MPLX (MPLX) 10-Q7 Nov 132013 Q3 Quarterly reportFinancial data

Company Profile

Document_and_Entity_Informatio

Document and Entity Information	9 Months Ended
	Sep. 30, 2013	Oct. 31, 2013	Oct. 31, 2013	Oct. 31, 2013
	Sep. 30, 2013	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-13	'	'	'
Document Fiscal Year Focus	'2013	'	'	'
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	'	36,951,515	36,951,515	1,508,225

Consolidated_Statements_of_Inc

Consolidated Statements of Income (USD $)	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended
In Millions, except Per Share data, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2012
			Limited Partners Common Units [Member]	Limited Partners Common Units [Member]	Limited Partners Common Units [Member]	Limited Partners Common Units [Member]	Limited Partners Subordinated Units [Member]	Limited Partners Subordinated Units [Member]	Predecessor [Member]	Predecessor [Member]
			MPC [Member]	MPC [Member]	Public [Member]	Public [Member]	MPC [Member]	MPC [Member]
Revenues and other income:	'	'	'	'	'	'	'	'	'	'
Sales and other operating revenues	$20.40	$60	'	'	'	'	'	'	$19.60	$53.50
Sales to related parties	97.8	284.2	'	'	'	'	'	'	96.6	265.8
Loss on sale of assets	0	0	'	'	'	'	'	'	0	-0.3
Other income	1	3.1	'	'	'	'	'	'	1.7	5
Other income - related parties	4.6	13.4	'	'	'	'	'	'	3.2	9.6
Total revenues and other income	123.8	360.7	'	'	'	'	'	'	121.1	333.6
Costs and expenses:	'	'	'	'	'	'	'	'	'	'
Cost of revenues (excludes items below)	33.1	99.2	'	'	'	'	'	'	53.5	136
Purchases from related parties	23.8	69.4	'	'	'	'	'	'	7.7	21.3
Depreciation	12.7	36.3	'	'	'	'	'	'	9.4	27.8
General and administrative expenses	13	40.4	'	'	'	'	'	'	11.2	38.1
Other taxes	1.4	4.9	'	'	'	'	'	'	3	9.7
Total costs and expenses	84	250.2	'	'	'	'	'	'	84.8	232.9
Income from operations	39.8	110.5	'	'	'	'	'	'	36.3	100.7
Related party interest and other financial income	0	0	'	'	'	'	'	'	0.5	1.3
Interest and other financial income (costs)	-0.2	-0.7	'	'	'	'	'	'	0	0
Income before income taxes	39.6	109.8	'	'	'	'	'	'	36.8	102
Provision (benefit) for income taxes	-0.4	-0.5	'	'	'	'	'	'	0.1	-0.1
Net income	39.2	109.3	'	'	'	'	'	'	36.9	101.9
Less: Net income attributable to MPC-retained interest	17.7	51.6	'	'	'	'	'	'	0	0
Net income attributable to MPLX LP	21.5	57.7	'	'	'	'	10.5	27.4	36.9	101.9
Less: General partnerbs interest in net income attributable to MPLX LP	0.5	1.2	'	'	'	'	'	'	'	'
Limited partnersb interest in net income attributable to MPLX LP	$21	$56.50	'	'	'	'	'	'	'	'
Net income attributable to MPLX LP per limited partner unit:	'	'	'	'	'	'	'	'	'	'
Common - basic (in USD per unit)	'	'	$0.29	$0.79	'	'	$0.29	$0.74	'	'
Common - diluted (in USD per unit)	'	'	$0.29	$0.79	'	'	$0.29	$0.74	'	'
Subordinated - basic and diluted (in USD per unit)	'	'	'	'	'	'	$0.29	$0.74	'	'
Weighted average limited partner units outstanding (basic and diluted):	'	'	'	'	'	'	'	'	'	'
Weighted average limited partner units outstanding basic and diluted (in shares)	'	'	17.1	17.1	19.9	19.9	37	37	'	'

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Current assets:	'	'
Cash and cash equivalents	$86.90	$216.70
Receivables	14.3	17.6
Receivables from related parties	40.5	38
Materials and supplies inventories	11.6	8.9
Other current assets	2.3	2.6
Total current assets	155.6	283.8
Property, plant and equipment, net	956.2	910
Goodwill	104.7	104.7
Other noncurrent assets	2.2	2.8
Total assets	1,218.70	1,301.30
Current liabilities:	'	'
Accounts payable	41.6	39
Payables to related parties	9.9	13.4
Deferred revenue - related parties	27.1	4.2
Accrued taxes	4.7	3
Long-term debt due within one year	0.7	0.7
Other current liabilities	1.6	3
Total current liabilities	85.6	63.3
Long-term debt	10	10.6
Deferred credits and other liabilities	1.3	0.6
Total liabilities	96.9	74.5
Commitments and contingencies (see Note 12)	'	'
Equity	'	'
Total MPLX LP partnersb capital	647.8	691.1
Noncontrolling interest retained by MPC	474	535.7
Total equity	1,121.80	1,226.80
Total liabilities and equity	1,218.70	1,301.30
MPC [Member]	'	'
Current assets:	'	'
Receivables from related parties	39	37.6
Current liabilities:	'	'
Payables to related parties	9.8	13.4
Deferred revenue - related parties	27.1	4.2
Limited Partners Common Units [Member] \| MPC [Member]	'	'
Equity	'	'
Total MPLX LP partnersb capital	57.9	57.4
Limited Partners Subordinated Units [Member] \| MPC [Member]	'	'
Equity	'	'
Total MPLX LP partnersb capital	210.4	209.3
General Partner [Member] \| MPC [Member]	'	'
Equity	'	'
Total MPLX LP partnersb capital	-32.5	13.7
Public [Member] \| Limited Partners Common Units [Member]	'	'
Equity	'	'
Total MPLX LP partnersb capital	$412	$410.70

