MPLX (MPLX) 10-Q 2 Nov 15 2015 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2015	Oct. 30, 2015
Document Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2015
Document Fiscal Year Focus	2,015
Document Fiscal Period Focus	Q3
Trading Symbol	MPLX
Entity Registrant Name	MPLX LP
Entity Central Index Key	1,552,000
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Common Units [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		80,336,711
General Partner Units [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		1,639,525

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) shares in Millions, $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Revenues and other income:
Sales and other operating revenues		$ 18.2	$ 16.6	$ 50.2	$ 52.2
Sales to related parties		122.7	114.1	360.7	336
Loss on sale of assets		0	0	(0.2)	0
Other income		1.4	1.5	4.2	4.1
Other income - related parties		6.3	5.8	18.8	16.9
Total revenues and other income		148.6	138	433.7	409.2
Costs and expenses:
Cost of revenues (excludes items below)		41.9	40.6	101.4	102.9
Purchases from related parties		26.5	23.7	75.1	71.4
Depreciation		12.7	12.5	38.1	37.5
General and administrative expenses		21.1	15.3	57.7	47.1
Other taxes		(0.7)	1.7	5.5	5.5
Total costs and expenses		101.5	93.8	277.8	264.4
Income from operations		47.1	44.2	155.9	144.8
Net interest and other financial costs		5.2	1.1	16.7	3
Income before income taxes		41.9	43.1	139.2	141.8
Provision for income taxes		(0.1)	0	(0.1)	(0.1)
Net income		41.8	43.1	139.1	141.7
Less: Net income attributable to MPC-retained interest		0.3	14	0.8	49.6
Net income attributable to MPLX LP	[1]	41.5	29.1	138.3	92.1
Less: General partner’s interest in net income attributable to MPLX LP		8.6	1.5	19.4	3.7
Limited partners’ interest in net income attributable to MPLX LP		$ 32.9	$ 27.6	$ 118.9	$ 88.4
Weighted average limited partner units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic		82	75.5	82	75.4
Weighted Average Limited Partnership Units Outstanding, Diluted		82	75.5	82	75.5
MPC [Member]
Revenues and other income:
Sales to related parties		$ 122.7	$ 114.1	$ 360.7	$ 336
Other income - related parties		6.1	5.5	17.9	16
Limited Partners Common Units [Member]
Costs and expenses:
Net income attributable to MPLX LP	[1]	$ 32.8	$ 13.7	$ 79.2	$ 42.8
Per Unit Data (See Note 5)
Common - basic (in USD per unit)		$ 0.41	$ 0.37	$ 1.42	$ 1.16
Common - diluted (in USD per unit)		$ 0.41	$ 0.37	$ 1.42	$ 1.16
Weighted average limited partner units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic		80.4	37.1	55.9	37
Weighted Average Limited Partnership Units Outstanding, Diluted		80.4	37.1	55.9	37.1
Cash distributions declared per limited partner common unit		$ 0.4700	$ 0.3575	$ 1.3200	$ 1.0275
Limited Partners Subordinated Units [Member] | MPC [Member]
Costs and expenses:
Net income attributable to MPLX LP	[1]	$ 0	$ 13.6	$ 33.5	$ 42.7
Per Unit Data (See Note 5)
Common - basic (in USD per unit)		$ 0	$ 0.37	$ 1.36	$ 1.16
Common - diluted (in USD per unit)		0	0.37	1.36	1.16
Subordinated - basic and diluted (in USD per unit)		$ 0	$ 0.37	$ 1.36	$ 1.16
Weighted average limited partner units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic		0	37	24.5	37
Weighted Average Limited Partnership Units Outstanding, Diluted		0	37	24.5	37
Weighted average limited partner units outstanding basic and diluted (in shares)		0	37	24.5	37
[1]	Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 90.4	$ 27.3
Receivables	11.8	10.2
Receivables from related parties	54.9	41
Materials and supplies inventories	11.8	11.7
Other current assets	4.1	7
Total current assets	173	97.2
Property, plant and equipment, net	1,109.2	1,008.6
Goodwill	104.7	104.7
Other noncurrent assets	4.2	4
Total assets	1,391.1	1,214.5
Current liabilities:
Accounts payable	67.6	42.2
Payables to related parties	25.1	20.2
Deferred revenue - related parties	30.5	30.5
Accrued taxes	6.3	5.2
Long-term debt due within one year	0.8	0.8
Accrued interest payable	2.5	0.6
Other current liabilities	2.7	1.1
Total current liabilities	135.5	100.6
Long-term deferred revenue - related parties	8.4	4
Long-term debt	752.5	644
Deferred credits and other liabilities	1.6	2.2
Total liabilities	$ 898	$ 750.8
Commitments and contingencies (see Note 13)
Equity
Total MPLX LP partners’ capital	$ 487.3	$ 458.2
Noncontrolling interest retained by MPC	5.8	5.5
Total equity	493.1	463.7
Total liabilities and equity	1,391.1	1,214.5
MPC [Member]
Current assets:
Receivables from related parties	54.3	40.5
Current liabilities:
Payables to related parties	24.6	20.1
Limited Partners Common Units [Member] | MPC [Member]
Equity
Total MPLX LP partners’ capital	492.7	261.1
Limited Partners Subordinated Units [Member] | MPC [Member]
Equity
Total MPLX LP partners’ capital	0	217.5
General Partner [Member] | MPC [Member]
Equity
Total MPLX LP partners’ capital	(652.5)	(659.4)
Public [Member] | Limited Partners Common Units [Member]
Equity
Total MPLX LP partners’ capital	$ 647.1	$ 639

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - shares	Sep. 30, 2015	Dec. 31, 2014
Units outstanding	1,639,525
Limited Partners Common Units [Member] | Public [Member]
Units issued	23,400,000	23,400,000
Units outstanding	23,400,000	23,400,000
MPC [Member] | Limited Partners Common Units [Member]
Units issued	57,000,000	20,000,000
Units outstanding	57,000,000	20,000,000
MPC [Member] | Limited Partners Subordinated Units [Member]
Units issued	0	37,000,000
Units outstanding	0	37,000,000
MPC [Member] | General Partner [Member]
Units issued	1,600,000	1,600,000
Units outstanding	1,600,000	1,600,000

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Operating activities:
Net income	$ 139.1	$ 141.7
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	38.1	37.5
Asset retirement expenditures	(0.7)	(1)
Net loss on disposal of assets	(0.2)	0
Changes in:
Current receivables	(1.6)	3.3
Materials and supplies inventories	(0.1)	0.7
Current accounts payable and accrued liabilities	12.7	0.9
Receivables from / liabilities to related parties	(4.6)	7.2
All other, net	1.8	0
Net cash provided by operating activities	184.9	190.3
Investing activities:
Additions to property, plant and equipment	(121.1)	(47.1)
Disposal of assets	0.4	0
All other, net	1.6	4
Net cash used in investing activities	(119.1)	(43.1)
Financing activities:
Long-term debt - borrowings	528.2	270
Long-term debt - repayments	(415.7)	(15.6)
Debt issuance costs	4.4	0
Net proceeds from equity offerings	1.3	0
Quarterly distributions to unitholders and general partner	(111.6)	(75.2)
Quarterly distributions to noncontrolling interest retained by MPC	(0.5)	(38.3)
Distributions related to purchase of additional interest in Pipe Line Holdings	0	(310)
Net cash used in financing activities	(2.7)	(169.1)
Net increase (decrease) in cash and cash equivalents	63.1	(21.9)
Cash and cash equivalents at beginning of period	27.3	54.1
Cash and cash equivalents at end of period	$ 90.4	$ 32.2

Consolidated Statements of Equi

Consolidated Statements of Equity - USD ($) $ in Millions	Total	Limited Partners Common Units [Member]Public [Member]	Limited Partners Common Units [Member]MPC [Member]	Limited Partners Subordinated Units [Member]MPC [Member]	General Partner [Member]MPC [Member]	Noncontrolling Interest Retained by MPC [Member]MPC [Member]
Beginning Balance at Dec. 31, 2013	$ 1,114.1	$ 412	$ 57.4	$ 209.3	$ (32.5)	$ 467.9
Purchase of additional interest in Pipe Line Holdings	(310)				(172.5)	(137.5)
Net income	141.7	23.8	20.4	44.2	3.7	49.6
Quarterly distributions to unitholders and general partner	(75.2)	(19.6)	(16.8)	(36.4)	(2.4)
Quarterly distributions to noncontrolling interest retained by MPC	(38.3)					(38.3)
Non-cash contribution from MPC	0.1					0.1
Equity-based compensation	0.9	0.9
Ending Balance at Sep. 30, 2014	833.3	417.1	61	217.1	(203.7)	341.8
Beginning Balance at Dec. 31, 2014	463.7	639	261.1	217.5	(659.4)	5.5
Issuance of units under ATM program	1.3	1.3
Net income	139.1	34.6	36.2	48.1	19.4	0.8
Quarterly distributions to unitholders and general partner	(111.6)	(28.9)	(24.7)	(45.5)	(12.5)
Quarterly distributions to noncontrolling interest retained by MPC	(0.5)					(0.5)
Subordinated unit conversion	0		220.1	(220.1)
Equity-based compensation	1.1	1.1
Ending Balance at Sep. 30, 2015	$ 493.1	$ 647.1	$ 492.7	$ 0	$ (652.5)	$ 5.8

Description of the Business and

Description of the Business and Basis of Presentation	9 Months Ended
	Sep. 30, 2015
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 fee-based, growth-oriented master limited partnership formed to own, operate, develop and acquire pipelines and other midstream assets related to the transportation and storage of crude oil, refined products and other hydrocarbon-based products. As of September 30, 2015 , our assets consisted of a 99.5 percent indirect interest in a network of common carrier crude oil and product pipeline systems and associated storage assets in the Midwest and Gulf Coast regions of the United States. We also own a 100 percent interest in a butane cavern in Neal, West Virginia with approximately one million barrels of natural gas liquids storage capacity. Our operations consist of one reportable segment. Basis of Presentation —Our financial position, results of operations and cash flows consist of consolidated MPLX LP activities and balances. All significant intercompany transactions and accounts have been eliminated. The accompanying interim consolidated financial statements are unaudited; however, in the opinion of our 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 Securities and Exchange Commission(“SEC”) applicable to interim period financial statements and do not include all of the information and disclosures required by accounting principles generally accepted in the United States (“U.S. GAAP”) for complete financial statements. These interim consolidated financial statements should be read in conjunction with the audited consolidated financial statements and notes thereto included in our Annual Report on Form 10-K for the year ended December 31, 2014 . The results of operations for the three months and nine months ended September 30, 2015 are not necessarily indicative of the results to be expected for the full year. In preparing our consolidated statements of equity, net income attributable to MPLX LP is allocated to unitholders in accordance with their respective ownership percentages. However, when distributions related to the incentive distribution rights are made, earnings equal to the amount of those distributions are first allocated to the general partner before the remaining earnings are allocated to the unitholders based on their respective ownership percentages. The allocation of net income attributable to MPLX LP for purposes of calculating net income per limited partner unit is described in Note 5 .

