MPLX (MPLX) 10-Q 4 May 15 2015 Q1 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	3 Months Ended
	Mar. 31, 2015	Apr. 30, 2015
Document Information [Line Items]
Document Type	10-Q
Amendment Flag	FALSE
Document Period End Date	31-Mar-15
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q1
Trading Symbol	MPLX
Entity Registrant Name	MPLX LP
Entity Central Index Key	1552000
Current Fiscal Year End Date	-19
Entity Filer Category	Non-accelerated Filer
Common Units [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		43,358,428
Subordinated Units [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		36,951,515
General Partner Units [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		1,638,978

Consolidated_Statements_of_Inc

Consolidated Statements of Income (USD $)	3 Months Ended
In Millions, except Per Share data, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014
Revenues and other income:
Sales and other operating revenues	$15.60		$16.90
Sales to related parties	114.4		113.8
Other income	1.4		1.5
Other income - related parties	6.1		5.1
Total revenues and other income	137.5		137.3
Costs and expenses:
Cost of revenues (excludes items below)	27.8		26.6
Purchases from related parties	23.9		24
Depreciation	12.7		12.6
General and administrative expenses	18.7		15.9
Other taxes	3.2		1.9
Total costs and expenses	86.3		81
Income from operations	51.2		56.3
Net interest and other financial costs	5.3		0.6
Income before income taxes	45.9		55.7
Provision for income taxes	0		0
Net income	45.9		55.7
Less: Net income attributable to MPC-retained interest	0.3		21.5
Net income attributable to MPLX LP	45.6	[1]	34.2	[1]
Less: General partner’s interest in net income attributable to MPLX LP	4		1
Limited partners’ interest in net income attributable to MPLX LP	41.6		33.2
Weighted average limited partner units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic	81.9		75.4
Weighted Average Limited Partnership Units Outstanding, Diluted	82		75.4
MPC [Member]
Revenues and other income:
Sales to related parties	114.4		113.8
Other income - related parties	5.8		4.8
Limited Partners Common Units [Member]
Costs and expenses:
Net income attributable to MPLX LP	20.1	[1]	15.2	[1]
Per Unit Data (See Note 5)
Common - basic (in USD per unit)	$0.46		$0.41
Common - diluted (in USD per unit)	$0.46		$0.41
Weighted average limited partner units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic	43.3		37
Weighted Average Limited Partnership Units Outstanding, Diluted	43.4		37
Cash distributions declared per limited partner common unit	$0.41		$0.33
Limited Partners Subordinated Units [Member] | MPC [Member]
Costs and expenses:
Net income attributable to MPLX LP	$17.20	[1]	$15.20	[1]
Per Unit Data (See Note 5)
Common - basic (in USD per unit)	$0.46		$0.41
Common - diluted (in USD per unit)	$0.46		$0.41
Subordinated - basic and diluted (in USD per unit)	$0.46		$0.41
Weighted average limited partner units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic	37		37
Weighted Average Limited Partnership Units Outstanding, Diluted	37		37
Weighted average limited partner units outstanding basic and diluted (in shares)	37		37
[1]	Allocation of net income attributable to MPLX LP is based on the current period distribution priorities as if all earnings for the period had been distributed.

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Current assets:
Cash and cash equivalents	$132.50	$27.30
Receivables	8.6	10.2
Receivables from related parties	51.2	41
Materials and supplies inventories	11.5	11.7
Other current assets	6.1	7
Total current assets	209.9	97.2
Property, plant and equipment, net	1,031	1,008.60
Goodwill	104.7	104.7
Other noncurrent assets	8.4	4
Total assets	1,354	1,214.50
Current liabilities:
Accounts payable	44.9	42.2
Payables to related parties	21.8	20.2
Deferred revenue - related parties	33.4	30.5
Accrued taxes	7.1	5.2
Long-term debt due within one year	0.8	0.8
Other current liabilities	7	1.7
Total current liabilities	115	100.6
Long-term deferred revenue - related parties	4.6	4
Long-term debt	757	644
Deferred credits and other liabilities	1.4	2.2
Total liabilities	878	750.8
Commitments and contingencies (see Note 13)
Equity
Total MPLX LP partners’ capital	470.3	458.2
Noncontrolling interest retained by MPC	5.7	5.5
Total equity	476	463.7
Total liabilities and equity	1,354	1,214.50
MPC [Member]
Current assets:
Receivables from related parties	50.7	40.5
Current liabilities:
Payables to related parties	21.8	20.1
Limited Partners Common Units [Member] | MPC [Member]
Equity
Total MPLX LP partners’ capital	263.8	261.1
Limited Partners Subordinated Units [Member] | MPC [Member]
Equity
Total MPLX LP partners’ capital	222.5	217.5
General Partner [Member] | MPC [Member]
Equity
Total MPLX LP partners’ capital	-657.7	-659.4
Public [Member] | Limited Partners Common Units [Member]
Equity
Total MPLX LP partners’ capital	$641.70	$639

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical)	Mar. 31, 2015	Dec. 31, 2014
Units outstanding	1,638,976
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	20,000,000	20,000,000
Units outstanding	20,000,000	20,000,000
MPC [Member] | Limited Partners Subordinated Units [Member]
Units issued	37,000,000	37,000,000
Units outstanding	37,000,000	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_Cas

Consolidated Statements of Cash Flows (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Operating activities:
Net income	$45.90	$55.70
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	12.7	12.6
Asset retirement expenditures	-0.1	-0.2
Changes in:
Current receivables	1.6	0
Materials and supplies inventories	-0.1	0.3
Current accounts payable and accrued liabilities	5	-2.3
Receivables from / liabilities to related parties	-5.1	6.3
All other, net	-1.4	-0.5
Net cash provided by operating activities	58.5	71.9
Investing activities:
Additions to property, plant and equipment	-29.7	-5.1
All other, net	0.4	-1.5
Net cash used in investing activities	-29.3	-6.6
Financing activities:
Long-term debt - borrowings	528.2	270
Long-term debt - repayments	-415.2	-0.2
Debt issuance costs	3.8	0
Quarterly distributions to unitholders and general partner	-33.1	-23.7
Quarterly distributions to noncontrolling interest retained by MPC	-0.1	-14.9
Distributions related to purchase of additional interest in Pipe Line Holdings	0	-310
Net cash provided by (used in) financing activities	76	-78.8
Net increase (decrease) in cash and cash equivalents	105.2	-13.5
Cash and cash equivalents at beginning of period	27.3	54.1
Cash and cash equivalents at end of period	$132.50	$40.60