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Limited Partners Common Units [Member] \| Public [Member]	'	'
Units issued	19.9	19.9
Units outstanding	19.9	19.9
MPC [Member] \| Limited Partners Common Units [Member]	'	'
Units issued	17.1	17.1
Units outstanding	17.1	17.1
MPC [Member] \| Limited Partners Subordinated Units [Member]	'	'
Units issued	37	37
Units outstanding	37	37
MPC [Member] \| General Partner [Member]	'	'
Units issued	1.5	1.5
Units outstanding	1.5	1.5

Consolidated_Statements_of_Cas

Consolidated Statements of Cash Flows (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
	Sep. 30, 2013	Predecessor [Member]
Operating activities:	'	'
Net income	$109.30	$101.90
Adjustments to reconcile net income to net cash provided by operating activities:	'	'
Depreciation	36.3	27.8
Deferred income taxes	0.5	-0.1
Asset retirement expenditures	-3.4	-2.6
Net loss on disposal of assets	0	0.3
Changes in:	'	'
Current receivables	3.3	1.9
Materials and supplies inventories	1.1	-0.4
Current accounts payable and accrued liabilities	2.7	0.8
Receivables from / liabilities to related parties	16.9	-4.5
All other, net	0.4	-3.3
Net cash provided by operating activities	167.1	121.8
Investing activities:	'	'
Additions to property, plant and equipment	-81	-109.2
Disposal of assets	0	0.1
Investments-repayments of loans receivable from a related party	0	221.7
Net cash provided by (used in) investing activities	-81	112.6
Financing activities:	'	'
Long-term debt-repayments	-0.6	-0.5
Quarterly distributions to unitholders and general partner	-55.4	0
Quarterly distributions to noncontrolling interest retained by MPC	-59.9	0
Distributions related to purchase of additional interest in Pipe Line Holdings	-100	0
Distributions to MPC	0	-165.5
Net cash used in financing activities	-215.9	-166
Net increase (decrease) in cash and cash equivalents	-129.8	68.4
Cash and cash equivalents at beginning of period	216.7	0.1
Cash and cash equivalents at end of period	$86.90	$68.50

Consolidated_Statements_of_Equ

Consolidated Statements of Equity/Net Investment (USD $)	Total	Predecessor [Member]	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]	Net Investment [Member]
In Millions	Total	Predecessor [Member]	Public [Member]	MPC [Member]	MPC [Member]	MPC [Member]	Noncontrolling Interest Retained by MPC [Member]	MPC [Member]	Predecessor [Member]
Beginning Balance at Dec. 31, 2011	'	$1,239.20	'	'	'	'	'	'	$1,239.20
Net income	'	101.9	'	'	'	'	'	'	101.9
Distributions to MPC, net	'	-165.5	'	'	'	'	'	'	-165.5
Non-Cash Contribution from MPC	0	'	'	'	'	'	'	'	'
Ending Balance at Sep. 30, 2012	'	1,175.60	'	'	'	'	'	'	1,175.60
Beginning Balance at Jun. 30, 2012	'	'	'	'	'	'	'	'	'
Net income	'	36.9	'	'	'	'	'	'	'
Ending Balance at Sep. 30, 2012	'	1,175.60	'	'	'	'	'	'	'
Beginning Balance at Dec. 31, 2012	1,226.80	'	410.7	57.4	209.3	13.7	'	535.7	'
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	'
Non-Cash Contribution from MPC	0.2	'	'	'	'	'	'	0.2	'
Equity-based compensation	0.8	'	0.8	'	'	'	'	'	'
Ending Balance at Sep. 30, 2013	$1,121.80	'	$412	$57.90	$210.40	($32.50)	'	$474	'

Description_of_the_Business_an

Description of the Business and Basis of Presentation	9 Months Ended
Description of the Business and Basis of Presentation	Sep. 30, 2013
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, 2013, the Partnership’s assets consisted of a 56.0 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 100.0 percent interest in a butane cavern in Neal, West Virginia with approximately one million barrels of storage capacity.

	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 used in the present tense or for periods starting on or after October 31, 2012, 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 56.0 percent general partner interest at September 30, 2013. Pipe Line Holdings owns 100.0 percent of Marathon Pipe Line LLC (“MPL”) and Ohio River Pipe Line LLC (“ORPL”). References in this report to the “Predecessor,” “we,” “our,” “us,” or like terms, when used for periods prior to October 31, 2012, refer to MPLX LP Predecessor, our predecessor for accounting purposes. References to “MPC” refer collectively to Marathon Petroleum Corporation and its subsidiaries, other than the Partnership. The Partnership was formed on March 27, 2012, as a Delaware limited partnership.

	MPLX LP Predecessor included the assets, liabilities and results of operations of certain crude oil and product pipeline systems and associated storage assets of MPC operated and held by MPL and ORPL prior to their contribution to the Partnership in connection with the Offering. Prior to the Offering, MPLX LP Predecessor results also included minority undivided joint interests in two crude oil pipeline systems that were not contributed to MPLX LP at the Offering.

	Net income attributable to MPLX LP for the three months and nine months ended September 30, 2012 included 100.0 percent of the net income related to the assets that were contributed to MPLX LP, while net income attributable to MPLX LP for the three months and nine months ended September 30, 2013 reflect the 51.0 percent general partner interest in Pipe Line Holdings initially contributed to MPLX LP through April 30, 2013 and thereafter the 56.0 percent general partner interest beginning May 1, 2013. For the periods subsequent to the Offering through April 30, 2013, we consolidated the results of operations of Pipe Line Holdings and then recorded a 49.0 percent noncontrolling interest deduction for the limited partner interest in Pipe Line Holdings retained by MPC. On May 1, 2013, we acquired an additional 5.0 percent interest in Pipe Line Holdings from MPC for consideration of $100.0 million in cash. Following the acquisition, we continued to consolidate the results of operations of Pipe Line Holdings and then recorded a 44.0 percent noncontrolling interest deduction for the limited partner interest in Pipe Line Holdings retained by MPC. The change in the Pipe Line Holdings general partner ownership interest is discussed in Note 2. The Neal, West Virginia butane cavern financial results are reflected only in the three months and nine months ended September 30, 2013. Additional differences in revenues and expenses are attributable to changes in agreements and activities, as detailed in Note 3. Due to these factors, our results of operations subsequent to the Offering are not comparable to our Predecessor’s historical results of operations.