Accounting Standards

Accounting Standards	9 Months Ended
	Sep. 30, 2015
Accounting Standards [Abstract]
Accounting Standards	Accounting Standards Recently Adopted In April 2015, the Financial Accounting Standards Board (“FASB”) issued an accounting standards update to simplify the presentation of debt issuance costs. The update requires that debt issue costs for term debt are to be presented on the balance sheet as a direct reduction of the term debt liability as opposed to a deferred charge within other noncurrent assets. The change is effective for fiscal years and interim periods within those fiscal years beginning after December 15, 2015. Retrospective application is required and early adoption is permitted. Our early adoption of this standard in the second quarter of 2015 did not have a material impact on our consolidated results of operations, financial position or cash flows. In August 2015, the FASB subsequently issued a clarification as to the handling of debt issuance costs related to line-of-credit arrangements that allows for the presentation of these costs as an asset which would be amortized over the term of the line-of-credit arrangements. This clarification did not have any impact on our consolidated results of operations, financial position or cash flows. In April 2014, the FASB issued an accounting standards update that redefines the criteria for determining discontinued operations and introduces new disclosures related to these disposals. The updated definition of a discontinued operation is the disposal of a component (or components) of an entity or the classification of a component (or components) of an entity as held for sale that represents a strategic shift for an entity and has (or will have) a major impact on an entity’s operations and financial results. The standard requires disclosure of additional financial information for discontinued operations and individually material components not qualifying for discontinued operation presentation, as well as information regarding an entity’s continuing involvement with the discontinued operation. The accounting standards update was effective prospectively for annual periods beginning on or after December 15, 2014, and interim periods within those years. Adoption of this standards update in the first quarter of 2015 did not impact our consolidated results of operations, financial position or cash flows. Not Yet Adopted In September 2015, the FASB issued an accounting standards update that eliminates the requirement to restate prior period financial statements for measurement period adjustments. This update requires that the cumulative impact of a measurement period adjustment be recognized in the reporting period in which the adjustment is identified. The standard is effective for interim and annual periods beginning after December 15, 2015 with early application permitted. Adoption of this standard is not expected to have a material impact on our consolidated results of operations, financial position or cash flows. In April 2015, the FASB issued an accounting standards update requiring that the earnings of transferred net assets prior to the dropdown date of the net assets to a master limited partnership be allocated entirely to the general partner when calculating earnings per unit under the two class method. Under this guidance, previously reported earnings per unit of the limited partners will not change as a result of a dropdown transaction. The change is effective for fiscal years and interim periods within those fiscal years beginning after December 15, 2015. Retrospective application is required and early adoption is permitted. In April 2015, the FASB issued an accounting standards update clarifying whether a customer should account for a cloud computing arrangement as an acquisition of a software license or as a service arrangement by providing characteristics that a cloud computing arrangement must have in order to be accounted for as a software license acquisition. The change is effective for fiscal years and interim periods within those fiscal years beginning after December 15, 2015. Retrospective or prospective application is allowed and early adoption is permitted. Adoption of this standard is not expected to have a material impact on our consolidated results of operations, financial position or cash flows. In February 2015, the FASB issued an accounting standards update making targeted changes to the current consolidation guidance. The new standard changes the considerations related to substantive rights, related parties, and decision making fees when applying the variable interest entity consolidation model and eliminates certain guidance for limited partnerships and similar entities under the voting interest consolidation model. The update is effective for fiscal years and interim periods within those fiscal years beginning after December 15, 2015. Early adoption is permitted. Adoption of this standard is not expected to have a material impact on our consolidated results of operations, financial position or cash flows. In August 2014, the FASB issued an accounting standards update requiring management to assess an entity’s ability to continue as a going concern and to provide related footnote disclosures in certain circumstances. Management will be required to assess if there is substantial doubt about an entity’s ability to continue as a going concern within one year after the date that the financial statements are issued. Disclosures will be required if conditions give rise to substantial doubt and the type of disclosure will be determined based on whether management’s plans will be able to alleviate the substantial doubt. The accounting standards update will be effective for the first annual period ending after December 15, 2016, and for annual periods and interim periods thereafter with early application permitted. In May 2014, the FASB issued an accounting standards update for revenue recognition that is aligned with the International Accounting Standards Board’s revenue recognition standard. The guidance in the update states that revenue is recognized when a customer obtains control of a good or service. Recognition of the revenue will involve a multiple step approach including identifying the contract, identifying the separate performance obligations, determining the transaction price, allocating the price to the performance obligations and then recognizing the revenue as the obligations are satisfied. Additional disclosures will be required to provide adequate information to understand the nature, amount, timing and uncertainty of reported revenues and revenues expected to be recognized. The accounting standards update will be effective on a retrospective or modified retrospective basis for annual reporting periods beginning after December 15, 2017, and interim periods within those years, with early adoption permitted no earlier than January 1, 2017. We are in the process of determining the impact of the new standard on our consolidated financial statements.

Acquisitions

Acquisitions	9 Months Ended
	Sep. 30, 2015
Equity [Abstract]
Acquisitions	Acquisitions Pending Merger with Markwest Energy Partners, L.P. On July 11, 2015 , MPLX LP and MarkWest Energy Partners, L.P. (“MWE”) entered into a definitive merger agreement (the “Merger Agreement”) whereby MWE would become a wholly owned subsidiary of MPLX LP (the “Merger”). The Merger is structured as a unit-for-unit transaction plus a one-time cash payment to MWE unitholders. At the effective time of the Merger, each common unit of MWE (each, a “Common Unit”) issued and outstanding will be converted into the right to receive 1.09 common units of MPLX LP representing limited partner interests in MPLX LP and cash in an amount obtained by dividing $675.0 million by the number of Common Units (including certain converted equity awards) plus the number of Class B Units of MWE. The Class A Units of MWE, which are all owned by wholly owned subsidiaries of MWE, issued and outstanding immediately prior to the effective time of the Merger will be converted into newly created Class A Units of MPLX LP in accordance with the Class A exchange ratio provided in the Merger Agreement. Each Class B Unit of MWE issued and outstanding immediately prior to the effective time of the Merger will be converted into a newly created Class B Unit of MPLX LP. As of September 30, 2015 , the implied total enterprise value for MWE was approximately $13.7 billion , including the assumption of debt of approximately $4.6 billion . MPC would contribute $675.0 million of cash to us to fund the one-time cash payment to MWE unitholders. The transaction between MPLX LP and MWE, which is subject to approval by MWE unitholders and to customary closing conditions and regulatory approvals, is expected to close in the fourth quarter of 2015. MPLX LP has incurred $4.3 million of acquisition costs through September 30, 2015, which are recorded in general and administrative expenses. Acquisitions of Interests in Pipe Line Holdings Effective December 1, 2014 , we purchased a 22.875 percent interest in Pipe Line Holdings from subsidiaries of MPC for consideration of $600.0 million , which was financed through borrowings under our bank revolving credit facility. In addition, we accepted a contribution of a 7.625 percent interest in Pipe Line Holdings from a subsidiary of MPC in exchange for the issuance of equity valued at $200.0 million . We recorded the combined 30.5 percent interest at its historical carrying value of $334.8 million and the excess cash paid and equity contributed over historical carrying value of $465.2 million as a decrease to general partner equity. Beginning December 1, 2014, our consolidated financial statements reflect the 99.5 percent general partner interest in Pipe Line Holdings owned by MPLX LP, while the 0.5 percent limited partner interest held by MPC is reflected as a noncontrolling interest. On March 1, 2014 , we acquired a 13 percent interest in Pipe Line Holdings from MPC for consideration of $310.0 million , which was funded with $40.0 million of cash on hand and $270.0 million of borrowings on our bank revolving credit facility. We recorded the 13 percent interest in Pipe Line Holdings at its historical carrying value of $137.5 million and the excess cash paid over historical carrying value of $172.5 million as a decrease to general partner equity. These acquisitions were approved by the conflicts committee of the board of directors of our general partner, which is comprised entirely of independent directors. Changes in MPLX LP’s equity resulting from changes in its ownership interest in Pipe Line Holdings for the three months and nine months ended September 30, 2015 and 2014 were as follows: Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2015 2014 2015 2014 Net income attributable to MPLX LP $ 41.5 $ 29.1 $ 138.3 $ 92.1 Transfer to noncontrolling interest retained by MPC: Decrease in general partner-MPC equity for purchases of additional interest in Pipe Line Holdings — — — (172.5 ) Change from net income attributable to MPLX LP and transfer to noncontrolling interest retained by MPC $ 41.5 $ 29.1 $ 138.3 $ (80.4 )