Consolidated_Statements_of_Equ

Consolidated Statements of Equity (USD $)	Total	Limited Partners Common Units [Member]	Limited Partners Common Units [Member]	Limited Partners Subordinated Units [Member]	General Partner [Member]	Noncontrolling Interest Retained by MPC [Member]	Noncontrolling Interest Retained by MPC [Member]
In Millions		Public [Member]	MPC [Member]	MPC [Member]	MPC [Member]		MPC [Member]
Beginning Balance at Dec. 31, 2013	$1,114.10	$412	$57.40	$209.30	($32.50)		$467.90
Purchase of additional interest in Pipe Line Holdings	-310				-172.5		-137.5
Net income	55.7	8.9	7.7	16.6	1		21.5
Quarterly distributions to unitholders and general partner	-23.7	-6.2	-5.4	-11.6	-0.5
Quarterly distributions to noncontrolling interest retained by MPC	-14.9						-14.9
Non-cash contribution from MPC	0.1						0.1
Equity-based compensation	-0.2	-0.2
Ending Balance at Mar. 31, 2014	821.1	414.5	59.7	214.3	-204.5		337.1
Beginning Balance at Dec. 31, 2014	463.7	639	261.1	217.5	-659.4		5.5
Net income	45.9	12.1	10.4	19.1	4		0.3
Quarterly distributions to unitholders and general partner	-33.1	-9	-7.7	-14.1	-2.3
Quarterly distributions to noncontrolling interest retained by MPC	-0.1						-0.1
Equity-based compensation	-0.4	-0.4
Ending Balance at Mar. 31, 2015	$476	$641.70	$263.80	$222.50	($657.70)		$5.70

Description_of_the_Business_an

Description of the Business and Basis of Presentation	3 Months Ended
	Mar. 31, 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 (the “Partnership”) 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 March 31, 2015, the Partnership’s 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. The Partnership also owns 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 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 ended March 31, 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 is described in Note 5.

Accounting_Standards

Accounting Standards	3 Months Ended
	Mar. 31, 2015
Accounting Standards [Abstract]
Accounting Standards	Accounting Standards
	Recently Adopted
	In April 2014, the Financial Accounting Standards Board (“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 standard update in the first quarter of 2015 did not have an impact on our consolidated results of operations, financial position or cash flows.
	Not Yet Adopted
	In April 2015, the FASB issued an 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. As a result, 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 April 2015, the FASB issued an update to simplify the presentation of debt issuance costs. The update requires that all debt issue costs be presented on the balance sheet as a direct reduction of the liability. 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. 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 way certain decisions are made 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. At this point, we have not determined the impact of the new standards update on our consolidated financial statements and related disclosures.
	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 issued on the same day. 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, 2016, and interim periods within those years, with no early adoption permitted. At this point, we have not determined the impact of the new standard on our consolidated financial statements.

Acquisitions

Acquisitions	3 Months Ended
	Mar. 31, 2015
Equity [Abstract]
Acquisitions	Acquisitions
	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 ended March 31, 2015 and 2014 were as follows:
		Three Months Ended March 31,
	(In millions)	2015		2014
	Net income attributable to MPLX LP	$	45.6	$	34.2
	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	$	45.6	$	(138.3	)

Related_Party_Agreements_and_T

Related Party Agreements and Transactions	3 Months Ended
	Mar. 31, 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 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 detailed descriptions of 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
	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 twelve months 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 March 31,
	(In millions)	2015		2014
	MPC	$	114.4	$	113.8
	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. Related party sales to MPC also consist of revenue related to volume deficiency credits.
	The fees received for operating pipelines for related parties included in other income-related parties on the consolidated statements of income were as follows:
		Three Months Ended March 31,
	(In millions)	2015		2014
	MPC	$	5.8	$	4.8
	Centennial	0.3		0.3
	Total	$	6.1	$	5.1
	MPC provides executive management services and certain general and administrative services to us under 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 March 31,
	(In millions)	2015		2014
	MPC	$	5.4	$	6
	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 March 31,
	(In millions)	2015		2014
	MPC	$	8.2	$	7.7
	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 March 31,
	(In millions)	2015		2014
	MPC	$	2.1	$	0.8
	MPLX 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 March 31,
	(In millions)	2015		2014
	Purchases from related parties	$	18.5	$	18
	General and administrative expenses	7.4		5.6
	Total	$	25.9	$	23.6
	Receivables from related parties were as follows:
	(In millions)	31-Mar-15		31-Dec-14
	MPC	$	50.7	$	40.5
	Centennial	0.3		0.3
	Muskegon	0.2		0.2
	Total	$	51.2	$	41
	Payables to related parties were as follows:
	(In millions)	31-Mar-15		31-Dec-14
	MPC	$	21.8	$	20.1
	Centennial	—		0.1
	Total	$	21.8	$	20.2
	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.
	During the three months ended March 31, 2015 and 2014, MPC did not ship its minimum committed volumes on certain 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)	31-Mar-15		31-Dec-14
	Minimum volume deficiencies - MPC	$	32.6	$	29.9
	Project reimbursements - MPC	5.4		4.6
	Total	$	38	$	34.5