	Our operations consist of one reportable segment.

	Basis of Presentation—Prior to the Offering on October 31, 2012, our financial position, results of operations and cash flows consisted of MPLX LP Predecessor, which represented a combined reporting entity. Subsequent to the Offering, our financial position, results of operations and cash flows consist of consolidated MPLX LP activities and balances.

	The accompanying interim consolidated financial statements are unaudited; however, in the opinion of our management, these statements reflect all adjustments necessary for a fair statement of the results for the periods reported. All such adjustments are of a normal, recurring nature unless otherwise disclosed. These interim consolidated financial statements, including the notes, have been prepared in accordance with the rules and regulations of the Securities and Exchange Commission (“SEC”) applicable to interim period financial statements and do not include all of the information and disclosures required by accounting principles generally accepted in the United States (“U.S. GAAP”) for complete financial statements.

	These interim consolidated financial statements should be read in conjunction with the audited consolidated financial statements and notes thereto for the year ended December 31, 2012 included in our Annual Report on Form 10-K filed with the SEC on March 25, 2013. The results of operations for the three months and nine months ended September 30, 2013 are not necessarily indicative of the results to be expected for the full year.

	The interim consolidated statements of income for periods prior to the Offering also included expense allocations for certain corporate functions historically performed by MPC, including allocations of general corporate expenses related to information technology, engineering, legal, human resources and other financial and administrative services. Those allocations were based primarily on specific identification, capital employed, wages or headcount. Our management believes the assumptions underlying the interim consolidated financial statements, including the assumptions regarding allocating general corporate expenses from MPC, are reasonable. However, these interim consolidated financial statements for periods prior to the Offering do not include all of the actual expenses that would have been incurred had we been a stand-alone publicly traded partnership during those periods and may not reflect our consolidated results of operations and cash flows had we been a stand-alone publicly traded partnership during those periods.

	Net income attributable to MPLX LP is allocated to the unitholders differently for preparation of the consolidated statements of equity/net investment and the calculation of net income per limited partner unit. In preparing our consolidated statements of equity/net investment, 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 4.

	Certain reclassifications of prior periods’ data have been made to conform to current classifications. On the December 31, 2012 consolidated balance sheet, deferred revenue-related parties of $4.2 million has been reclassified from payables to related parties to a separate line. On the consolidated statement of cash flows for the nine months ended September 30, 2012, within operating activities, asset retirement expenditures of $2.6 million have been reclassified from all other, net to a separate line, and changes in crude oil inventories of $0.7 million have been reclassified from changes in inventories to all other, net.

Acquisition

Acquisition	9 Months Ended
	Sep. 30, 2013
Equity [Abstract]	'
Acquisition	'
	Acquisition

	On May 1, 2013, we acquired an additional 5.0 percent interest in Pipe Line Holdings from MPC for consideration of $100.0 million, which was funded with cash on hand. We recorded the additional 5.0 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. Prior to this transaction, the 5.0 percent interest was held by MPC and was reflected as part of the noncontrolling interest retained by MPC in our consolidated financial statements. The acquisition of the additional 5.0 percent interest was accounted for on a prospective basis as we acquired an additional interest in an already-consolidated entity. Beginning May 1, 2013, our consolidated financial statements reflect the 56.0 percent general partner interest in Pipe Line Holdings owned by MPLX LP, while the 44.0 percent limited partner interest held by MPC is reflected as a noncontrolling interest. The terms of the acquisition 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, 2013 and 2012 were as follows:



		Three Months Ended September 30,				Nine Months Ended September 30,
	(In millions)	2013		2012		2013			2012
				Predecessor					Predecessor
	Net income attributable to MPLX LP	$	21.5	$	36.9	$	57.7		$	101.9

	Transfer to noncontrolling interest retained by MPC:
	Decrease in general partner-MPC equity for purchase of additional interest in Pipe Line Holdings	—		—		(46.4		)	—

	Change from net income attributable to MPLX LP and transfer to noncontrolling interest retained by MPC	$	21.5	$	36.9	$	11.3		$	101.9

Related_Party_Agreements_and_T

Related Party Agreements and Transactions	9 Months Ended
	Sep. 30, 2013
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 and Southeast regions of the United States.

	•	Centennial Pipeline LLC (“Centennial”), in which MPC has a 50.0 percent interest. Centennial owns a products pipeline and storage facility.

	•	Muskegon Pipeline LLC (“Muskegon”), in which MPC has a 60.0 percent interest. Muskegon owns a common carrier products pipeline.


	Commercial Agreements

	At the closing of the Offering, the Partnership entered into long-term, fee-based transportation services agreements with MPC. On October 1, 2012, MPL entered into long-term, fee-based storage services agreements with MPC. Under these agreements, we provide transportation and storage services to MPC, and MPC has committed to provide us with minimum quarterly throughput and storage volumes of crude oil and products and minimum storage volumes of butane. We believe the terms and conditions under these agreements, as well as our initial agreements with MPC described below, are generally no less favorable to either party than those that could have been negotiated with unaffiliated parties with respect to similar services.

	These commercial agreements with MPC include:

	•	three separate 10-year transportation services agreements and one five-year transportation services agreement under which MPC pays the Partnership fees for transporting crude oil on each of our crude oil pipeline systems;

	•	four separate 10-year transportation services agreements under which MPC pays the Partnership fees for transporting products on each of our product pipeline systems;

	•	a five-year transportation services agreement under which MPC pays the Partnership fees for handling crude oil and products at our Wood River, Illinois barge dock;

	•	a 10-year storage services agreement under which MPC pays the Partnership fees for providing storage services at our Neal, West Virginia butane cavern; and

	•	four separate three-year storage services agreements under which MPC pays the Partnership fees for providing storage services at our tank farms.

