MPLX (MPLX) 10-Q 3 Nov 14 2014 Q3 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	9 Months Ended
	Sep. 30, 2014	Oct. 31, 2014	Oct. 31, 2014	Oct. 31, 2014
		Common Units [Member]	Limited Partners Subordinated Units [Member]	General Partner Units [Member]
Document Information [Line Items]	'	'	'	'
Document Type	'10-Q	'	'	'
Amendment Flag	'false	'	'	'
Document Period End Date	30-Sep-14	'	'	'
Document Fiscal Year Focus	'2014	'	'	'
Document Fiscal Period Focus	'Q3	'	'	'
Trading Symbol	'MPLX	'	'	'
Entity Registrant Name	'MPLX LP	'	'	'
Entity Central Index Key	'0001552000	'	'	'
Current Fiscal Year End Date	'--12-31	'	'	'
Entity Filer Category	'Non-accelerated Filer	'	'	'
Entity Common Stock, Shares Outstanding	'	36,966,994	36,951,515	1,508,541

Consolidated_Statements_of_Inc

Consolidated Statements of Income (USD $)	3 Months Ended		9 Months Ended
In Millions, except Per Share data, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Revenues and other income:	'	'	'	'
Sales and other operating revenues	$16.60	$20.40	$52.20	$60
Sales to related parties	114.1	97.8	336	284.2
Other income	1.5	1	4.1	3.1
Other income - related parties	5.8	4.6	16.9	13.4
Total revenues and other income	138	123.8	409.2	360.7
Costs and expenses:	'	'	'	'
Cost of revenues (excludes items below)	40.6	33.1	102.9	99.2
Purchases from related parties	23.7	23.8	71.4	69.4
Depreciation	12.5	12.7	37.5	36.3
General and administrative expenses	15.3	13	47.1	40.4
Other taxes	1.7	1.4	5.5	4.9
Total costs and expenses	93.8	84	264.4	250.2
Income from operations	44.2	39.8	144.8	110.5
Net interest and other financial costs	1.1	0.2	3	0.7
Income before income taxes	43.1	39.6	141.8	109.8
Provision for income taxes	0	-0.4	-0.1	-0.5
Net income	43.1	39.2	141.7	109.3
Less: Net income attributable to MPC-retained interest	14	17.7	49.6	51.6
Net income attributable to MPLX LP	29.1	21.5	92.1	57.7
Less: General partnerbs interest in net income attributable to MPLX LP	1.5	0.5	3.7	1.2
Limited partnersb interest in net income attributable to MPLX LP	27.6	21	88.4	56.5
Weighted average limited partner units outstanding:	'	'	'	'
Weighted Average Limited Partnership Units Outstanding, Basic	75.5	75.4	75.4	75.4
Weighted Average Limited Partnership Units Outstanding, Diluted	75.5	75.4	75.5	75.4
MPC [Member]	'	'	'	'
Revenues and other income:	'	'	'	'
Sales to related parties	114.1	97.8	336	284.2
Other income - related parties	5.5	4.3	16	12.5
Limited Partners Common Units [Member]	'	'	'	'
Costs and expenses:	'	'	'	'
Net income attributable to MPLX LP	13.7	10.5	42.8	29.1
Per Unit Data (See Note 5)	'	'	'	'
Common - basic (in USD per unit)	$0.37	$0.29	$1.16	$0.79
Common - diluted (in USD per unit)	$0.37	$0.29	$1.16	$0.79
Weighted average limited partner units outstanding:	'	'	'	'
Weighted Average Limited Partnership Units Outstanding, Basic	37.1	37	37	37
Weighted Average Limited Partnership Units Outstanding, Diluted	37.1	37	37.1	37
Cash distributions declared per limited partner common unit	$0.36	$0.30	$1.03	$0.86
Limited Partners Subordinated Units [Member] | MPC [Member]	'	'	'	'
Costs and expenses:	'	'	'	'
Net income attributable to MPLX LP	$13.60	$10.50	$42.70	$27.40
Per Unit Data (See Note 5)	'	'	'	'
Common - basic (in USD per unit)	$0.37	$0.29	$1.16	$0.74
Common - diluted (in USD per unit)	$0.37	$0.29	$1.16	$0.74
Subordinated - basic and diluted (in USD per unit)	$0.37	$0.29	$1.16	$0.74
Weighted average limited partner units outstanding:	'	'	'	'
Weighted Average Limited Partnership Units Outstanding, Basic	37	37	37	37
Weighted Average Limited Partnership Units Outstanding, Diluted	37	37	37	37
Weighted average limited partner units outstanding basic and diluted (in shares)	37	37	37	37

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Sep. 30, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Current assets:	'	'
Cash and cash equivalents	$32.20	$54.10
Receivables	8.9	12.2
Receivables from related parties	45.9	48.3
Materials and supplies inventories	11.6	11.6
Other current assets	6.9	8.9
Total current assets	105.5	135.1
Property, plant and equipment, net	982.6	966.6
Goodwill	104.7	104.7
Other noncurrent assets	1.5	2.1
Total assets	1,194.30	1,208.50
Current liabilities:	'	'
Accounts payable	34.9	30.5
Payables to related parties	17.3	12.8
Deferred revenue - related parties	30.1	34
Accrued taxes	5.9	4
Long-term debt due within one year	0.8	0.7
Other current liabilities	1.8	1.4
Total current liabilities	90.8	83.4
Long-term deferred revenue - related parties	4.2	0
Long-term debt	264.2	9.8
Deferred credits and other liabilities	1.8	1.2
Total liabilities	361	94.4
Commitments and contingencies (see Note 14)	'	'
Equity	'	'
Total MPLX LP partnersb capital	491.5	646.2
Noncontrolling interest retained by MPC	341.8	467.9
Total equity	833.3	1,114.10
Total liabilities and equity	1,194.30	1,208.50
MPC [Member]	'	'
Current assets:	'	'
Receivables from related parties	45.4	47.4
Current liabilities:	'	'
Payables to related parties	17.3	12.8
Limited Partners Common Units [Member] | MPC [Member]	'	'
Equity	'	'
Total MPLX LP partnersb capital	61	57.4
Limited Partners Subordinated Units [Member] | MPC [Member]	'	'
Equity	'	'
Total MPLX LP partnersb capital	217.1	209.3
General Partner [Member] | MPC [Member]	'	'
Equity	'	'
Total MPLX LP partnersb capital	-203.7	-32.5
Public [Member] | Limited Partners Common Units [Member]	'	'
Equity	'	'
Total MPLX LP partnersb capital	$417.10	$412

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical)	Sep. 30, 2014	Dec. 31, 2013
Units outstanding	1,508,541	'
Limited Partners Common Units [Member] | Public [Member]	'	'
Units issued	19,900,000	19,900,000
Units outstanding	19,900,000	19,900,000
MPC [Member] | Limited Partners Common Units [Member]	'	'
Units issued	17,100,000	17,100,000
Units outstanding	17,100,000	17,100,000
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	1,500,000	1,500,000
Units outstanding	1,500,000	1,500,000

Consolidated_Statements_of_Cas

Consolidated Statements of Cash Flows (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013
Operating activities:	'	'
Net income	$141.70	$109.30
Adjustments to reconcile net income to net cash provided by operating activities:	'	'
Depreciation	37.5	36.3
Deferred income taxes	0	0.5
Asset retirement expenditures	-1	-3.4
Changes in:	'	'
Current receivables	3.3	3.3
Materials and supplies inventories	0.7	1.1
Current accounts payable and accrued liabilities	0.9	2.7
Receivables from / liabilities to related parties	7.2	16.9
All other, net	0	0.4
Net cash provided by operating activities	190.3	167.1
Investing activities:	'	'
Additions to property, plant and equipment	-47.1	-81
All other, net	4	0
Net cash used in investing activities	-43.1	-81
Financing activities:	'	'
Long-term debt - borrowings	270	0
Long-term debt - repayments	-15.6	-0.6
Quarterly distributions to unitholders and general partner	-75.2	-55.4
Quarterly distributions to noncontrolling interest retained by MPC	-38.3	-59.9
Distributions related to purchase of additional interest in Pipe Line Holdings	-310	-100
Net cash used in financing activities	-169.1	-215.9
Net decrease in cash and cash equivalents	-21.9	-129.8
Cash and cash equivalents at beginning of period	54.1	216.7
Cash and cash equivalents at end of period	$32.20	$86.90

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, 2012	$1,226.80	$410.70	$57.40	$209.30	$13.70	'	$535.70
Purchase of additional interest in Pipe Line Holdings	-100	'	'	'	-46.4	'	-53.6
Net income	109.3	15.2	13.1	28.2	1.2	'	51.6
Quarterly distributions to unitholders and general partner	-55.4	-14.7	-12.6	-27.1	-1	'	'
Quarterly distributions to noncontrolling interest retained by MPC	-59.9	'	'	'	'	'	-59.9
Property, plant and equipment contributed by MPC	0.2	'	'	'	'	'	0.2
Equity-based compensation	0.8	0.8	'	'	'	'	'
Ending Balance at Sep. 30, 2013	1,121.80	412	57.9	210.4	-32.5	'	474
Beginning Balance at Dec. 31, 2013	1,114.10	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
Property, plant and equipment contributed by MPC	0.1	'	'	'	'	'	0.1
Equity-based compensation	0.9	0.9	'	'	'	'	'
Ending Balance at Sep. 30, 2014	$833.30	$417.10	$61	$217.10	($203.70)	'	$341.80

Description_of_the_Business_an

Description of the Business and Basis of Presentation	9 Months Ended
	Sep. 30, 2014
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 September 30, 2014, the Partnership’s assets consisted of a 69 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 storage capacity.
	The Partnership was formed on March 27, 2012, as a Delaware limited partnership. On October 31, 2012, the Partnership completed its initial public offering (the “Offering”) of 19,895,000 common units (including 2,595,000 common units issued pursuant to the exercise of the underwriters’ over-allotment option), representing limited partner interests. Unless the context otherwise requires, references in this report to “MPLX LP,” the “Partnership,” “we,” “our,” “us,” or like terms refer to MPLX LP and its subsidiaries, including MPLX Operations LLC (“MPLX Operations”) and MPLX Terminal and Storage LLC (“MPLX Terminal and Storage”), both wholly-owned subsidiaries, and MPLX Pipe Line Holdings LP (“Pipe Line Holdings”), of which MPLX LP owned a 69 percent general partner interest at September 30, 2014. Initially, 51 percent of Pipe Line Holdings was contributed to the Partnership. Due to subsequent acquisitions, this interest increased to 56 percent as of May 1, 2013 and 69 percent as of March 1, 2014, as discussed in Note 3. Pipe Line Holdings owns 100 percent of Marathon Pipe Line LLC (“MPL”) and Ohio River Pipe Line LLC (“ORPL”). References to “MPC” refer collectively to Marathon Petroleum Corporation and its subsidiaries, other than the Partnership.
	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, 2013. The results of operations for the three months and nine months ended September 30, 2014 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	9 Months Ended
	Sep. 30, 2014
Accounting Standards [Abstract]	'
Accounting Standards	'
	Accounting Standards
	Not Yet Adopted
	In August 2014, the Financial Accounting Standards Board ("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, 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, a final determination has not been made as to the impact of the adoption of this standards update in the first quarter of 2017. However, we do not expect it to have a material 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 which 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 is effective prospectively for annual periods beginning on or after December 15, 2014, and interim periods within those years. Early adoption is permitted. Adoption of this standards update in the first quarter of 2015 is not expected to have an impact on our consolidated results of operations, financial position or cash flows.

Acquisitions

Acquisitions	9 Months Ended
	Sep. 30, 2014
Equity [Abstract]	'
Acquisitions	'
	Acquisitions
	Effective 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 revolver. We recorded the 13 percent interest 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. Prior to this transaction, the 13 percent interest was held by MPC and was reflected as part of the noncontrolling interest retained by MPC in our consolidated financial statements. Beginning March 1, 2014, our consolidated financial statements reflect the 69 percent general partner interest in Pipe Line Holdings owned by MPLX LP, while the 31 percent limited partner interest held by MPC is reflected as a noncontrolling interest.
	On May 1, 2013, we acquired a five percent interest in Pipe Line Holdings from MPC for consideration of $100.0 million, which was funded with cash on hand. We recorded the five percent interest in Pipe Line Holdings at its historical carrying value of $53.6 million and the excess cash paid over historical carrying value of $46.4 million as a decrease to general partner equity.
	These acquisitions were accounted for on a prospective basis and the terms of the 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, 2014 and 2013 were as follows:
		Three Months Ended September 30,				Nine Months Ended September 30,
	(In millions)	2014		2013		2014			2013
	Net income attributable to MPLX LP	$	29.1	$	21.5	$	92.1		$	57.7
	Transfer to noncontrolling interest retained by MPC:
	Decrease in general partner-MPC equity for purchases of additional interest in Pipe Line Holdings	—		—		(172.5		)	(46.4		)
	Change from net income attributable to MPLX LP and transfer to noncontrolling interest retained by MPC	$	29.1	$	21.5	$	(80.4	)	$	11.3

