)			6				11
Other financial costs			0				1
Income before income taxes			52				98
Net (loss) income			52				98
Net income attributable to noncontrolling interest			1				1
Less: Net income attributable to Predecessor			0				0
Net income attributable to MPLX LP			51				97
Less: General partner’s interest in net income attributable to MPLX LP			7				11
Limited partners’ interest in net (loss) income attributable to MPLX LP			44				86
Changes in:
Amortization of deferred financing costs							1
Deferred income taxes							0
Current receivables							2
Current accounts payable and accrued liabilities							14
Receivables from / liabilities to related parties							8
All other, net							0
Net Cash Provided by (Used in) Operating Activities							128
Investing activities:
Additions to property, plant and equipment							64
Investments - loans from (to) related parties							0
All other, net							0
Net cash used in investing activities							(64)
Financing activities:
Long-term debt - borrowings							528
Repayments of Long-term Debt							415
Debt issuance costs							4
Net proceeds from equity offerings							1
Distributions to unitholders and general partner							70
Net cash (used in) provided by financing activities							39
Cash and Cash Equivalents, Period Increase (Decrease)							103
Cash and cash equivalents			130				130			27
Hardin Street Marine Business [Member] | HSM
Revenues and other income:
Service revenue			0				0
Service revenue - related parties			33				64
Rental income - related parties			21				42
Other income			0				0
Other income - related parties			12				23
Total revenues and other income			66				129
Costs and expenses:
Cost of revenues (excludes items below)			15				29
Purchases - related parties			16				32
Depreciation and amortization			7				14
General and administrative expenses			3				6
Other taxes			1				2
Total costs and expenses			42				83
Income from operations			24				46
Interest expense (net of amounts capitalized of $7 million, $1 million, $14 million and $1 million, respectively)			0				0
Other financial costs			0				0
Income before income taxes			24				46
Net (loss) income			24				46
Net income attributable to noncontrolling interest			0				0
Less: Net income attributable to Predecessor			24				46
Net income attributable to MPLX LP			0				0
Less: General partner’s interest in net income attributable to MPLX LP			0				0
Limited partners’ interest in net (loss) income attributable to MPLX LP			0				0
Changes in:
Amortization of deferred financing costs							0
Deferred income taxes							(1)
Current receivables							0
Current accounts payable and accrued liabilities							(2)
Receivables from / liabilities to related parties							11
All other, net							(1)
Net Cash Provided by (Used in) Operating Activities							45
Investing activities:
Additions to property, plant and equipment							6
Investments - loans from (to) related parties							(38)
All other, net							1
Net cash used in investing activities							(45)
Financing activities:
Long-term debt - borrowings							0
Repayments of Long-term Debt							0
Debt issuance costs							0
Net proceeds from equity offerings							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
Cash and cash equivalents			0				0			$ 0
MPC [Member]
Revenues and other income:
Service revenue - related parties		145	152		295		294
Rental income - related parties		29	25		55		50
Other income - related parties		$ 16	$ 18		33		34
Financing activities:
Distributions to MPC from Predecessor					$ 104		1
MPC [Member] | MPLX LP (Previously Reported)
Financing activities:
Distributions to MPC from Predecessor							1
MPC [Member] | Hardin Street Marine Business [Member] | HSM
Financing activities:
Distributions to MPC from Predecessor							$ 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]	llocation of net income (loss) attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.

Acquisitions - Purchase of HS47

Acquisitions - Purchase of HSM retrospective adjustments (Parenthetical) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Business Combinations [Abstract]
Interest Costs Capitalized	$ 7	$ 1	$ 14	$ 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 ($)	Jun. 30, 2016USD ($)	Jun. 30, 2015USD ($)	[1]	Jun. 30, 2016USD ($)	Jun. 30, 2015USD ($)	[1],[2]
Business Acquisition [Line Items]
Goodwill				$ 2,570	$ 2,199			$ 2,199
Depreciation and amortization					$ 137	$ 20		269	$ 39
Limited Partners Subordinated Units [Member]
Business Acquisition [Line Items]
Equity Instruments Conversion Date		Jul. 1, 2016
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
Paid/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
Paid/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
[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.

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
Paid/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	Jun. 30, 2016	Dec. 31, 2015	Dec. 04, 2015
Business Acquisition [Line Items]
Goodwill	$ 2,199	$ 2,570
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 | 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

Acquisitions - Unaudited Pro Fo

Acquisitions - Unaudited Pro Forma Financial Information (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2015
Business Combinations [Abstract]
Revenues and other income	$ 668	$ 1,332
Net (loss) income attributable to MPLX LP	$ (11)	$ 53
Net income attributable to MPLX LP per unit - basic	$ (0.19)	$ (0.10)
Net income attributable to MPLX LP per unit - diluted	$ (0.19)	$ (0.10)

Acquisitions - Pro Forma Narrat

Acquisitions - Pro Forma Narrative (Details)	Dec. 04, 2015	Jun. 30, 2016
Business Acquisition [Line Items]
Business Acquisition, Pro Forma Information, Description		The unaudited pro forma financial information includes adjustments primarily to align accounting policies, adjust depreciation expense to reflect the fair value of property, plant and equipment, increase amortization expense related to identifiable intangible assets and 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			6 Months Ended
		Jun. 30, 2016	Jun. 30, 2015		Jun. 30, 2016	Jun. 30, 2015
Business Acquisition [Line Items]
Total revenues and other income		$ 564	$ 213	[1]	$ 1,173	$ 414	[1]
Net income attributable to noncontrolling interest		1	1	[1]	1	1	[1]
Net income attributable to MPLX LP	[2]	$ 19	51	[1]	$ (41)	97	[1]
EMG Utica, LLC [Member]
Business Acquisition [Line Items]
Noncontrolling Owners Accrual of Preference Amount			11			21
MarkWest Utica EMG [Member]
Business Acquisition [Line Items]
Total revenues and other income			34			67
Operating Expenses			7			14
Depreciation and amortization			16			32
Net income attributable to noncontrolling interest			15			29
Net income attributable to MPLX LP			$ (5)			$ (9)
[1]	(1)Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.
[2]	llocation of net income (loss) attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.