	All of our transportation services agreements for our crude oil and product pipeline systems (other than our Wood River, Illinois to Patoka, Illinois crude system) automatically renew for up to two additional five-year terms unless terminated by either party. The transportation services agreements for our Wood River to Patoka crude system and our barge dock automatically renew for up to four additional two-year terms unless terminated by either party. Our butane cavern storage services agreement does not automatically renew. Our storage services agreements for our tank farms automatically renew for additional one-year terms unless terminated by either party.

	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. If the minimum capacity of the pipeline falls below the level of MPC’s commitment at any time, or if capacity on the pipeline is required to be allocated among shippers because volume nominations exceed available capacity, depending on the cause of the reduction in capacity, MPC’s commitment may be reduced or MPC will receive a credit for its minimum volume commitment for that period. In addition to MPC’s minimum volume commitment, MPC is also responsible for any loading, handling, transfer and other charges with respect to volumes we transport for MPC. If we agree to make any capital expenditures at MPC’s request, MPC will reimburse us for, or we will have the right in certain circumstances to file for an increased tariff rate to recover, the actual cost of such capital expenditures. Our transportation services agreements include provisions that permit MPC to suspend, reduce or terminate its obligations under the applicable agreement if certain events occur. These events include MPC deciding to permanently or indefinitely suspend refining operations at one or more of its refineries for at least twelve consecutive months and certain force majeure events that would prevent us or MPC from performing required services under the applicable agreement.

	Under the storage services agreements, we are obligated to make available to MPC on a firm basis the available storage capacity at our tank farms and butane cavern, and MPC pays us a per-barrel fee for such storage capacity, regardless of whether MPC fully utilizes the available capacity. Beginning on January 1, 2014, the per-barrel fees in our storage services agreements will be adjusted annually based on changes in the producer price index.

	Operating Agreements

	At the closing of the Offering, the Partnership entered into an operating services agreement with MPC under which we operate various pipeline systems owned by MPC. In addition, under existing operating services agreements that MPL had previously entered into with MPC and third parties, MPL continues to operate various pipeline systems owned by MPC and third parties. Under these operating services agreements, the Partnership receives an operating fee for operating the assets and is reimbursed for direct and indirect costs associated with operating the assets. Most of these agreements are indexed for inflation. These agreements range from one to five years and automatically renew unless terminated by either party.

	Effective February 1, 2013, we entered into an operating agreement with Blanchard Pipe Line Company LLC (“Blanchard”), a wholly-owned subsidiary of MPC, under which we operate various pipeline systems in Texas owned by Blanchard that it acquired in conjunction with MPC’s acquisition of the Galveston Bay refinery. Under the operating agreement, we receive an initial annual fee of $1.0 million, subject to adjustment for inflation, and are reimbursed for specific costs associated with operating the pipeline systems. The term of the operating agreement is until December 31, 2014, and it is automatically extended from year to year thereafter unless terminated by either party at least three months prior to the end of the term.

	Management Services Agreements

	Prior to the closing of the Offering, MPL entered into two management services agreements with MPC under which we provide certain management services to MPC with respect to certain of MPC’s retained pipeline assets. We receive fixed annual fees under the agreements for providing the required management services, initially in the amount of $0.7 million and thereafter adjusted annually for inflation and based on changes in the scope of management services provided.

	Omnibus Agreement

	Upon the closing of the Offering, the Partnership entered into an omnibus agreement with MPC that addresses our payment of a fixed annual fee to MPC for the provision of executive management services by certain executive officers of our general partner and our reimbursement of MPC for the provision of certain general and administrative services to us, as well as MPC’s indemnification of us for certain matters, including environmental, title and tax matters.

	Employee Services Agreements

	Effective October 1, 2012, the Partnership entered into two employee services agreements with MPC under which we reimburse MPC for the provision of certain operational and management services to us in support of our pipelines, barge dock, butane cavern and tank farms.

	Related Party Transactions

	We believe that transactions with related parties, other than certain transactions with MPC related to the provision of administrative services for periods prior to the Offering, were conducted on terms comparable to those with unrelated parties. See below for additional information on the transactions with MPC.

	Sales to related parties were as follows:



		Three Months Ended September 30,						Nine Months Ended September 30,
	(In millions)	2013			2012			2013			2012
					Predecessor						Predecessor
	MPC	$	97.8		$	96.6		$	284.2		$	265.8


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.

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)	2013			2012			2013			2012
				Predecessor						Predecessor
MPC	$	4.3		$	3		$	12.5		$	8.8

Centennial	0.3			0.1			0.8			0.7

Muskegon	—			0.1			0.1			0.1

Total	$	4.6		$	3.2		$	13.4		$	9.6


Subsequent to the Offering, MPC provides executive management services and certain general and administrative services to us under terms of the omnibus agreement. For periods prior to the Offering, MPC performed certain services related to information technology, engineering, legal, human resources and other financial and administrative services. Rates for shared services were negotiated between us and the service providers. Where costs incurred on our behalf could not practically be determined by specific identification, these costs were primarily allocated to us based on capital employed, wages or headcount. Our management believes those allocations were a reasonable reflection of the utilization of services provided. However, those allocations may not have fully reflected the expenses that would have been incurred had we been a stand-alone publicly traded partnership during those periods.

Charges for services included in purchases from related parties primarily relate to services that support our operations and maintenance activities. These charges were as follows:



	Three Months Ended September 30,						Nine Months Ended September 30,
(In millions)	2013			2012			2013			2012
				Predecessor						Predecessor
MPC	$	4.2		$	4.4		$	13.3		$	11.3


Charges for services included in general and administrative expenses primarily relate to services that support our executive management, accounting and human resources activities, and allocations of corporate overhead costs from MPC for periods prior to the Offering. These charges were as follows:



	Three Months Ended September 30,						Nine Months Ended September 30,
(In millions)	2013			2012			2013			2012
				Predecessor						Predecessor
MPC	$	7.7		$	5.9		$	23.5		$	17.4


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)	2013			2012			2013			2012
				Predecessor						Predecessor
MPC	$	2.4		$	1.4		$	5.9		$	4


Effective October 1, 2012, MPL’s employees transferred to MPC, and we entered into two employee services agreements with MPC for the provision of certain operational and management services. Expenses incurred under the employee services agreements for periods subsequent to October 1, 2012 are shown in the table below by the income statement line where they were recorded. For periods prior to October 1, 2012, MPL employees were considered to participate in multiemployer benefit plans of MPC. Our allocated share of MPC’s employee benefit plan expenses for periods prior to October 1, 2012, including costs related to stock-based compensation plans, is shown in the table below by income statement line and was based upon a percentage of the salaries and wages of employees whose costs were recorded in each income statement line. 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. Expenses for employee benefit plans other than stock-based compensation plans were allocated to us primarily as a percentage of headcount. For the stock-based compensation plans, we were charged with the expenses directly attributed to our Predecessor’s employees, which were $0.2 million and $0.8 million for the three months and nine months ended September 30, 2012.