Related_Party_Agreements_and_T

Related Party Agreements and Transactions	9 Months Ended
	Sep. 30, 2014
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 noncontrolling 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, 2013.
	On March 31, 2014, Pipe Line Holdings entered into a credit agreement with MPL Investment LLC, a subsidiary of MPC, providing for a $50.0 million revolving credit facility which is scheduled to terminate on March 31, 2019. The agreement requires that we remain in compliance with the covenants, terms and conditions to which we are subject under our bank revolving credit agreement. This facility allows more efficient use of our bank revolving credit agreement. Borrowings of revolving loans under this credit facility bear interest at the one-month term LIBO Rate plus 1.375 percent. As of September 30, 2014, there were no borrowings outstanding under this facility.
	We believe the terms and conditions under our agreements with MPC are generally comparable to those with unrelated parties.
	Related Party Transactions
	Sales to related parties were as follows:
		Three Months Ended September 30,				Nine Months Ended September 30,
	(In millions)	2014		2013		2014		2013
	MPC	$	114.1	$	97.8	$	336	$	284.2
	Related party sales to MPC consist of crude oil and product 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 September 30,				Nine Months Ended September 30,
	(In millions)	2014		2013		2014		2013
	MPC	$	5.5	$	4.3	$	16	$	12.5
	Centennial	0.3		0.3		0.8		0.8
	Muskegon	—		—		0.1		0.1
	Total	$	5.8	$	4.6	$	16.9	$	13.4
	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 September 30,				Nine Months Ended September 30,
	(In millions)	2014		2013		2014		2013
	MPC	$	6	$	4.2	$	18.1	$	13.3
	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)	2014		2013		2014		2013
	MPC	$	7.8	$	7.7	$	23.1	$	23.5
	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)	2014		2013		2014		2013
	MPC	$	1.9	$	2.4	$	4.1	$	5.9
	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 September 30,				Nine Months Ended September 30,
	(In millions)	2014		2013		2014		2013
	Purchases from related parties	$	17.7	$	19.6	$	53.3	$	56.1
	General and administrative expenses	5.6		3.5		16.6		12
	Total	$	23.3	$	23.1	$	69.9	$	68.1
	Receivables from related parties were as follows:
	(In millions)	30-Sep-14		31-Dec-13
	MPC	$	45.4	$	47.4
	Centennial	0.4		0.6
	Muskegon	0.1		0.3
	Total	$	45.9	$	48.3
	Prepaid assets with related parties included in other current assets on the consolidated balance sheets were as follows:
	(In millions)	30-Sep-14		31-Dec-13
	MPC	$	—	$	0.4
	Long-term receivables related to indemnifications provided by MPC, included in other noncurrent assets on the consolidated balance sheets, were as follows:
	(In millions)	30-Sep-14		31-Dec-13
	MPC	$	—	$	0.2
	Payables to related parties were as follows:
	(In millions)	30-Sep-14		31-Dec-13
	MPC	$	17.3	$	12.8
	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 applicable four or eight quarter period. The use or expiration of the credits is a decrease in deferred revenue-related parties.
	During the nine months ended September 30, 2014 and 2013, MPC did not ship its minimum committed volumes on certain of our pipeline systems. The resulting deficiencies were related to transportation services agreements which have four quarter make-up periods. 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)	30-Sep-14		31-Dec-13
	Minimum volume deficiencies - MPC	$	29.5	$	34
	Project reimbursements - MPC	4.8		—
	Total	$	34.3	$	34

Net_Income_Per_Limited_Partner

Net Income Per Limited Partner Unit	9 Months Ended
	Sep. 30, 2014
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 September 30,						Nine Months Ended September 30,
	(In millions)	2014			2013			2014			2013
	Net income attributable to MPLX LP	$	29.1		$	21.5		$	92.1		$	57.7
	Less: General partner's distributions	1.5			0.5			3.4			1.3
	declared (including IDRs) (1)
	Limited partners’ distributions declared	13.2			11			38			31.6
	on common units (1)
	Limited partner's distributions declared	13.2			11			38			31.6
	on subordinated units (1)
	Distributions less than (in excess of) net income attributable to MPLX LP	$	1.2		$	(1.0	)	$	12.7		$	(6.8	)
	(1)	See Note 6 for distribution information.
		Three Months Ended September 30, 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)	$	1.5		$	13.2		$	13.2		$	27.9
	Distributions less than net income attributable to MPLX LP	0.3			0.5			0.4			1.2
	Net income attributable to MPLX LP	$	1.8		$	13.7		$	13.6		$	29.1
	Weighted average units outstanding:
	Basic	1.4			37.1			37			75.5
	Diluted	1.4			37.1			37			75.5
	Net income attributable to MPLX LP per limited partner unit:
	Basic				$	0.37		$	0.37
	Diluted				$	0.37		$	0.37
		Three Months Ended September 30, 2013
	(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	$	0.5		$	11		$	11		$	22.5
	Distributions in excess of net income attributable to MPLX LP	—			(0.5		)	(0.5		)	(1.0		)
	Net income attributable to MPLX LP	$	0.5		$	10.5		$	10.5		$	21.5
	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				$	0.29		$	0.29
	Diluted				$	0.29		$	0.29
		Nine Months Ended September 30, 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	$	3.4		$	38		$	38		$	79.4
	Distributions less than net income attributable to MPLX LP	3.2			4.8			4.7			12.7
	Net income attributable to MPLX LP	$	6.6		$	42.8		$	42.7		$	92.1
	Weighted average units outstanding:
	Basic	1.4			37			37			75.4
	Diluted	1.4			37.1			37			75.5
	Net income attributable to MPLX LP per limited partner unit:
	Basic				$	1.16		$	1.16
	Diluted				$	1.16		$	1.16
		Nine Months Ended September 30, 2013
	(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	$	1.3		$	31.6		$	31.6		$	64.5
Distributions in excess of net income attributable to MPLX LP	(0.1		)	(2.5		)	(4.2		)	(6.8		)
Net income attributable to MPLX LP	$	1.2		$	29.1		$	27.4		$	57.7
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				$	0.79		$	0.74
Diluted				$	0.79		$	0.74

Equity_Notes

Equity (Notes)	9 Months Ended
	Sep. 30, 2014
Equity [Abstract]	'
Equity	'
	Equity
	Units Outstanding - We had 36,966,994 common units outstanding as of September 30, 2014. Of that number, 17,056,515 were owned by MPC, which also owned 36,951,515 subordinated units and the two percent general partner interest, represented by 1,508,541 general partner units.
	The changes in the number of units outstanding from December 31, 2013 through September 30, 2014 are summarized below:
	(In units)	Common		Subordinated			General Partner		Total
	Balance at December 31, 2013	36,951,515		36,951,515			1,508,225		75,411,255
	Issuance of units	—		—			316		316
	Unit-based compensation awards	15,479		—			—		15,479
	Balance at September 30, 2014	36,966,994		36,951,515			1,508,541		75,427,050
	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 - 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)	2014			2013			2014		2013
	Net income attributable to MPLX LP	$	29.1		$	21.5		$	92.1	$	57.7
	Less: General partner's incentive distribution rights	1			—			1.9		—
	Net income attributable to MPLX LP available to general and limited partners	$	28.1		$	21.5		$	90.2	$	57.7
	General partner's 2% interest in net income attributable to MPLX LP	$	0.5		$	0.5		$	1.8	$	1.2
	General partner's incentive distribution rights	1			—			1.9		—
	General partner's interest in net income attributable to MPLX LP	$	1.5		$	0.5		$	3.7	$	1.2
	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 21, 2014, we declared a quarterly cash distribution of $0.3575 per unit, totaling $27.9 million. This distribution will be paid on November 14, 2014 to unitholders of record on November 4, 2014.
	The allocation of total quarterly cash distributions to general and limited partners is as follows for the three and nine months ended September 30, 2014 and 2013. 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)	2014			2013			2014		2013
	General partner's distributions:
	General partner's distributions	$	0.5		$	0.5		$	1.5	$	1.3
	General partner's incentive distribution rights	1			—			1.9		—
	Total general partner's distributions	$	1.5		$	0.5		$	3.4	$	1.3
	Limited partners' distributions:
	Common unitholders	$	13.2		$	11		$	38	$	31.6
	Subordinated unitholders	13.2			11			38		31.6
	Total limited partners' distributions	26.4			22			76		63.2
	Total cash distributions declared	$	27.9		$	22.5		$	79.4	$	64.5

Other_Items

Other Items	9 Months Ended
	Sep. 30, 2014
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)	2014			2013			2014			2013
	Interest income	$	—		$	—		$	—		$	(0.3	)
	Interest expense	1.2			0.2			3.1			0.8
	Interest capitalized	(0.2		)	(0.2		)	(0.7		)	(0.6		)
	Other financial costs	0.1			0.2			0.6			0.8
	Net interest and other financial costs	$	1.1		$	0.2		$	3		$	0.7

Income_Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2014
Income Tax Disclosure [Abstract]	'
Income Taxes	'
	Income Taxes
	We are not a taxable entity for United States federal income tax purposes or for the majority of states that impose an income tax. Taxes on our net income generally are borne by our partners through the allocation of taxable income. Our income tax provision results from partnership activity in the states of Kentucky, Tennessee and Texas.
	Our income tax provision was less than $0.1 million and $0.4 million for the three months ended September 30, 2014 and 2013 and $0.1 million and $0.5 million for the nine months ended September 30, 2014 and 2013. Our effective tax rate was less than 0.1 percent and 1.0 percent for the three months ended September 30, 2014 and 2013 and 0.1 percent and 0.5 percent for the nine months ended September 30, 2014 and 2013.
	As of September 30, 2014 and December 31, 2013, we had unrecognized tax benefits of less than $0.1 million.

Property_Plant_and_Equipment

Property, Plant and Equipment	9 Months Ended
	Sep. 30, 2014
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)	30-Sep-14		31-Dec-13
	Land	$	5.3	$	5.3
	Pipelines and related assets	1,068.20		1,060.70
	Storage and delivery facilities	165.1		165.7
	Other	23.6		22.8
	Assets under construction	64.5		21.9
	Total	1,326.70		1,276.40
	Less accumulated depreciation	344.1		309.8
	Property, plant and equipment, net	$	982.6	$	966.6

Fair_Value_Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2014
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, 2014 and December 31, 2013.
	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) MPC’s 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 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 September 30, 2014 and December 31, 2013.
		September 30, 2014				December 31, 2013
	(In millions)	Fair Value		Carrying Value		Fair Value		Carrying Value
	Long-term debt	$	255.3	$	255	$	—	$	—

Debt

Debt	9 Months Ended
	Sep. 30, 2014
Debt Disclosure [Abstract]	'
Debt	'
	Debt
	Our outstanding borrowings at September 30, 2014 and December 31, 2013 consisted of the following:
	(In millions)	30-Sep-14		31-Dec-13
	MPLX Operations - bank revolving credit agreement due 2017	$	255	$	—
	MPL - capital lease obligations due 2020	10		10.5
	Total	265		10.5
	Amounts due within one year	0.8		0.7
	Total long-term debt due after one year	$	264.2	$	9.8
	On March 31, 2014, Pipe Line Holdings entered into a credit agreement with MPL Investment LLC, a subsidiary of MPC. As of September 30, 2014, there were no borrowings outstanding under this facility. A description of this agreement is discussed in detail in Note 4.
	During the nine months ended September 30, 2014, we borrowed $270.0 million under our bank revolving credit agreement, at an average interest rate of 1.5 percent, per annum, and repaid $15.0 million of those borrowings. The borrowing was used to fund our acquisition of the additional interest in Pipe Line Holdings on March 1, 2014. At September 30, 2014, we had $255.0 million of borrowings and no letters of credit outstanding under the bank revolving credit agreement, resulting in a total unused revolving credit availability of $245.0 million, or 49.0 percent of the borrowing capacity. The bank revolving credit agreement is scheduled to mature on October 31, 2017.

Supplemental_Cash_Flow_Informa

Supplemental Cash Flow Information	9 Months Ended
	Sep. 30, 2014
Supplemental Cash Flow Elements [Abstract]	'
Supplemental Cash Flow Information	'
	Supplemental Cash Flow Information
		Nine Months Ended September 30,
	(In millions)	2014		2013
	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 materials and supplies inventories	$	0.7	$	3.8
	Property, plant and equipment contributed by MPC	0.1		0.2
	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)	2014		2013
	Additions to property, plant and equipment	$	47.1	$	81
	Plus: Increase in capital accruals	5.9		1.6
	Asset retirement expenditures	1		3.4
	Total capital expenditures	$	54	$	86

EquityBased_Compensation_Plan

Equity-Based Compensation Plan	9 Months Ended
	Sep. 30, 2014
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, 2014:
		Number		Weighted
		of Units		Average
				Fair Value
	Outstanding at December 31, 2013	77,754		$	33.84
	Granted	49,465		49.09
	Issued	(27,360	)	34.45
	Outstanding at September 30, 2014	99,859		41.22
	Performance 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 $1.16 for 2014, 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 nine months ended September 30, 2014:
		Number of
		Units
	Outstanding at December 31, 2013	436,917
	Granted	500,507
	Outstanding at September 30, 2014	937,424

Commitments_and_Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2014
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, 2014 and December 31, 2013, accrued liabilities for remediation totaled $1.0 million and $1.1 million, respectively. At September 30, 2014 and December 31, 2013, it is reasonably possible that an estimated loss existed of up to $0.4 million in excess of the amount accrued for remediation. However, 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 the Offering were $0.1 million and $0.3 million at September 30, 2014 and December 31, 2013, respectively.
	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 $1.7 million as of September 30, 2014, 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 the underlying triggering event has little or no past experience upon which a reasonable prediction of the outcome can be based.
	Contractual commitments—At September 30, 2014, our contractual commitments to acquire property, plant and equipment totaled $29.2 million. Our contractual commitments at September 30, 2014 were primarily related to a tank expansion project in Patoka, Illinois and other pipeline related projects.