Equity Method Investments (Deta

Equity Method Investments (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	[1]	Jun. 30, 2016	Jun. 30, 2015	[1]	Dec. 31, 2015	Feb. 28, 2013
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 2,485			$ 2,485			$ 2,458
Other income - related parties	28	$ 18		52	$ 35
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity	1,100			1,100
Equity Method Investment Difference Between Carrying Amount And Underlying Equity Portion Related To Goodwill Not Amortized	$ 459			$ 459
MarkWest Utica EMG [Member]
Schedule of Equity Method Investments [Line Items]
Minimum Capital Contributed by Noncontrolling Owners								$ 950
Percentage of Required Capital Contribution by Reporting Entity after NCI								100.00%
Aggregate Contributions to VIE Threshold								$ 2,000
Percentage of Ownership Interest in Joint Venture, Maximum	70.00%			70.00%
Threshold Percentage of Noncontrolling Ownership Interest in Joint Venture	30.00%			30.00%
Maximum Percentage of Additional Noncontrolling Capital Contribution before Threshold Ownership				10.00%
Actual Capital Contributed by Noncontrolling Owners	$ 998			$ 998
Actual Capital Contribution to Date	1,500			1,500
Noncontrolling Owners Preference Threshold								$ 500
Noncontrolling Owners Accrual of Preference Amount	$ 4			$ 8
Percentage of Cash Generation Received	60.00%			60.00%
Percentage Investment Balance Target for Distribution of Available Cash	60.00%			60.00%
Percentage of aggregate investment balance	56.00%			56.00%
Equity method investments	$ 2,300			$ 2,300
Other income - related parties	$ 5			$ 7
Equity Method Investment, Ownership Percentage	60.00%			60.00%
Ohio Gathering Company, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Other income - related parties	$ 3			$ 7
Equity Method Investment, Ownership Percentage	36.00%			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%			60.00%
Equity Method Investment, Other than Temporary Impairment				$ 96
Income (Loss) from Equity Method Investments from Asset Impairment	$ 58
Income (Loss) from Equity Method Investments from Elimination of Basis Differential	$ 31
Sensitivity Analysis of Fair Value, Impact to Income Statement				$ 1
Income Approach Valuation Technique [Member] | Equity Method Investments [Member] | Ohio Condensate Company, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Fair Value Inputs, Discount Rate				11.20%
[1]	(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			6 Months Ended
		Jun. 30, 2016	Jun. 30, 2015	[1]	Jun. 30, 2016		Jun. 30, 2015	[1],[3]	Dec. 31, 2015
Schedule of Equity Method Investments [Line Items]
Revenue					$ 191
Gross margin					155
Income from operations					(8)
Net income					(8)
Loss from equity method investments		$ (83)	$ 0		(78)	[2]	$ 0
Current assets		183			183				$ 150
Noncurrent assets		2,664			2,664				2,619
Current liabilities		140			140				102
Noncurrent liabilities		16			16				13
MarkWest Utica EMG [Member]
Schedule of Equity Method Investments [Line Items]
Revenue	[4]				113
Gross margin	[4]				113
Income from operations	[4]				68
Net income	[4]				68
Loss from equity method investments	[2],[4]				7
Current assets		138			138				113	[4]
Noncurrent assets		2,193			2,193				2,207	[4]
Current liabilities		108			108				77	[4]
Noncurrent liabilities		2			2				1	[4]
Ohio Condensate Company, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Revenue					10
Gross margin					10
Income from operations					(94)
Net income					(94)
Loss from equity method investments	[2]				(88)
Current assets		7			7				7
Noncurrent assets		31			31				127
Current liabilities		6			6				6
Noncurrent liabilities		14			14				12
Other VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Revenue					0
Gross margin					0
Income from operations					0
Net income					0
Loss from equity method investments	[2]				0
Current assets		0			0				0
Noncurrent assets		55			55				42
Current liabilities		0			0				1
Noncurrent liabilities		0			0				0
Non-VIEs [Member]
Schedule of Equity Method Investments [Line Items]
Revenue					68
Gross margin					32
Income from operations					18
Net income					18
Loss from equity method investments	[2]				3
Current assets		38			38				30
Noncurrent assets		385			385				243
Current liabilities		26			26				18
Noncurrent liabilities		0			0				0
Utica Condensate [Member]
Schedule of Equity Method Investments [Line Items]
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures		$ 788			$ 788				$ 781
[1]	(1)Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.
[2]	(2)Income (loss) from equity method investments includes the impact of any basis differential amortization or accretion.
[3]	(1)Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.
[4]	(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 $788 million and $781 million as of June 30, 2016 and December 31, 2015, respectively.

Related Party Agreements and 56

Related Party Agreements and Transactions - Additional Information (Detail) $ in Millions	6 Months Ended
	Jun. 30, 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
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	$ 1,900
Repayments of Long-term Lines of Credit	1,900
Long-term Debt, Gross	$ 0
Debt Instrument, Interest Rate, Effective Percentage	1.93%

Related Party Agreements and 57

Related Party Agreements and Transactions - Sales to Related Parties (Detail) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015		Jun. 30, 2016	Jun. 30, 2015
Related Party Transaction [Line Items]
Service revenue - related parties	$ 145	$ 152	[1]	$ 295	$ 294	[1]
Rental income - related parties	29	25	[1]	55	50	[1]
Product sales - related parties	3	0	[1]	6	0	[1]
MPC [Member]
Related Party Transaction [Line Items]
Service revenue - related parties	145	152		295	294
Rental income - related parties	29	25		55	50
Product sales - related parties	3	$ 0		6	$ 0
Sales Revenue, Goods, Related Party, Net Zero	$ 7			$ 12
[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 Transactions - Other Income - Related Parties (Detail) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015		Jun. 30, 2016	Jun. 30, 2015
Related Party Transaction [Line Items]
Other income - related parties	$ 28	$ 18	[1]	$ 52	$ 35	[1]
MPC [Member]
Related Party Transaction [Line Items]
Other income - related parties	16	18		33	34
MarkWest Utica EMG [Member]
Related Party Transaction [Line Items]
Other income - related parties	5	0		7	0
Ohio Gathering Company, LLC [Member]
Related Party Transaction [Line Items]
Other income - related parties	3	0		7	0
Ohio Condensate Company, LLC [Member]
Related Party Transaction [Line Items]
Other income - related parties	1	0		2	0
Other [Member]
Related Party Transaction [Line Items]
Other income - related parties	$ 3	$ 0		$ 3	$ 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 59