Employee services expenses from related parties were as follows:



	Three Months Ended September 30,						Nine Months Ended September 30,
(In millions)	2013			2012			2013			2012
				Predecessor						Predecessor
Purchases from related parties	$	19.6		$	3.3		$	56.1		$	10

General and administrative expenses	3.5			5			12			19.5

Total	$	23.1		$	8.3		$	68.1		$	29.5


Receivables from related parties were as follows:



(In millions)	30-Sep-13			31-Dec-12
MPC	$	39		$	37.6

Centennial	1			0.3

Muskegon	0.5			0.1

Total	$	40.5		$	38


At September 30, 2013 and December 31, 2012, we also had long-term receivables of $0.3 million from MPC, which were included in other noncurrent assets on the consolidated balance sheets, related to indemnifications provided under the omnibus agreement.

Payables to related parties were as follows:



(In millions)	30-Sep-13			31-Dec-12
MPC	$	9.8		$	13.4

Centennial	0.1			—

Total	$	9.9		$	13.4


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, 2013 and the period October 31, 2012 through December 31, 2012, MPC did not transport its minimum committed volumes. The resulting deficiencies were all related to transportation services agreements which have four quarter make-up periods. The deferred revenue-related parties associated with the deficiencies was as follows:



(In millions)	30-Sep-13			31-Dec-12
MPC	$	27.1		$	4.2


To centralize cash management activities for MPC, MPC Investment Fund, Inc. (“MPCIF”), a wholly-owned subsidiary of MPC, was established and an agreement was executed on June 21, 2011 among MPCIF, MPL and ORPL. On a daily basis, we sent our excess cash to MPCIF as an advance or requested cash from MPCIF as a draw. Our net cash balance with MPCIF on the last day of each quarter was classified as loans receivable from related party or as loans payable to related party. The loan balance remained constant until the last day of the next quarter. Loans receivable earned interest at the three-month London Interbank Offered Rate (“LIBOR”) plus 10 basis points. Loans payable bore interest at the three-month LIBOR plus 50 basis points. At the end of each quarter, the net balance of the daily advances and draws and the accrued interest was rolled into the loan balance for the subsequent quarter. This agreement was terminated on September 28, 2012, in connection with the Offering.

Related party interest and other financial income were as follows:



(In millions)	Three Months Ended September 30,						Nine Months Ended September 30,
	2013			2012			2013			2012
				Predecessor						Predecessor
Interest income on loans receivable from MPCIF	$	—		$	0.5		$	—		$	1.3

Net_Income_Per_Limited_Partner

Net Income Per Limited Partner Unit	9 Months Ended
	Sep. 30, 2013
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.
	Net income per limited partner unit is only calculated for the three months and nine months ended September 30, 2013 as no units were outstanding in the corresponding periods of 2012.



		Three Months Ended				Nine Months Ended
	(In millions)	30-Sep-13				30-Sep-13
	Net income attributable to MPLX LP	$	21.5			$	57.7

	Less: General partner’s distributions declared (1)	0.5				1.3

	Limited partners’ distributions declared on common units (1)	11				31.6

	Limited partner’s distributions declared on subordinated units (1)	11				31.6

	Distributions in excess of net income attributable to MPLX LP	$	(1.0	)		$	(6.8	)


	(1)	On October 22, 2013, we announced the board of directors of our general partner had declared a quarterly cash distribution of $0.2975 per unit, totaling $22.5 million. This distribution will be paid on November 14, 2013, to unitholders of record on November 4, 2013.



		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					37				37

	Diluted					37				37

	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, 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					37				37

Diluted					37				37

Net income attributable to MPLX LP per limited partner unit:
Basic					$	0.79			$	0.74

Diluted					$	0.79			$	0.74

Other_Items

Other Items	9 Months Ended
	Sep. 30, 2013
Other Income and Expenses [Abstract]	'
Other Items	'
	Other Items

	Interest and other financial income (costs) were:



		Three Months Ended September 30,						Nine Months Ended September 30,
	(In millions)	2013			2012			2013			2012
					Predecessor						Predecessor
	Interest income	$	—		$	—		$	0.3		$	—

	Interest expense	(0.2		)	(0.2		)	(0.8		)	(0.5		)
	Interest capitalized	0.2			0.2			0.6			0.5

	Other financial costs	(0.2		)	—			(0.8		)	—

	Interest and other financial income (costs)	$	(0.2	)	$	—		$	(0.7	)	$	—

Income_Taxes

Income Taxes	9 Months Ended
Income Taxes	Sep. 30, 2013
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, Texas and Tennessee.

	Our income tax provision was expense of $0.4 million and a benefit of $0.1 million for the three months ended September 30, 2013 and 2012 and expenses of $0.5 million and $0.1 million for the nine months ended September 30, 2013 and 2012, respectively. Our effective tax rate was 1.0 percent and (0.3) percent for the three months ended September 30, 2013 and 2012 and 0.5 percent and 0.1 percent for the nine months ended September 30, 2013 and 2012, respectively.

	As of September 30, 2013 and December 31, 2012, we had unrecognized tax benefits of $0.2 million.