Condensed_Consolidating_Financ

Condensed Consolidating Financial Statements (Notes)	9 Months Ended
	Sep. 30, 2014
Condensed Financial Information of Parent Company Only Disclosure [Abstract]	'
Condensed Financial Information of Parent Company Only Disclosure [Text Block]	'
	Condensed Consolidating Financial Statements
	For purposes of the following footnote, MPLX LP is referred to as "Parent Guarantor" and MPLX Operations is referred to as "Subsidiary Issuer." All other consolidated subsidiaries of the Partnership are collectively referred to as "Non-Guarantor Subsidiaries." The condensed consolidating financial information is provided in connection with the potential issuance of debt securities by the Subsidiary Issuer, which may be fully and unconditionally guaranteed by the Parent Guarantor.
	The following condensed consolidating financial information reflects the Partnership’s stand-alone accounts, the accounts of the Subsidiary Issuer, the combined accounts of the Non-Guarantor Subsidiaries, consolidating adjustments and the Partnership’s consolidated financial information. The condensed consolidating financial information should be read in conjunction with the Partnership’s accompanying consolidated financial statements and related notes. The Parent Guarantor's and the Subsidiary Issuer’s investment in and equity income from their consolidated subsidiaries are presented in accordance with the equity method of accounting in which the equity income from consolidated subsidiaries includes the results of operations of the Partnership assets.
	Condensed Consolidating Statements of Income
		Three Months Ended September 30, 2014
	(In millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
	Revenues and other income:
	Sales and other operating revenues	$	—		$	—		$	16.6		$	—		$	16.6
	Sales to related parties	—			—			114.1			—			114.1
	Other income	—			—			1.5			—			1.5
	Other income - related parties	—			—			5.8			—			5.8
	Equity in earnings of subsidiaries	30.4			31.9			—			(62.3		)	—
	Total revenues and other income	30.4			31.9			138			(62.3		)	138
	Costs and expenses:
	Cost of revenues (excludes items below)	—			—			40.6			—			40.6
	Purchases from related parties	—			—			23.7			—			23.7
	Depreciation	—			—			12.5			—			12.5
	General and administrative expenses	1.3			—			14			—			15.3
	Other taxes	—			—			1.7			—			1.7
	Total costs and expenses	1.3			—			92.5			—			93.8
	Income from operations	29.1			31.9			45.5			(62.3		)	44.2
	Net interest and other financial costs (income)	—			1.5			(0.4		)	—			1.1
	Income before income taxes	29.1			30.4			45.9			(62.3		)	43.1
	Provision for income taxes	—			—			—			—			—
	Net income	29.1			30.4			45.9			(62.3		)	43.1
	Less: Net income attributable to MPC-retained interest	—			—			—			14			14
	Net income attributable to MPLX LP	$	29.1		$	30.4		$	45.9		$	(76.3	)	$	29.1
	Condensed Consolidating Statements of Income
		Three Months Ended September 30, 2013
	(In millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
	Revenues and other income:
	Sales and other operating revenues	$	—		$	—		$	20.4		$	—		$	20.4
	Sales to related parties	—			—			97.8			—			97.8
	Other income	—			—			1			—			1
	Other income - related parties	—			—			4.6			—			4.6
	Equity in earnings of subsidiaries	22.8			23.5			—			(46.3		)	—
	Total revenues and other income	22.8			23.5			123.8			(46.3		)	123.8
	Costs and expenses:
	Cost of revenues (excludes items below)	—			—			33.1			—			33.1
	Purchases from related parties	—			—			23.8			—			23.8
	Depreciation	—			—			12.7			—			12.7
	General and administrative expenses	1.2			—			11.8			—			13
	Other taxes	0.1			—			1.3			—			1.4
	Total costs and expenses	1.3			—			82.7			—			84
	Income from operations	21.5			23.5			41.1			(46.3		)	39.8
	Net interest and other financial costs (income)	—			0.7			(0.5		)	—			0.2
	Income before income taxes	21.5			22.8			41.6			(46.3		)	39.6
	Provision for income taxes	—			—			0.4			—			0.4
	Net income	21.5			22.8			41.2			(46.3		)	39.2
	Less: Net income attributable to MPC-retained interest	—			—			—			17.7			17.7
	Net income attributable to MPLX LP	$	21.5		$	22.8		$	41.2		$	(64.0	)	$	21.5
	Condensed Consolidating Statements of Income
		Nine Months Ended September 30, 2014
	(In millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
	Revenues and other income:
	Sales and other operating revenues	$	—		$	—		$	52.2		$	—		$	52.2
	Sales to related parties	—			—			336			—			336
	Other income	—			—			4.1			—			4.1
	Other income - related parties	—			—			16.9			—			16.9
	Equity in earnings of subsidiaries	97.2			101.5			—			(198.7		)	—
	Total revenues and other income	97.2			101.5			409.2			(198.7		)	409.2
	Costs and expenses:
	Cost of revenues (excludes items below)	—			—			102.9			—			102.9
Purchases from related parties	—			—			71.4			—			71.4
Depreciation	—			—			37.5			—			37.5
General and administrative expenses	5.1			—			42			—			47.1
Other taxes	—			—			5.5			—			5.5
Total costs and expenses	5.1			—			259.3			—			264.4
Income from operations	92.1			101.5			149.9			(198.7		)	144.8
Net interest and other financial costs (income)	—			4.3			(1.3		)	—			3
Income before income taxes	92.1			97.2			151.2			(198.7		)	141.8
Provision for income taxes	—			—			0.1			—			0.1
Net income	92.1			97.2			151.1			(198.7		)	141.7
Less: Net income attributable to MPC-retained interest	—			—			—			49.6			49.6
Net income attributable to MPLX LP	$	92.1		$	97.2		$	151.1		$	(248.3	)	$	92.1
Condensed Consolidating Statements of Income
	Nine Months Ended September 30, 2013
(In millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
Revenues and other income:
Sales and other operating revenues	$	—		$	—		$	60		$	—		$	60
Sales to related parties	—			—			284.2			—			284.2
Other income	—			—			3.1			—			3.1
Other income - related parties	—			—			13.4			—			13.4
Equity in earnings of subsidiaries	61.5			63.1			—			(124.6		)	—
Total revenues and other income	61.5			63.1			360.7			(124.6		)	360.7
Costs and expenses:
Cost of revenues (excludes items below)	—			—			99.2			—			99.2
Purchases from related parties	—			—			69.4			—			69.4
Depreciation	—			—			36.3			—			36.3
General and administrative expenses	3.6			—			36.8			—			40.4
Other taxes	0.2			—			4.7			—			4.9
Total costs and expenses	3.8			—			246.4			—			250.2
Income from operations	57.7			63.1			114.3			(124.6		)	110.5
Net interest and other financial costs (income)	—			1.6			(0.9		)	—			0.7
Income before income taxes	57.7			61.5			115.2			(124.6		)	109.8
Provision for income taxes	—			—			0.5			—			0.5
Net income	57.7			61.5			114.7			(124.6		)	109.3
Less: Net income attributable to MPC-retained interest	—			—			—			51.6			51.6
Net income attributable to MPLX LP	$	57.7		$	61.5		$	114.7		$	(176.2	)	$	57.7
Condensed Consolidating Balance Sheets
	30-Sep-14
(In millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
Assets
Current assets:
Cash and cash equivalents	$	0.8		$	0.5		$	30.9		$	—		$	32.2
Receivables	0.4			—			8.5			—			8.9
Receivables from related parties	—			4.2			186.8			(145.1		)	45.9
Materials and supplies inventories	—			—			11.6			—			11.6
Other current assets	—			—			6.9			—			6.9
Total current assets	1.2			4.7			244.7			(145.1		)	105.5
Property, plant and equipment, net	—			—			982.6			—			982.6
Investment in subsidiaries	496.7			886.8			—			(1,383.5		)	—
Goodwill	—			—			104.7			—			104.7
Other noncurrent assets	—			1.5			—			—			1.5
Total assets	$	497.9		$	893		$	1,332.00		$	(1,528.6	)	$	1,194.30
Liabilities
Current liabilities:
Accounts payable	$	0.4		$	0.1		$	34.4		$	—		$	34.9
Payables to related parties	4.3			140.9			17.2			(145.1		)	17.3
Deferred revenue - related parties	—			—			30.1			—			30.1
Accrued taxes	0.4			—			5.5			—			5.9
Long-term debt due within one year	—			—			0.8			—			0.8
Other current liabilities	—			0.3			1.5			—			1.8
Total current liabilities	5.1			141.3			89.5			(145.1		)	90.8
Long-term deferred revenue - related parties	—			—			4.2			—			4.2
Long-term debt	—			255			9.2			—			264.2
Deferred credits and other liabilities	1.3			—			0.5			—			1.8
Total liabilities	6.4			396.3			103.4			(145.1		)	361
Equity
MPLX LP partners’ capital	491.5			496.7			1,228.60			(1,725.3		)	491.5
Noncontrolling interest retained by MPC	—			—			—			341.8			341.8
Total equity	491.5			496.7			1,228.60			(1,383.5		)	833.3
Total liabilities and equity	$	497.9		$	893		$	1,332.00		$	(1,528.6	)	$	1,194.30
Condensed Consolidating Balance Sheets
	31-Dec-13
(In millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
Assets
Current assets:
Cash and cash equivalents	$	—		$	7.2		$	46.9		$	—		$	54.1
Receivables	0.4			—			11.8			—			12.2
Receivables from related parties	—			—			132.2			(83.9		)	48.3
Materials and supplies inventories	—			—			11.6			—			11.6
Other current assets	0.3			—			8.6			—			8.9
Total current assets	0.7			7.2			211.1			(83.9		)	135.1
Property, plant and equipment, net	—			—			966.6			—			966.6
Investment in subsidiaries	647.1			721.6			—			(1,368.7		)	—
Goodwill	—			—			104.7			—			104.7
Other noncurrent assets	—			1.8			0.3			—			2.1
Total assets	$	647.8		$	730.6		$	1,282.70		$	(1,452.6	)	$	1,208.50
Liabilities
Current liabilities:
Accounts payable	$	0.2		$	0.3		$	30		$	—		$	30.5
Payables to related parties	0.7			83.2			12.8			(83.9		)	12.8
Deferred revenue - related parties	—			—			34			—			34
Accrued taxes	0.3			—			3.7			—			4
Long-term debt due within one year	—			—			0.7			—			0.7
Other current liabilities	—			—			1.4			—			1.4
Total current liabilities	1.2			83.5			82.6			(83.9		)	83.4
Long-term debt	—			—			9.8			—			9.8
Deferred credits and other liabilities	0.4			—			0.8			—			1.2
Total liabilities	1.6			83.5			93.2			(83.9		)	94.4
Equity
MPLX LP partners’ capital	646.2			647.1			1,189.50			(1,836.6		)	646.2
Noncontrolling interest retained by MPC	—			—			—			467.9			467.9
Total equity	646.2			647.1			1,189.50			(1,368.7		)	1,114.10
Total liabilities and equity	$	647.8		$	730.6		$	1,282.70		$	(1,452.6	)	$	1,208.50
Condensed Consolidating Statements of Cash Flow
	Nine Months Ended September 30, 2014
(In millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
Net cash provided by (used in) operating activities	$	76		$	72.7		$	191		$	(149.4	)	$	190.3
Investing activities:
Additions to property, plant and equipment	—			(0.2		)	(46.9		)	—			(47.1		)
Investment in Pipe Line Holdings	—			(310.0		)	—			310			—
Loans to affiliates	—			—			(51.0		)	51			—
All other, net	—			—			4			—			4
Net cash provided by (used in) investing activities	—			(310.2		)	(93.9		)	361			(43.1		)
Financing activities:
Proceeds from borrowings from affiliates	—			51			—			(51.0		)	—
Long-term debt - borrowings	—			270			—			—			270
- repayments	—			(15.0		)	(0.6		)	—			(15.6		)
Quarterly distributions	(75.2		)	(75.2		)	(74.2		)	149.4			(75.2		)
Quarterly distributions to noncontrolling interest retained by MPC	—			—			(38.3		)	—			(38.3		)
Distributions related to purchase of additional interest in Pipe Line Holdings	—			—			—			(310.0		)	(310.0		)
Net cash provided by (used in) financing activities	(75.2		)	230.8			(113.1		)	(211.6		)	(169.1		)
Net increase (decrease) in cash and cash equivalents	0.8			(6.7		)	(16.0		)	—			(21.9		)
Cash and cash equivalents at beginning of period	—			7.2			46.9			—			54.1
Cash and cash equivalents at end of period	$	0.8		$	0.5		$	30.9		$	—		$	32.2
Condensed Consolidating Statements of Cash Flow
	Nine Months Ended September 30, 2013
(In millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
Net cash provided by (used in) operating activities	$	54.8		$	68.9		$	167		$	(123.6	)	$	167.1
Investing activities:
Additions to property, plant and equipment	—			(0.2		)	(80.8		)	—			(81.0		)
Investment in Pipe Line Holdings	—			(100.0		)	—			100			—
Loans to affiliates	—			—			(100.0		)	100			—
Repayments of loans from affiliates	—			—			16			(16.0		)	—
Net cash provided by (used in) investing activities	—			(100.2		)	(164.8		)	184			(81.0		)
Financing activities:
Proceeds from borrowings from affiliates	—			100			—			(100.0		)	—
Payments on borrowings from affiliates	—			(16.0		)	—			16			—
Long-term debt - repayments	—			—			(0.6		)	—			(0.6		)
Quarterly distributions	(55.4		)	(55.4		)	(68.2		)	123.6			(55.4		)
Quarterly distributions to noncontrolling interest retained by MPC	—			—			(59.9		)	—			(59.9		)
Distributions related to purchase of additional interest in Pipe Line Holdings	—			—			—			(100.0		)	(100.0		)
Net cash provided by (used in) financing activities	(55.4		)	28.6			(128.7		)	(60.4		)	(215.9		)
Net decrease in cash and cash equivalents	(0.6		)	(2.7		)	(126.5		)	—			(129.8		)
Cash and cash equivalents at beginning of period	1			9.7			206			—			216.7
Cash and cash equivalents at end of period	$	0.4		$	7		$	79.5		$	—		$	86.9

Subsequent_Event

Subsequent Event	9 Months Ended
	Sep. 30, 2014
Subsequent Event [Line Items]	'
Subsequent Events [Text Block]	'
	Subsequent Event
	On October 29, 2014, the board of directors of MPC authorized the sale of its remaining 31 percent interest in Pipe Line Holdings to MPLX.