Related Party Agreements and Tr

Related Party Agreements and Transactions	9 Months Ended
	Sep. 30, 2015
Related Party Transactions [Abstract]
Related Party Agreements and Transactions	Related Party Agreements and Transactions Our 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 common carrier products pipeline and storage facility. • Muskegon Pipeline LLC (“Muskegon”), in which MPC has a 60 percent interest. Muskegon owns a common carrier products pipeline. Related Party Agreements We have various long-term, fee-based commercial agreements with MPC under which we provide pipeline transportation and storage services to MPC, and MPC has committed to provide us with minimum quarterly throughput and storage volumes of crude oil and refined products and minimum storage volumes of butane. For additional information regarding our commercial and other agreements with MPC, see Item 1. Business in our Annual Report on Form 10-K for the year ended December 31, 2014 . We believe the terms and conditions under our agreements with MPC are generally comparable to those with unrelated parties. New Agreements On September 17, 2015, MPL entered into an amendment (the “First Amendment”) to its existing Patoka, Illinois tank farm Storage Services Agreement with MPC dated September 24, 2012 (the “Storage Services Agreement”). Under the Storage Services Agreement, MPC pays a monthly fee to store crude oil at MPL’s Patoka, Illinois tank farm. MPC’s fees under the Storage Services Agreement are for the use of the available shell capacity of MPL’s Patoka, Illinois tank farm, regardless of whether MPC fully utilizes all of its contractual capacity. The First Amendment provides for an increase in available shell capacity at the Patoka, Illinois tank farm from 1,386,000 barrels to 2,626,000 barrels due to the addition of four newly constructed tanks at the facility. Effective June 11, 2015, MPL entered into a transportation services agreement with MPC pursuant to which MPL will charge fees to MPC, at applicable Federal Energy Regulatory Commission tariff rates, for transporting products on the Cornerstone pipeline system and related Utica build-out projects. MPC will be obligated to transport certain minimum quarterly volumes of products on the associated pipeline systems. If MPC fails to transport its minimum volume during any quarter, then MPC will pay MPL a quarterly deficiency payment. The amount of any quarterly deficiency payment paid by MPC may be applied as a credit for any volumes transported on the applicable pipeline system in excess of MPC’s minimum volume commitment during any of the succeeding four quarters, after which time any unused credits will expire. Upon the expiration or termination of this agreement, MPC will have the opportunity to apply any remaining credit amounts until the completion of the succeeding four quarter period, without regard to the minimum volume commitment under the agreement. This agreement has an initial term of 15 years after the project’s in-service date and automatically renews for up to two renewal terms of five years each unless either party provides the other party with written notice of its intent to terminate at least six months prior to the end of the primary term or any renewal term, as applicable. We operate various pipeline systems owned by MPC under operating services agreements. Under the agreements the Partnership receives a fee for operating the assets and is generally reimbursed for all direct and indirect costs associated with operating the assets. On January 1, 2015, MPC and MPL amended the Amended and Restated Operating Agreement to reflect the transfer of certain assets from MPC to Hardin Street Transportation LLC (“HST”), an indirect, wholly-owned subsidiary of MPC. This amended agreement, with an annual operating fee of $0.6 million , now covers only the assets not transferred to HST. Also on January 1, 2015, MPL entered into a new agreement with HST (the “HST Operating Agreement”) for operation of the transferred assets with an annual fee of $12.2 million . The HST Operating Agreement has an initial term of one year and automatically renews for additional one -year terms, unless either party provides the other party with written notice of its intent to terminate the agreement at least six months prior to the end of the initial term or any renewal term. In combination, the amended and new agreement did not change the fees received by MPL or the services provided under the agreements. Related Party Transactions Sales to related parties were as follows: Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2015 2014 2015 2014 MPC $ 122.7 $ 114.1 $ 360.7 $ 336.0 Related party sales to MPC consist of crude oil and refined products pipeline transportation services based on regulated tariff rates and storage services based on contracted rates. Under our transportation services agreements, if MPC fails to transport its minimum throughput volumes during any quarter, then MPC will pay us 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. We recognize 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 for operating pipelines for related parties is included in other income-related parties on the consolidated statements of income were as follows: Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2015 2014 2015 2014 MPC $ 6.1 $ 5.5 $ 17.9 $ 16.0 Centennial 0.2 0.3 0.8 0.8 Muskegon — — 0.1 0.1 Total $ 6.3 $ 5.8 $ 18.8 $ 16.9 MPC provides executive management services and certain general and administrative services to us under the terms of the omnibus agreement. Charges for services included in purchases from related parties primarily relate to services that support our operations and maintenance activities, as well as compensation expenses. These charges were as follows: Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2015 2014 2015 2014 MPC $ 6.6 $ 6.0 $ 18.4 $ 18.1 Charges for services included in general and administrative expenses primarily relate to services that support our executive management, accounting and human resources activities. These charges were as follows: Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2015 2014 2015 2014 MPC $ 8.1 $ 7.8 $ 24.2 $ 23.1 In addition, some service costs related to engineering services are associated with assets under construction. These costs added to property, plant and equipment were as follows: Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2015 2014 2015 2014 MPC $ 3.2 $ 1.9 $ 9.4 $ 4.1 MPLX LP contracts 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 from 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 September 30, Nine Months Ended September 30, (In millions) 2015 2014 2015 2014 Purchases from related parties $ 19.9 $ 17.7 $ 56.7 $ 53.3 General and administrative expenses 6.1 5.6 20.2 16.6 Total $ 26.0 $ 23.3 $ 76.9 $ 69.9 Receivables from related parties were as follows: (In millions) September 30, 2015 December 31, 2014 MPC $ 54.3 $ 40.5 Centennial 0.5 0.3 Muskegon 0.1 0.2 Total $ 54.9 $ 41.0 Payables to related parties were as follows: (In millions) September 30, 2015 December 31, 2014 MPC $ 24.6 $ 20.1 Centennial 0.5 0.1 Total $ 25.1 $ 20.2 During the nine months ended September 30, 2015 and 2014 , MPC did not ship its minimum committed volumes on certain of our pipeline systems. In addition, capital projects we are 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) September 30, 2015 December 31, 2014 Minimum volume deficiencies - MPC $ 33.7 $ 29.9 Project reimbursements - MPC 5.2 4.6 Total $ 38.9 $ 34.5

Net Income Per Limited Partner

Net Income Per Limited Partner Unit	9 Months Ended
	Sep. 30, 2015
Earnings Per Share [Abstract]
Net Income Per Limited Partner Unit	Net Income Per Limited Partner Unit Net income 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 attributable to MPLX LP by the weighted average number of common units and subordinated units outstanding. Because we have more than one class of participating securities, we use the two-class method when calculating the net income per unit applicable to limited partners. The classes of participating securities include common units, subordinated units, general partner units, certain equity-based compensation awards and incentive distribution rights. As discussed further in Note 6 , the subordinated units, all of which were owned by MPC, were converted into common units during the third quarter. For purposes of calculating net income per unit, the subordinated units were treated as if they converted to common units on July 1, 2015. Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2015 2014 2015 2014 Net income attributable to MPLX LP $ 41.5 $ 29.1 $ 138.3 $ 92.1 Less: General partner’s distributions declared (including IDRs) (1) 8.8 1.5 19.0 3.4 Limited partners’ distributions declared on common units (1) 37.7 13.2 74.6 38.0 Limited partner’s distributions declared on subordinated units (1) — 13.2 31.4 38.0 Undistributed net income (loss) attributable to MPLX LP $ (5.0 ) $ 1.2 $ 13.3 $ 12.7 (1) See Note 6 for distribution information. Three Months Ended September 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) $ 8.8 $ 37.7 $ — $ 46.5 Undistributed net loss attributable to MPLX LP (0.1 ) (4.9 ) — (5.0 ) Net income attributable to MPLX LP (1) $ 8.7 $ 32.8 $ — $ 41.5 Weighted average units outstanding: Basic 1.6 80.4 — 82.0 Diluted 1.6 80.4 — 82.0 Net income attributable to MPLX LP per limited partner unit: Basic $ 0.41 $ — Diluted $ 0.41 $ — Three Months Ended September 30, 2014 (In millions, except per unit data) General Partner Limited Partners’ Common Units Limited Partner’s Subordinated Units Total Basic and diluted net income attributable to MPLX LP per unit: Net income attributable to MPLX LP: Distributions declared (including IDRs) $ 1.5 $ 13.2 $ 13.2 $ 27.9 Undistributed net income attributable to MPLX LP 0.3 0.5 0.4 1.2 Net income attributable to MPLX LP (1) $ 1.8 $ 13.7 $ 13.6 $ 29.1 Weighted average units outstanding: Basic 1.4 37.1 37.0 75.5 Diluted 1.4 37.1 37.0 75.5 Net income attributable to MPLX LP per limited partner unit: Basic $ 0.37 $ 0.37 Diluted $ 0.37 $ 0.37 Nine Months Ended September 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) $ 19.0 $ 74.6 $ 31.4 $ 125.0 Undistributed net income attributable to MPLX LP 6.6 4.6 2.1 13.3 Net income attributable to MPLX LP (1) $ 25.6 $ 79.2 $ 33.5 $ 138.3 Weighted average units outstanding: Basic 1.6 55.9 24.5 82.0 Diluted 1.6 55.9 24.5 82.0 Net income attributable to MPLX LP per limited partner unit: Basic $ 1.42 $ 1.36 Diluted $ 1.42 $ 1.36 Nine Months Ended September 30, 2014 (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) $ 3.4 $ 38.0 $ 38.0 $ 79.4 Undistributed net income attributable to MPLX LP 3.2 4.8 4.7 12.7 Net income attributable to MPLX LP (1) $ 6.6 $ 42.8 $ 42.7 $ 92.1 Weighted average units outstanding: Basic 1.4 37.0 37.0 75.4 Diluted 1.4 37.1 37.0 75.5 Net income attributable to MPLX LP per limited partner unit: Basic $ 1.16 $ 1.16 Diluted $ 1.16 $ 1.16 (1) Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.

Equity

Equity	9 Months Ended
	Sep. 30, 2015
Equity [Abstract]
Equity	Equity Units Outstanding - We had 80,336,711 common units outstanding as of September 30, 2015 . Of that number, 56,932,134 were owned by MPC, which also owned the two percent general partner interest, represented by 1,639,525 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 our 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 May 18, 2015 , we filed a prospectus supplement to our shelf registration statement filed with the SEC on March 27, 2015, authorizing the continuous issuance of up to an aggregate of $500.0 million of common units, in amounts, at prices and on terms to be determined by market conditions and other factors at the time of our offerings (such continuous offering program, or at-the-market program, referred to as our “ATM Program”). We expect the net proceeds from sales under the ATM Program will be used for general partnership purposes. During the nine months ended September 30, 2015 , we issued an aggregate of 25,166 common units under our ATM Program, generating net proceeds of approximately $1.3 million . The changes in the number of units outstanding from December 31, 2014 through September 30, 2015 are summarized below: (In units) Common Subordinated General Partner Total Balance at December 31, 2014 43,341,098 36,951,515 1,638,625 81,931,238 Unit-based compensation awards (1) 18,932 — 386 19,318 Issuance of units under the ATM Program (2) 25,166 — 514 25,680 Subordinated unit conversion 36,951,515 (36,951,515 ) — — Balance at September 30, 2015 80,336,711 — 1,639,525 81,976,236 (1) As a result of the unit-based compensation awards issued during the period, MPLX GP contributed less than $0.1 million in exchange for 386 general partner units to maintain its two percent general partner interest. (2) As a result of common units issued under the ATM Program during the period, MPLX GP contributed less than $0.1 million in exchange for 514 general partner units to maintain its two percent general partner interest. Issuance of Additional Securities - Our partnership agreement authorizes us to issue an unlimited number of additional partnership securities for the consideration and on the terms and conditions determined by our general partner without the approval of the unitholders. Net Income Allocation - In preparing our consolidated statements of equity, net income attributable to MPLX LP is allocated to the unitholders in accordance with their respective ownership percentages. However, when distributions related to the incentive distribution rights are made, earnings equal to the amount of those distributions are first allocated to the general partner before the remaining earnings are allocated to the unitholders based on their respective ownership percentages. The following table presents the allocation of the general partner’s interest in net income attributable to MPLX LP: Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2015 2014 2015 2014 Net income attributable to MPLX LP $ 41.5 $ 29.1 $ 138.3 $ 92.1 Less: General partner's incentive distribution rights and other 7.9 1.0 16.9 1.9 Net income attributable to MPLX LP available to general and limited partners $ 33.6 $ 28.1 $ 121.4 $ 90.2 General partner's two percent interest in net income attributable to MPLX LP $ 0.7 $ 0.5 $ 2.5 $ 1.8 General partner's incentive distribution rights and other 7.9 1.0 16.9 1.9 General partner's interest in net income attributable to MPLX LP $ 8.6 $ 1.5 $ 19.4 $ 3.7 Cash distributions - Our 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 our partnership agreement, on October 20, 2015 , we declared a quarterly cash distribution of $0.47 per unit, resulting in total distributions to our limited and general partners of $46.5 million . These distributions will be paid on November 13, 2015 to unitholders of record on November 3, 2015 . The allocation of total quarterly cash distributions to general and limited partners is as follows for the three and nine months ended September 30, 2015 and 2014 . Our distributions are declared subsequent to quarter end; therefore, the following table represents total cash distributions applicable to the period in which the distributions were earned. Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2015 2014 2015 2014 General partner's distributions: General partner's distributions $ 1.0 $ 0.5 $ 2.5 $ 1.5 General partner's incentive distribution rights distributions 7.8 1.0 16.5 1.9 Total general partner's distributions $ 8.8 $ 1.5 $ 19.0 $ 3.4 Limited partners' distributions: Common unitholders $ 37.7 $ 13.2 $ 74.6 $ 38.0 Subordinated unitholders — 13.2 31.4 38.0 Total limited partners' distributions 37.7 26.4 106.0 76.0 Total cash distributions declared $ 46.5 $ 27.9 $ 125.0 $ 79.4