Property_Plant_and_Equipment

Property, Plant and Equipment	9 Months Ended
	Sep. 30, 2013
Property, Plant and Equipment [Abstract]	'
Property, Plant and Equipment	'
	Property, Plant and Equipment



	(In millions)	30-Sep-13		December 31, 2012
	Land	$	4.6	$	4.6

	Pipelines and related assets	1,020.50		971.8

	Storage and delivery facilities	165		165.3

	Other	22.8		22.7

	Assets under construction	48.4		21.9

	Total	1,261.30		1,186.30

	Less accumulated depreciation	305.1		276.3

	Net property, plant and equipment	$	956.2	$	910

Fair_Value_Measurements

Fair Value Measurements	9 Months Ended
Fair Value Measurements	Sep. 30, 2013
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, 2013 or December 31, 2012.

	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.

Debt

Debt	9 Months Ended
	Sep. 30, 2013
Debt Disclosure [Abstract]	'
Debt	'
	Debt

	Our outstanding borrowings at September 30, 2013 and December 31, 2012 consisted of the following:



	(In millions)	30-Sep-13		31-Dec-12
	MPLX Operations - $500.0 million revolving credit agreement due 2017	$	—	$	—

	MPL - capital lease obligation due 2020	10.7		11.3

	Total	10.7		11.3

	Amounts due within one year	0.7		0.7

	Total long-term debt due after one year	$	10	$	10.6


	There were no borrowings or letters of credit outstanding under our revolving credit agreement at September 30, 2013 or December 31, 2012.

Supplemental_Cash_Flow_Informa

Supplemental Cash Flow Information	9 Months Ended
	Sep. 30, 2013
Supplemental Cash Flow Elements [Abstract]	'
Supplemental Cash Flow Information	'
	Supplemental Cash Flow Information


		Nine Months Ended September 30,
	(In millions)	2013		2012
				Predecessor
	Net cash provided by operating activities included:
	Income taxes paid through MPC	$	—	$	0.2

	Non-cash investing and financing activities:
	Net transfers of property, plant and equipment to materials and supplies inventories	3.8		—

	Property, plant and equipment contributed by MPC	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)	2013		2012
				Predecessor
	Additions to property, plant and equipment	$	81	$	109.2

	Plus: Increase in capital accruals	1.6		9

	Asset retirement expenditures	3.4		2.6

	Total capital expenditures	$	86	$	120.8

EquityBased_Compensation_Plan

Equity-Based Compensation Plan	9 Months Ended
	Sep. 30, 2013
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]	'
Equity-Based Compensation Plan	'
	Equity-Based Compensation Plan

	Phantom Units—In the nine months ended September 30, 2013, grants of 9,729 phantom units were made under the MPLX LP 2012 Incentive Compensation Plan (the “MPLX 2012 Plan”) to non-employee directors of MPLX LP's general partner and MPC. Awards to non-employee directors were accounted for as non-employee awards. Phantom units granted to non-employee directors vest immediately at the time of the grant, as they are non-forfeitable, but are not issued until the director’s departure from the board of directors. Prior to issuance, non-employee directors do not have the right to vote such units and cash distribution equivalents accrue in the form of additional phantom units and will be issued when the director departs from the board of directors.

	In the nine months ended September 30, 2013, grants of 62,682 phantom units were made under the MPLX 2012 Plan to certain officers of MPLX LP’s general partner and certain eligible MPC officers who make significant contributions to our business. These grants were accounted for as employee awards. In general, these phantom units will vest over a requisite service period of three years. Prior to vesting, these phantom unit recipients do not have the right to vote such units and cash distributions declared will be accrued and paid upon vesting. The accrued distributions at September 30, 2013 were less than $0.1 million.

	The fair values of phantom units are based on the fair value of MPLX LP common limited partner units on the grant date.

	The following is a summary of phantom unit award activity of MPLX LP common limited partner units for the nine months ended September 30, 2013:



		Phantom Units
		Number	Weighted
		of Units	Average
			Fair Value
	Outstanding at December 31, 2012	1,951	$	26.69

	Granted	72,411	33.35

	Outstanding at September 30, 2013	74,362	33.18


	Performance Units—In the nine months ended September 30, 2013, grants of 1,747,670 performance units were made under the MPLX 2012 Plan to certain officers of MPLX LP’s general partner and certain eligible MPC officers who make significant contributions to our business. These awards are intended to have a per unit payout determined by the total unitholder return of MPLX LP common units as compared to the total unitholder return of a selected group of peer partnerships. The final per-unit payout will be the average of the results of four measurement periods during the 36 months requisite service period. These performance units will pay out 75.0 percent in cash and 25.0 percent in MPLX LP common units. The performance units paying out in cash are accounted for as liability awards and recorded at fair value with a mark-to-market adjustment made each quarter. The performance units paying out in units are accounted for as equity awards and have a grant date fair value of $0.74 per unit, 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, 2013:



		Number of
		Units
	Outstanding at December 31, 2012	—

	Granted	436,917

	Outstanding at September 30, 2013	436,917

Commitments_and_Contingencies

Commitments and Contingencies	9 Months Ended
Commitments and Contingencies	Sep. 30, 2013
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, 2013 and December 31, 2012, accrued liabilities for remediation totaled $1.5 million. At September 30, 2013 and December 31, 2012, it is reasonably possible that an estimated loss existed of up to $0.4 million and $0.5 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 the penalties, if any, which may be imposed. Receivables from MPC for indemnification of environmental costs related to incidents occurring prior to the Offering were $0.7 million and $2.0 million at September 30, 2013 and December 31, 2012, 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 a third-party complaint against numerous owners and operators of petroleum products facilities in the Hartford/Wood River, Illinois area, including MPL, asserting claims of contribution under the Illinois Contribution Act 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 litigation 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 the lawsuit.

	Guarantees—We have entered into guarantees with maximum potential undiscounted payments totaling $1.4 million and $1.5 million as of September 30, 2013 and December 31, 2012, respectively, which consist of leases of vehicles extending through 2018 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, 2013, our contractual commitments to acquire property, plant and equipment totaled $13.3 million. Our contractual commitments at September 30, 2013 were primarily related to a multi-year SCADA (supervisory control and data acquisition) system upgrade project.