Description_of_the_Business_an1

Description of the Business and Basis of Presentation (Policies)	9 Months Ended
	Sep. 30, 2014
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)	9 Months Ended
	Sep. 30, 2014
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, 2014 and 2013 were as follows:
		Three Months Ended September 30,				Nine Months Ended September 30,
	(In millions)	2014		2013		2014			2013
	Net income attributable to MPLX LP	$	29.1	$	21.5	$	92.1		$	57.7
	Transfer to noncontrolling interest retained by MPC:
	Decrease in general partner-MPC equity for purchases of additional interest in Pipe Line Holdings	—		—		(172.5		)	(46.4		)
	Change from net income attributable to MPLX LP and transfer to noncontrolling interest retained by MPC	$	29.1	$	21.5	$	(80.4	)	$	11.3

Related_Party_Agreements_and_T1

Related Party Agreements and Transactions (Tables)	9 Months Ended
	Sep. 30, 2014
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)	2014		2013		2014		2013
	MPC	$	114.1	$	97.8	$	336	$	284.2
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 September 30,				Nine Months Ended September 30,
	(In millions)	2014		2013		2014		2013
	MPC	$	5.5	$	4.3	$	16	$	12.5
	Centennial	0.3		0.3		0.8		0.8
	Muskegon	—		—		0.1		0.1
	Total	$	5.8	$	4.6	$	16.9	$	13.4
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)	2014		2013		2014		2013
	MPC	$	6	$	4.2	$	18.1	$	13.3
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)	2014		2013		2014		2013
	MPC	$	7.8	$	7.7	$	23.1	$	23.5
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)	2014		2013		2014		2013
	MPC	$	1.9	$	2.4	$	4.1	$	5.9
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)	2014		2013		2014		2013
	Purchases from related parties	$	17.7	$	19.6	$	53.3	$	56.1
	General and administrative expenses	5.6		3.5		16.6		12
	Total	$	23.3	$	23.1	$	69.9	$	68.1
Receivables from Related Parties	'
	Receivables from related parties were as follows:
	(In millions)	30-Sep-14		31-Dec-13
	MPC	$	45.4	$	47.4
	Centennial	0.4		0.6
	Muskegon	0.1		0.3
	Total	$	45.9	$	48.3
Schedule of Prepaid Assets With Related Parties [Table Text Block]	'
	Prepaid assets with related parties included in other current assets on the consolidated balance sheets were as follows:
	(In millions)	30-Sep-14		31-Dec-13
	MPC	$	—	$	0.4
Schedule of Long Term Receivables with Related Parties [Table Text Block]	'
	Long-term receivables related to indemnifications provided by MPC, included in other noncurrent assets on the consolidated balance sheets, were as follows:
	(In millions)	30-Sep-14		31-Dec-13
	MPC	$	—	$	0.2
Payables to Related Parties	'
	Payables to related parties were as follows:
	(In millions)	30-Sep-14		31-Dec-13
	MPC	$	17.3	$	12.8
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)	30-Sep-14		31-Dec-13
	Minimum volume deficiencies - MPC	$	29.5	$	34
	Project reimbursements - MPC	4.8		—
	Total	$	34.3	$	34

Net_Income_Per_Limited_Partner1

Net Income Per Limited Partner Unit (Tables)	9 Months Ended
	Sep. 30, 2014
Earnings Per Share [Abstract]	'
Schedule of Distributions By Partner By Class	'
		Three Months Ended September 30,						Nine Months Ended September 30,
	(In millions)	2014			2013			2014			2013
	Net income attributable to MPLX LP	$	29.1		$	21.5		$	92.1		$	57.7
	Less: General partner's distributions	1.5			0.5			3.4			1.3
	declared (including IDRs) (1)
	Limited partners’ distributions declared	13.2			11			38			31.6
	on common units (1)
	Limited partner's distributions declared	13.2			11			38			31.6
	on subordinated units (1)
	Distributions less than (in excess of) net income attributable to MPLX LP	$	1.2		$	(1.0	)	$	12.7		$	(6.8	)
	(1)	See Note 6 for distribution information.
Schedule of Basic and Diluted Earnings Per Unit	'
		Three Months Ended September 30, 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)	$	1.5		$	13.2		$	13.2		$	27.9
	Distributions less than net income attributable to MPLX LP	0.3			0.5			0.4			1.2
	Net income attributable to MPLX LP	$	1.8		$	13.7		$	13.6		$	29.1
	Weighted average units outstanding:
	Basic	1.4			37.1			37			75.5
	Diluted	1.4			37.1			37			75.5
	Net income attributable to MPLX LP per limited partner unit:
	Basic				$	0.37		$	0.37
	Diluted				$	0.37		$	0.37
		Three Months Ended September 30, 2013
	(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	$	0.5		$	11		$	11		$	22.5
	Distributions in excess of net income attributable to MPLX LP	—			(0.5		)	(0.5		)	(1.0		)
	Net income attributable to MPLX LP	$	0.5		$	10.5		$	10.5		$	21.5
	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				$	0.29		$	0.29
	Diluted				$	0.29		$	0.29
		Nine Months Ended September 30, 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	$	3.4		$	38		$	38		$	79.4
	Distributions less than net income attributable to MPLX LP	3.2			4.8			4.7			12.7
	Net income attributable to MPLX LP	$	6.6		$	42.8		$	42.7		$	92.1
	Weighted average units outstanding:
	Basic	1.4			37			37			75.4
	Diluted	1.4			37.1			37			75.5
	Net income attributable to MPLX LP per limited partner unit:
	Basic				$	1.16		$	1.16
	Diluted				$	1.16		$	1.16
		Nine Months Ended September 30, 2013
	(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	$	1.3		$	31.6		$	31.6		$	64.5
	Distributions in excess of net income attributable to MPLX LP	(0.1		)	(2.5		)	(4.2		)	(6.8		)
	Net income attributable to MPLX LP	$	1.2		$	29.1		$	27.4		$	57.7
	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				$	0.79		$	0.74
Diluted				$	0.79		$	0.74

Equity_Tables

Equity (Tables)	9 Months Ended
	Sep. 30, 2014
Equity [Abstract]	'
Schedule of Stockholders Equity [Table Text Block]	'
	The changes in the number of units outstanding from December 31, 2013 through September 30, 2014 are summarized below:
	(In units)	Common		Subordinated			General Partner		Total
	Balance at December 31, 2013	36,951,515		36,951,515			1,508,225		75,411,255
	Issuance of units	—		—			316		316
	Unit-based compensation awards	15,479		—			—		15,479
	Balance at September 30, 2014	36,966,994		36,951,515			1,508,541		75,427,050
Schedule Of Calculation Of Net Income Applicable to Partners [Table Text Block]	'
	Net Income Allocation - 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)	2014			2013			2014		2013
	Net income attributable to MPLX LP	$	29.1		$	21.5		$	92.1	$	57.7
	Less: General partner's incentive distribution rights	1			—			1.9		—
	Net income attributable to MPLX LP available to general and limited partners	$	28.1		$	21.5		$	90.2	$	57.7
	General partner's 2% interest in net income attributable to MPLX LP	$	0.5		$	0.5		$	1.8	$	1.2
	General partner's incentive distribution rights	1			—			1.9		—
	General partner's interest in net income attributable to MPLX LP	$	1.5		$	0.5		$	3.7	$	1.2
Distributions Made to Limited Partner, by Distribution [Table Text Block]	'
	The allocation of total quarterly cash distributions to general and limited partners is as follows for the three and nine months ended September 30, 2014 and 2013. 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)	2014			2013			2014		2013
	General partner's distributions:
	General partner's distributions	$	0.5		$	0.5		$	1.5	$	1.3
	General partner's incentive distribution rights	1			—			1.9		—
	Total general partner's distributions	$	1.5		$	0.5		$	3.4	$	1.3
	Limited partners' distributions:
	Common unitholders	$	13.2		$	11		$	38	$	31.6
	Subordinated unitholders	13.2			11			38		31.6
	Total limited partners' distributions	26.4			22			76		63.2
	Total cash distributions declared	$	27.9		$	22.5		$	79.4	$	64.5

Other_Items_Tables

Other Items (Tables)	9 Months Ended
	Sep. 30, 2014
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)	2014			2013			2014			2013
	Interest income	$	—		$	—		$	—		$	(0.3	)
	Interest expense	1.2			0.2			3.1			0.8
	Interest capitalized	(0.2		)	(0.2		)	(0.7		)	(0.6		)
	Other financial costs	0.1			0.2			0.6			0.8
	Net interest and other financial costs	$	1.1		$	0.2		$	3		$	0.7

Property_Plant_and_Equipment_T

Property, Plant and Equipment (Tables)	9 Months Ended
	Sep. 30, 2014
Property, Plant and Equipment [Abstract]	'
Property, Plant and Equipment	'
	Our investment in property, plant and equipment with associated accumulated depreciation was:
	(In millions)	30-Sep-14		31-Dec-13
	Land	$	5.3	$	5.3
	Pipelines and related assets	1,068.20		1,060.70
	Storage and delivery facilities	165.1		165.7
	Other	23.6		22.8
	Assets under construction	64.5		21.9
	Total	1,326.70		1,276.40
	Less accumulated depreciation	344.1		309.8
	Property, plant and equipment, net	$	982.6	$	966.6

Fair_Value_Measurements_Fair_V

Fair Value Measurements Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2014
Fair Value Disclosures [Abstract]	'
Fair Value, by Balance Sheet Grouping [Table Text Block]	'
	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) MPC’s 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 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 September 30, 2014 and December 31, 2013.
		September 30, 2014				December 31, 2013
	(In millions)	Fair Value		Carrying Value		Fair Value		Carrying Value
	Long-term debt	$	255.3	$	255	$	—	$	—

Debt_Tables

Debt (Tables)	9 Months Ended
	Sep. 30, 2014
Debt Disclosure [Abstract]	'
Summary of Outstanding Borrowings	'
	Our outstanding borrowings at September 30, 2014 and December 31, 2013 consisted of the following:
	(In millions)	30-Sep-14		31-Dec-13
	MPLX Operations - bank revolving credit agreement due 2017	$	255	$	—
	MPL - capital lease obligations due 2020	10		10.5
	Total	265		10.5
	Amounts due within one year	0.8		0.7
	Total long-term debt due after one year	$	264.2	$	9.8

Supplemental_Cash_Flow_Informa1

Supplemental Cash Flow Information (Tables)	9 Months Ended
	Sep. 30, 2014
Supplemental Cash Flow Elements [Abstract]	'
Summary of Supplemental Cash Flow Information	'
		Nine Months Ended September 30,
	(In millions)	2014		2013
	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 materials and supplies inventories	$	0.7	$	3.8
	Property, plant and equipment contributed by MPC	0.1		0.2
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)	2014		2013
	Additions to property, plant and equipment	$	47.1	$	81
	Plus: Increase in capital accruals	5.9		1.6
	Asset retirement expenditures	1		3.4
	Total capital expenditures	$	54	$	86

EquityBased_Compensation_Plan_

Equity-Based Compensation Plan (Tables)	9 Months Ended
	Sep. 30, 2014
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, 2014:
		Number		Weighted
		of Units		Average
				Fair Value
	Outstanding at December 31, 2013	77,754		$	33.84
	Granted	49,465		49.09
	Issued	(27,360	)	34.45
	Outstanding at September 30, 2014	99,859		41.22
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 nine months ended September 30, 2014:
		Number of
		Units
	Outstanding at December 31, 2013	436,917
	Granted	500,507
	Outstanding at September 30, 2014	937,424