Related Party Agreements and Transaction - Summary of Charges for Omnibus Agreement (Detail) - MPC [Member] - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Related Party Transaction [Line Items]
Charges for services included in purchases from related party	$ 12	$ 18	$ 26	$ 36
Purchases From Related Parties [Member]
Related Party Transaction [Line Items]
Charges for services included in purchases from related party	5	7	11	14
General and Administrative Expense [Member]
Related Party Transaction [Line Items]
Charges for services included in purchases from related party	$ 7	$ 11	$ 15	$ 22

Related Party Agreements and 60

Related Party Agreements and Transactions - Summary of Related Party Costs Added to Property, Plant and Equipment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
MPC [Member] | Assets Under Construction [Member]
Property, Plant and Equipment [Line Items]
Costs added to property, plant and equipment	$ 9	$ 4	$ 18	$ 6

Related Party Agreements and 61

Related Party Agreements and Transactions - Employee Services Agreements (Detail) - MPC [Member] - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	$ 92	$ 41	$ 183	$ 81
Purchases From Related Parties [Member]
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	73	33	143	66
General and Administrative Expense [Member]
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	$ 19	$ 8	$ 40	$ 15

Related Party Agreements and 62

Related Party Agreements and Transactions - Receivables from Related Parties (Detail) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Receivables - related parties	$ 113	$ 187
MPC [Member]
Related Party Transaction [Line Items]
Receivables - related parties	104	175
MarkWest Utica EMG [Member]
Related Party Transaction [Line Items]
Receivables - related parties	5	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	$ 1	$ 3

Related Party Agreements and 63

Related Party Agreements and Transactions Related Party Agreements and Transactions - Long-term Receivables from Related Parties (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Long-term receivables - related parties	$ 26	$ 25
MPC [Member]
Related Party Transaction [Line Items]
Long-term receivables - related parties	$ 26	$ 25

Related Party Agreements and 64

Related Party Agreements and Transactions - Payables to Related Parties (Detail) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Payables - related parties	$ 65	$ 54
MPC [Member]
Related Party Transaction [Line Items]
Payables - related parties	51	33
MarkWest Utica EMG [Member]
Related Party Transaction [Line Items]
Payables - related parties	$ 14	$ 21

Related Party Agreements and 65

Related Party Agreements and Transactions - Summary of Deferred Revenue - Related Parties (Detail) - MPC [Member] - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Deferred Revenue Related Parties	$ 48	$ 41
Minimum volume deficiencies [Member]
Related Party Transaction [Line Items]
Deferred Revenue Related Parties	43	36
Project Reimbursements [Member]
Related Party Transaction [Line Items]
Deferred Revenue Related Parties	$ 5	$ 5

Net Income Per Limited Partne66

Net Income Per Limited Partner Unit - Schedule of Distributions by Partner by Class (Detail) - USD ($) shares in Millions, $ in Millions		3 Months Ended			6 Months Ended
		Jun. 30, 2016	Jun. 30, 2015		Jun. 30, 2016	Jun. 30, 2015
Net Income Per Share [Line Items]
Earnings Per Unit, Potentially Dilutive Units					10
Net income attributable to MPLX LP	[1]	$ 19	$ 51	[2]	$ (41)	$ 97	[2]
Less: Distribution declared		(231)	(42)		(431)	(79)
Undistributed net income attributable to MPLX LP		(212)	9		(472)	18
Preferred Units [Member]
Net Income Per Share [Line Items]
Net income attributable to MPLX LP	[1]	9			9
Less: Distribution declared	[3]	(9)	0		(9)	0
Undistributed net income attributable to MPLX LP		0			0
General Partner [Member] | MPC [Member]
Net Income Per Share [Line Items]
Net income attributable to MPLX LP	[1]	45	11		85	19
Less: Distribution declared	[3]	(50)	(6)		(94)	(10)
Undistributed net income attributable to MPLX LP		(5)	5		(9)	9
Limited Partners Common Units [Member]
Net Income Per Share [Line Items]
Net income attributable to MPLX LP	[1]	(35)	21		(135)	42
Less: Distribution declared	[3]	(172)	(19)		(328)	(37)
Undistributed net income attributable to MPLX LP		(207)	2		(463)	5
Limited Partners Subordinated Units [Member] | MPC [Member]
Net Income Per Share [Line Items]
Net income attributable to MPLX LP	[1]		19			36
Less: Distribution declared	[3]	$ 0	(17)		$ 0	(32)
Undistributed net income attributable to MPLX LP			$ 2			$ 4
[1]	llocation of net income (loss) attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.
[2]	(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 Partne67