Description_of_the_Business_an1

Description of the Business and Basis of Presentation (Policies)	9 Months Ended
	Sep. 30, 2013
Accounting Policies [Abstract]	'
Use of Estimates	'
Use of Estimates	The accompanying interim consolidated financial statements are unaudited; however, in the opinion of our management, these statements reflect all adjustments necessary for a fair statement of the results for the periods reported. All such adjustments are of a normal, recurring nature unless otherwise disclosed. These interim consolidated financial statements, including the notes, have been prepared in accordance with the rules and regulations of the Securities and Exchange Commission (“SEC”) applicable to interim period financial statements and do not include all of the information and disclosures required by accounting principles generally accepted in the United States (“U.S. GAAP”) for complete financial statements.
Reclassification	'
Reclassification	Certain reclassifications of prior periods’ data have been made to conform to current classifications. On the December 31, 2012 consolidated balance sheet, deferred revenue-related parties of $4.2 million has been reclassified from payables to related parties to a separate line. On the consolidated statement of cash flows for the nine months ended September 30, 2012, within operating activities, asset retirement expenditures of $2.6 million have been reclassified from all other, net to a separate line, and changes in crude oil inventories of $0.7 million have been reclassified from changes in inventories to all other, net.
Net Income per Unit	'
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.
Equity Based Compensation Arrangements	'
Equity Based Compensation Arrangements	The fair values of phantom units are based on the fair value of MPLX LP common limited partner units on the grant date.

Acquisition_Tables

Acquisition (Tables)	9 Months Ended
	Sep. 30, 2013
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, 2013 and 2012 were as follows:



		Three Months Ended September 30,				Nine Months Ended September 30,
	(In millions)	2013		2012		2013			2012
				Predecessor					Predecessor
	Net income attributable to MPLX LP	$	21.5	$	36.9	$	57.7		$	101.9

	Transfer to noncontrolling interest retained by MPC:
	Decrease in general partner-MPC equity for purchase of additional interest in Pipe Line Holdings	—		—		(46.4		)	—

	Change from net income attributable to MPLX LP and transfer to noncontrolling interest retained by MPC	$	21.5	$	36.9	$	11.3		$	101.9

Related_Party_Agreements_and_T1

Related Party Agreements and Transactions (Tables)	9 Months Ended
	Sep. 30, 2013
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)	2013		2012		2013		2012
				Predecessor				Predecessor
	MPC	$	97.8	$	96.6	$	284.2	$	265.8

Summary of Fees Received for Operating Pipelines for Related Parties Included in Other Income - Related Parties	'
	The fees received for operating pipelines for related parties included in other income-related parties on the consolidated statements of income were as follows:



		Three Months Ended September 30,				Nine Months Ended September 30,
	(In millions)	2013		2012		2013		2012
				Predecessor				Predecessor
	MPC	$	4.3	$	3	$	12.5	$	8.8

	Centennial	0.3		0.1		0.8		0.7

	Muskegon	—		0.1		0.1		0.1

	Total	$	4.6	$	3.2	$	13.4	$	9.6

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. These charges were as follows:



		Three Months Ended September 30,				Nine Months Ended September 30,
	(In millions)	2013		2012		2013		2012
				Predecessor				Predecessor
	MPC	$	4.2	$	4.4	$	13.3	$	11.3

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, and allocations of corporate overhead costs from MPC for periods prior to the Offering. These charges were as follows:



		Three Months Ended September 30,				Nine Months Ended September 30,
	(In millions)	2013		2012		2013		2012
				Predecessor				Predecessor
	MPC	$	7.7	$	5.9	$	23.5	$	17.4

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)	2013		2012		2013		2012
				Predecessor				Predecessor
	MPC	$	2.4	$	1.4	$	5.9	$	4

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)	2013		2012		2013		2012
				Predecessor				Predecessor
	Purchases from related parties	$	19.6	$	3.3	$	56.1	$	10

	General and administrative expenses	3.5		5		12		19.5

	Total	$	23.1	$	8.3	$	68.1	$	29.5

Receivables from Related Parties	'
	Receivables from related parties were as follows:



	(In millions)	30-Sep-13		31-Dec-12
	MPC	$	39	$	37.6

	Centennial	1		0.3

	Muskegon	0.5		0.1

	Total	$	40.5	$	38

Payables to Related Parties	'
	Payables to related parties were as follows:



	(In millions)	30-Sep-13		31-Dec-12
	MPC	$	9.8	$	13.4

	Centennial	0.1		—

	Total	$	9.9	$	13.4

Summary of Deferred Revenue - Related Parties	'
	he deferred revenue-related parties associated with the deficiencies was as follows:



	(In millions)	30-Sep-13		31-Dec-12
	MPC	$	27.1	$	4.2

Related Party Net Interest and Other Financial Income	'
	Related party interest and other financial income were as follows:



	(In millions)	Three Months Ended September 30,				Nine Months Ended September 30,
		2013		2012		2013		2012
				Predecessor				Predecessor
	Interest income on loans receivable from MPCIF	$	—	$	0.5	$	—	$	1.3

Net_Income_Per_Limited_Partner1

Net Income Per Limited Partner Unit (Tables)	9 Months Ended
	Sep. 30, 2013
Earnings Per Share [Abstract]	'
Schedule of Net Income Per Limited Partner Unit	'
	Net income per limited partner unit is only calculated for the three months and nine months ended September 30, 2013 as no units were outstanding in the corresponding periods of 2012.