Condensed_Consolidating_Financ1

Condensed Consolidating Financial Statements (Tables)	9 Months Ended
	Sep. 30, 2014
Condensed Financial Information of Parent Company Only Disclosure [Abstract]	'
Condensed Financial Statements [Table Text Block]	'
	Condensed Consolidating Statements of Income
		Three Months Ended September 30, 2014
	(In millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
	Revenues and other income:
	Sales and other operating revenues	$	—		$	—		$	16.6		$	—		$	16.6
	Sales to related parties	—			—			114.1			—			114.1
	Other income	—			—			1.5			—			1.5
	Other income - related parties	—			—			5.8			—			5.8
	Equity in earnings of subsidiaries	30.4			31.9			—			(62.3		)	—
	Total revenues and other income	30.4			31.9			138			(62.3		)	138
	Costs and expenses:
	Cost of revenues (excludes items below)	—			—			40.6			—			40.6
	Purchases from related parties	—			—			23.7			—			23.7
	Depreciation	—			—			12.5			—			12.5
	General and administrative expenses	1.3			—			14			—			15.3
	Other taxes	—			—			1.7			—			1.7
	Total costs and expenses	1.3			—			92.5			—			93.8
	Income from operations	29.1			31.9			45.5			(62.3		)	44.2
	Net interest and other financial costs (income)	—			1.5			(0.4		)	—			1.1
	Income before income taxes	29.1			30.4			45.9			(62.3		)	43.1
	Provision for income taxes	—			—			—			—			—
	Net income	29.1			30.4			45.9			(62.3		)	43.1
	Less: Net income attributable to MPC-retained interest	—			—			—			14			14
	Net income attributable to MPLX LP	$	29.1		$	30.4		$	45.9		$	(76.3	)	$	29.1
	Condensed Consolidating Statements of Income
		Three Months Ended September 30, 2013
	(In millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
	Revenues and other income:
	Sales and other operating revenues	$	—		$	—		$	20.4		$	—		$	20.4
	Sales to related parties	—			—			97.8			—			97.8
	Other income	—			—			1			—			1
	Other income - related parties	—			—			4.6			—			4.6
	Equity in earnings of subsidiaries	22.8			23.5			—			(46.3		)	—
	Total revenues and other income	22.8			23.5			123.8			(46.3		)	123.8
	Costs and expenses:
	Cost of revenues (excludes items below)	—			—			33.1			—			33.1
	Purchases from related parties	—			—			23.8			—			23.8
	Depreciation	—			—			12.7			—			12.7
	General and administrative expenses	1.2			—			11.8			—			13
	Other taxes	0.1			—			1.3			—			1.4
	Total costs and expenses	1.3			—			82.7			—			84
	Income from operations	21.5			23.5			41.1			(46.3		)	39.8
	Net interest and other financial costs (income)	—			0.7			(0.5		)	—			0.2
	Income before income taxes	21.5			22.8			41.6			(46.3		)	39.6
	Provision for income taxes	—			—			0.4			—			0.4
	Net income	21.5			22.8			41.2			(46.3		)	39.2
	Less: Net income attributable to MPC-retained interest	—			—			—			17.7			17.7
	Net income attributable to MPLX LP	$	21.5		$	22.8		$	41.2		$	(64.0	)	$	21.5
	Condensed Consolidating Statements of Income
		Nine Months Ended September 30, 2014
	(In millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
	Revenues and other income:
	Sales and other operating revenues	$	—		$	—		$	52.2		$	—		$	52.2
	Sales to related parties	—			—			336			—			336
	Other income	—			—			4.1			—			4.1
	Other income - related parties	—			—			16.9			—			16.9
	Equity in earnings of subsidiaries	97.2			101.5			—			(198.7		)	—
	Total revenues and other income	97.2			101.5			409.2			(198.7		)	409.2
	Costs and expenses:
	Cost of revenues (excludes items below)	—			—			102.9			—			102.9
	Purchases from related parties	—			—			71.4			—			71.4
	Depreciation	—			—			37.5			—			37.5
	General and administrative expenses	5.1			—			42			—			47.1
Other taxes	—			—			5.5			—			5.5
Total costs and expenses	5.1			—			259.3			—			264.4
Income from operations	92.1			101.5			149.9			(198.7		)	144.8
Net interest and other financial costs (income)	—			4.3			(1.3		)	—			3
Income before income taxes	92.1			97.2			151.2			(198.7		)	141.8
Provision for income taxes	—			—			0.1			—			0.1
Net income	92.1			97.2			151.1			(198.7		)	141.7
Less: Net income attributable to MPC-retained interest	—			—			—			49.6			49.6
Net income attributable to MPLX LP	$	92.1		$	97.2		$	151.1		$	(248.3	)	$	92.1
Condensed Consolidating Statements of Income
	Nine Months Ended September 30, 2013
(In millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
Revenues and other income:
Sales and other operating revenues	$	—		$	—		$	60		$	—		$	60
Sales to related parties	—			—			284.2			—			284.2
Other income	—			—			3.1			—			3.1
Other income - related parties	—			—			13.4			—			13.4
Equity in earnings of subsidiaries	61.5			63.1			—			(124.6		)	—
Total revenues and other income	61.5			63.1			360.7			(124.6		)	360.7
Costs and expenses:
Cost of revenues (excludes items below)	—			—			99.2			—			99.2
Purchases from related parties	—			—			69.4			—			69.4
Depreciation	—			—			36.3			—			36.3
General and administrative expenses	3.6			—			36.8			—			40.4
Other taxes	0.2			—			4.7			—			4.9
Total costs and expenses	3.8			—			246.4			—			250.2
Income from operations	57.7			63.1			114.3			(124.6		)	110.5
Net interest and other financial costs (income)	—			1.6			(0.9		)	—			0.7
Income before income taxes	57.7			61.5			115.2			(124.6		)	109.8
Provision for income taxes	—			—			0.5			—			0.5
Net income	57.7			61.5			114.7			(124.6		)	109.3
Less: Net income attributable to MPC-retained interest	—			—			—			51.6			51.6
Net income attributable to MPLX LP	$	57.7		$	61.5		$	114.7		$	(176.2	)	$	57.7
Condensed Consolidating Balance Sheets
	30-Sep-14
(In millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
Assets
Current assets:
Cash and cash equivalents	$	0.8		$	0.5		$	30.9		$	—		$	32.2
Receivables	0.4			—			8.5			—			8.9
Receivables from related parties	—			4.2			186.8			(145.1		)	45.9
Materials and supplies inventories	—			—			11.6			—			11.6
Other current assets	—			—			6.9			—			6.9
Total current assets	1.2			4.7			244.7			(145.1		)	105.5
Property, plant and equipment, net	—			—			982.6			—			982.6
Investment in subsidiaries	496.7			886.8			—			(1,383.5		)	—
Goodwill	—			—			104.7			—			104.7
Other noncurrent assets	—			1.5			—			—			1.5
Total assets	$	497.9		$	893		$	1,332.00		$	(1,528.6	)	$	1,194.30
Liabilities
Current liabilities:
Accounts payable	$	0.4		$	0.1		$	34.4		$	—		$	34.9
Payables to related parties	4.3			140.9			17.2			(145.1		)	17.3
Deferred revenue - related parties	—			—			30.1			—			30.1
Accrued taxes	0.4			—			5.5			—			5.9
Long-term debt due within one year	—			—			0.8			—			0.8
Other current liabilities	—			0.3			1.5			—			1.8
Total current liabilities	5.1			141.3			89.5			(145.1		)	90.8
Long-term deferred revenue - related parties	—			—			4.2			—			4.2
Long-term debt	—			255			9.2			—			264.2
Deferred credits and other liabilities	1.3			—			0.5			—			1.8
Total liabilities	6.4			396.3			103.4			(145.1		)	361
Equity
MPLX LP partners’ capital	491.5			496.7			1,228.60			(1,725.3		)	491.5
Noncontrolling interest retained by MPC	—			—			—			341.8			341.8
Total equity	491.5			496.7			1,228.60			(1,383.5		)	833.3
Total liabilities and equity	$	497.9		$	893		$	1,332.00		$	(1,528.6	)	$	1,194.30
Condensed Consolidating Balance Sheets
	31-Dec-13
(In millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
Assets
Current assets:
Cash and cash equivalents	$	—		$	7.2		$	46.9		$	—		$	54.1
Receivables	0.4			—			11.8			—			12.2
Receivables from related parties	—			—			132.2			(83.9		)	48.3
Materials and supplies inventories	—			—			11.6			—			11.6
Other current assets	0.3			—			8.6			—			8.9
Total current assets	0.7			7.2			211.1			(83.9		)	135.1
Property, plant and equipment, net	—			—			966.6			—			966.6
Investment in subsidiaries	647.1			721.6			—			(1,368.7		)	—
Goodwill	—			—			104.7			—			104.7
Other noncurrent assets	—			1.8			0.3			—			2.1
Total assets	$	647.8		$	730.6		$	1,282.70		$	(1,452.6	)	$	1,208.50
Liabilities
Current liabilities:
Accounts payable	$	0.2		$	0.3		$	30		$	—		$	30.5
Payables to related parties	0.7			83.2			12.8			(83.9		)	12.8
Deferred revenue - related parties	—			—			34			—			34
Accrued taxes	0.3			—			3.7			—			4
Long-term debt due within one year	—			—			0.7			—			0.7
Other current liabilities	—			—			1.4			—			1.4
Total current liabilities	1.2			83.5			82.6			(83.9		)	83.4
Long-term debt	—			—			9.8			—			9.8
Deferred credits and other liabilities	0.4			—			0.8			—			1.2
Total liabilities	1.6			83.5			93.2			(83.9		)	94.4
Equity
MPLX LP partners’ capital	646.2			647.1			1,189.50			(1,836.6		)	646.2
Noncontrolling interest retained by MPC	—			—			—			467.9			467.9
Total equity	646.2			647.1			1,189.50			(1,368.7		)	1,114.10
Total liabilities and equity	$	647.8		$	730.6		$	1,282.70		$	(1,452.6	)	$	1,208.50
Condensed Consolidating Statements of Cash Flow
	Nine Months Ended September 30, 2014
(In millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
Net cash provided by (used in) operating activities	$	76		$	72.7		$	191		$	(149.4	)	$	190.3
Investing activities:
Additions to property, plant and equipment	—			(0.2		)	(46.9		)	—			(47.1		)
Investment in Pipe Line Holdings	—			(310.0		)	—			310			—
Loans to affiliates	—			—			(51.0		)	51			—
All other, net	—			—			4			—			4
Net cash provided by (used in) investing activities	—			(310.2		)	(93.9		)	361			(43.1		)
Financing activities:
Proceeds from borrowings from affiliates	—			51			—			(51.0		)	—
Long-term debt - borrowings	—			270			—			—			270
- repayments	—			(15.0		)	(0.6		)	—			(15.6		)
Quarterly distributions	(75.2		)	(75.2		)	(74.2		)	149.4			(75.2		)
Quarterly distributions to noncontrolling interest retained by MPC	—			—			(38.3		)	—			(38.3		)
Distributions related to purchase of additional interest in Pipe Line Holdings	—			—			—			(310.0		)	(310.0		)
Net cash provided by (used in) financing activities	(75.2		)	230.8			(113.1		)	(211.6		)	(169.1		)
Net increase (decrease) in cash and cash equivalents	0.8			(6.7		)	(16.0		)	—			(21.9		)
Cash and cash equivalents at beginning of period	—			7.2			46.9			—			54.1
Cash and cash equivalents at end of period	$	0.8		$	0.5		$	30.9		$	—		$	32.2
Condensed Consolidating Statements of Cash Flow
	Nine Months Ended September 30, 2013
(In millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
Net cash provided by (used in) operating activities	$	54.8		$	68.9		$	167		$	(123.6	)	$	167.1
Investing activities:
Additions to property, plant and equipment	—			(0.2		)	(80.8		)	—			(81.0		)
Investment in Pipe Line Holdings	—			(100.0		)	—			100			—
Loans to affiliates	—			—			(100.0		)	100			—
Repayments of loans from affiliates	—			—			16			(16.0		)	—
Net cash provided by (used in) investing activities	—			(100.2		)	(164.8		)	184			(81.0		)
Financing activities:
Proceeds from borrowings from affiliates	—			100			—			(100.0		)	—
Payments on borrowings from affiliates	—			(16.0		)	—			16			—
Long-term debt - repayments	—			—			(0.6		)	—			(0.6		)
Quarterly distributions	(55.4		)	(55.4		)	(68.2		)	123.6			(55.4		)
Quarterly distributions to noncontrolling interest retained by MPC	—			—			(59.9		)	—			(59.9		)
Distributions related to purchase of additional interest in Pipe Line Holdings	—			—			—			(100.0		)	(100.0		)
Net cash provided by (used in) financing activities	(55.4		)	28.6			(128.7		)	(60.4		)	(215.9		)
Net decrease in cash and cash equivalents	(0.6		)	(2.7		)	(126.5		)	—			(129.8		)
Cash and cash equivalents at beginning of period	1			9.7			206			—			216.7
Cash and cash equivalents at end of period	$	0.4		$	7		$	79.5		$	—		$	86.9

Description_of_Business_and_Ba

Description of Business and Basis of Presentation - Additional Information (Detail)

0 Months Ended

9 Months Ended

0 Months Ended

4 Months Ended

7 Months Ended

10 Months Ended

7 Months Ended

9 Months Ended

Oct. 31, 2012

Mar. 27, 2012

Sep. 30, 2014

Oct. 31, 2012

Oct. 31, 2012

Mar. 01, 2014

Apr. 30, 2013

Sep. 30, 2014

Feb. 28, 2014

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

bbl

Limited Partners Common Units [Member]

Other Ownership Interest Common Units [Member]

MPLX Pipe Line Holdings LP [Member]

MPLX Pipe Line Holdings LP [Member]

MPLX Pipe Line Holdings LP [Member]

MPLX Pipe Line Holdings LP [Member]

Crude Oil and Product Pipeline Systems [Member]

Storage Services Butane Cavern [Member]

Marathon Pipe Line Llc And Ohio River Pipe Line Llc [Member]

Segment

MPLX Pipe Line Holdings LP [Member]

Description Of Business And Basis Of Presentation [Line Items]

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Percent of the net income related to assets contributed to MPLX LP

'

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'

69.00%

51.00%

69.00%

56.00%

69.00%

100.00%

100.00%

Storage capacity (in number of barrels)

'

'

1,000,000

'

'

'

'

'

'

'

'

'

Date of Partnership formation

'

27-Mar-12

'

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'

'

'

'

'

Date for initial public offering Completed

'OctoberB 31, 2012

'

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'

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'

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Common units for initial public offering

'

'

'

19,895,000

2,595,000

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'

Number of segments

'

'

1

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'

'

'

'

'

'

'

'

Acquisitions_Additional_Inform

Acquisitions - Additional Information (Detail) (USD $)

0 Months Ended

9 Months Ended

0 Months Ended

4 Months Ended

7 Months Ended

10 Months Ended

0 Months Ended

3 Months Ended

9 Months Ended

0 Months Ended

9 Months Ended

0 Months Ended

In Millions, unless otherwise specified

Mar. 01, 2014

1-May-13

Sep. 30, 2014

Sep. 30, 2013

Mar. 01, 2014

Apr. 30, 2013

Sep. 30, 2014

Feb. 28, 2014

Mar. 01, 2014

1-May-13

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Mar. 01, 2014

1-May-13

Mar. 01, 2014

Sep. 30, 2014

Sep. 30, 2013

Mar. 01, 2014

1-May-13

Mar. 01, 2014

Mar. 01, 2014

MPLX Pipe Line Holdings LP [Member]

MPLX Pipe Line Holdings LP [Member]

MPLX Pipe Line Holdings LP [Member]

MPLX Pipe Line Holdings LP [Member]

General Partner [Member]

General Partner [Member]

General Partner [Member]

General Partner [Member]

General Partner [Member]

General Partner [Member]