Net_Income_Per_Limited_Partner

Net Income Per Limited Partner Unit	3 Months Ended
	Mar. 31, 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.
		Three Months Ended March 31,
	(In millions)	2015		2014
	Net income attributable to MPLX LP	$	45.6	$	34.2
	Less: General partner’s distributions declared (including IDRs) (1)	3.9		0.8
	Limited partners’ distributions declared on common units (1)	17.8		12.1
	Limited partner’s distributions declared on subordinated units (1)	15.2		12.1
	Undistributed net income attributable to MPLX LP	$	8.7	$	9.2
	(1)	See Note 6 for distribution information.
		Three Months Ended March 31, 2015
	(In millions, except per unit data)	General		Limited		Limited		Total
		Partner		Partners’		Partner’s
				Common		Subordinated
				Units		Units
	Basic and diluted net income attributable to MPLX LP per unit:
	Net income attributable to MPLX LP:
	Distributions declared (including IDRs)	$	3.9	$	17.8	$	15.2	$	36.9
	Undistributed net income attributable to MPLX LP	4.4		2.3		2		8.7
	Net income attributable to MPLX LP (1)	$	8.3	$	20.1	$	17.2	$	45.6
	Weighted average units outstanding:
	Basic	1.6		43.3		37		81.9
	Diluted	1.6		43.4		37		82
	Net income attributable to MPLX LP per limited partner unit:
	Basic			$	0.46	$	0.46
	Diluted			$	0.46	$	0.46
		Three Months Ended March 31, 2014
	(In millions, except per unit data)	General		Limited		Limited		Total
		Partner		Partners’		Partner’s
				Common		Subordinated
				Units		Units
	Basic and diluted net income attributable to MPLX LP per unit:
	Net income attributable to MPLX LP:
	Distributions declared (including IDRs)	$	0.8	$	12.1	$	12.1	$	25
	Undistributed net income attributable to MPLX LP	3		3.1		3.1		9.2
	Net income attributable to MPLX LP (1)	$	3.8	$	15.2	$	15.2	$	34.2
	Weighted average units outstanding:
	Basic	1.4		37		37		75.4
	Diluted	1.4		37		37		75.4
	Net income attributable to MPLX LP per limited partner unit:
	Basic			$	0.41	$	0.41
	Diluted			$	0.41	$	0.41
	(1)	Allocation of net income attributable to MPLX LP is based on the current period distribution priorities as if all earnings for the period had been distributed.

Equity_Notes

Equity (Notes)	3 Months Ended
	Mar. 31, 2015
Equity [Abstract]
Equity	Equity
	Units Outstanding - We had 43,358,322 common units outstanding as of March 31, 2015. Of that number, 19,980,619 were owned by MPC, which also owned 36,951,515 subordinated units and the two percent general partner interest, represented by 1,638,976 general partner units.
	The changes in the number of units outstanding from December 31, 2014 through March 31, 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)	17,224		—			351	17,575
	Balance at March 31, 2015	43,358,322		36,951,515			1,638,976	81,948,813
	(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 351 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 unitholders in accordance with their respective ownership percentages. However, when distributions related to the incentive distribution rights are made, earnings equal to the amount of those distributions are first allocated to the general partner before the remaining earnings are allocated to the unitholders based on their respective ownership percentages. The following table presents the allocation of the general partner’s interest in net income attributable to MPLX LP:
		Three Months Ended March 31,
	(In millions)	2015			2014
	Net income attributable to MPLX LP	$	45.6		$	34.2
	Less: General partner's incentive distribution rights	3.2			0.3
	Net income attributable to MPLX LP available to general and limited partners	$	42.4		$	33.9
	General partner's two percent interest in net income attributable to MPLX LP	$	0.8		$	0.7
	General partner's incentive distribution rights	3.2			0.3
	General partner's interest in net income attributable to MPLX LP	$	4		$	1
	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 April 20, 2015, we declared a quarterly cash distribution of $0.41 per unit, totaling $36.9 million. This distribution will be paid on May 15, 2015 to unitholders of record on May 5, 2015.
	The allocation of total quarterly cash distributions to general and limited partners is as follows for the three months ended March 31, 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 March 31,
	(In millions)	2015			2014
	General partner's distributions:
	General partner's distributions	$	0.7		$	0.5
	General partner's incentive distribution rights	3.2			0.3
	Total general partner's distributions	$	3.9		$	0.8
	Limited partners' distributions:
	Common unitholders	$	17.8		$	12.1
	Subordinated unitholders	15.2			12.1
	Total limited partners' distributions	33			24.2
	Total cash distributions declared	$	36.9		$	25

Other_Items

Other Items	3 Months Ended
	Mar. 31, 2015
Other Income and Expenses [Abstract]
Other Items	Other Items
	Net interest and other financial costs were:
		Three Months Ended March 31,
	(In millions)	2015			2014
	Interest expense	5.5			0.6
	Interest capitalized	(0.6		)	(0.2		)
	Other financial costs	0.4			0.2
	Net interest and other financial costs	$	5.3		$	0.6

Property_Plant_and_Equipment

Property, Plant and Equipment	3 Months Ended
	Mar. 31, 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)	31-Mar-15		31-Dec-14
	Land	$	5.5	$	5.5
	Pipelines and related assets	1,081.80		1,081.40
	Storage and delivery facilities	166.8		165.8
	Other	23.9		23.7
	Assets under construction	118.3		85.3
	Total	1,396.30		1,361.70
	Less accumulated depreciation	365.3		353.1
	Property, plant and equipment, net	$	1,031.00	$	1,008.60

Fair_Value_Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 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 March 31, 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, at March 31, 2015 and December 31, 2014.
		March 31, 2015				December 31, 2014
	(In millions)	Fair Value		Carrying Value		Fair Value		Carrying Value
	Long-term debt	$	756.7	$	748.2	$	635.6	$	635

Debt

Debt	3 Months Ended
	Mar. 31, 2015
Debt Disclosure [Abstract]
Debt	Debt
	Our outstanding borrowings at March 31, 2015 and December 31, 2014 consisted of the following:
	(In millions)	31-Mar-15		31-Dec-14
	MPLX LP:
	Bank revolving credit facility due 2019	—		385
	Term loan facility due 2019	250		250
	4.000% senior notes due 2025	500		—
	Consolidated subsidiaries:
	Pipe Line Holdings - revolving credit agreement due 2019	—		—
	MPL - capital lease obligations due 2020	9.6		9.8
	Total	759.6		644.8
	Unamortized discount	1.8		—
	Amounts due within one year	0.8		0.8
	Total long-term debt due after one year	$	757	$	644
	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 three months ended March 31, 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 March 31, 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 three months ended March 31, 2015 were at an average interest rate of 1.566 percent.
	On March 31, 2014, Pipe Line Holdings entered into a $50.0 million credit agreement with MPL Investment LLC, a subsidiary of MPC. As of March 31, 2015, there were no borrowings outstanding under this facility.