Other Items

Other Items	9 Months Ended
	Sep. 30, 2015
Other Income and Expenses [Abstract]
Other Items	Other Items Net interest and other financial costs were: Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2015 2014 2015 2014 Interest income $ (0.1 ) $ — $ (0.2 ) $ — Interest expense 5.9 1.2 18.0 3.1 Interest capitalized (1.2 ) (0.2 ) (2.6 ) (0.7 ) Other financial costs 0.6 0.1 1.5 0.6 Net interest and other financial costs $ 5.2 $ 1.1 $ 16.7 $ 3.0

Property, Plant and Equipment

Property, Plant and Equipment	9 Months Ended
	Sep. 30, 2015
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment	Property, Plant and Equipment Our investment in property, plant and equipment with associated accumulated depreciation was: (In millions) September 30, 2015 December 31, 2014 Land $ 5.5 $ 5.5 Pipelines and related assets 1,085.0 1,081.4 Storage and delivery facilities 197.7 165.8 Assets under construction 184.3 85.3 Other 24.2 23.7 Total 1,496.7 1,361.7 Less accumulated depreciation 387.5 353.1 Property, plant and equipment, net $ 1,109.2 $ 1,008.6

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2015
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements Fair Values—Recurring There were no assets accounted for at fair value on a recurring basis at September 30, 2015 and December 31, 2014 . Fair Values—Reported Our primary financial instruments are trade receivables and payables. We believe the carrying values of our current assets and liabilities approximate fair value. Our fair value assessment incorporates a variety of considerations, including (1) the short-term duration of the instruments, (2) our investment-grade credit rating and (3) our historical incurrence of and expected future insignificance of bad debt expense, which includes an evaluation of counterparty credit risk. Fair value of fixed-rate long-term debt is measured using a market approach, based upon the average of quotes from major financial institutions and a third-party service for our debt. Because these quotes cannot be independently verified to the market, they are considered Level 3 inputs. Fair value of our variable-rate long-term debt approximates the carrying value. The following table summarizes the fair value and carrying value of our long-term debt, excluding capital leases and debt issuance costs, at September 30, 2015 and December 31, 2014 . September 30, 2015 December 31, 2014 (In millions) Fair Value Carrying Value Fair Value Carrying Value Long-term debt $ 711.1 $ 748.3 $ 635.6 $ 635.0

Debt

Debt	9 Months Ended
	Sep. 30, 2015
Debt Disclosure [Abstract]
Debt	Debt Our outstanding borrowings at September 30, 2015 and December 31, 2014 consisted of the following: (In millions) September 30, 2015 December 31, 2014 MPLX LP: Bank revolving credit facility due 2019 — 385.0 Term loan facility due 2019 250.0 250.0 4.000% senior notes due 2025 500.0 — Consolidated subsidiaries: Pipe Line Holdings - revolving credit agreement due 2019 — — MPL - capital lease obligations due 2020 9.1 9.8 Total 759.1 644.8 Unamortized debt issuance costs (1) (4.1 ) — Unamortized discount (1.7 ) — Amounts due within one year (0.8 ) (0.8 ) Total long-term debt due after one year $ 752.5 $ 644.0 (1) We adopted the updated FASB debt issuance cost standard as of June 30, 2015. This has been applied retrospectively and there was no effect to the prior period presented. Senior Notes On February 12, 2015, we completed a public offering of $500 million aggregate principal amount of four percent unsecured senior notes due February 15, 2025 (the “Senior Notes”). The net proceeds from the offering of the Senior Notes were approximately $495.0 million , after deducting underwriting discounts. The net proceeds were used to repay the amounts outstanding under the bank revolving credit facility, as well as for general partnership purposes. Interest is payable semi-annually in arrears on February 15 and August 15 of each year, commencing on August 15, 2015. Credit Agreements During the nine months ended September 30, 2015 , we borrowed $30.0 million under the bank revolving credit facility, at an average interest rate of 1.457 percent , per annum, and repaid $415.0 million of these borrowings. At September 30, 2015 , we had no borrowings or letters of credit outstanding under this facility, resulting in total unused loan availability of $1.0 billion , or 100 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 nine months ended September 30, 2015 were at an average interest rate of 1.651 percent .

Supplemental Cash Flow Informat

Supplemental Cash Flow Information	9 Months Ended
	Sep. 30, 2015
Supplemental Cash Flow Elements [Abstract]
Supplemental Cash Flow Information	Supplemental Cash Flow Information Nine Months Ended September 30, (In millions) 2015 2014 Net cash provided by operating activities included: Interest paid (net of amounts capitalized) $ 12.4 $ 1.8 Income taxes paid $ 0.1 $ — Non-cash investing and financing activities: Net transfers of property, plant and equipment to materials and supplies inventories $ — $ 0.7 Property, plant and equipment contributed by MPC — 0.1 The consolidated statements of cash flows exclude changes to the consolidated balance sheets that did not affect cash. The following is a reconciliation of additions to property, plant and equipment to total capital expenditures: Nine Months Ended September 30, (In millions) 2015 2014 Additions to property, plant and equipment $ 121.1 $ 47.1 Plus: Increase in capital accruals 17.5 5.9 Asset retirement expenditures 0.7 1.0 Total capital expenditures $ 139.3 $ 54.0

Equity-Based Compensation Plan

Equity-Based Compensation Plan	9 Months Ended
	Sep. 30, 2015
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 nine months ended September 30, 2015 : Number Weighted Outstanding at December 31, 2014 100,769 $ 41.66 Granted 35,630 80.16 Settled (32,313 ) 40.18 Outstanding at September 30, 2015 104,086 55.30 Performance Units —We grant 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 our 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 $1.03 for 2015, as calculated using a Monte Carlo valuation model. The following is a summary of the equity-classified performance unit award activity for the nine months ended September 30, 2015 : Number of Outstanding at December 31, 2014 924,143 Granted 597,249 Outstanding at September 30, 2015 1,521,392

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2015
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies We are 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 we have not recorded an accrued liability, we are 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 —We are 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 noncompliance. At September 30, 2015 and December 31, 2014 , accrued liabilities for remediation totaled $1.1 million and $0.6 million , respectively. It is not presently possible to estimate the ultimate amount of all remediation costs that might be incurred or penalties, if any, which may be imposed. Receivables from MPC for indemnification of environmental costs related to incidents occurring prior to our initial public offering completed on October 31, 2012 were less than $0.1 million at September 30, 2015 . There were no receivables from MPC for indemnification at December 31, 2014 . Litigation Relating to the Pending Merger —In July 2015, a purported class action lawsuit asserting claims challenging the Merger was filed in the Court of Chancery of the State of Delaware by a purported unitholder of MWE. In August 2015, two similar putative class action lawsuits were filed in the Court of Chancery of the State of Delaware by plaintiffs who purport to be unitholders of MWE. On September 9, 2015, these lawsuits were consolidated into one action pending in the Court of Chancery of the State of Delaware, now captioned In re MarkWest Energy Partners, L.P. Unitholder Litigation . On October 1, 2015, the plaintiffs filed a consolidated complaint against the individual members of the board of directors of MarkWest Energy GP, L.L.C. (the “MWE GP Board”), MPLX, MPLX GP LLC, the general partner of MPLX, MPC and Sapphire Holdco LLC, a subsidiary of MPLX, asserting in connection with the Merger and related disclosures that, among other things, (i) the MWE GP Board breached its duties in approving the Merger with MPLX and (ii) MPC, MPLX, MPLX GP, and Sapphire Holdco LLC aided and abetted such breaches. The complaint seeks, among other relief, to enjoin the Merger, or in the event the Merger is consummated, rescission of the Merger or monetary damages. An adverse judgment for rescission of the Merger or for monetary damages following the completion of the Merger could have an adverse effect on MPLX. The parties believe these claims are without merit and intend to vigorously defend the consolidated lawsuit and therefore, at this time, we do not believe the ultimate resolution of this lawsuit will have a material adverse effect. Other Lawsuits —In 2003, the State of Illinois brought an action against the Premcor Refining Group, Inc. (“Premcor”) and Apex Refining Company (“Apex”) asserting claims for environmental cleanup related to the refinery owned by these entities in the Hartford/Wood River, Illinois area. In 2006, Premcor and Apex filed third-party complaints against numerous owners and operators of petroleum products facilities in the Hartford/Wood River, Illinois area, including MPL. These complaints, which have been amended since filing, assert claims of common law nuisance and contribution under the Illinois Contribution Act and other laws for environmental cleanup costs that may be imposed on Premcor and Apex by the State of Illinois. There are several third-party defendants in the litigation and MPL has asserted cross-claims in contribution against the various third-party defendants. This litigation is currently pending in the Third Judicial Circuit Court, Madison County, Illinois. While the ultimate outcome of these litigated matters remains uncertain, neither the likelihood of an unfavorable outcome nor the ultimate liability, if any, with respect to this matter can be determined at this time and we are unable to estimate a reasonably possible loss (or range of loss) for this litigation. Under our omnibus agreement, MPC will indemnify us for the full cost of any losses should MPL be deemed responsible for any damages in this lawsuit. Guarantees —We have entered into guarantees with maximum potential undiscounted payments totaling $1.9 million as of September 30, 2015 , which consist of leases of vehicles that contain general lease indemnities and guaranteed residual values. Over the years, we have sold various assets in the normal course of our 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 us to perform upon the occurrence of a triggering event or condition. These guarantees and indemnifications are part of the normal course of selling assets. We are 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 we have little or no past experience with the underlying triggering event upon which a reasonable prediction of the outcome can be based. Contractual commitments —At September 30, 2015 , our contractual commitments to acquire property, plant and equipment totaled $49.6 million . Our contractual commitments at September 30, 2015 were primarily related to various projects including a butane cavern in Robinson, Illinois, a tank expansion in Patoka, Illinois connecting to MPC’s Southern Access Extension pipeline project and the Cornerstone Pipeline project.