Noncontrolling Interest Retained by MPC [Member]

Noncontrolling Interest Retained by MPC [Member]

Noncontrolling Interest Retained by MPC [Member]

Noncontrolling Interest Retained by MPC [Member]

Noncontrolling Interest Retained by MPC [Member]

Pipe Line Holding [Member]

Pipe Line Holding [Member]

Cash [Member]

Revolving Credit Agreement [Member]

MPC [Member]

MPC [Member]

MPC [Member]

Pipe Line Holding [Member]

Pipe Line Holding [Member]

Business Acquisition [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Business Acquisition, Effective Date of Acquisition

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

1-Mar-14

1-May-13

'

'

Additional interest in Pipe Line Holdings

13.00%

5.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Purchase of additional interest in Pipe Line Holdings

$310

$100

$310

$100

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

$40

$270

Purchase of additional interest in Pipe Line Holdings, historical value

'

'

-310

-100

'

'

'

'

'

'

'

'

'

'

137.5

53.6

'

-137.5

-53.6

'

'

'

'

Purchase of additional interest in Pipe Line Holdings, excess cash paid

'

'

'

'

'

'

'

'

$172.50

$46.40

$0

$0

$172.50

$46.40

'

'

'

'

'

'

'

'

'

Percent of the net income related to assets contributed to MPLX LP

'

'

'

'

69.00%

51.00%

69.00%

56.00%

'

'

'

'

'

'

'

'

31.00%

'

'

'

'

'

'

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		9 Months Ended
In Millions, unless otherwise specified	Mar. 01, 2014	1-May-13	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]	'	'	'	'	'	'
Net income attributable to MPLX LP	'	'	$29.10	$21.50	$92.10	$57.70
Transfer to noncontrolling interest retained by MPC:	'	'	'	'	'	'
Change from net income attributable to MPLX LP and transfer to noncontrolling interest retained by MPC	'	'	29.1	21.5	-80.4	11.3
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	($172.50)	($46.40)	$0	$0	($172.50)	($46.40)

Related_Party_Agreements_and_T2

Related Party Agreements and Transactions - Additional Information (Detail) (USD $)	0 Months Ended	9 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Sep. 30, 2013	Sep. 30, 2014
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%
Intercompany Revolving Credit Agreement [Member]	'	'	'
Related Party Transaction [Line Items]	'	'	'
Line of Credit Facility, Initiation Date	31-Mar-14	'	'
Line of Credit Facility, Current Borrowing Capacity	'	'	$50
Line of Credit Facility, Expiration Date	'	31-Mar-19	'
Debt Instrument, Description of Variable Rate Basis	'one-month term LIBO Rate plus 1.375 percent	'	'
Borrowings outstanding	'	'	$0

Related_Party_Agreements_and_T3

Related Party Agreements and Transactions - Sales to Related Parties (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Related Party Transaction [Line Items]	'	'	'	'
Sales to related parties	$114.10	$97.80	$336	$284.20
MPC [Member]	'	'	'	'
Related Party Transaction [Line Items]	'	'	'	'
Sales to related parties	$114.10	$97.80	$336	$284.20

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		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Related Party Transaction [Line Items]	'	'	'	'
Other income - related parties	$5.80	$4.60	$16.90	$13.40
MPC [Member]	'	'	'	'
Related Party Transaction [Line Items]	'	'	'	'
Other income - related parties	5.5	4.3	16	12.5
Centennial [Member]	'	'	'	'
Related Party Transaction [Line Items]	'	'	'	'
Other income - related parties	0.3	0.3	0.8	0.8
Muskegon [Member]	'	'	'	'
Related Party Transaction [Line Items]	'	'	'	'
Other income - related parties	$0	$0	$0.10	$0.10

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		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
MPC [Member]	'	'	'	'
Related Party Transaction [Line Items]	'	'	'	'
Charges for services included in purchases from related party	$6	$4.20	$18.10	$13.30

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		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
MPC [Member]	'	'	'	'
Related Party Transaction [Line Items]	'	'	'	'
Charges for services included in general and administrative expenses	$7.80	$7.70	$23.10	$23.50

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		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
MPC [Member] | Assets Under Construction [Member]	'	'	'	'
Property, Plant and Equipment [Line Items]	'	'	'	'
Costs added to property, plant and equipment	$1.90	$2.40	$4.10	$5.90

Related_Party_Agreements_and_T8

Related Party Agreements and Transactions - Employee Services Expenses from Related Parties (Detail) (MPC [Member], USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Related Party Transaction [Line Items]	'	'	'	'
Expenses incurred under employee services agreements	$23.30	$23.10	$69.90	$68.10
Purchases from Related Parties [Member]	'	'	'	'
Related Party Transaction [Line Items]	'	'	'	'
Expenses incurred under employee services agreements	17.7	19.6	53.3	56.1
General and Administrative Expenses [Member]	'	'	'	'
Related Party Transaction [Line Items]	'	'	'	'
Expenses incurred under employee services agreements	$5.60	$3.50	$16.60	$12

Related_Party_Agreements_and_T9

Related Party Agreements and Transactions - Receivables from Related Parties (Detail) (USD $)	Sep. 30, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Related Party Transaction [Line Items]	'	'
Current receivables from related parties	$45.90	$48.30
MPC [Member]	'	'
Related Party Transaction [Line Items]	'	'
Current receivables from related parties	45.4	47.4
Centennial [Member]	'	'
Related Party Transaction [Line Items]	'	'
Current receivables from related parties	0.4	0.6
Muskegon [Member]	'	'
Related Party Transaction [Line Items]	'	'
Current receivables from related parties	$0.10	$0.30

Recovered_Sheet1

Related Party Agreements and Transactions Related Party Agreements and Transactions - Prepaid Assets with Related Parties (Details) (MPC [Member], USD $)	Sep. 30, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
MPC [Member]	'	'
Related Party Transaction [Line Items]	'	'
Prepaid Assets, Related Parties	$0	$0.40

Recovered_Sheet2

Related Party Agreements and Transactions Related Party Agreements and Transactions - Long Term Receivables from Related Parties (Details) (MPC [Member], USD $)	Sep. 30, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
MPC [Member]	'	'
Related Party Transaction [Line Items]	'	'
Due from Related Parties, Noncurrent	$0	$0.20

Recovered_Sheet3

Related Party Agreements and Transactions - Payables to Related Parties (Detail) (USD $)	Sep. 30, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Related Party Transaction [Line Items]	'	'
Payables to related parties	$17.30	$12.80
MPC [Member]	'	'
Related Party Transaction [Line Items]	'	'
Payables to related parties	$17.30	$12.80

Recovered_Sheet4

Related Party Agreements and Transactions - Summary of Deferred Revenue - Related Parties (Detail) (MPC [Member], USD $)	Sep. 30, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Related Party Transaction [Line Items]	'	'
Deferred Revenue Related Parties	$34.30	$34
Minimum volume deficiencies [Member]	'	'
Related Party Transaction [Line Items]	'	'
Deferred Revenue Related Parties	29.5	34
Project Reimbursements [Member]	'	'
Related Party Transaction [Line Items]	'	'
Deferred Revenue Related Parties	$4.80	$0

Net_Income_Per_Limited_Partner2

Net Income Per Limited Partner Unit - Schedule of Distributions by Partner by Class (Detail) (USD $)	3 Months Ended				9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Net Income Per Share [Line Items]	'		'		'		'
Net income attributable to MPLX LP	$29.10		$21.50		$92.10		$57.70
Less: Distribution declared	-27.9		-22.5		-79.4		-64.5
Distributions less than (in excess of) net income attributable to MPLX LP	1.2		-1		12.7		-6.8
General Partner [Member] | MPC [Member]	'		'		'		'
Net Income Per Share [Line Items]	'		'		'		'
Net income attributable to MPLX LP	1.8		0.5		6.6		1.2
Less: Distribution declared	-1.5	[1]	-0.5	[1]	-3.4	[1]	-1.3	[1]
Distributions less than (in excess of) net income attributable to MPLX LP	0.3		0		3.2		-0.1
Limited Partners Common Units [Member]	'		'		'		'
Net Income Per Share [Line Items]	'		'		'		'
Net income attributable to MPLX LP	13.7		10.5		42.8		29.1
Less: Distribution declared	-13.2	[1]	-11	[1]	-38	[1]	-31.6	[1]
Distributions less than (in excess of) net income attributable to MPLX LP	0.5		-0.5		4.8		-2.5
Limited Partners Subordinated Units [Member] | MPC [Member]	'		'		'		'
Net Income Per Share [Line Items]	'		'		'		'
Net income attributable to MPLX LP	13.6		10.5		42.7		27.4
Less: Distribution declared	-13.2	[1]	-11	[1]	-38	[1]	-31.6	[1]
Distributions less than (in excess of) net income attributable to MPLX LP	$0.40		($0.50)		$4.70		($4.20)
[1]	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				9 Months Ended
In Millions, except Per Share data, unless otherwise specified	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Net income attributable to MPLX LP:	'		'		'		'
Distributions declared (including IDRs)	$27.90		$22.50		$79.40		$64.50
Distributions less than (in excess of) net income attributable to MPLX LP	1.2		-1		12.7		-6.8
Net income attributable to MPLX LP	29.1		21.5		92.1		57.7
Weighted average units outstanding:	'		'		'		'
Weighted Average Limited Partnership Units Outstanding, Basic	75.5		75.4		75.4		75.4
Weighted Average Limited Partnership Units Outstanding, Diluted	75.5		75.4		75.5		75.4
Limited Partners Common Units [Member]	'		'		'		'
Net income attributable to MPLX LP:	'		'		'		'
Distributions declared (including IDRs)	13.2	[1]	11	[1]	38	[1]	31.6	[1]
Distributions less than (in excess of) net income attributable to MPLX LP	0.5		-0.5		4.8		-2.5
Net income attributable to MPLX LP	13.7		10.5		42.8		29.1
Weighted average units outstanding:	'		'		'		'
Weighted Average Limited Partnership Units Outstanding, Basic	37.1		37		37		37
Weighted Average Limited Partnership Units Outstanding, Diluted	37.1		37		37.1		37
Net income attributable to MPLX LP per limited partner unit:	'		'		'		'
Basic (in USD per unit)	$0.37		$0.29		$1.16		$0.79
Diluted (in USD per unit)	$0.37		$0.29		$1.16		$0.79
MPC [Member] | General Partner [Member]	'		'		'		'
Net income attributable to MPLX LP:	'		'		'		'
Distributions declared (including IDRs)	1.5	[1]	0.5	[1]	3.4	[1]	1.3	[1]
Distributions less than (in excess of) net income attributable to MPLX LP	0.3		0		3.2		-0.1
Net income attributable to MPLX LP	1.8		0.5		6.6		1.2
Weighted average units outstanding:	'		'		'		'
Weighted Average Limited Partnership Units Outstanding, Basic	1.4		1.4		1.4		1.4
Weighted Average Limited Partnership Units Outstanding, Diluted	1.4		1.4		1.4		1.4
MPC [Member] | Limited Partners Subordinated Units [Member]	'		'		'		'
Net income attributable to MPLX LP:	'		'		'		'
Distributions declared (including IDRs)	13.2	[1]	11	[1]	38	[1]	31.6	[1]
Distributions less than (in excess of) net income attributable to MPLX LP	0.4		-0.5		4.7		-4.2
Net income attributable to MPLX LP	$13.60		$10.50		$42.70		$27.40
Weighted average units outstanding:	'		'		'		'
Weighted Average Limited Partnership Units Outstanding, Basic	37		37		37		37
Weighted Average Limited Partnership Units Outstanding, Diluted	37		37		37		37
Net income attributable to MPLX LP per limited partner unit:	'		'		'		'
Basic (in USD per unit)	$0.37		$0.29		$1.16		$0.74
Diluted (in USD per unit)	$0.37		$0.29		$1.16		$0.74
[1]	See Note 6 for distribution information.