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			6 Months Ended
		Jun. 30, 2016	Jun. 30, 2015		Jun. 30, 2016	Jun. 30, 2015
Net income attributable to MPLX LP:
Distributions declared (including IDRs)		$ 231	$ 42		$ 431	$ 79
Undistributed net income attributable to MPLX LP		(212)	9		(472)	18
Net income attributable to MPLX LP	[1]	$ 19	$ 51	[2]	$ (41)	$ 97	[2]
Weighted average units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic		355	82		331	82
Weighted Average Limited Partnership Units Outstanding, Diluted		355	82		331	82
Limited Partners Common Units [Member]
Net income attributable to MPLX LP:
Distributions declared (including IDRs)	[3]	$ 172	$ 19		$ 328	$ 37
Undistributed net income attributable to MPLX LP		(207)	2		(463)	5
Net income attributable to MPLX LP	[1]	$ (35)	$ 21		$ (135)	$ 42
Weighted average units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic		331	43		316	43
Weighted Average Limited Partnership Units Outstanding, Diluted		331	43		316	43
Net income attributable to MPLX LP per limited partner unit:
Basic (in USD per unit)		$ (0.11)	$ 0.50		$ (0.43)	$ 0.96
Diluted (in USD per unit)		$ (0.11)	$ 0.50		$ (0.43)	$ 0.96
Preferred Units [Member]
Net income attributable to MPLX LP:
Distributions declared (including IDRs)	[3]	$ 9	$ 0		$ 9	$ 0
Undistributed net income attributable to MPLX LP		0			0
Net income attributable to MPLX LP	[1]	$ 9			$ 9
Weighted average units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic		17			8
Weighted Average Limited Partnership Units Outstanding, Diluted		17			8
MPC [Member] | General Partner [Member]
Net income attributable to MPLX LP:
Distributions declared (including IDRs)	[3]	$ 50	6		$ 94	10
Undistributed net income attributable to MPLX LP		(5)	5		(9)	9
Net income attributable to MPLX LP	[1]	$ 45	$ 11		$ 85	$ 19
Weighted average units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic		7	2		7	2
Weighted Average Limited Partnership Units Outstanding, Diluted		7	2		7	2
MPC [Member] | Limited Partners Subordinated Units [Member]
Net income attributable to MPLX LP:
Distributions declared (including IDRs)	[3]	$ 0	$ 17		$ 0	$ 32
Undistributed net income attributable to MPLX LP			2			4
Net income attributable to MPLX LP	[1]		$ 19			$ 36
Weighted average units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic			37			37
Weighted Average Limited Partnership Units Outstanding, Diluted			37			37
Net income attributable to MPLX LP per limited partner unit:
Basic (in USD per unit)			$ 0.50			$ 0.96
Diluted (in USD per unit)			$ 0.50			$ 0.96
[1]	llocation of net income (loss) attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.
[2]	(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		Aug. 12, 2016	Aug. 02, 2016	Jul. 22, 2016	Jul. 01, 2016	Mar. 04, 2016	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 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							7,506,520		7,506,520
Unit-based compensation awards(1)	[1]								38,012
Issuance of units under ATM Program									$ 321	$ 1
Distributions declared (including IDRs)							$ 231	$ 42	$ 431	$ 79
Limited Partners Common Units [Member]
Stockholders Equity Note [Line Items]
Units outstanding							331,283,429		331,283,429
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		79,466,136
Subsequent Event [Member]
Stockholders Equity Note [Line Items]
Distribution Made to Limited Partner, Declaration Date				Jul. 22, 2016
Cash distributions declared per limited partner common unit				$ 0.5100
Distributions declared (including IDRs)				$ 222
Distribution Made to Limited Partner, Distribution Date		Aug. 12, 2016
Distribution Made to Limited Partner, Date of Record			Aug. 2, 2016
ATM Program [Member]
Stockholders Equity Note [Line Items]
Equity Instrument Regulatory Filing Date						Mar. 4, 2016
Common Units Aggregate Value							$ 500		$ 500
Partners' Capital Account, Public Sale of Units Net of Offering Costs									$ 315
Class B Units [Member] | Subsequent Event [Member]
Stockholders Equity Note [Line Items]
Unit-based compensation awards(1)					3,990,878
Limited Partners Common Units [Member]
Stockholders Equity Note [Line Items]
Unit-based compensation awards(1)	[1]								37,251
Cash distributions declared per limited partner common unit							$ 0.5100	$ 0.4400	$ 1.0150	$ 0.8500
Distributions declared (including IDRs)	[2]						$ 172	$ 19	$ 328	$ 37
Limited Partners Common Units [Member] | MPC [Member]
Stockholders Equity Note [Line Items]
Units outstanding							79,000,000		79,000,000		57,000,000
Limited Partners Common Units [Member] | Subsequent Event [Member]
Stockholders Equity Note [Line Items]
Conversion of Stock, Shares Issued					4,350,057
General Partner Units [Member]
Stockholders Equity Note [Line Items]
Unit-based compensation awards(1)	[1]								761
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]	(2)As a result of the unit-based compensation awards issued during the period, MPLX GP contributed less than $1 million in exchange for 761 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)	6 Months Ended
	Jun. 30, 2016shares
Stockholders Equity [Line Items]
Balance at December 31, 2015	311,469,407
Units converted	38,012	[1]
Balance at June 30, 2016	346,771,705
Limited Partners Common Units [Member]
Stockholders Equity [Line Items]
Balance at December 31, 2015	296,687,176
Units converted	37,251	[1]
Balance at June 30, 2016	331,283,429
Class B Units [Member]
Stockholders Equity [Line Items]
Balance at December 31, 2015	7,981,756	[2]
Balance at June 30, 2016	7,981,756	[2]
General Partner Units [Member]
Stockholders Equity [Line Items]
Balance at December 31, 2015	6,800,475
Units converted	761	[1]
Balance at June 30, 2016	7,506,520
ATM Program [Member]
Stockholders Equity [Line Items]
Issuance of units under the ATM Program(3)	12,270,406	[3]
ATM Program [Member] | Limited Partners Common Units [Member]
Stockholders Equity [Line Items]
Issuance of units under the ATM Program(3)	12,025,000	[3]
ATM Program [Member] | General Partner Units [Member]
Stockholders Equity [Line Items]
Issuance of units under the ATM Program(3)	245,406	[3]
Hardin Street Marine [Member]
Stockholders Equity [Line Items]
Partners' Capital Account, Units, Acquisitions	22,993,880	[4]
Hardin Street Marine [Member] | Limited Partners Common Units [Member]
Stockholders Equity [Line Items]
Partners' Capital Account, Units, Acquisitions	22,534,002	[4]
Hardin Street Marine [Member] | General Partner Units [Member]
Stockholders Equity [Line Items]
Partners' Capital Account, Units, Acquisitions	459,878	[4]
[1]	(2)As a result of the unit-based compensation awards issued during the period, MPLX GP contributed less than $1 million in exchange for 761 general partner units to maintain its two percent general partner interest.
[2]	(1)On July 1, 2016, 3,990,878 Class B units converted to 4,350,057 common units and will be eligible to receive the second quarter distribution.
[3]	(3)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.
[4]	(4)See Note 3 for information regarding the HSM acquisition.