Supplemental_Cash_Flow_Informa

Supplemental Cash Flow Information	3 Months Ended
	Mar. 31, 2015
Supplemental Cash Flow Elements [Abstract]
Supplemental Cash Flow Information	Supplemental Cash Flow Information
		Three Months Ended March 31,
	(In millions)	2015			2014
	Net cash provided by operating activities included:
	Interest paid (net of amounts capitalized)	$	1.8		$	—
	Non-cash investing and financing activities:
	Net transfers of property, plant and equipment to (from) materials and supplies inventories	$	(0.3	)	$	0.4
	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:
		Three Months Ended March 31,
	(In millions)	2015			2014
	Additions to property, plant and equipment	$	29.7		$	5.1
	Plus: Increase in capital accruals	5			0.1
	Asset retirement expenditures	0.1			0.2
	Total capital expenditures	$	34.8		$	5.4

EquityBased_Compensation_Plan

Equity-Based Compensation Plan	3 Months Ended
	Mar. 31, 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 three months ended March 31, 2015:
		Number		Weighted
		of Units		Average
				Fair Value
	Outstanding at December 31, 2014	100,769		$	41.66
	Granted	30,659		81.83
	Settled	(30,548	)	39.87
	Outstanding at March 31, 2015	100,880		54.41
	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 $1.04 for 2015, as calculated using a Monte Carlo valuation model.
	The following is a summary of the equity-classified performance unit award activity of MPLX LP common limited partner units for the three months ended March 31, 2015:
		Number of
		Units
	Outstanding at December 31, 2014	924,143
	Granted	590,687
	Outstanding at March 31, 2015	1,514,830

Commitments_and_Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 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 March 31, 2015 and December 31, 2014, accrued liabilities for remediation totaled $2.1 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 $0.2 million at March 31, 2015. There were no receivables from MPC for indemnification at December 31, 2014.
	Legal Proceedings—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 March 31, 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 March 31, 2015, our contractual commitments to acquire property, plant and equipment totaled $61.5 million. Our contractual commitments at March 31, 2015 were primarily related to various projects including a butane cavern in Robinson, Illinois and tank expansion in Patoka, Illinois connecting to MPC’s Southern Access Extension.

Subsequent_Event

Subsequent Event	3 Months Ended
	Mar. 31, 2015
Subsequent Event [Abstract]
Subsequent Events [Text Block]	Subsequent Event
	On April 29, 2015, the board of directors of MPC authorized the sale of its marine business to MPLX. Subject to the approval of our board of directors, the negotiation of a definitive agreement and the satisfaction of customary closing conditions, the transaction is expected to close in the next several months.

Description_of_the_Business_an1

Description of the Business and Basis of Presentation Description of Business and Basis of Presentation (Policies)	3 Months Ended
	Mar. 31, 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 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)	3 Months Ended
	Mar. 31, 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 ended March 31, 2015 and 2014 were as follows:
		Three Months Ended March 31,
	(In millions)	2015		2014
	Net income attributable to MPLX LP	$	45.6	$	34.2
	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	$	45.6	$	(138.3	)

Related_Party_Agreements_and_T1

Related Party Agreements and Transactions (Tables)	3 Months Ended
	Mar. 31, 2015
Related Party Transactions [Abstract]
Sales to Related Parties	Sales to related parties were as follows:
		Three Months Ended March 31,
	(In millions)	2015		2014
	MPC	$	114.4	$	113.8
Summary of Fees Received for Operating Pipelines for Related Parties Included in Other Income - Related Parties	The fees received for operating pipelines for related parties included in other income-related parties on the consolidated statements of income were as follows:
		Three Months Ended March 31,
	(In millions)	2015		2014
	MPC	$	5.8	$	4.8
	Centennial	0.3		0.3
	Total	$	6.1	$	5.1
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 March 31,
	(In millions)	2015		2014
	MPC	$	5.4	$	6
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 March 31,
	(In millions)	2015		2014
	MPC	$	8.2	$	7.7
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 March 31,
	(In millions)	2015		2014
	MPC	$	2.1	$	0.8
Employee Services Expenses from Related Parties	Employee services expenses from related parties were as follows:
		Three Months Ended March 31,
	(In millions)	2015		2014
	Purchases from related parties	$	18.5	$	18
	General and administrative expenses	7.4		5.6
	Total	$	25.9	$	23.6
Receivables from Related Parties	Receivables from related parties were as follows:
	(In millions)	31-Mar-15		31-Dec-14
	MPC	$	50.7	$	40.5
	Centennial	0.3		0.3
	Muskegon	0.2		0.2
	Total	$	51.2	$	41
Payables to Related Parties	Payables to related parties were as follows:
	(In millions)	31-Mar-15		31-Dec-14
	MPC	$	21.8	$	20.1
	Centennial	—		0.1
	Total	$	21.8	$	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)	31-Mar-15		31-Dec-14
	Minimum volume deficiencies - MPC	$	32.6	$	29.9
	Project reimbursements - MPC	5.4		4.6
	Total	$	38	$	34.5

Net_Income_Per_Limited_Partner1

Net Income Per Limited Partner Unit (Tables)	3 Months Ended
	Mar. 31, 2015
Earnings Per Share [Abstract]
Schedule of Distributions By Partner By Class
		Three Months Ended March 31,
	(In millions)	2015		2014
	Net income attributable to MPLX LP	$	45.6	$	34.2
	Less: General partner’s distributions declared (including IDRs) (1)	3.9		0.8
	Limited partners’ distributions declared on common units (1)	17.8		12.1
	Limited partner’s distributions declared on subordinated units (1)	15.2		12.1
	Undistributed net income attributable to MPLX LP	$	8.7	$	9.2
	(1)	See Note 6 for distribution information.
Schedule of Basic and Diluted Earnings Per Unit
		Three Months Ended March 31, 2015
	(In millions, except per unit data)	General		Limited		Limited		Total
		Partner		Partners’		Partner’s
				Common		Subordinated
				Units		Units
	Basic and diluted net income attributable to MPLX LP per unit:
	Net income attributable to MPLX LP:
	Distributions declared (including IDRs)	$	3.9	$	17.8	$	15.2	$	36.9
	Undistributed net income attributable to MPLX LP	4.4		2.3		2		8.7
	Net income attributable to MPLX LP (1)	$	8.3	$	20.1	$	17.2	$	45.6
	Weighted average units outstanding:
	Basic	1.6		43.3		37		81.9
	Diluted	1.6		43.4		37		82
	Net income attributable to MPLX LP per limited partner unit:
	Basic			$	0.46	$	0.46
	Diluted			$	0.46	$	0.46
		Three Months Ended March 31, 2014
	(In millions, except per unit data)	General		Limited		Limited		Total
		Partner		Partners’		Partner’s
				Common		Subordinated
				Units		Units
	Basic and diluted net income attributable to MPLX LP per unit:
	Net income attributable to MPLX LP:
	Distributions declared (including IDRs)	$	0.8	$	12.1	$	12.1	$	25
	Undistributed net income attributable to MPLX LP	3		3.1		3.1		9.2
	Net income attributable to MPLX LP (1)	$	3.8	$	15.2	$	15.2	$	34.2
	Weighted average units outstanding:
	Basic	1.4		37		37		75.4
	Diluted	1.4		37		37		75.4
	Net income attributable to MPLX LP per limited partner unit:
	Basic			$	0.41	$	0.41
	Diluted			$	0.41	$	0.41
	(1)	Allocation of net income attributable to MPLX LP is based on the current period distribution priorities as if all earnings for the period had been distributed.