Equity_Details

Equity (Details) (USD $)	3 Months Ended		9 Months Ended					0 Months Ended
In Millions, except Share data, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2014	Sep. 30, 2014	Nov. 14, 2014	Nov. 04, 2014	Oct. 21, 2014
					Limited Partners Common Units [Member]	Subordinated Units [Member]	MPC [Member]	Subsequent Event [Member]	Subsequent Event [Member]	Subsequent Event [Member]
							Limited Partners Common Units [Member]
Stockholders Equity Note [Line Items]	'	'	'	'	'	'	'	'	'	'
Units outstanding	'	'	'	'	36,966,994	'	17,056,515	'	'	'
Other Ownership Interests, Units Outstanding	'	'	'	'	'	36,951,515	'	'	'	'
Limited Liability Company (LLC) or Limited Partnership (LP), Managing Member or General Partner, Ownership Interest	'	'	2.00%	'	'	'	'	'	'	'
Units outstanding	1,508,541	'	1,508,541	'	'	'	'	'	'	'
Distribution Made to Limited Partner, Declaration Date	'	'	'	'	'	'	'	'	'	21-Oct-14
Cash distributions declared per limited partner common unit	'	'	'	'	'	'	'	'	'	$0.36
Distributions declared (including IDRs)	$27.90	$22.50	$79.40	$64.50	'	'	'	'	'	$27.90
Distribution Made to Limited Partner, Distribution Date	'	'	'	'	'	'	'	14-Nov-14	'	'
Distribution Made to Limited Partner, Date of Record	'	'	'	'	'	'	'	'	4-Nov-14	'

Equity_Changes_in_Partners_Cap

Equity - Changes in Partners Capital, Unit Rollforward (Details)	9 Months Ended				9 Months Ended
	Sep. 30, 2014	Sep. 30, 2014	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014
		Limited Partners Common Units [Member]	Limited Partners Subordinated Units [Member]	Limited Partners Subordinated Units [Member]	General Partner Units [Member]
Stockholders Equity [Line Items]	'	'	'	'	'
Balance at December 31, 2013	75,411,255	36,951,515	36,951,515	36,951,515	1,508,225
Issuance of units	316	'	'	'	316
Unit-based compensation awards	15,479	15,479	'	'	'
Balance at September 30, 2014	75,427,050	36,966,994	36,951,515	36,951,515	1,508,541

Equity_Net_Income_Allocation_D

Equity - Net Income Allocation (Details) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Equity [Abstract]	'	'	'	'
Net income attributable to MPLX LP	$29.10	$21.50	$92.10	$57.70
General partner's incentive distribution rights	1	0	1.9	0
Net income attributable to MPLX LP available to general and limited partners	28.1	21.5	90.2	57.7
General partner's 2% interest in net income attributable to MPLX LP	0.5	0.5	1.8	1.2
General partner's interest in net income attributable to MPLX LP	($1.50)	($0.50)	($3.70)	($1.20)

Equity_Cash_Distributions_Deta

Equity - Cash Distributions (Details) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Incentive Distribution Made to Managing Member or General Partner [Line Items]	'	'	'	'
General partner's distributions	$0.50	$0.50	$1.50	$1.30
General partner's incentive distribution rights	1	0	1.9	0
Total general partner's distributions	1.5	0.5	3.4	1.3
Limited partners' distributions	26.4	22	76	63.2
Total cash distributions declared	27.9	22.5	79.4	64.5
Limited Partners Common Units [Member]	'	'	'	'
Incentive Distribution Made to Managing Member or General Partner [Line Items]	'	'	'	'
Limited partners' distributions	13.2	11	38	31.6
Subordinated Units [Member]	'	'	'	'
Incentive Distribution Made to Managing Member or General Partner [Line Items]	'	'	'	'
Limited partners' distributions	$13.20	$11	$38	$31.60

Other_Items_Summary_of_Interes

Other Items - Summary of Interest and Other Financial (Costs) (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Other Income and Expenses [Abstract]	'	'	'	'
Interest income	$0	$0	$0	($0.30)
Interest expense	-1.2	-0.2	-3.1	-0.8
Interest capitalized	-0.2	-0.2	-0.7	-0.6
Other financial costs	-0.1	-0.2	-0.6	-0.8
Net interest and other financial costs	$1.10	$0.20	$3	$0.70

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended		3 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Dec. 31, 2013
					Maximum [Member]	Maximum [Member]
Income Tax Disclosure [Line Items]	'	'	'	'	'	'
Income tax expense	$0	$0.40	$0.10	$0.50	$0.10	'
Effective tax rate	'	1.00%	0.10%	0.50%	0.10%	'
Unrecognized tax benefits	'	'	'	'	$0.10	$0.10

Property_Plant_and_Equipment_P

Property, Plant and Equipment - Property, Plant and Equipment (Detail) (USD $)	Sep. 30, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Property, Plant and Equipment [Line Items]	'	'
Gross property, plant and equipment	$1,326.70	$1,276.40
Less accumulated depreciation	344.1	309.8
Net property, plant and equipment	982.6	966.6
Land [Member]	'	'
Property, Plant and Equipment [Line Items]	'	'
Gross property, plant and equipment	5.3	5.3
Pipelines and Related Assets [Member]	'	'
Property, Plant and Equipment [Line Items]	'	'
Gross property, plant and equipment	1,068.20	1,060.70
Storage and Delivery Facilities [Member]	'	'
Property, Plant and Equipment [Line Items]	'	'
Gross property, plant and equipment	165.1	165.7
Other [Member]	'	'
Property, Plant and Equipment [Line Items]	'	'
Gross property, plant and equipment	23.6	22.8
Assets Under Construction [Member]	'	'
Property, Plant and Equipment [Line Items]	'	'
Gross property, plant and equipment	$64.50	$21.90

Fair_Value_Measurements_Fair_V1

Fair Value Measurements Fair Value Measurements - Fair Values - Reported (Details) (USD $)	Sep. 30, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Fair Value [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Long-term debt	$255.30	$0
Carrying Value [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Long-term debt	$255	$0

Debt_Summary_of_Outstanding_Bo

Debt - Summary of Outstanding Borrowings (Detail) (USD $)	Sep. 30, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Debt Instrument [Line Items]	'	'
Total	$265	$10.50
Amounts due within one year	0.8	0.7
Total long-term debt due after one year	264.2	9.8
MPLX Operations LLC [Member]	'	'
Debt Instrument [Line Items]	'	'
Amounts due within one year	0	0
Total long-term debt due after one year	255	0
Capital Lease Obligations [Member] | Marathon Pipe Line Llc [Member]	'	'
Debt Instrument [Line Items]	'	'
MPL - capital lease obligations due 2020	10	10.5
MPLX Revolver [Member] | Line of Credit [Member] | MPLX Operations LLC [Member]	'	'
Debt Instrument [Line Items]	'	'
MPLX Operations - bank revolving credit agreement due 2017	$255	$0

Debt_Additional_Information_De

Debt - Additional Information (Detail) (USD $)	9 Months Ended			0 Months Ended	9 Months Ended		9 Months Ended	0 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Mar. 31, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2014	Mar. 01, 2014	1-May-13
			Revolving Credit Agreement [Member]	Intercompany Revolving Credit Agreement [Member]	Intercompany Revolving Credit Agreement [Member]	Intercompany Revolving Credit Agreement [Member]	Marathon Pipe Line Llc [Member]	Pipe Line Holding [Member]	Pipe Line Holding [Member]
Debt Instrument [Line Items]	'	'	'	'	'	'	'	'	'
Line of Credit Facility, Initiation Date	'	'	'	31-Mar-14	'	'	'	'	'
Borrowings outstanding	'	'	$255	'	'	$0	'	'	'
Proceeds from Lines of Credit	270	0	270	'	'	'	'	'	'
Debt Instrument, Interest Rate, Effective Percentage	'	'	1.50%	'	'	'	'	'	'
Extinguishment of Debt, Amount	'	'	15	'	'	'	'	'	'
Business Acquisition, Effective Date of Acquisition	'	'	'	'	'	'	'	1-Mar-14	1-May-13
Letters of Credit Outstanding, Amount	'	'	0	'	'	'	'	'	'
Line of Credit Facility, Remaining Borrowing Capacity	'	'	$245	'	'	'	'	'	'
Line of Credit Facility, Remaining Borrowing Capacity, Percentage	'	'	49.00%	'	'	'	'	'	'
Line of Credit Facility, Expiration Date	'	'	31-Oct-17	'	31-Mar-19	'	'	'	'
Capital Lease Due Date Year	'	'	'	'	'	'	'2020	'	'

Supplemental_Cash_Flow_Informa2

Supplemental Cash Flow Information - Summary of Supplemental Cash Flow Information (Detail) (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013
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 materials and supplies inventories	0.7	3.8
Property, plant and equipment contributed by MPC	$0.10	$0.20

Supplemental_Cash_Flow_Informa3

Supplemental Cash Flow Information - Summary of Reconciliation of Additions to Property, Plant and Equipment to Total Capital Expenditures (Detail) (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013
Supplemental Cash Flow Elements [Abstract]	'	'
Additions to property, plant and equipment	$47.10	$81
Plus: Increase in capital accruals	5.9	1.6
Asset retirement expenditures	1	3.4
Total capital expenditures	$54	$86

EquityBased_Compensation_Plan_1

Equity-Based Compensation Plan - Additional Information (Detail) (Performance Shares [Member], MPLX LP 2012 Incentive Compensation Plan [Member], USD $)	9 Months Ended
	Sep. 30, 2014
Performance Shares [Member] | MPLX LP 2012 Incentive Compensation Plan [Member]	'
Equity Transactions And Share Based Compensation [Line Items]	'
Performance units grant date fair value (in USD per unit)	$1.16

EquityBased_Compensation_Plan_2

Equity-Based Compensation Plan Equity-Based Compensation Plan - Summary of Phantom Unit Award Activity (Details) (Phantom Units [Member], USD $)	9 Months Ended
	Sep. 30, 2014
Phantom Units [Member]	'
Number of Units	'
Outstanding at December 31, 2013	77,754
Granted	49,465
Issued	27,360
Outstanding at September 30, 2014	99,859
Weighted Average Fair Value	'
Outstanding at December 31, 2013	$33.84
Granted (in USD per unit)	$49.09
Issued (in USD per unit)	$34.45
Outstanding at September 30, 2014	$41.22

EquityBased_Compensation_Plan_3

Equity-Based Compensation Plan - Summary of Performance Unit Award Activity (Detail) (Performance Shares [Member])	9 Months Ended
	Sep. 30, 2014
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, 2013	436,917
Granted	500,507
Outstanding at September 30, 2014	937,424

Commitments_and_Contingencies_

Commitments and Contingencies - Additional Information (Detail) (USD $)	9 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Commitments And Contingencies [Line Items]	'	'
Accrued liabilities for remediation	$1	$1.10
Maximum potential undiscounted payments	1.7	'
Contractual commitments to acquire property, plant and equipment	29.2	'
Maximum [Member]	'	'
Commitments And Contingencies [Line Items]	'	'
Site contingency, loss exposure in excess of accrual	0.4	0.4
Environmental Loss Contingency [Member] | MPC [Member]	'	'
Commitments And Contingencies [Line Items]	'	'
Receivables from MPC for indemnification of environmental costs	$0.10	$0.30

Condensed_Consolidating_Financ2

Condensed Consolidating Financial Statements - Condensed Income Statement (Details) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Revenues and other income:	'	'	'	'
Sales and other operating revenues	$16.60	$20.40	$52.20	$60
Sales to related parties	114.1	97.8	336	284.2
Other income	1.5	1	4.1	3.1
Other income - related parties	5.8	4.6	16.9	13.4
Equity in earnings of subsidiaries	0	0	0	0
Total revenues and other income	138	123.8	409.2	360.7
Costs and expenses:	'	'	'	'
Cost of revenues (excludes items below)	40.6	33.1	102.9	99.2
Purchases from related parties	23.7	23.8	71.4	69.4
Depreciation	12.5	12.7	37.5	36.3
General and administrative expenses	15.3	13	47.1	40.4
Other taxes	1.7	1.4	5.5	4.9
Total costs and expenses	93.8	84	264.4	250.2
Income from operations	44.2	39.8	144.8	110.5
Net interest and other financial costs	1.1	0.2	3	0.7
Income before income taxes	43.1	39.6	141.8	109.8
Provision for income taxes	0	-0.4	-0.1	-0.5
Net income	43.1	39.2	141.7	109.3
Less: Net income attributable to MPC-retained interest	14	17.7	49.6	51.6
Net income attributable to MPLX LP	29.1	21.5	92.1	57.7
Parent Guarantor [Member]	'	'	'	'
Revenues and other income:	'	'	'	'
Sales and other operating revenues	0	0	0	0
Sales to related parties	0	0	0	0
Other income	0	0	0	0
Other income - related parties	0	0	0	0
Equity in earnings of subsidiaries	30.4	22.8	97.2	61.5
Total revenues and other income	30.4	22.8	97.2	61.5
Costs and expenses:	'	'	'	'
Cost of revenues (excludes items below)	0	0	0	0
Purchases from related parties	0	0	0	0
Depreciation	0	0	0	0
General and administrative expenses	1.3	1.2	5.1	3.6
Other taxes	0	0.1	0	0.2
Total costs and expenses	1.3	1.3	5.1	3.8
Income from operations	29.1	21.5	92.1	57.7
Net interest and other financial costs	0	0	0	0
Income before income taxes	29.1	21.5	92.1	57.7
Provision for income taxes	0	0	0	0
Net income	29.1	21.5	92.1	57.7
Less: Net income attributable to MPC-retained interest	0	0	0	0
Net income attributable to MPLX LP	29.1	21.5	92.1	57.7
Subsidiary Issuer [Member]	'	'	'	'
Revenues and other income:	'	'	'	'
Sales and other operating revenues	0	0	0	0
Sales to related parties	0	0	0	0
Other income	0	0	0	0
Other income - related parties	0	0	0	0
Equity in earnings of subsidiaries	31.9	23.5	101.5	63.1
Total revenues and other income	31.9	23.5	101.5	63.1
Costs and expenses:	'	'	'	'
Cost of revenues (excludes items below)	0	0	0	0
Purchases from related parties	0	0	0	0
Depreciation	0	0	0	0
General and administrative expenses	0	0	0	0
Other taxes	0	0	0	0
Total costs and expenses	0	0	0	0
Income from operations	31.9	23.5	101.5	63.1
Net interest and other financial costs	1.5	0.7	4.3	1.6
Income before income taxes	30.4	22.8	97.2	61.5
Provision for income taxes	0	0	0	0
Net income	30.4	22.8	97.2	61.5
Less: Net income attributable to MPC-retained interest	0	0	0	0
Net income attributable to MPLX LP	30.4	22.8	97.2	61.5
Non-Guarantor Subsidiaries [Member]	'	'	'	'
Revenues and other income:	'	'	'	'
Sales and other operating revenues	16.6	20.4	52.2	60
Sales to related parties	114.1	97.8	336	284.2
Other income	1.5	1	4.1	3.1
Other income - related parties	5.8	4.6	16.9	13.4
Equity in earnings of subsidiaries	0	0	0	0
Total revenues and other income	138	123.8	409.2	360.7
Costs and expenses:	'	'	'	'
Cost of revenues (excludes items below)	40.6	33.1	102.9	99.2
Purchases from related parties	23.7	23.8	71.4	69.4
Depreciation	12.5	12.7	37.5	36.3
General and administrative expenses	14	11.8	42	36.8
Other taxes	1.7	1.3	5.5	4.7
Total costs and expenses	92.5	82.7	259.3	246.4
Income from operations	45.5	41.1	149.9	114.3
Net interest and other financial costs	-0.4	-0.5	-1.3	-0.9
Income before income taxes	45.9	41.6	151.2	115.2
Provision for income taxes	0	-0.4	-0.1	-0.5
Net income	45.9	41.2	151.1	114.7
Less: Net income attributable to MPC-retained interest	0	0	0	0
Net income attributable to MPLX LP	45.9	41.2	151.1	114.7
Consolidating Adjustments [Member]	'	'	'	'
Revenues and other income:	'	'	'	'
Sales and other operating revenues	0	0	0	0
Sales to related parties	0	0	0	0
Other income	0	0	0	0
Other income - related parties	0	0	0	0
Equity in earnings of subsidiaries	-62.3	-46.3	-198.7	-124.6
Total revenues and other income	-62.3	-46.3	-198.7	-124.6
Costs and expenses:	'	'	'	'
Cost of revenues (excludes items below)	0	0	0	0
Purchases from related parties	0	0	0	0
Depreciation	0	0	0	0
General and administrative expenses	0	0	0	0
Other taxes	0	0	0	0
Total costs and expenses	0	0	0	0
Income from operations	-62.3	-46.3	-198.7	-124.6
Net interest and other financial costs	0	0	0	0
Income before income taxes	-62.3	-46.3	-198.7	-124.6
Provision for income taxes	0	0	0	0
Net income	-62.3	-46.3	-198.7	-124.6
Less: Net income attributable to MPC-retained interest	14	17.7	49.6	51.6
Net income attributable to MPLX LP	($76.30)	($64)	($248.30)	($176.20)