Subsequent Event

Subsequent Event	9 Months Ended
	Sep. 30, 2015
Subsequent Event [Abstract]
Subsequent Events [Text Block]	Subsequent Events On October 27, 2015, in connection with the pending Merger, we amended our $1.0 billion bank revolving credit facility to, among other things, (i) extend the term of the bank revolving credit facility to a five -year term commencing on the date of the closing of the Merger and (ii) increase the borrowing capacity of the bank revolving credit facility to up to $2.0 billion . The amendment will only become effective upon the consummation of the Merger. Subject to consummation of the Merger, our board of directors approved the purchase of the remaining 0.5 percent interest in Pipe Line Holdings from MPC for $12.0 million . This transaction would close at the same time as the Merger.

Description of the Business a21

Description of the Business and Basis of Presentation Description of Business and Basis of Presentation (Policies)	9 Months Ended
	Sep. 30, 2015
Accounting Policies [Abstract]
Use of Estimates	The accompanying interim consolidated financial statements are unaudited; however, in the opinion of our 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 Securities and Exchange Commission(“SEC”) applicable to interim period financial statements and do not include all of the information and disclosures required by accounting principles generally accepted in the United States (“U.S. GAAP”) for complete financial statements.
Net Income per Unit	Net income 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 attributable to MPLX LP by the weighted average number of common units and subordinated units outstanding. Because we have more than one class of participating securities, we use the two-class method when calculating the net income per unit applicable to limited partners. The classes of participating securities include common units, subordinated units, general partner units, certain equity-based compensation awards and incentive distribution rights.

Acquisitions (Tables)

Acquisitions (Tables)	9 Months Ended
	Sep. 30, 2015
Equity [Abstract]
Summary Changes in MPLX LP's Ownership Interest in Pipe Line Holdings	Changes in MPLX LP’s equity resulting from changes in its ownership interest in Pipe Line Holdings for the three months and nine months ended September 30, 2015 and 2014 were as follows: Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2015 2014 2015 2014 Net income attributable to MPLX LP $ 41.5 $ 29.1 $ 138.3 $ 92.1 Transfer to noncontrolling interest retained by MPC: Decrease in general partner-MPC equity for purchases of additional interest in Pipe Line Holdings — — — (172.5 ) Change from net income attributable to MPLX LP and transfer to noncontrolling interest retained by MPC $ 41.5 $ 29.1 $ 138.3 $ (80.4 )

Related Party Agreements and 23

Related Party Agreements and Transactions (Tables)	9 Months Ended
	Sep. 30, 2015
Related Party Transactions [Abstract]
Sales to Related Parties	Sales to related parties were as follows: Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2015 2014 2015 2014 MPC $ 122.7 $ 114.1 $ 360.7 $ 336.0
Summary of Fees Received for Operating Pipelines for Related Parties Included in Other Income - Related Parties	The revenue received for operating pipelines for related parties is included in other income-related parties on the consolidated statements of income were as follows: Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2015 2014 2015 2014 MPC $ 6.1 $ 5.5 $ 17.9 $ 16.0 Centennial 0.2 0.3 0.8 0.8 Muskegon — — 0.1 0.1 Total $ 6.3 $ 5.8 $ 18.8 $ 16.9
Summary of Charges for Services Included in Purchases from Related Parties	Charges for services included in purchases from related parties primarily relate to services that support our operations and maintenance activities, as well as compensation expenses. These charges were as follows: Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2015 2014 2015 2014 MPC $ 6.6 $ 6.0 $ 18.4 $ 18.1
Summary of Related Party Charges for Services Included in General and Administrative Expenses	Charges for services included in general and administrative expenses primarily relate to services that support our executive management, accounting and human resources activities. These charges were as follows: Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2015 2014 2015 2014 MPC $ 8.1 $ 7.8 $ 24.2 $ 23.1
Summary of Related Party Costs Added to Property, Plant and Equipment	In addition, some service costs related to engineering services are associated with assets under construction. These costs added to property, plant and equipment were as follows: Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2015 2014 2015 2014 MPC $ 3.2 $ 1.9 $ 9.4 $ 4.1
Employee Services Expenses from Related Parties	Employee services expenses from related parties were as follows: Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2015 2014 2015 2014 Purchases from related parties $ 19.9 $ 17.7 $ 56.7 $ 53.3 General and administrative expenses 6.1 5.6 20.2 16.6 Total $ 26.0 $ 23.3 $ 76.9 $ 69.9
Receivables from Related Parties	Receivables from related parties were as follows: (In millions) September 30, 2015 December 31, 2014 MPC $ 54.3 $ 40.5 Centennial 0.5 0.3 Muskegon 0.1 0.2 Total $ 54.9 $ 41.0
Payables to Related Parties	Payables to related parties were as follows: (In millions) September 30, 2015 December 31, 2014 MPC $ 24.6 $ 20.1 Centennial 0.5 0.1 Total $ 25.1 $ 20.2
Summary of Deferred Revenue - Related Parties	The deferred revenue-related parties balance associated with the minimum volume deficiencies and project reimbursements were as follows: (In millions) September 30, 2015 December 31, 2014 Minimum volume deficiencies - MPC $ 33.7 $ 29.9 Project reimbursements - MPC 5.2 4.6 Total $ 38.9 $ 34.5

Net Income Per Limited Partne24

Net Income Per Limited Partner Unit (Tables)	9 Months Ended
	Sep. 30, 2015
Earnings Per Share [Abstract]
Schedule of Distributions By Partner By Class	Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2015 2014 2015 2014 Net income attributable to MPLX LP $ 41.5 $ 29.1 $ 138.3 $ 92.1 Less: General partner’s distributions declared (including IDRs) (1) 8.8 1.5 19.0 3.4 Limited partners’ distributions declared on common units (1) 37.7 13.2 74.6 38.0 Limited partner’s distributions declared on subordinated units (1) — 13.2 31.4 38.0 Undistributed net income (loss) attributable to MPLX LP $ (5.0 ) $ 1.2 $ 13.3 $ 12.7 (1) See Note 6 for distribution information.
Schedule of Basic and Diluted Earnings Per Unit	Three Months Ended September 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) $ 8.8 $ 37.7 $ — $ 46.5 Undistributed net loss attributable to MPLX LP (0.1 ) (4.9 ) — (5.0 ) Net income attributable to MPLX LP (1) $ 8.7 $ 32.8 $ — $ 41.5 Weighted average units outstanding: Basic 1.6 80.4 — 82.0 Diluted 1.6 80.4 — 82.0 Net income attributable to MPLX LP per limited partner unit: Basic $ 0.41 $ — Diluted $ 0.41 $ — Three Months Ended September 30, 2014 (In millions, except per unit data) General Partner Limited Partners’ Common Units Limited Partner’s Subordinated Units Total Basic and diluted net income attributable to MPLX LP per unit: Net income attributable to MPLX LP: Distributions declared (including IDRs) $ 1.5 $ 13.2 $ 13.2 $ 27.9 Undistributed net income attributable to MPLX LP 0.3 0.5 0.4 1.2 Net income attributable to MPLX LP (1) $ 1.8 $ 13.7 $ 13.6 $ 29.1 Weighted average units outstanding: Basic 1.4 37.1 37.0 75.5 Diluted 1.4 37.1 37.0 75.5 Net income attributable to MPLX LP per limited partner unit: Basic $ 0.37 $ 0.37 Diluted $ 0.37 $ 0.37 Nine Months Ended September 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) $ 19.0 $ 74.6 $ 31.4 $ 125.0 Undistributed net income attributable to MPLX LP 6.6 4.6 2.1 13.3 Net income attributable to MPLX LP (1) $ 25.6 $ 79.2 $ 33.5 $ 138.3 Weighted average units outstanding: Basic 1.6 55.9 24.5 82.0 Diluted 1.6 55.9 24.5 82.0 Net income attributable to MPLX LP per limited partner unit: Basic $ 1.42 $ 1.36 Diluted $ 1.42 $ 1.36 Nine Months Ended September 30, 2014 (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) $ 3.4 $ 38.0 $ 38.0 $ 79.4 Undistributed net income attributable to MPLX LP 3.2 4.8 4.7 12.7 Net income attributable to MPLX LP (1) $ 6.6 $ 42.8 $ 42.7 $ 92.1 Weighted average units outstanding: Basic 1.4 37.0 37.0 75.4 Diluted 1.4 37.1 37.0 75.5 Net income attributable to MPLX LP per limited partner unit: Basic $ 1.16 $ 1.16 Diluted $ 1.16 $ 1.16 (1) Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.

Equity (Tables)

Equity (Tables)	9 Months Ended
	Sep. 30, 2015
Equity [Abstract]
Schedule of Stockholders Equity [Table Text Block]	The changes in the number of units outstanding from December 31, 2014 through September 30, 2015 are summarized below: (In units) Common Subordinated General Partner Total Balance at December 31, 2014 43,341,098 36,951,515 1,638,625 81,931,238 Unit-based compensation awards (1) 18,932 — 386 19,318 Issuance of units under the ATM Program (2) 25,166 — 514 25,680 Subordinated unit conversion 36,951,515 (36,951,515 ) — — Balance at September 30, 2015 80,336,711 — 1,639,525 81,976,236 (1) As a result of the unit-based compensation awards issued during the period, MPLX GP contributed less than $0.1 million in exchange for 386 general partner units to maintain its two percent general partner interest. (2) As a result of common units issued under the ATM Program during the period, MPLX GP contributed less than $0.1 million in exchange for 514 general partner units to maintain its two percent general partner interest.
Schedule Of Calculation Of Net Income Applicable to Partners [Table Text Block]	Net Income Allocation - In preparing our consolidated statements of equity, net income attributable to MPLX LP is allocated to the unitholders in accordance with their respective ownership percentages. However, when distributions related to the incentive distribution rights are made, earnings equal to the amount of those distributions are first allocated to the general partner before the remaining earnings are allocated to the unitholders based on their respective ownership percentages. The following table presents the allocation of the general partner’s interest in net income attributable to MPLX LP: Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2015 2014 2015 2014 Net income attributable to MPLX LP $ 41.5 $ 29.1 $ 138.3 $ 92.1 Less: General partner's incentive distribution rights and other 7.9 1.0 16.9 1.9 Net income attributable to MPLX LP available to general and limited partners $ 33.6 $ 28.1 $ 121.4 $ 90.2 General partner's two percent interest in net income attributable to MPLX LP $ 0.7 $ 0.5 $ 2.5 $ 1.8 General partner's incentive distribution rights and other 7.9 1.0 16.9 1.9 General partner's interest in net income attributable to MPLX LP $ 8.6 $ 1.5 $ 19.4 $ 3.7
Distributions Made to Limited Partner, by Distribution [Table Text Block]	Our distributions are declared subsequent to quarter end; therefore, the following table represents total cash distributions applicable to the period in which the distributions were earned. Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2015 2014 2015 2014 General partner's distributions: General partner's distributions $ 1.0 $ 0.5 $ 2.5 $ 1.5 General partner's incentive distribution rights distributions 7.8 1.0 16.5 1.9 Total general partner's distributions $ 8.8 $ 1.5 $ 19.0 $ 3.4 Limited partners' distributions: Common unitholders $ 37.7 $ 13.2 $ 74.6 $ 38.0 Subordinated unitholders — 13.2 31.4 38.0 Total limited partners' distributions 37.7 26.4 106.0 76.0 Total cash distributions declared $ 46.5 $ 27.9 $ 125.0 $ 79.4