Equity - Net Income Allocation

Equity - Net Income Allocation (Details) - USD ($) $ in Millions		3 Months Ended			6 Months Ended
		Jun. 30, 2016	Jun. 30, 2015		Jun. 30, 2016	Jun. 30, 2015
Stockholders Equity [Line Items]
Net income attributable to MPLX LP	[1]	$ 19	$ 51	[2]	$ (41)	$ 97	[2]
Preferred unit distributions		231	42		431	79
General partner's incentive distribution rights and other		47	6		88	9
Net (loss) income attributable to MPLX LP available to general and limited partners		(37)	45		(138)	88
General partner's two percent interest in net (loss) income attributable to MPLX LP		(1)	1		(3)	2
Less: General partner’s interest in net income attributable to MPLX LP		46	7	[2]	85	11	[2]
Preferred Partner [Member]
Stockholders Equity [Line Items]
Net income attributable to MPLX LP	[1]	9			9
Preferred unit distributions	[3]	$ 9	$ 0		$ 9	$ 0
[1]	llocation of net income (loss) attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.
[2]	(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 - Cash Distributions (De

Equity - Cash Distributions (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Incentive Distribution Made to Managing Member or General Partner [Line Items]
General partner's distributions	$ 4	$ 1	$ 8	$ 2
General partner's incentive distribution rights	46	6	86	9
Total general partner's distributions	50	7	94	11
Partners' distributions	231	42	431	79
Limited Partners Common Units [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Partners' distributions	172	19	328	37
Subordinated Units [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Partners' distributions	0	16	0	31
Preferred Units [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Partners' distributions	9	0	9	0
Limited Partner [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Partners' distributions	$ 172	$ 35	$ 328	$ 68

Redeemable Preferred Units (Det

Redeemable Preferred Units (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		May 13, 2016	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Redeemable Noncontrolling Interest [Line Items]
Issuance of redeemable preferred units					$ 984	$ 0	[1]
Preferred unit distributions			$ 231	$ 42	431	79
Series A Convertible Preferred Units [Member]
Redeemable Noncontrolling Interest [Line Items]
Issuance of units under the ATM Program(3)		30.8
Preferred Stock, Dividend Rate, Percentage		6.50%
Issuance of redeemable preferred units		$ 984
Preferred Stock, Dividend Rate, Per-Dollar-Amount		$ 0.528125
Preferred Units, Description		The purchasers may convert their Preferred Units into common units, at any time after the third anniversary of the issuance date or prior to liquidation, dissolution or winding up of the Partnership, in full or in part, subject to minimum conversion amounts and conditions. After the fourth anniversary of the issuance date, the Partnership may convert the Preferred Units into common units at any time, in whole or in part, subject to certain minimum conversion amounts and conditions, if the closing price of MPLX common units is greater than $48.75 for the 20 day trading period immediately preceding the conversion notice date. The conversion rate for the Preferred Units shall be the quotient of (a) the sum of (i) $32.50, plus (ii) any unpaid cash distributions on the applicable Preferred Unit, divided by (b) $32.50. The holders of the Preferred Units are entitled to vote on an as-converted basis with the common unitholders and will have certain other class voting rights with respect to any amendment to the partnership agreement that would adversely affect any rights, preferences or privileges of the Preferred Units. In addition, upon certain events involving a change in control the holders of Preferred Units may elect, among other potential elections, to convert their Preferred Units to common units at the then change of control conversion rate.
Preferred Partner [Member]
Redeemable Noncontrolling Interest [Line Items]
Preferred unit distributions	[2]		$ 9	$ 0	$ 9	$ 0
Preferred Units [Member] | Series A Convertible Preferred Units [Member]
Redeemable Noncontrolling Interest [Line Items]
Shares Issued, Price Per Share		$ 32.50
[1]	(1)Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.
[2]	See Note 7 for distribution information.

Redeemable Preferred Units Rede

Redeemable Preferred Units Redeemable Preferred Units Rollforward (Details) - USD ($) $ in Millions	May 13, 2016	Jun. 30, 2016	Jun. 30, 2016	Jun. 30, 2015	[1]	Dec. 31, 2015
Redeemable Noncontrolling Interest [Line Items]
Issuance of MPLX LP redeemable preferred units on May 13, 2016			$ 984	$ 0
Net income allocated for May 13, 2016 through June 30, 2016		$ 9
Balance at June 30, 2016		$ 993	$ 993			$ 0
Series A Convertible Preferred Units [Member]
Redeemable Noncontrolling Interest [Line Items]
Issuance of MPLX LP redeemable preferred units on May 13, 2016	$ 984
[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 (Details)

Segment Information (Details)	Dec. 04, 2015	Jun. 30, 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			6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015		Jun. 30, 2016	Jun. 30, 2015
Segment Reporting Information [Line Items]
Segment revenues	$ 137	$ 0	[1]	$ 237	$ 0	[1]
Segment other income	1	2	[1]	3	3	[1]
Total segment revenues and other income	564	213	[1]	1,173	414	[1]
Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment revenues	723			1,413
Segment other income	18			37
Total segment revenues and other income	741	213		1,450	414
Segment cost of revenues	311			600
Segment operating income before portion attributable to noncontrolling interest	430			850
Segment portion attributable to noncontrolling interest and Predecessor	36	35		111	68
Segment operating income attributable to MPLX LP	394	88		739	170
Operating Segments [Member] | L&S [Member]
Segment Reporting Information [Line Items]
Segment revenues	193	193		385	376
Segment other income	18	20		37	38
Total segment revenues and other income	211	213		422	414
Segment cost of revenues	88	90		177	176
Segment operating income before portion attributable to noncontrolling interest	123	123		245	238
Segment portion attributable to noncontrolling interest and Predecessor	0	35		34	68
Segment operating income attributable to MPLX LP	123	$ 88		211	$ 170
Operating Segments [Member] | G&P [Member]
Segment Reporting Information [Line Items]
Segment revenues	530			1,028
Segment other income	0			0
Total segment revenues and other income	530			1,028
Segment cost of revenues	223			423
Segment operating income before portion attributable to noncontrolling interest	307			605
Segment portion attributable to noncontrolling interest and Predecessor	36			77
Segment operating income attributable to MPLX LP	$ 271			$ 528
[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				6 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2015		Jun. 30, 2016		Jun. 30, 2015
Segment Reporting, Revenue Reconciling Item [Line Items]
Loss from equity method investments	$ (83)		$ 0	[1]	$ (78)	[2]	$ 0	[1],[3]
Other income - related parties	28		18	[1]	52		35	[1]
Impairment expense	(1)	$ (129)	0	[1]	(130)		0	[1],[3]
Depreciation and amortization	(137)		(20)	[1]	(269)		(39)	[1],[3]
General and administrative expenses	(49)		(21)	[1]	(101)		(43)	[1]
Income from operations	76		82	[1]	103		156	[1]
Operating Segments [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Segment operating income attributable to MPLX LP	394		88		739		170
Segment portion attributable to noncontrolling interest and Predecessor	36		35		111		68
Segment Reconciling Items [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Segment portion attributable to unconsolidated affiliates	(83)		0		(166)		0
Loss from equity method investments	(83)		0		(78)		0
Other income - related parties	11		0		18		0
Unrealized derivative losses	$ (12)		$ 0		$ (21)		$ 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]	(2)Income (loss) from equity method investments includes the impact of any basis differential amortization or accretion.
[3]	(1)Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.