Equity_Tables

Equity (Tables)	3 Months Ended
	Mar. 31, 2015
Equity [Abstract]
Schedule of Stockholders Equity [Table Text Block]	The changes in the number of units outstanding from December 31, 2014 through March 31, 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)	17,224		—			351	17,575
	Balance at March 31, 2015	43,358,322		36,951,515			1,638,976	81,948,813
	(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 351 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 unitholders in accordance with their respective ownership percentages. However, when distributions related to the incentive distribution rights are made, earnings equal to the amount of those distributions are first allocated to the general partner before the remaining earnings are allocated to the unitholders based on their respective ownership percentages. The following table presents the allocation of the general partner’s interest in net income attributable to MPLX LP:
		Three Months Ended March 31,
	(In millions)	2015			2014
	Net income attributable to MPLX LP	$	45.6		$	34.2
	Less: General partner's incentive distribution rights	3.2			0.3
	Net income attributable to MPLX LP available to general and limited partners	$	42.4		$	33.9
	General partner's two percent interest in net income attributable to MPLX LP	$	0.8		$	0.7
	General partner's incentive distribution rights	3.2			0.3
	General partner's interest in net income attributable to MPLX LP	$	4		$	1
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 March 31,
	(In millions)	2015			2014
	General partner's distributions:
	General partner's distributions	$	0.7		$	0.5
	General partner's incentive distribution rights	3.2			0.3
	Total general partner's distributions	$	3.9		$	0.8
	Limited partners' distributions:
	Common unitholders	$	17.8		$	12.1
	Subordinated unitholders	15.2			12.1
	Total limited partners' distributions	33			24.2
	Total cash distributions declared	$	36.9		$	25

Other_Items_Tables

Other Items (Tables)	3 Months Ended
	Mar. 31, 2015
Other Income and Expenses [Abstract]
Summary of Interest and Other Financial (Costs)	Net interest and other financial costs were:
		Three Months Ended March 31,
	(In millions)	2015			2014
	Interest expense	5.5			0.6
	Interest capitalized	(0.6		)	(0.2		)
	Other financial costs	0.4			0.2
	Net interest and other financial costs	$	5.3		$	0.6

Property_Plant_and_Equipment_T

Property, Plant and Equipment (Tables)	3 Months Ended
	Mar. 31, 2015
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment	Our investment in property, plant and equipment with associated accumulated depreciation was:
	(In millions)	31-Mar-15		31-Dec-14
	Land	$	5.5	$	5.5
	Pipelines and related assets	1,081.80		1,081.40
	Storage and delivery facilities	166.8		165.8
	Other	23.9		23.7
	Assets under construction	118.3		85.3
	Total	1,396.30		1,361.70
	Less accumulated depreciation	365.3		353.1
	Property, plant and equipment, net	$	1,031.00	$	1,008.60

Fair_Value_Measurements_Fair_V

Fair Value Measurements Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 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, at March 31, 2015 and December 31, 2014.
		March 31, 2015				December 31, 2014
	(In millions)	Fair Value		Carrying Value		Fair Value		Carrying Value
	Long-term debt	$	756.7	$	748.2	$	635.6	$	635

Debt_Tables

Debt (Tables)	3 Months Ended
	Mar. 31, 2015
Debt Disclosure [Abstract]
Summary of Outstanding Borrowings	Our outstanding borrowings at March 31, 2015 and December 31, 2014 consisted of the following:
	(In millions)	31-Mar-15		31-Dec-14
	MPLX LP:
	Bank revolving credit facility due 2019	—		385
	Term loan facility due 2019	250		250
	4.000% senior notes due 2025	500		—
	Consolidated subsidiaries:
	Pipe Line Holdings - revolving credit agreement due 2019	—		—
	MPL - capital lease obligations due 2020	9.6		9.8
	Total	759.6		644.8
	Unamortized discount	1.8		—
	Amounts due within one year	0.8		0.8
	Total long-term debt due after one year	$	757	$	644

Supplemental_Cash_Flow_Informa1

Supplemental Cash Flow Information (Tables)	3 Months Ended
	Mar. 31, 2015
Supplemental Cash Flow Elements [Abstract]
Summary of Supplemental Cash Flow Information
		Three Months Ended March 31,
	(In millions)	2015			2014
	Net cash provided by operating activities included:
	Interest paid (net of amounts capitalized)	$	1.8		$	—
	Non-cash investing and financing activities:
	Net transfers of property, plant and equipment to (from) materials and supplies inventories	$	(0.3	)	$	0.4
	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:
		Three Months Ended March 31,
	(In millions)	2015			2014
	Additions to property, plant and equipment	$	29.7		$	5.1
	Plus: Increase in capital accruals	5			0.1
	Asset retirement expenditures	0.1			0.2
	Total capital expenditures	$	34.8		$	5.4

EquityBased_Compensation_Plan_

Equity-Based Compensation Plan (Tables)	3 Months Ended
	Mar. 31, 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 three months ended March 31, 2015:
		Number		Weighted
		of Units		Average
				Fair Value
	Outstanding at December 31, 2014	100,769		$	41.66
	Granted	30,659		81.83
	Settled	(30,548	)	39.87
	Outstanding at March 31, 2015	100,880		54.41
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 of MPLX LP common limited partner units for the three months ended March 31, 2015:
		Number of
		Units
	Outstanding at December 31, 2014	924,143
	Granted	590,687
	Outstanding at March 31, 2015	1,514,830