Other Items (Tables)

Other Items (Tables)	9 Months Ended
	Sep. 30, 2015
Other Income and Expenses [Abstract]
Summary of Interest and Other Financial (Costs)	Net interest and other financial costs were: Three Months Ended September 30, Nine Months Ended September 30, (In millions) 2015 2014 2015 2014 Interest income $ (0.1 ) $ — $ (0.2 ) $ — Interest expense 5.9 1.2 18.0 3.1 Interest capitalized (1.2 ) (0.2 ) (2.6 ) (0.7 ) Other financial costs 0.6 0.1 1.5 0.6 Net interest and other financial costs $ 5.2 $ 1.1 $ 16.7 $ 3.0

Property, Plant and Equipment (

Property, Plant and Equipment (Tables)	9 Months Ended
	Sep. 30, 2015
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment	Our investment in property, plant and equipment with associated accumulated depreciation was: (In millions) September 30, 2015 December 31, 2014 Land $ 5.5 $ 5.5 Pipelines and related assets 1,085.0 1,081.4 Storage and delivery facilities 197.7 165.8 Assets under construction 184.3 85.3 Other 24.2 23.7 Total 1,496.7 1,361.7 Less accumulated depreciation 387.5 353.1 Property, plant and equipment, net $ 1,109.2 $ 1,008.6

Fair Value Measurements Fair Va

Fair Value Measurements Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2015
Fair Value Disclosures [Abstract]
Fair Value, by Balance Sheet Grouping [Table Text Block]	The following table summarizes the fair value and carrying value of our long-term debt, excluding capital leases and debt issuance costs, at September 30, 2015 and December 31, 2014 . September 30, 2015 December 31, 2014 (In millions) Fair Value Carrying Value Fair Value Carrying Value Long-term debt $ 711.1 $ 748.3 $ 635.6 $ 635.0

Debt (Tables)

Debt (Tables)	9 Months Ended
	Sep. 30, 2015
Debt Disclosure [Abstract]
Summary of Outstanding Borrowings	Our outstanding borrowings at September 30, 2015 and December 31, 2014 consisted of the following: (In millions) September 30, 2015 December 31, 2014 MPLX LP: Bank revolving credit facility due 2019 — 385.0 Term loan facility due 2019 250.0 250.0 4.000% senior notes due 2025 500.0 — Consolidated subsidiaries: Pipe Line Holdings - revolving credit agreement due 2019 — — MPL - capital lease obligations due 2020 9.1 9.8 Total 759.1 644.8 Unamortized debt issuance costs (1) (4.1 ) — Unamortized discount (1.7 ) — Amounts due within one year (0.8 ) (0.8 ) Total long-term debt due after one year $ 752.5 $ 644.0 (1) We adopted the updated FASB debt issuance cost standard as of June 30, 2015. This has been applied retrospectively and there was no effect to the prior period presented.

Supplemental Cash Flow Inform30

Supplemental Cash Flow Information (Tables)	9 Months Ended
	Sep. 30, 2015
Supplemental Cash Flow Elements [Abstract]
Summary of Supplemental Cash Flow Information	Nine Months Ended September 30, (In millions) 2015 2014 Net cash provided by operating activities included: Interest paid (net of amounts capitalized) $ 12.4 $ 1.8 Income taxes paid $ 0.1 $ — Non-cash investing and financing activities: Net transfers of property, plant and equipment to materials and supplies inventories $ — $ 0.7 Property, plant and equipment contributed by MPC — 0.1
Summary of Reconciliation of Additions to Property, Plant and Equipment to Total Capital Expenditures	The following is a reconciliation of additions to property, plant and equipment to total capital expenditures: Nine Months Ended September 30, (In millions) 2015 2014 Additions to property, plant and equipment $ 121.1 $ 47.1 Plus: Increase in capital accruals 17.5 5.9 Asset retirement expenditures 0.7 1.0 Total capital expenditures $ 139.3 $ 54.0

Equity-Based Compensation Plan

Equity-Based Compensation Plan (Tables)	9 Months Ended
	Sep. 30, 2015
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 nine months ended September 30, 2015 : Number Weighted Outstanding at December 31, 2014 100,769 $ 41.66 Granted 35,630 80.16 Settled (32,313 ) 40.18 Outstanding at September 30, 2015 104,086 55.30
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 nine months ended September 30, 2015 : Number of Outstanding at December 31, 2014 924,143 Granted 597,249 Outstanding at September 30, 2015 1,521,392

Description of Business and Bas

Description of Business and Basis of Presentation - Additional Information (Detail) bbl in Millions	9 Months Ended
	Sep. 30, 2015Segmentbbl
Description Of Business And Basis Of Presentation [Line Items]
Storage capacity (in number of barrels)	1
Number of segments | Segment	1
Crude Oil and Product Pipeline Systems [Member]
Description Of Business And Basis Of Presentation [Line Items]
Percent of the net income related to assets contributed to MPLX LP	99.50%
Storage Services Butane Cavern [Member]
Description Of Business And Basis Of Presentation [Line Items]
Percent of the net income related to assets contributed to MPLX LP	100.00%

Acquisitions - Additional Infor

Acquisitions - Additional Information (Detail) $ in Millions	Jul. 11, 2015USD ($)	Dec. 01, 2014USD ($)	Mar. 01, 2014USD ($)	Sep. 30, 2015USD ($)	Sep. 30, 2014USD ($)	Sep. 30, 2015USD ($)	Sep. 30, 2014USD ($)
Business Acquisition [Line Items]
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned		$ 200
Additional interest in Pipe Line Holdings		22.875%	13.00%
Purchase of additional interest in Pipe Line Holdings		$ 600	$ 310			$ 0	$ 310
Percentage Of Partners Interest Contributed		7.625%
Percentage of Partners Interest Purchased and Contributed		30.50%
Purchase of additional interest in Pipe Line Holdings, historical value							(310)
MPLX Pipe Line Holdings LP [Member]
Business Acquisition [Line Items]
Percent of the net income related to assets contributed to MPLX LP		99.50%
General Partner [Member]
Business Acquisition [Line Items]
Purchase of additional interest in Pipe Line Holdings, excess cash paid		$ 465.2	172.5	$ 0	$ 0	0	172.5
Noncontrolling Interest Retained by MPC [Member]
Business Acquisition [Line Items]
Purchase of additional interest in Pipe Line Holdings, historical value		$ 334.8	$ 137.5
Noncontrolling Interest Retained by MPC [Member] | MPC [Member]
Business Acquisition [Line Items]
Purchase of additional interest in Pipe Line Holdings, historical value							$ (137.5)
Percent of the net income related to assets contributed to MPLX LP		0.50%
Pipe Line Holding [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition		Dec. 1, 2014	Mar. 1, 2014
MarkWest [Member]
Business Acquisition [Line Items]
Business Acquisition, Date of Acquisition Agreement	Jul. 11, 2015
Common Units Conversion Ratio	1.09
General Partners' Contributed Capital	$ 675			675		675
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned				13,700		13,700
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt				$ 4,600		4,600
Business Combination, Acquisition Related Costs						$ 4.3
Cash [Member] | Pipe Line Holding [Member]
Business Acquisition [Line Items]
Purchase of additional interest in Pipe Line Holdings			$ 40
Revolving Credit Agreement [Member] | Pipe Line Holding [Member]
Business Acquisition [Line Items]
Purchase of additional interest in Pipe Line Holdings			$ 270

Acquisitions - Summary Changes

Acquisitions - Summary Changes in MPLX LP's Ownership Interest in Pipe Line Holdings (Detail) - USD ($) $ in Millions		Dec. 01, 2014	Mar. 01, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Net income attributable to MPLX LP	[1]			$ 41.5	$ 29.1	$ 138.3	$ 92.1
Transfer to noncontrolling interest retained by MPC:
Change from net income attributable to MPLX LP and transfer to noncontrolling interest retained by MPC				41.5	29.1	138.3	(80.4)
General Partner [Member]
Transfer to noncontrolling interest retained by MPC:
Decrease in general partner-MPC equity for purchases of additional interest in Pipe Line Holdings		$ (465.2)	$ (172.5)	$ 0	$ 0	$ 0	$ (172.5)
[1]	Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.

Related Party Agreements and 35

Related Party Agreements and Transactions - Additional Information (Detail) $ in Millions	Sep. 30, 2015bbl	Sep. 30, 2015USD ($)	Sep. 30, 2014USD ($)	Sep. 30, 2015USD ($)bbl	Sep. 30, 2014USD ($)	Sep. 16, 2015bbl
Related Party Transaction [Line Items]
Storage capacity (in number of barrels) | bbl				1,000,000
Revenue from Related Parties		$ 122.7	$ 114.1	$ 360.7	$ 336
MPC [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties		$ 122.7	$ 114.1	$ 360.7	$ 336
Centennial [Member] | MPC [Member]
Related Party Transaction [Line Items]
Equity method investments, ownership percentage	50.00%	50.00%		50.00%
Muskegon [Member] | MPC [Member]
Related Party Transaction [Line Items]
Equity method investments, ownership percentage	60.00%	60.00%		60.00%
Operating Agreements [Member] | MPC [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties				$ 0.6
Operating Agreements [Member] | Hardin Street Transportation LLC [Member]
Related Party Transaction [Line Items]
Term Of Agreements				1 year
Renewal Term Agreement				1 year
Revenue from Related Parties				$ 12.2
Storage Services Tank Farm [Member] | Commercial Agreements [Member] | MPC [Member]
Related Party Transaction [Line Items]
Storage capacity (in number of barrels) | bbl	2,626,000					1,386,000
Number of Tanks	4
Transportation Services Agreements [Member] | Cornerstone pipeline system and Utica build out projects [Member] | Commercial Agreements [Member] | MPC [Member]
Related Party Transaction [Line Items]
Term Of Agreements				15 years
Number of Renewals	2	2		2
Renewal Term Agreement				5 years

Related Party Agreements and 36

Related Party Agreements and Transactions - Sales to Related Parties (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Related Party Transaction [Line Items]
Sales to related parties	$ 122.7	$ 114.1	$ 360.7	$ 336
MPC [Member]
Related Party Transaction [Line Items]
Sales to related parties	$ 122.7	$ 114.1	$ 360.7	$ 336