Segment Information - Reconci77

Segment Information - Reconciliation to Total Revenues and Other Income (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015		Jun. 30, 2016		Jun. 30, 2015
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues and other income	$ 564	$ 213	[1]	$ 1,173		$ 414	[1]
Loss from equity method investments	(83)	0	[1]	(78)	[2]	0	[1],[3]
Other income - related parties	28	18	[1]	52		35	[1]
Operating Segments [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues and other income	741	213		1,450		414
Segment Reconciling Items [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue adjustment from unconsolidated affiliates	(99)	0		(203)		0
Loss from equity method investments	(83)	0		(78)		0
Other income - related parties	11	0		18		0
Unrealized derivative loss	$ (6)	$ 0		$ (14)		$ 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]	(2)Income (loss) from equity method investments includes the impact of any basis differential amortization or accretion.
[3]	(1)Financial information has been retrospectively adjusted for the acquisition of Hardin Street Marine LLC from MPC. See Notes 1 and 3.

Segment Information - Reconci78

Segment Information - Reconciliation to Net Income Attributable to Noncontrolling Interests (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Reconciliation to Net Income Attributable to Noncontrolling Interests [Line Items]
Net income attributable to noncontrolling interests and Predecessor	$ 1	$ 25	$ 24	$ 47
Operating Segments [Member]
Reconciliation to Net Income Attributable to Noncontrolling Interests [Line Items]
Segment portion attributable to noncontrolling interest and Predecessor	36	35	111	68
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	(56)	(10)	(85)	(21)
Portion of operating income attributable to noncontrolling interest of unconsolidated affiliates	$ 21	$ 0	$ (2)	$ 0

Segment Information - Reconci79

Segment Information - Reconciliation of Capital Expenditures (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Segment Reporting, Other Significant Reconciling Item [Line Items]
Payments to Acquire Property, Plant, and Equipment		$ (278)	$ (35)	$ (569)	$ (70)	[1]
Operating Segments [Member]
Segment Reporting, Other Significant Reconciling Item [Line Items]
Payments to Acquire Property, Plant, and Equipment		(294)	(35)	(629)	(70)
Operating Segments [Member] | L&S [Member]
Segment Reporting, Other Significant Reconciling Item [Line Items]
Payments to Acquire Property, Plant, and Equipment		(82)	(35)	(144)	(70)
Operating Segments [Member] | G&P [Member]
Segment Reporting, Other Significant Reconciling Item [Line Items]
Payments to Acquire Property, Plant, and Equipment	[2]	(212)	0	(485)	0
Segment Reconciling Items [Member]
Segment Reporting, Other Significant Reconciling Item [Line Items]
Payments to Acquire Property, Plant, and Equipment		$ 16	$ 0	$ 60	$ 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 $16 million and $60 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	Jun. 30, 2016	Dec. 31, 2015
Segment Reporting, Other Significant Reconciling Item [Line Items]
Assets	$ 16,079	$ 16,104
L&S [Member] | Operating Segments [Member]
Segment Reporting, Other Significant Reconciling Item [Line Items]
Assets	1,952	1,858
G&P [Member] | Operating Segments [Member]
Segment Reporting, Other Significant Reconciling Item [Line Items]
Assets	$ 14,127	$ 14,246

Income Taxes (Details)

Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	[1]	Jun. 30, 2016	[2]	Jun. 30, 2015
Income Taxes [Line Items]
Benefit for income taxes	$ 8	$ 0		$ 12		$ 0	[1]
Maximum [Member]
Income Taxes [Line Items]
Benefit for income taxes	$ (1)					$ (1)
[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 HSM from MPC. See Notes 1 and 3. Prior to this acquisition, MPC paid all income taxes related to HSM.

Income Taxes Income Taxes - Rec

Income Taxes Income Taxes - Reconciliation of Federal Statutory Income Tax Rate (Details) - USD ($) $ in Millions		3 Months Ended			6 Months Ended
		Jun. 30, 2016	Jun. 30, 2015	[1]	Jun. 30, 2016		Jun. 30, 2015	[1]
Entity Information [Line Items]
Income before income taxes		$ 12	$ 76		$ (29)	[2]	$ 144
Federal statutory rate					35.00%
Federal income tax at statutory rate(2)	[2],[3]				$ (12)
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount	[2]				(1)
State income taxes net of federal benefit - MarkWest Hydrocarbon	[2]				(1)
Provision on income from Class A units(2)	[2],[3]				0
Other	[2]				2
(Benefit) provision for income tax		$ (8)	$ 0		(12)	[2]	$ 0
Reportable Legal Entities [Member] | Mark West Hydrocarbon [Member]
Entity Information [Line Items]
Income before income taxes					(35)
Federal income tax at statutory rate(2)	[3]				(12)
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount					(1)
State income taxes net of federal benefit - MarkWest Hydrocarbon					(1)
Provision on income from Class A units(2)	[3]				0
Other					1
(Benefit) provision for income tax					(13)
Reportable Legal Entities [Member] | Partnership [Member]
Entity Information [Line Items]
Income before income taxes					4
Federal income tax at statutory rate(2)	[3]				0
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount					0
State income taxes net of federal benefit - MarkWest Hydrocarbon					0
Provision on income from Class A units(2)	[3]				0
Other					1
(Benefit) provision for income tax					1
Eliminations [Member]
Entity Information [Line Items]
Income before income taxes					2
Federal income tax at statutory rate(2)	[3]				0
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount					0
State income taxes net of federal benefit - MarkWest Hydrocarbon					0
Provision on income from Class A units(2)	[3]				0
Other					0
(Benefit) provision for income tax					$ 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 HSM from MPC. See Notes 1 and 3. Prior to this acquisition, MPC paid all income taxes related to HSM.
[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	Jun. 30, 2016	Dec. 31, 2015
Inventory Disclosure [Abstract]
NGLs	$ 2	$ 3
Line fill	6	5
Spare parts, materials and supplies	41	43
Inventories	$ 49	$ 51