Description_of_Business_and_Ba

Description of Business and Basis of Presentation - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2015
	bbl
	Segment
Description Of Business And Basis Of Presentation [Line Items]
Storage capacity (in number of barrels)	1,000,000
Number of segments	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_Inform

Acquisitions - Additional Information (Detail) (USD $)	0 Months Ended		3 Months Ended
In Millions, unless otherwise specified	Dec. 01, 2014	Mar. 02, 2014	Mar. 31, 2015	Mar. 31, 2014	Dec. 01, 2014
Business Acquisition [Line Items]
Additional interest in Pipe Line Holdings	22.88%	13.00%
Purchase of additional interest in Pipe Line Holdings	$600	$310	$0	$310
Percentage Of Partners Interest Contributed	7.63%
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned	200				200
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	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	1-Dec-14	1-Mar-14
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_i

Acquisitions - Summary Changes in MPLX LP's Ownership Interest in Pipe Line Holdings (Detail) (USD $)	0 Months Ended		3 Months Ended
In Millions, unless otherwise specified	Dec. 01, 2014	Mar. 02, 2014	Mar. 31, 2015		Mar. 31, 2014
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Net income attributable to MPLX LP			$45.60	[1]	$34.20	[1]
Transfer to noncontrolling interest retained by MPC:
Change from net income attributable to MPLX LP and transfer to noncontrolling interest retained by MPC			45.6		-138.3
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	($465.20)	($172.50)	$0		($172.50)
[1]	Allocation of net income attributable to MPLX LP is based on the current period distribution priorities as if all earnings for the period had been distributed.

Related_Party_Agreements_and_T2

Related Party Agreements and Transactions - Additional Information (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Related Party Transaction [Line Items]
Revenue from Related Parties	$114.40	$113.80
MPC [Member]
Related Party Transaction [Line Items]
Revenue from Related Parties	114.4	113.8
Centennial [Member] | MPC [Member]
Related Party Transaction [Line Items]
Equity method investments, ownership percentage	50.00%
Muskegon [Member] | MPC [Member]
Related Party Transaction [Line Items]
Equity method investments, ownership percentage	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]
Revenue from Related Parties	$12.20

Related_Party_Agreements_and_T3

Related Party Agreements and Transactions - Sales to Related Parties (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Related Party Transaction [Line Items]
Sales to related parties	$114.40	$113.80
MPC [Member]
Related Party Transaction [Line Items]
Sales to related parties	$114.40	$113.80

Related_Party_Agreements_and_T4

Related Party Agreements and Transactions - Summary of Fees Received for Operating Pipelines for Related Parties Included in Other Income - Related Parties (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Related Party Transaction [Line Items]
Other income - related parties	$6.10	$5.10
MPC [Member]
Related Party Transaction [Line Items]
Other income - related parties	5.8	4.8
Centennial [Member]
Related Party Transaction [Line Items]
Other income - related parties	$0.30	$0.30

Related_Party_Agreements_and_T5

Related Party Agreements and Transaction - Summary of Charges for Service Included in Purchases from Related Parties (Detail) (MPC [Member], USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
MPC [Member]
Related Party Transaction [Line Items]
Charges for services included in purchases from related party	$5.40	$6

Related_Party_Agreements_and_T6

Related Party Agreements and Transactions - Summary of Related Party Charges for Services Included in General and Administrative Expenses (Detail) (MPC [Member], USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
MPC [Member]
Related Party Transaction [Line Items]
Charges for services included in general and administrative expenses	$8.20	$7.70

Related_Party_Agreements_and_T7

Related Party Agreements and Transactions - Summary of Related Party Costs Added to Property, Plant and Equipment (Detail) (MPC [Member], Assets Under Construction [Member], USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
MPC [Member] | Assets Under Construction [Member]
Property, Plant and Equipment [Line Items]
Costs added to property, plant and equipment	$2.10	$0.80

Related_Party_Agreements_and_T8

Related Party Agreements and Transactions - Employee Services Expenses from Related Parties (Detail) (MPC [Member], USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	$25.90	$23.60
Purchases from Related Parties [Member]
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	18.5	18
General and Administrative Expenses [Member]
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	$7.40	$5.60

Related_Party_Agreements_and_T9

Related Party Agreements and Transactions - Receivables from Related Parties (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Related Party Transaction [Line Items]
Current receivables from related parties	$51.20	$41
MPC [Member]
Related Party Transaction [Line Items]
Current receivables from related parties	50.7	40.5
Centennial [Member]
Related Party Transaction [Line Items]
Current receivables from related parties	0.3	0.3
Muskegon [Member]
Related Party Transaction [Line Items]
Current receivables from related parties	$0.20	$0.20

Recovered_Sheet1

Related Party Agreements and Transactions - Payables to Related Parties (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Related Party Transaction [Line Items]
Payables to related parties	$21.80	$20.20
MPC [Member]
Related Party Transaction [Line Items]
Payables to related parties	21.8	20.1
Centennial [Member]
Related Party Transaction [Line Items]
Payables to related parties	$0	$0.10

Recovered_Sheet2

Related Party Agreements and Transactions - Summary of Deferred Revenue - Related Parties (Detail) (MPC [Member], USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Related Party Transaction [Line Items]
Deferred Revenue Related Parties	$38	$34.50
Minimum volume deficiencies [Member]
Related Party Transaction [Line Items]
Deferred Revenue Related Parties	32.6	29.9
Project Reimbursements [Member]
Related Party Transaction [Line Items]
Deferred Revenue Related Parties	$5.40	$4.60

Net_Income_Per_Limited_Partner2

Net Income Per Limited Partner Unit - Schedule of Distributions by Partner by Class (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014
Net Income Per Share [Line Items]
Net income attributable to MPLX LP	$45.60	[1]	$34.20	[1]
Less: Distribution declared	-36.9		-25
Undistributed net income attributable to MPLX LP	8.7		9.2
General Partner [Member] | MPC [Member]
Net Income Per Share [Line Items]
Net income attributable to MPLX LP	8.3	[1]	3.8	[1]
Less: Distribution declared	-3.9	[2]	-0.8	[2]
Undistributed net income attributable to MPLX LP	4.4		3
Limited Partners Common Units [Member]
Net Income Per Share [Line Items]
Net income attributable to MPLX LP	20.1	[1]	15.2	[1]
Less: Distribution declared	-17.8	[2]	-12.1	[2]
Undistributed net income attributable to MPLX LP	2.3		3.1
Limited Partners Subordinated Units [Member] | MPC [Member]
Net Income Per Share [Line Items]
Net income attributable to MPLX LP	17.2	[1]	15.2	[1]
Less: Distribution declared	-15.2	[2]	-12.1	[2]
Undistributed net income attributable to MPLX LP	$2		$3.10
[1]	Allocation of net income attributable to MPLX LP is based on the current period distribution priorities as if all earnings for the period had been distributed.
[2]	See Note 6 for distribution information.