Related Party Agreements and 37

Related Party Agreements and Transactions - Summary of Fees Received for Operating Pipelines for Related Parties Included in Other Income - Related Parties (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Related Party Transaction [Line Items]
Other income - related parties	$ 6.3	$ 5.8	$ 18.8	$ 16.9
MPC [Member]
Related Party Transaction [Line Items]
Other income - related parties	6.1	5.5	17.9	16
Centennial [Member]
Related Party Transaction [Line Items]
Other income - related parties	0.2	0.3	0.8	0.8
Muskegon [Member]
Related Party Transaction [Line Items]
Other income - related parties	$ 0	$ 0	$ 0.1	$ 0.1

Related Party Agreements and 38

Related Party Agreements and Transaction - Summary of Charges for Service Included in Purchases from Related Parties (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
MPC [Member]
Related Party Transaction [Line Items]
Charges for services included in purchases from related party	$ 6.6	$ 6	$ 18.4	$ 18.1

Related Party Agreements and 39

Related Party Agreements and Transactions - Summary of Related Party Charges for Services Included in General and Administrative Expenses (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
MPC [Member]
Related Party Transaction [Line Items]
Charges for services included in general and administrative expenses	$ 8.1	$ 7.8	$ 24.2	$ 23.1

Related Party Agreements and 40

Related Party Agreements and Transactions - Summary of Related Party Costs Added to Property, Plant and Equipment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
MPC [Member] | Assets Under Construction [Member]
Property, Plant and Equipment [Line Items]
Costs added to property, plant and equipment	$ 3.2	$ 1.9	$ 9.4	$ 4.1

Related Party Agreements and 41

Related Party Agreements and Transactions - Employee Services Expenses from Related Parties (Detail) - MPC [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	$ 26	$ 23.3	$ 76.9	$ 69.9
Purchases from Related Parties [Member]
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	19.9	17.7	56.7	53.3
General and Administrative Expenses [Member]
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	$ 6.1	$ 5.6	$ 20.2	$ 16.6

Related Party Agreements and 42

Related Party Agreements and Transactions - Receivables from Related Parties (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Related Party Transaction [Line Items]
Current receivables from related parties	$ 54.9	$ 41
MPC [Member]
Related Party Transaction [Line Items]
Current receivables from related parties	54.3	40.5
Centennial [Member]
Related Party Transaction [Line Items]
Current receivables from related parties	0.5	0.3
Muskegon [Member]
Related Party Transaction [Line Items]
Current receivables from related parties	$ 0.1	$ 0.2

Related Party Agreements and 43

Related Party Agreements and Transactions - Payables to Related Parties (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Related Party Transaction [Line Items]
Payables to related parties	$ 25.1	$ 20.2
MPC [Member]
Related Party Transaction [Line Items]
Payables to related parties	24.6	20.1
Centennial [Member]
Related Party Transaction [Line Items]
Payables to related parties	$ 0.5	$ 0.1

Related Party Agreements and 44

Related Party Agreements and Transactions - Summary of Deferred Revenue - Related Parties (Detail) - MPC [Member] - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Related Party Transaction [Line Items]
Deferred Revenue Related Parties	$ 38.9	$ 34.5
Minimum volume deficiencies [Member]
Related Party Transaction [Line Items]
Deferred Revenue Related Parties	33.7	29.9
Project Reimbursements [Member]
Related Party Transaction [Line Items]
Deferred Revenue Related Parties	$ 5.2	$ 4.6

Net Income Per Limited Partne45

Net Income Per Limited Partner Unit - Schedule of Distributions by Partner by Class (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Net Income Per Share [Line Items]
Net income attributable to MPLX LP	[1]	$ 41.5	$ 29.1	$ 138.3	$ 92.1
Less: Distribution declared		(46.5)	(27.9)	(125)	(79.4)
Undistributed net income attributable to MPLX LP		(5)	1.2	13.3	12.7
General Partner [Member] | MPC [Member]
Net Income Per Share [Line Items]
Net income attributable to MPLX LP	[1]	8.7	1.8	25.6	6.6
Less: Distribution declared	[2]	(8.8)	(1.5)	(19)	(3.4)
Undistributed net income attributable to MPLX LP		(0.1)	0.3	6.6	3.2
Limited Partners Common Units [Member]
Net Income Per Share [Line Items]
Net income attributable to MPLX LP	[1]	32.8	13.7	79.2	42.8
Less: Distribution declared	[2]	(37.7)	(13.2)	(74.6)	(38)
Undistributed net income attributable to MPLX LP		(4.9)	0.5	4.6	4.8
Limited Partners Subordinated Units [Member] | MPC [Member]
Net Income Per Share [Line Items]
Net income attributable to MPLX LP	[1]	0	13.6	33.5	42.7
Less: Distribution declared	[2]	0	(13.2)	(31.4)	(38)
Undistributed net income attributable to MPLX LP		$ 0	$ 0.4	$ 2.1	$ 4.7
[1]	Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.
[2]	See Note 6 for distribution information.

Net Income Per Limited Partne46

Net Income Per Limited Partner Unit - Basic and Diluted Earnings Per Unit (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Net income attributable to MPLX LP:
Distributions declared (including IDRs)		$ 46.5	$ 27.9	$ 125	$ 79.4
Undistributed net income attributable to MPLX LP		(5)	1.2	13.3	12.7
Net income attributable to MPLX LP	[1]	$ 41.5	$ 29.1	$ 138.3	$ 92.1
Weighted average units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic		82	75.5	82	75.4
Weighted Average Limited Partnership Units Outstanding, Diluted		82	75.5	82	75.5
Limited Partners Common Units [Member]
Net income attributable to MPLX LP:
Distributions declared (including IDRs)	[2]	$ 37.7	$ 13.2	$ 74.6	$ 38
Undistributed net income attributable to MPLX LP		(4.9)	0.5	4.6	4.8
Net income attributable to MPLX LP	[1]	$ 32.8	$ 13.7	$ 79.2	$ 42.8
Weighted average units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic		80.4	37.1	55.9	37
Weighted Average Limited Partnership Units Outstanding, Diluted		80.4	37.1	55.9	37.1
Net income attributable to MPLX LP per limited partner unit:
Basic (in USD per unit)		$ 0.41	$ 0.37	$ 1.42	$ 1.16
Diluted (in USD per unit)		$ 0.41	$ 0.37	$ 1.42	$ 1.16
MPC [Member] | General Partner [Member]
Net income attributable to MPLX LP:
Distributions declared (including IDRs)	[2]	$ 8.8	$ 1.5	$ 19	$ 3.4
Undistributed net income attributable to MPLX LP		(0.1)	0.3	6.6	3.2
Net income attributable to MPLX LP	[1]	$ 8.7	$ 1.8	$ 25.6	$ 6.6
Weighted average units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic		1.6	1.4	1.6	1.4
Weighted Average Limited Partnership Units Outstanding, Diluted		1.6	1.4	1.6	1.4
MPC [Member] | Limited Partners Subordinated Units [Member]
Net income attributable to MPLX LP:
Distributions declared (including IDRs)	[2]	$ 0	$ 13.2	$ 31.4	$ 38
Undistributed net income attributable to MPLX LP		0	0.4	2.1	4.7
Net income attributable to MPLX LP	[1]	$ 0	$ 13.6	$ 33.5	$ 42.7
Weighted average units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic		0	37	24.5	37
Weighted Average Limited Partnership Units Outstanding, Diluted		0	37	24.5	37
Net income attributable to MPLX LP per limited partner unit:
Basic (in USD per unit)		$ 0	$ 0.37	$ 1.36	$ 1.16
Diluted (in USD per unit)		$ 0	$ 0.37	$ 1.36	$ 1.16
[1]	Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.
[2]	See Note 6 for distribution information.

Equity (Details)

Equity (Details) - USD ($) $ / shares in Units, $ in Millions		Nov. 13, 2015	Nov. 03, 2015	Oct. 20, 2015	May. 18, 2015	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Stockholders Equity Note [Line Items]
Limited Liability Company (LLC) or Limited Partnership (LP), Managing Member or General Partner, Ownership Interest								2.00%
Units outstanding						1,639,525		1,639,525
Unit-based compensation awards(1)	[1]							19,318
Issuance of units under ATM program								$ 1.3
Distributions declared (including IDRs)						$ 46.5	$ 27.9	$ 125	$ 79.4
Limited Partners Common Units [Member]
Stockholders Equity Note [Line Items]
Units outstanding						80,336,711		80,336,711
Subordinated Units [Member]
Stockholders Equity Note [Line Items]
Equity Instruments Conversion Date						Aug. 17, 2015
Unit-based compensation awards(1)						36,951,515		0
MPC [Member] | Limited Partners Common Units [Member]
Stockholders Equity Note [Line Items]
Units outstanding						56,932,134		56,932,134
Subsequent Event [Member]
Stockholders Equity Note [Line Items]
Distribution Made to Limited Partner, Declaration Date				Oct. 20, 2015
Cash distributions declared per limited partner common unit				$ 0.47
Distributions declared (including IDRs)				$ 46.5
Distribution Made to Limited Partner, Distribution Date		Nov. 13, 2015
Distribution Made to Limited Partner, Date of Record			Nov. 3, 2015
ATM Program [Member]
Stockholders Equity Note [Line Items]
Equity Instrument Regulatory Filing Date					May 18, 2015
Common Units Aggregate Value					$ 500
Partners' Capital Account, Units, Sale of Units	[2]							25,680
Partners' Capital Account, Public Sale of Units Net of Offering Costs								$ 1.3
Limited Partners Common Units [Member]
Stockholders Equity Note [Line Items]
Unit-based compensation awards(1)	[1]							18,932
Cash distributions declared per limited partner common unit						$ 0.4700	$ 0.3575	$ 1.3200	$ 1.0275
Distributions declared (including IDRs)	[3]					$ 37.7	$ 13.2	$ 74.6	$ 38
Limited Partners Common Units [Member] | Subordinated Units [Member]
Stockholders Equity Note [Line Items]
Unit-based compensation awards(1)								36,951,515
Limited Partners Common Units [Member] | MPC [Member]
Stockholders Equity Note [Line Items]
Units outstanding						57,000,000		57,000,000		20,000,000
Limited Partners Common Units [Member] | ATM Program [Member]
Stockholders Equity Note [Line Items]
Partners' Capital Account, Units, Sale of Units	[2]							25,166
General Partner Units [Member]
Stockholders Equity Note [Line Items]
Unit-based compensation awards(1)	[1]							386
Issuance of units under ATM program								$ 0.1
General Partner Units [Member] | ATM Program [Member]
Stockholders Equity Note [Line Items]
Partners' Capital Account, Units, Sale of Units	[2]							514
Issuance of units under ATM program								$ 0.1
[1]	As a result of the unit-based compensation awards issued during the period, MPLX GP contributed less than $0.1 million in exchange for 386 general partner units to maintain its two percent general partner interest.
[2]	As a result of common units issued under the ATM Program during the period, MPLX GP contributed less than $0.1 million in exchange for 514 general partner units to maintain its two percent general partner interest.
[3]	See Note 6 for distribution information.