Property, Plant and Equipment -

Property, Plant and Equipment - Property, Plant and Equipment (Detail) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	$ 11,253	$ 10,647
Less accumulated depreciation	893	650
Net property, plant and equipment	10,360	9,997
Gas Gathering And Transmission Equipment And Facilities [Member]
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	4,573	4,307
Processing, Fractionation And Storage Facilities [Member]
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	3,456	3,185
Pipelines and Related Assets [Member]
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	1,186	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	662	606
Assets Under Construction [Member]
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	$ 898	$ 946

Fair Value Measurements - Recur

Fair Value Measurements - Recurring - Financial Instruments by Valuation Hierarchy (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross Amount	$ 3
Gross Amount	(47)
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross Amount	1	$ 9
Gross Amount	(45)	(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	1	0
Gross Amount	(41)	(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	0	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	0	7
Gross Amount	$ (4)	$ 0

Fair Value Measurments - Recurr

Fair Value Measurments - Recurring - Significant Unobservable Inputs in Level 3 Valuation (Details) - Fair Value, Inputs, Level 3 [Member]	6 Months Ended
	Jun. 30, 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] | ERCOT Pricing [Member] | Embedded derivatives in commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Commodity Price	$ 26.47
Minimum [Member] | Liability [Member] | Ethanol prices [Member] | Commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price	0.24	[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	0.52	[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	0.52	[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	0.69	[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	0.67	[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	0.63	[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	0.62	[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	0.99	[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	0.99	[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	2.61	[3]
Maximum [Member] | Assets [Member] | ERCOT Pricing [Member] | Embedded derivatives in commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Commodity Price	57.95
Maximum [Member] | Liability [Member] | Ethanol prices [Member] | Commodity contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Forward Commodity Price	0.28	[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	0.57	[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	0.58	[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	0.72	[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	0.73	[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	0.69	[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	0.71	[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	1.03	[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	1.10	[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	$ 3.35	[3]
[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 - Rec87

Fair Value Measurements - Recurring - Changes in Level 3 Measurements (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016
Commodity Derivative Contracts (net)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value at beginning of period		$ 0		$ 7
Total loss (realized and unrealized) included in earnings(1)	[1]	(6)		(7)
Settlements		1		(5)
Netting adjustment (2)	[1]	1		1
Fair value at end of period		(4)		(4)
Fair Value Measurement with Unobservable Inputs Reconciliation, Recurring Basis Asset and Liabilities Net, Change in Unrealized Gain (Loss) Included in Earnings Contract Held		(5)		(6)
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		(34)		(32)
Total loss (realized and unrealized) included in earnings(1)	[1]	(7)		(11)
Settlements		1		3
Netting adjustment (2)	[1]	0		0
Fair value at end of period		$ (40)		(40)
Fair Value Measurement with Unobservable Inputs Reconciliation, Recurring Basis Asset and Liabilities Net, Change in Unrealized Gain (Loss) Included in Earnings Contract Held			$ (8)	$ (11)
[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)	Jun. 30, 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	Jun. 30, 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	$ 4,400	$ 5,255
Other Liabilities, Fair Value Disclosure	98	100
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	4,748	5,212
Other Liabilities, Fair Value Disclosure	$ 105	$ 99
MarkWest [Member] | Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Gross			$ 4,100

Derivative Financial Instrume90

Derivative Financial Instruments - Volume of Commodity Derivative Activity (Details) - Not Designated as Hedging Instrument [Member] - Commodity contracts	Jun. 30, 2016MMBTUbblgal
Short [Member]
Derivative [Line Items]
Nonmonetary Notional Amount of Price Risk Derivatives of Crude Oil | bbl	184,000
Nonmonetary Notional Amount of Price Risk Derivatives of Natural Gas Liquids | gal	64,810,176
Long [Member]
Derivative [Line Items]
Nonmonetary Notional Amount Of Price Risk Derivatives Of Natural Gas | MMBTU	1,088,484

Derivative Financial Instrume91

Derivative Financial Instruments - Embedded Derivatives in Commodity Contracts (Details) - Embedded derivatives in commodity contracts $ in Millions	6 Months Ended
	Jun. 30, 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	$ 41
Electricity [Member]
Derivative [Line Items]
Embedded Derivative, Fair Value of Embedded Derivative Liability	$ 1

Derivative Financial Instrume92

Derivative Financial Instruments - Derivatives Balance Sheet Location (Details) - Commodity contracts - USD ($) $ in Millions		Jun. 30, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	$ 1	$ 9
Derivative Liability, Fair Value, Gross Liability	[1]	45	32
Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	1	9
Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	[1]	9	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]	$ 36	$ 27
[1]	(1)Includes embedded derivatives in commodity contracts as discussed above.

Derivative Financial Instrume93

Derivative Financial Instruments - Net Derivatives (Details) $ in Millions	Jun. 30, 2016USD ($)
Assets
Gross Amount	$ 3
Gross Amounts Offset in the Consolidated Balance Sheets	(2)
Net Amount of Assets in the Consolidated Balance Sheets	1
Liabilities
Gross Amount	(47)
Gross Amounts Offset in the Consolidated Balance Sheets	2
Net Amount of Liabilities in the Consolidated Balance Sheets	(45)
Current Assets [Member] | Commodity contracts
Assets
Gross Amount	2
Gross Amounts Offset in the Consolidated Balance Sheets	(2)
Net Amount of Assets in the Consolidated Balance Sheets	0
Current Liabilities [Member] | Commodity contracts
Liabilities
Gross Amount	(6)
Gross Amounts Offset in the Consolidated Balance Sheets	2
Net Amount of Liabilities in the Consolidated Balance Sheets	(4)
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	(2)
Net Amount of Assets in the Consolidated Balance Sheets	1
Not Designated as Hedging Instrument [Member] | Current Assets [Member] | Embedded derivatives in commodity contracts
Assets
Gross Amount	1
Gross Amounts Offset in the Consolidated Balance Sheets	0
Net Amount of Assets in the Consolidated Balance Sheets	1
Not Designated as Hedging Instrument [Member] | Current Liabilities [Member]
Liabilities
Gross Amount	(11)
Gross Amounts Offset in the Consolidated Balance Sheets	2
Net Amount of Liabilities in the Consolidated Balance Sheets	(9)
Not Designated as Hedging Instrument [Member] | Current Liabilities [Member] | Embedded derivatives in commodity contracts
Liabilities
Gross Amount	(5)
Gross Amounts Offset in the Consolidated Balance Sheets	0
Net Amount of Liabilities in the Consolidated Balance Sheets	(5)
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	(36)
Gross Amounts Offset in the Consolidated Balance Sheets	0
Net Amount of Liabilities in the Consolidated Balance Sheets	(36)
Not Designated as Hedging Instrument [Member] | Non Current Liabilities [Member] | Embedded derivatives in commodity contracts
Liabilities
Gross Amount	(36)
Gross Amounts Offset in the Consolidated Balance Sheets	0
Net Amount of Liabilities in the Consolidated Balance Sheets	$ (36)