Net_Income_Per_Limited_Partner3

Net Income Per Limited Partner Unit - Basic and Diluted Earnings Per Unit (Detail) (USD $)	3 Months Ended
In Millions, except Per Share data, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014
Net income attributable to MPLX LP:
Distributions declared (including IDRs)	$36.90		$25
Undistributed net income attributable to MPLX LP	8.7		9.2
Net income attributable to MPLX LP	45.6	[1]	34.2	[1]
Weighted average units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic	81.9		75.4
Weighted Average Limited Partnership Units Outstanding, Diluted	82		75.4
Limited Partners Common Units [Member]
Net income attributable to MPLX LP:
Distributions declared (including IDRs)	17.8	[2]	12.1	[2]
Undistributed net income attributable to MPLX LP	2.3		3.1
Net income attributable to MPLX LP	20.1	[1]	15.2	[1]
Weighted average units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic	43.3		37
Weighted Average Limited Partnership Units Outstanding, Diluted	43.4		37
Net income attributable to MPLX LP per limited partner unit:
Basic (in USD per unit)	$0.46		$0.41
Diluted (in USD per unit)	$0.46		$0.41
MPC [Member] | General Partner [Member]
Net income attributable to MPLX LP:
Distributions declared (including IDRs)	3.9	[2]	0.8	[2]
Undistributed net income attributable to MPLX LP	4.4		3
Net income attributable to MPLX LP	8.3	[1]	3.8	[1]
Weighted average units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic	1.6		1.4
Weighted Average Limited Partnership Units Outstanding, Diluted	1.6		1.4
MPC [Member] | Limited Partners Subordinated Units [Member]
Net income attributable to MPLX LP:
Distributions declared (including IDRs)	15.2	[2]	12.1	[2]
Undistributed net income attributable to MPLX LP	2		3.1
Net income attributable to MPLX LP	$17.20	[1]	$15.20	[1]
Weighted average units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic	37		37
Weighted Average Limited Partnership Units Outstanding, Diluted	37		37
Net income attributable to MPLX LP per limited partner unit:
Basic (in USD per unit)	$0.46		$0.41
Diluted (in USD per unit)	$0.46		$0.41
[1]	Allocation of net income attributable to MPLX LP is based on the current period distribution priorities as if all earnings for the period had been distributed.
[2]	See Note 6 for distribution information.

Equity_Details

Equity (Details) (USD $)	3 Months Ended		0 Months Ended
In Millions, except Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	15-May-15	5-May-15	Apr. 20, 2015
Stockholders Equity Note [Line Items]
Limited Liability Company (LLC) or Limited Partnership (LP), Managing Member or General Partner, Ownership Interest	2.00%
Units outstanding	1,638,976
Distributions declared (including IDRs)	$36.90	$25
Limited Partners Common Units [Member]
Stockholders Equity Note [Line Items]
Units outstanding	43,358,322
Subordinated Units [Member]
Stockholders Equity Note [Line Items]
Other Ownership Interests, Units Outstanding	36,951,515
MPC [Member] | Limited Partners Common Units [Member]
Stockholders Equity Note [Line Items]
Units outstanding	19,980,619
Subsequent Event [Member]
Stockholders Equity Note [Line Items]
Distribution Made to Limited Partner, Declaration Date					20-Apr-15
Cash distributions declared per limited partner common unit					$0.41
Distributions declared (including IDRs)					36.9
Distribution Made to Limited Partner, Distribution Date			15-May-15
Distribution Made to Limited Partner, Date of Record				5-May-15
General Partner Units [Member]
Stockholders Equity Note [Line Items]
Partners' Capital Account, Sale of Units	$0.10
Partners' Capital Account, Units, Sale of Units	351

Equity_Changes_in_Partners_Cap

Equity - Changes in Partners Capital, Unit Rollforward (Details)	3 Months Ended
	Mar. 31, 2015		Dec. 31, 2014
Stockholders Equity [Line Items]
Balance at December 31, 2014	81,931,238
Unit-based compensation awards(1)	17,575	[1]
Balance at March 31, 2015	81,948,813
Limited Partners Common Units [Member]
Stockholders Equity [Line Items]
Balance at December 31, 2014	43,341,098
Unit-based compensation awards(1)	17,224	[1]
Balance at March 31, 2015	43,358,322
Limited Partners Subordinated Units [Member]
Stockholders Equity [Line Items]
Balance at December 31, 2014			36,951,515
Balance at March 31, 2015	36,951,515		36,951,515
General Partner Units [Member]
Stockholders Equity [Line Items]
Balance at December 31, 2014	1,638,625
Unit-based compensation awards(1)	351	[1]
Balance at March 31, 2015	1,638,976
[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 351 general partner units to maintain its two percent general partner interest.

Equity_Net_Income_Allocation_D

Equity - Net Income Allocation (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014
Equity [Abstract]
Net income attributable to MPLX LP	$45.60	[1]	$34.20	[1]
General partner's incentive distribution rights	3.2		0.3
Net income attributable to MPLX LP available to general and limited partners	42.4		33.9
General partner's two percent interest in net income attributable to MPLX LP	0.8		0.7
Less: General partner’s interest in net income attributable to MPLX LP	$4		$1
[1]	Allocation of net income attributable to MPLX LP is based on the current period distribution priorities as if all earnings for the period had been distributed.