Condensed_Consolidating_Financ3

Condensed Consolidating Financial Statements - Condensed Balance Sheet (Details) (USD $)	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Current assets:	'	'	'	'
Cash and cash equivalents	$32.20	$54.10	$86.90	$216.70
Receivables	8.9	12.2	'	'
Receivables from related parties	45.9	48.3	'	'
Materials and supplies inventories	11.6	11.6	'	'
Other current assets	6.9	8.9	'	'
Total current assets	105.5	135.1	'	'
Property, plant and equipment, net	982.6	966.6	'	'
Investment in subsidiaries	0	0	'	'
Goodwill	104.7	104.7	'	'
Other noncurrent assets	1.5	2.1	'	'
Total assets	1,194.30	1,208.50	'	'
Current liabilities:	'	'	'	'
Accounts payable	34.9	30.5	'	'
Payables to related parties	17.3	12.8	'	'
Deferred revenue - related parties	30.1	34	'	'
Accrued taxes	5.9	4	'	'
Long-term debt due within one year	0.8	0.7	'	'
Other current liabilities	1.8	1.4	'	'
Total current liabilities	90.8	83.4	'	'
Long-term deferred revenue - related parties	4.2	0	'	'
Long-term debt	264.2	9.8	'	'
Deferred credits and other liabilities	1.8	1.2	'	'
Total liabilities	361	94.4	'	'
Equity [Abstract]	'	'	'	'
Total MPLX LP partnersb capital	491.5	646.2	'	'
Noncontrolling interest retained by MPC	341.8	467.9	'	'
Total equity	833.3	1,114.10	1,121.80	1,226.80
Total liabilities and equity	1,194.30	1,208.50	'	'
Parent Guarantor [Member]	'	'	'	'
Current assets:	'	'	'	'
Cash and cash equivalents	0.8	0	0.4	1
Receivables	0.4	0.4	'	'
Receivables from related parties	0	0	'	'
Materials and supplies inventories	0	0	'	'
Other current assets	0	0.3	'	'
Total current assets	1.2	0.7	'	'
Property, plant and equipment, net	0	0	'	'
Investment in subsidiaries	496.7	647.1	'	'
Goodwill	0	0	'	'
Other noncurrent assets	0	0	'	'
Total assets	497.9	647.8	'	'
Current liabilities:	'	'	'	'
Accounts payable	0.4	0.2	'	'
Payables to related parties	4.3	0.7	'	'
Deferred revenue - related parties	0	0	'	'
Accrued taxes	0.4	0.3	'	'
Long-term debt due within one year	0	0	'	'
Other current liabilities	0	0	'	'
Total current liabilities	5.1	1.2	'	'
Long-term deferred revenue - related parties	0	'	'	'
Long-term debt	0	0	'	'
Deferred credits and other liabilities	1.3	0.4	'	'
Total liabilities	6.4	1.6	'	'
Equity [Abstract]	'	'	'	'
Total MPLX LP partnersb capital	491.5	646.2	'	'
Noncontrolling interest retained by MPC	0	0	'	'
Total equity	491.5	646.2	'	'
Total liabilities and equity	497.9	647.8	'	'
Subsidiary Issuer [Member]	'	'	'	'
Current assets:	'	'	'	'
Cash and cash equivalents	0.5	7.2	7	9.7
Receivables	0	0	'	'
Receivables from related parties	4.2	0	'	'
Materials and supplies inventories	0	0	'	'
Other current assets	0	0	'	'
Total current assets	4.7	7.2	'	'
Property, plant and equipment, net	0	0	'	'
Investment in subsidiaries	886.8	721.6	'	'
Goodwill	0	0	'	'
Other noncurrent assets	1.5	1.8	'	'
Total assets	893	730.6	'	'
Current liabilities:	'	'	'	'
Accounts payable	0.1	0.3	'	'
Payables to related parties	140.9	83.2	'	'
Deferred revenue - related parties	0	0	'	'
Accrued taxes	0	0	'	'
Long-term debt due within one year	0	0	'	'
Other current liabilities	0.3	0	'	'
Total current liabilities	141.3	83.5	'	'
Long-term deferred revenue - related parties	0	'	'	'
Long-term debt	255	0	'	'
Deferred credits and other liabilities	0	0	'	'
Total liabilities	396.3	83.5	'	'
Equity [Abstract]	'	'	'	'
Total MPLX LP partnersb capital	496.7	647.1	'	'
Noncontrolling interest retained by MPC	0	0	'	'
Total equity	496.7	647.1	'	'
Total liabilities and equity	893	730.6	'	'
Non-Guarantor Subsidiaries [Member]	'	'	'	'
Current assets:	'	'	'	'
Cash and cash equivalents	30.9	46.9	79.5	206
Receivables	8.5	11.8	'	'
Receivables from related parties	186.8	132.2	'	'
Materials and supplies inventories	11.6	11.6	'	'
Other current assets	6.9	8.6	'	'
Total current assets	244.7	211.1	'	'
Property, plant and equipment, net	982.6	966.6	'	'
Investment in subsidiaries	0	0	'	'
Goodwill	104.7	104.7	'	'
Other noncurrent assets	0	0.3	'	'
Total assets	1,332	1,282.70	'	'
Current liabilities:	'	'	'	'
Accounts payable	34.4	30	'	'
Payables to related parties	17.2	12.8	'	'
Deferred revenue - related parties	30.1	34	'	'
Accrued taxes	5.5	3.7	'	'
Long-term debt due within one year	0.8	0.7	'	'
Other current liabilities	1.5	1.4	'	'
Total current liabilities	89.5	82.6	'	'
Long-term deferred revenue - related parties	4.2	'	'	'
Long-term debt	9.2	9.8	'	'
Deferred credits and other liabilities	0.5	0.8	'	'
Total liabilities	103.4	93.2	'	'
Equity [Abstract]	'	'	'	'
Total MPLX LP partnersb capital	1,228.60	1,189.50	'	'
Noncontrolling interest retained by MPC	0	0	'	'
Total equity	1,228.60	1,189.50	'	'
Total liabilities and equity	1,332	1,282.70	'	'
Consolidating Adjustments [Member]	'	'	'	'
Current assets:	'	'	'	'
Cash and cash equivalents	0	0	0	0
Receivables	0	0	'	'
Receivables from related parties	-145.1	-83.9	'	'
Materials and supplies inventories	0	0	'	'
Other current assets	0	0	'	'
Total current assets	-145.1	-83.9	'	'
Property, plant and equipment, net	0	0	'	'
Investment in subsidiaries	-1,383.50	-1,368.70	'	'
Goodwill	0	0	'	'
Other noncurrent assets	0	0	'	'
Total assets	-1,528.60	-1,452.60	'	'
Current liabilities:	'	'	'	'
Accounts payable	0	0	'	'
Payables to related parties	-145.1	-83.9	'	'
Deferred revenue - related parties	0	0	'	'
Accrued taxes	0	0	'	'
Long-term debt due within one year	0	0	'	'
Other current liabilities	0	0	'	'
Total current liabilities	-145.1	-83.9	'	'
Long-term deferred revenue - related parties	0	'	'	'
Long-term debt	0	0	'	'
Deferred credits and other liabilities	0	0	'	'
Total liabilities	-145.1	-83.9	'	'
Equity [Abstract]	'	'	'	'
Total MPLX LP partnersb capital	-1,725.30	-1,836.60	'	'
Noncontrolling interest retained by MPC	341.8	467.9	'	'
Total equity	-1,383.50	-1,368.70	'	'
Total liabilities and equity	($1,528.60)	($1,452.60)	'	'

Condensed_Consolidating_Financ4

Condensed Consolidating Financial Statements - Condensed Cash Flow Statement (Details) (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013
Condensed Cash Flow Statements, Captions [Line Items]	'	'
Net Cash Provided by (Used in) Operating Activities	$190.30	$167.10
Investing activities:	'	'
Additions to property, plant and equipment	-47.1	-81
All other, net	4	0
Net cash used in investing activities	-43.1	-81
Financing activities:	'	'
Long-term debt - borrowings	270	0
Long-term debt - repayments	-15.6	-0.6
Quarterly distributions to unitholders and general partner	-75.2	-55.4
Quarterly distributions to noncontrolling interest retained by MPC	-38.3	-59.9
Distributions related to purchase of additional interest in Pipe Line Holdings	-310	-100
Net cash used in financing activities	-169.1	-215.9
Net increase (decrease) in cash and cash equivalents	-21.9	-129.8
Cash and cash equivalents at beginning of period	54.1	216.7
Cash and cash equivalents at end of period	32.2	86.9
Parent Guarantor [Member]	'	'
Condensed Cash Flow Statements, Captions [Line Items]	'	'
Net Cash Provided by (Used in) Operating Activities	76	54.8
Investing activities:	'	'
Net cash used in investing activities	0	0
Financing activities:	'	'
Quarterly distributions to unitholders and general partner	-75.2	-55.4
Net cash used in financing activities	-75.2	-55.4
Net increase (decrease) in cash and cash equivalents	0.8	-0.6
Cash and cash equivalents at beginning of period	0	1
Cash and cash equivalents at end of period	0.8	0.4
Subsidiary Issuer [Member]	'	'
Condensed Cash Flow Statements, Captions [Line Items]	'	'
Net Cash Provided by (Used in) Operating Activities	72.7	68.9
Investing activities:	'	'
Additions to property, plant and equipment	-0.2	-0.2
Investment in Pipe Line Holdings	-310	-100
Net cash used in investing activities	-310.2	-100.2
Financing activities:	'	'
Proceeds from borrowing from affiliates	51	100
Payments on borrowings from affiliates	'	-16
Long-term debt - borrowings	270	'
Long-term debt - repayments	-15	'
Quarterly distributions to unitholders and general partner	-75.2	-55.4
Net cash used in financing activities	230.8	28.6
Net increase (decrease) in cash and cash equivalents	-6.7	-2.7
Cash and cash equivalents at beginning of period	7.2	9.7
Cash and cash equivalents at end of period	0.5	7
Non-Guarantor Subsidiaries [Member]	'	'
Condensed Cash Flow Statements, Captions [Line Items]	'	'
Net Cash Provided by (Used in) Operating Activities	191	167
Investing activities:	'	'
Additions to property, plant and equipment	-46.9	-80.8
Investment in Pipe Line Holdings	0	0
Loans to affiliates	-51	-100
All other, net	4	'
Net cash used in investing activities	-93.9	-164.8
Proceeds from Related Party Debt	'	16
Financing activities:	'	'
Long-term debt - repayments	-0.6	-0.6
Quarterly distributions to unitholders and general partner	-74.2	-68.2
Quarterly distributions to noncontrolling interest retained by MPC	-38.3	-59.9
Net cash used in financing activities	-113.1	-128.7
Net increase (decrease) in cash and cash equivalents	-16	-126.5
Cash and cash equivalents at beginning of period	46.9	206
Cash and cash equivalents at end of period	30.9	79.5
Consolidating Adjustments [Member]	'	'
Condensed Cash Flow Statements, Captions [Line Items]	'	'
Net Cash Provided by (Used in) Operating Activities	-149.4	-123.6
Investing activities:	'	'
Investment in Pipe Line Holdings	310	100
Loans to affiliates	51	100
Net cash used in investing activities	361	184
Proceeds from Related Party Debt	'	-16
Financing activities:	'	'
Proceeds from borrowing from affiliates	-51	-100
Payments on borrowings from affiliates	'	16
Quarterly distributions to unitholders and general partner	149.4	123.6
Distributions related to purchase of additional interest in Pipe Line Holdings	-310	-100
Net cash used in financing activities	-211.6	-60.4
Net increase (decrease) in cash and cash equivalents	0	0
Cash and cash equivalents at beginning of period	0	0
Cash and cash equivalents at end of period	$0	$0

Subsequent_Event_Additional_In

Subsequent Event - Additional Information (Details)	0 Months Ended
	Oct. 29, 2014	Mar. 01, 2014	Oct. 29, 2014
	Subsequent Event [Member]	MPC [Member]	MPC [Member]
		Noncontrolling Interest Retained by MPC [Member]	Noncontrolling Interest Retained by MPC [Member]
			Subsequent Event [Member]
Subsequent Event [Line Items]	'	'	'
Subsequent Event, Date	29-Oct-14	'	'
Percent of the net income related to assets contributed to MPLX LP	'	31.00%	31.00%