Derivatives Financial Instrumen

Derivatives Financial Instruments - Derivative Income Statement Location (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2016
Derivative [Line Items]
Total gain (loss)	$ (13)	$ (15)
Product Sales [Member]
Derivative [Line Items]
Realized gain (loss)	(1)	6
Unrealized gain (loss)	6	14
Total gain (loss)	(7)	(8)
Purchased product costs [Member]
Derivative [Line Items]
Unrealized gain (loss)	8	9
Cost of Sales [Member]
Derivative [Line Items]
Unrealized gain (loss)	$ 2	$ 2

Debt - Summary of Outstanding B

Debt - Summary of Outstanding Borrowings (Detail) - USD ($) $ in Millions		Jun. 30, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Total		$ 4,859	$ 5,736
Unamortized debt issuance costs		(8)	(8)
Unamortized discount(1)	[1]	(450)	(472)
Amounts due within one year		(1)	(1)
Total long-term debt due after one year		4,400	5,255
Senior Notes [Member] | MarkWest [Member]
Debt Instrument [Line Items]
Unamortized discount(1)			(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		0	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 $442 million and $465 million discount as of June 30, 2016 and December 31, 2015, respectively, related to the difference between the fair value and the principal amount of the assumed MarkWest debt.

Debt - Summary of Outstanding96

Debt - Summary of Outstanding Borrowings Interest Rates and Table Due Dates (Details)	6 Months Ended
	Jun. 30, 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	6 Months Ended
	Jun. 30, 2016	Dec. 31, 2015
MPLX Revolver [Member]
Debt Instrument [Line Items]
Proceeds from Lines of Credit	$ 434
Debt Instrument, Interest Rate, Effective Percentage	1.899%
Repayments of Long-term Lines of Credit	$ 1,300
Letters of Credit Outstanding, Amount	8
Line of Credit Facility, Remaining Borrowing Capacity	$ 1,990
Line of Credit Facility, Remaining Borrowing Capacity, Percentage	99.60%
MPLX Term Loan [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage	1.931%
Line of Credit [Member] | MPLX Revolver [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 0	$ 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	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2015	[1]	Jun. 30, 2016	Jun. 30, 2015	[1],[2]
Goodwill [Line Items]
Number of Reporting Units						3
Number of Reporting Units Impaired						2
Impairment expense		$ 1	$ 129	$ 0		$ 130	$ 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) - USD ($) $ in Millions		6 Months Ended
		Jun. 30, 2016	Dec. 31, 2015
Goodwill [Line Items]
Goodwill, Gross		$ 2,329	$ 2,570
Accumulated impairment losses		(130)	0
Balance as of December 31, 2015		2,570
Purchase price allocation adjustments(1)	[1]	(241)
Impairment losses		(130)
Balance as of June 30, 2016		2,199
L&S [Member]
Goodwill [Line Items]
Goodwill, Gross		116	116
Accumulated impairment losses		0	0
Balance as of December 31, 2015		116
Purchase price allocation adjustments(1)	[1]	0
Impairment losses		0
Balance as of June 30, 2016		116
G&P [Member]
Goodwill [Line Items]
Goodwill, Gross		2,213	2,454
Accumulated impairment losses		(130)	$ 0
Balance as of December 31, 2015		2,454
Purchase price allocation adjustments(1)	[1]	(241)
Impairment losses		(130)
Balance as of June 30, 2016		$ 2,083
[1]	(1)See Note 3 for further discussion on purchase price allocation adjustments.

Supplemental Cash Flow Infor100

Supplemental Cash Flow Information - Summary of Supplemental Cash Flow Information (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Net cash provided by operating activities included:
Interest paid (net of amounts capitalized)	$ 109	$ 2
Non-cash investing and financing activities:
Net transfers of property, plant and equipment from materials and supplies inventories	$ (5)	$ 0

Supplemental Cash Flow Infor101

Supplemental Cash Flow Information - Summary of Reconciliation of Additions to Property, Plant and Equipment to Total Capital Expenditures (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Supplemental Cash Flow Elements [Abstract]
(Decrease) increase in capital accruals	$ (6)	$ 13

Supplemental Cash Flow Infor102

Supplemental Cash Flow Information Supplemental Cash Flow Information - Narrative (Details) $ in Millions	6 Months Ended
	Jun. 30, 2016USD ($)
Supplemental Cash Flow Elements [Abstract]
Contribution from MPC	$ 15

Equity-Based Compensation Pl103

Equity-Based Compensation Plan - Summary of Phantom Unit Award Activity (Details) - Phantom Units [Member]	6 Months Ended
	Jun. 30, 2016$ / sharesshares
Number of Units
Outstanding at December 31, 2015 | shares	1,031,219
Granted | shares	445,555
Settled | shares	43,660
Forfeited | shares	19,490
Outstanding at June 30, 2016 | shares	1,413,624
Weighted Average Fair Value
Outstanding at December 31, 2015 | $ / shares	$ 35.49
Granted (in USD per unit) | $ / shares	29.31
Settled (in USD per unit) | $ / shares	51.21
Forfeited (in USD per unit) | $ / shares	31.36
Outstanding at June 30, 2016 | $ / shares	$ 33.11

Equity-Based Compensation Pl104

Equity-Based Compensation Plan - Additional Information (Detail) - Performance Shares [Member] - MPLX LP 2012 Incentive Compensation Plan [Member]	6 Months Ended
	Jun. 30, 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 Pl105

Equity-Based Compensation Plan - Summary of Performance Unit Award Activity (Detail) - Performance Shares [Member]	6 Months Ended
	Jun. 30, 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	789,375
Settled	458,011
Forfeited	(53,507)
Outstanding at June 30, 2016	1,799,249

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2016	Dec. 31, 2015
Commitments And Contingencies [Line Items]
Accrued liabilities for remediation	$ 7	$ 1
Contractual commitments to acquire property, plant and equipment	190
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