Equity - Changes in Partners Ca

Equity - Changes in Partners Capital, Unit Rollforward (Details) - shares		3 Months Ended	9 Months Ended
		Sep. 30, 2015	Sep. 30, 2015
Stockholders Equity [Line Items]
Balance at December 31, 2014			81,931,238
Units converted	[1]		19,318
Balance at September 30, 2015		81,976,236	81,976,236
Limited Partners Common Units [Member]
Stockholders Equity [Line Items]
Balance at December 31, 2014			43,341,098
Units converted	[1]		18,932
Balance at September 30, 2015		80,336,711	80,336,711
Limited Partners Subordinated Units [Member]
Stockholders Equity [Line Items]
Balance at December 31, 2014			36,951,515
Balance at September 30, 2015		0	0
General Partner Units [Member]
Stockholders Equity [Line Items]
Balance at December 31, 2014			1,638,625
Units converted	[1]		386
Balance at September 30, 2015		1,639,525	1,639,525
ATM Program [Member]
Stockholders Equity [Line Items]
Issuance of units under the ATM Program(2)	[2]		25,680
ATM Program [Member] | Limited Partners Common Units [Member]
Stockholders Equity [Line Items]
Issuance of units under the ATM Program(2)	[2]		25,166
ATM Program [Member] | General Partner Units [Member]
Stockholders Equity [Line Items]
Issuance of units under the ATM Program(2)	[2]		514
Subordinated Units [Member]
Stockholders Equity [Line Items]
Units converted		36,951,515	0
Subordinated Units [Member] | Limited Partners Common Units [Member]
Stockholders Equity [Line Items]
Units converted			36,951,515
Subordinated Units [Member] | Limited Partners Subordinated Units [Member]
Stockholders Equity [Line Items]
Units converted			(36,951,515)
[1]	As a result of the unit-based compensation awards issued during the period, MPLX GP contributed less than $0.1 million in exchange for 386 general partner units to maintain its two percent general partner interest.
[2]	As a result of common units issued under the ATM Program during the period, MPLX GP contributed less than $0.1 million in exchange for 514 general partner units to maintain its two percent general partner interest.

Equity - Net Income Allocation

Equity - Net Income Allocation (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Equity [Abstract]
Net income attributable to MPLX LP	[1]	$ 41.5	$ 29.1	$ 138.3	$ 92.1
General partner's incentive distribution rights and other		7.9	1	16.9	1.9
Net income attributable to MPLX LP available to general and limited partners		33.6	28.1	121.4	90.2
General partner's two percent interest in net income attributable to MPLX LP		0.7	0.5	2.5	1.8
Less: General partner’s interest in net income attributable to MPLX LP		$ 8.6	$ 1.5	$ 19.4	$ 3.7
[1]	Allocation of net income attributable to MPLX LP assumes all earnings for the period had been distributed based on the current period distribution priorities.

Equity - Cash Distributions (De

Equity - Cash Distributions (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Incentive Distribution Made to Managing Member or General Partner [Line Items]
General partner's distributions	$ 1	$ 0.5	$ 2.5	$ 1.5
General partner's incentive distribution rights	7.8	1	16.5	1.9
Total general partner's distributions	8.8	1.5	19	3.4
Limited partners' distributions	37.7	26.4	106	76
Total cash distributions declared	46.5	27.9	125	79.4
Limited Partners Common Units [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Limited partners' distributions	37.7	13.2	74.6	38
Subordinated Units [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Limited partners' distributions	$ 0	$ 13.2	$ 31.4	$ 38

Other Items - Summary of Intere

Other Items - Summary of Interest and Other Financial (Costs) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Other Income and Expenses [Abstract]
Interest income	$ 0.1	$ 0	$ 0.2	$ 0
Interest expense	(5.9)	(1.2)	(18)	(3.1)
Interest capitalized	(1.2)	(0.2)	(2.6)	(0.7)
Other financial costs	(0.6)	(0.1)	(1.5)	(0.6)
Net interest and other financial costs	$ 5.2	$ 1.1	$ 16.7	$ 3

Property, Plant and Equipment -

Property, Plant and Equipment - Property, Plant and Equipment (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	$ 1,496.7	$ 1,361.7
Less accumulated depreciation	387.5	353.1
Net property, plant and equipment	1,109.2	1,008.6
Land [Member]
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	5.5	5.5
Pipelines and Related Assets [Member]
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	1,085	1,081.4
Storage and Delivery Facilities [Member]
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	197.7	165.8
Assets Under Construction [Member]
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	184.3	85.3
Other [Member]
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	$ 24.2	$ 23.7

Fair Value Measurements Fair 53

Fair Value Measurements Fair Value Measurements - Fair Values - Reported (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	$ 711.1	$ 635.6
Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	$ 748.3	$ 635

Debt - Summary of Outstanding B

Debt - Summary of Outstanding Borrowings (Detail) - USD ($) $ in Millions		Sep. 30, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Total		$ 759.1	$ 644.8
Unamortized debt issuance costs(1)	[1]	(4.1)	0
Unamortized discount		(1.7)	0
Amounts due within one year		(0.8)	(0.8)
Total long-term debt due after one year		752.5	644
Capital Lease Obligations [Member] | Marathon Pipe Line Llc [Member]
Debt Instrument [Line Items]
MPL - capital lease obligations due 2020		9.1	9.8
MPLX Revolver [Member] | Line of Credit [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		0	385
MPLX Term Loan [Member] | Unsecured Debt [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		250	250
Senior Notes Due Two Thousand Twenty Five [Member] | Senior Notes [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		500	0
Related Party Revolving Credit Agreement [Member] | MPLX Pipe Line Holdings LP [Member]
Debt Instrument [Line Items]
Pipe Line Holdings - revolving credit agreement due 2019		$ 0	$ 0
[1]	(1) We adopted the updated FASB debt issuance cost standard as of June 30, 2015. This has been applied retrospectively and there was no effect to the prior period presented.

Debt - Additional Information (

Debt - Additional Information (Detail) - USD ($) $ in Millions		Feb. 12, 2015	Sep. 30, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Unamortized Debt Issuance Expense	[1]		$ 4.1	$ 0
Marathon Pipe Line Llc [Member]
Debt Instrument [Line Items]
Capital Lease Due Date Year			2,020
MPLX Pipe Line Holdings LP [Member] | Related Party Revolving Credit Agreement [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date			Mar. 31, 2019
Senior Notes Due Two Thousand Twenty Five [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount		$ 500
Debt Instrument, Interest Rate, Stated Percentage		4.00%
Debt Instrument, Maturity Date		Feb. 15, 2025	Feb. 15, 2025
Proceeds from (Repayments of) Debt			$ 495
MPLX Revolver [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Expiration Date			Nov. 20, 2019
Proceeds from Lines of Credit			$ 30
Debt Instrument, Interest Rate, Effective Percentage			1.457%
Extinguishment of Debt, Amount			$ 415
Letters of Credit Outstanding, Amount			0
Line of Credit Facility, Remaining Borrowing Capacity			$ 1,000
Line of Credit Facility, Remaining Borrowing Capacity, Percentage			100.00%
MPLX Term Loan [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date			Nov. 20, 2019
Debt Instrument, Interest Rate, Effective Percentage			1.651%
Line of Credit [Member] | MPLX Revolver [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross			$ 0	$ 385
Maximum [Member] | MPLX Term Loan [Member] | Mplx Lp [Member] | Letter of Credit [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity			$ 250
[1]	(1) We adopted the updated FASB debt issuance cost standard as of June 30, 2015. This has been applied retrospectively and there was no effect to the prior period presented.

Supplemental Cash Flow Inform56

Supplemental Cash Flow Information - Summary of Supplemental Cash Flow Information (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Net cash provided by operating activities included:
Interest paid (net of amounts capitalized)	$ 12.4	$ 1.8
Income taxes paid	0.1	0
Non-cash investing and financing activities:
Net transfers of property, plant and equipment to materials and supplies inventories	0	0.7
Property, plant and equipment contributed by MPC	$ 0	$ 0.1

Supplemental Cash Flow Inform57

Supplemental Cash Flow Information - Summary of Reconciliation of Additions to Property, Plant and Equipment to Total Capital Expenditures (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Supplemental Cash Flow Elements [Abstract]
Additions to property, plant and equipment	$ 121.1	$ 47.1
Plus: Increase in capital accruals	17.5	5.9
Asset retirement expenditures	0.7	1
Total capital expenditures	$ 139.3	$ 54

Equity-Based Compensation Pla58

Equity-Based Compensation Plan Equity-Based Compensation Plan - Summary of Phantom Unit Award Activity (Details) - Phantom Units [Member]	9 Months Ended
	Sep. 30, 2015$ / sharesshares
Number of Units
Outstanding at December 31, 2014 | shares	100,769
Granted | shares	35,630
Settled | shares	32,313
Outstanding at September 30, 2015 | shares	104,086
Weighted Average Fair Value
Outstanding at December 31, 2014	$ 41.66
Granted (in USD per unit)	80.16
Settled (in USD per unit)	40.18
Outstanding at September 30, 2015	$ 55.30

Equity-Based Compensation Pla59

Equity-Based Compensation Plan - Additional Information (Detail) - Performance Shares [Member] - MPLX LP 2012 Incentive Compensation Plan [Member]	9 Months Ended
	Sep. 30, 2015$ / shares
Equity Transactions And Share Based Compensation [Line Items]
Performance units grant date fair value (in USD per unit)	$ 1.03
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 Pla60

Equity-Based Compensation Plan - Summary of Performance Unit Award Activity (Detail) - Performance Shares [Member]	9 Months Ended
	Sep. 30, 2015shares
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Outstanding at December 31, 2014	924,143
Granted	597,249
Outstanding at September 30, 2015	1,521,392

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Commitments And Contingencies [Line Items]
Accrued liabilities for remediation	$ 1.1	$ 0.6
Maximum potential undiscounted payments	1.9
Contractual commitments to acquire property, plant and equipment	49.6
Environmental Loss Contingency [Member] | MPC [Member]
Commitments And Contingencies [Line Items]
Receivables from MPC for indemnification of environmental costs	$ 0.1	$ 0

Subsequent Event - Additional I

Subsequent Event - Additional Information (Details) - Subsequent Event [Member] - USD ($) $ in Millions	Oct. 28, 2015	Oct. 27, 2015
Pipe Line Holding [Member]
Subsequent Event [Line Items]
Business Acquisition, Percentage of Voting Interests Acquired	0.50%
Business Combination, Consideration Transferred	$ 12
MPLX Revolver [Member] | Mplx Lp [Member]
Subsequent Event [Line Items]
Line of Credit Facility, Current Borrowing Capacity		$ 1,000
Debt Instrument, Term		5 years
Line of Credit Facility, Maximum Borrowing Capacity		$ 2,000