Equity_Cash_Distributions_Deta

Equity - Cash Distributions (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Incentive Distribution Made to Managing Member or General Partner [Line Items]
General partner's distributions	$0.70	$0.50
General partner's incentive distribution rights	3.2	0.3
Total general partner's distributions	3.9	0.8
Limited partners' distributions	33	24.2
Total cash distributions declared	36.9	25
Limited Partners Common Units [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Limited partners' distributions	17.8	12.1
Subordinated Units [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Limited partners' distributions	$15.20	$12.10

Other_Items_Summary_of_Interes

Other Items - Summary of Interest and Other Financial (Costs) (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Other Income and Expenses [Abstract]
Interest expense	($5.50)	($0.60)
Interest capitalized	-0.6	-0.2
Other financial costs	-0.4	-0.2
Net interest and other financial costs	$5.30	$0.60

Property_Plant_and_Equipment_P

Property, Plant and Equipment - Property, Plant and Equipment (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	$1,396.30	$1,361.70
Less accumulated depreciation	365.3	353.1
Net property, plant and equipment	1,031	1,008.60
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	1,081.80	1,081.40
Storage and Delivery Facilities [Member]
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	166.8	165.8
Other [Member]
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	23.9	23.7
Assets Under Construction [Member]
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	$118.30	$85.30

Fair_Value_Measurements_Fair_V1

Fair Value Measurements Fair Value Measurements - Fair Values - Reported (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	$756.70	$635.60
Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	$748.20	$635

Debt_Summary_of_Outstanding_Bo

Debt - Summary of Outstanding Borrowings (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Debt Instrument [Line Items]
Total	$759.60	$644.80
Unamortized discount	1.8	0
Amounts due within one year	0.8	0.8
Total long-term debt due after one year	757	644
Capital Lease Obligations [Member] | Marathon Pipe Line Llc [Member]
Debt Instrument [Line Items]
4.000% senior notes due 2025	9.6	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]
Long-term Line of Credit	$0	$0

Debt_Additional_Information_De

Debt - Additional Information (Detail) (USD $)	3 Months Ended	0 Months Ended
	Mar. 31, 2015	Feb. 12, 2015	Dec. 31, 2014
Related Party Revolving Credit Agreement [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity	50,000,000
Marathon Pipe Line Llc [Member]
Debt Instrument [Line Items]
Capital Lease Due Date Year	2020
MPLX Pipe Line Holdings LP [Member] | Related Party Revolving Credit Agreement [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	31-Mar-19
Long-term Line of Credit	0		0
Senior Notes Due Two Thousand Twenty Five [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount		500,000,000
Debt Instrument, Interest Rate, Stated Percentage		4.00%
Debt Instrument, Maturity Date	15-Feb-25	15-Feb-25
Proceeds from (Repayments of) Debt	495,000,000
MPLX Revolver [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Expiration Date	20-Nov-19
Proceeds from Lines of Credit	30,000,000
Debt Instrument, Interest Rate, Effective Percentage	1.46%
Extinguishment of Debt, Amount	415,000,000
Letters of Credit Outstanding, Amount	0
Line of Credit Facility, Remaining Borrowing Capacity	1,000,000,000
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	20-Nov-19
Debt Instrument, Interest Rate, Effective Percentage	1.57%
Line of Credit [Member] | MPLX Revolver [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	0		385,000,000
Maximum [Member] | MPLX Term Loan [Member] | Mplx Lp [Member] | Letter of Credit [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity	250,000,000

Supplemental_Cash_Flow_Informa2

Supplemental Cash Flow Information - Summary of Supplemental Cash Flow Information (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Net cash provided by operating activities included:
Interest paid (net of amounts capitalized)	$1.80	$0
Non-cash investing and financing activities:
Net transfers of property, plant and equipment to (from) materials and supplies inventories	-0.3	0.4
Property, plant and equipment contributed by MPC	$0	$0.10

Supplemental_Cash_Flow_Informa3

Supplemental Cash Flow Information - Summary of Reconciliation of Additions to Property, Plant and Equipment to Total Capital Expenditures (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Supplemental Cash Flow Elements [Abstract]
Additions to property, plant and equipment	$29.70	$5.10
Plus: Increase in capital accruals	5	0.1
Asset retirement expenditures	0.1	0.2
Total capital expenditures	$34.80	$5.40

EquityBased_Compensation_Plan_1

Equity-Based Compensation Plan Equity-Based Compensation Plan - Summary of Phantom Unit Award Activity (Details) (Phantom Units [Member], USD $)	3 Months Ended
	Mar. 31, 2015
Phantom Units [Member]
Number of Units
Outstanding at December 31, 2014	100,769
Granted	30,659
Settled	30,548
Outstanding at March 31, 2015	100,880
Weighted Average Fair Value
Outstanding at December 31, 2014	$41.66
Granted (in USD per unit)	$81.83
Settled (in USD per unit)	$39.87
Outstanding at March 31, 2015	$54.41

EquityBased_Compensation_Plan_2

Equity-Based Compensation Plan - Additional Information (Detail) (Performance Shares [Member], MPLX LP 2012 Incentive Compensation Plan [Member], USD $)	3 Months Ended
	Mar. 31, 2015
Equity Transactions And Share Based Compensation [Line Items]
Performance units grant date fair value (in USD per unit)	$1.04
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%

EquityBased_Compensation_Plan_3

Equity-Based Compensation Plan - Summary of Performance Unit Award Activity (Detail) (Performance Shares [Member])	3 Months Ended
	Mar. 31, 2015
Performance Shares [Member]
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	590,687
Outstanding at March 31, 2015	1,514,830

Commitments_and_Contingencies_

Commitments and Contingencies - Additional Information (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Commitments And Contingencies [Line Items]
Accrued liabilities for remediation	$2.10	$0.60
Maximum potential undiscounted payments	1.9
Contractual commitments to acquire property, plant and equipment	61.5
Environmental Loss Contingency [Member] | MPC [Member]
Commitments And Contingencies [Line Items]
Receivables from MPC for indemnification of environmental costs	$0.20	$0

Subsequent_Event_Additional_In

Subsequent Event - Additional Information (Details) (Subsequent Event [Member])	0 Months Ended
	Apr. 29, 2015
Subsequent Event [Member]
Subsequent Event [Line Items]
Subsequent Event, Date	29-Apr-15