		Three Months Ended			Nine Months Ended
	(In millions)	30-Sep-13			30-Sep-13
	Net income attributable to MPLX LP	$	21.5		$	57.7

	Less: General partner’s distributions declared (1)	0.5			1.3

	Limited partners’ distributions declared on common units (1)	11			31.6

	Limited partner’s distributions declared on subordinated units (1)	11			31.6

	Distributions in excess of net income attributable to MPLX LP	$	(1.0	)	$	(6.8	)


	(1)	On October 22, 2013, we announced the board of directors of our general partner had declared a quarterly cash distribution of $0.2975 per unit, totaling $22.5 million. This distribution will be paid on November 14, 2013, to unitholders of record on November 4, 2013.
Schedule of Basic and Diluted Earnings Per Unit	'


		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				37			37

	Diluted				37			37

	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, 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				37			37

	Diluted				37			37

	Net income attributable to MPLX LP per limited partner unit:
	Basic				$	0.79		$	0.74

	Diluted				$	0.79		$	0.74

Other_Items_Tables

Other Items (Tables)	9 Months Ended
	Sep. 30, 2013
Other Income and Expenses [Abstract]	'
Summary of Interest and Other Financial (Costs)	'
	Interest and other financial income (costs) were:



		Three Months Ended September 30,						Nine Months Ended September 30,
	(In millions)	2013			2012			2013			2012
					Predecessor						Predecessor
	Interest income	$	—		$	—		$	0.3		$	—

	Interest expense	(0.2		)	(0.2		)	(0.8		)	(0.5		)
	Interest capitalized	0.2			0.2			0.6			0.5

	Other financial costs	(0.2		)	—			(0.8		)	—

	Interest and other financial income (costs)	$	(0.2	)	$	—		$	(0.7	)	$	—

Property_Plant_and_Equipment_T

Property, Plant and Equipment (Tables)	9 Months Ended
	Sep. 30, 2013
Property, Plant and Equipment [Abstract]	'
Property, Plant and Equipment	'


	(In millions)	30-Sep-13		December 31, 2012
	Land	$	4.6	$	4.6

	Pipelines and related assets	1,020.50		971.8

	Storage and delivery facilities	165		165.3

	Other	22.8		22.7

	Assets under construction	48.4		21.9

	Total	1,261.30		1,186.30

	Less accumulated depreciation	305.1		276.3

	Net property, plant and equipment	$	956.2	$	910

Debt_Tables

Debt (Tables)	9 Months Ended
	Sep. 30, 2013
Debt Disclosure [Abstract]	'
Summary of Outstanding Borrowings	'
	Our outstanding borrowings at September 30, 2013 and December 31, 2012 consisted of the following:



	(In millions)	30-Sep-13		31-Dec-12
	MPLX Operations - $500.0 million revolving credit agreement due 2017	$	—	$	—

	MPL - capital lease obligation due 2020	10.7		11.3

	Total	10.7		11.3

	Amounts due within one year	0.7		0.7

	Total long-term debt due after one year	$	10	$	10.6

Supplemental_Cash_Flow_Informa1

Supplemental Cash Flow Information (Tables)	9 Months Ended
	Sep. 30, 2013
Supplemental Cash Flow Elements [Abstract]	'
Summary of Supplemental Cash Flow Information	'


		Nine Months Ended September 30,
	(In millions)	2013		2012
				Predecessor
	Net cash provided by operating activities included:
	Income taxes paid through MPC	$	—	$	0.2

	Non-cash investing and financing activities:
	Net transfers of property, plant and equipment to materials and supplies inventories	3.8		—

	Property, plant and equipment contributed by MPC	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)	2013		2012
				Predecessor
	Additions to property, plant and equipment	$	81	$	109.2

	Plus: Increase in capital accruals	1.6		9

	Asset retirement expenditures	3.4		2.6

	Total capital expenditures	$	86	$	120.8

EquityBased_Compensation_Plan_

Equity-Based Compensation Plan (Tables)	9 Months Ended
	Sep. 30, 2013
Phantom Units [Member]	'
Summary of Equity Based Unit Award Activity	'
	The following is a summary of phantom unit award activity of MPLX LP common limited partner units for the nine months ended September 30, 2013:



		Phantom Units
		Number	Weighted
		of Units	Average
			Fair Value
	Outstanding at December 31, 2012	1,951	$	26.69

	Granted	72,411	33.35

	Outstanding at September 30, 2013	74,362	33.18

Performance Units [Member]	'
Summary of Equity Based Unit Award Activity	'
	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, 2013:



		Number of
		Units
	Outstanding at December 31, 2012	—

	Granted	436,917

	Outstanding at September 30, 2013	436,917

Description_of_Business_and_Ba

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

1 Months Ended

2 Months Ended

9 Months Ended

1 Months Ended

3 Months Ended

9 Months Ended

2 Months Ended

4 Months Ended

9 Months Ended

1 Months Ended

9 Months Ended

2 Months Ended

4 Months Ended

9 Months Ended

In Millions, except Share data, unless otherwise specified

Oct. 31, 2012

Jun. 30, 2013

Sep. 30, 2013

Oct. 31, 2012

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Jun. 30, 2013

Apr. 30, 2013

Sep. 30, 2013

Sep. 30, 2012

Dec. 31, 2012

Sep. 30, 2013

Jun. 30, 2013

Apr. 30, 2013

Sep. 30, 2013

bbl

Limited Partners Common Units [Member]

Other Ownership Interest Common Units [Member]

Predecessor [Member]

MPLX Pipe Line Holdings LP [Member]

Asset retirement expenditures [Member]

All other net [Member]

Deferred revenue related party [Member]

MPLX Operations LLC [Member]

MPLX Terminal and Storage LLC [Member]

MPL [Member]

ORPL [Member]

MPC [Member]

Storage Services Butane Cavern [Member]

Crude Oil and Product Pipeline Systems [Member]

Segment

MPLX Pipe Line Holdings LP [Member]

Retained Interest [Member]

Description Of Business And Basis Of Presentation [Line Items]

Ownership interest (in percentage)

56.00%

51.00%

56.00%

100.00%

56.00%

Storage capacity (in number of barrels)

1,000,000

Date for initial public offering Completed

31-Oct-12

Common units for initial public offering

19,895,000

2,595,000

Date of Partnership formation

27-Mar-12

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

100.00%

56.00%

44.00%

49.00%

Additional interest in Pipe Line Holdings

5.00%

Purchase of additional interest in Pipe Line Holdings

$100

Number of segments

Prior period reclassification adjustment

$2.60

$0.70

$4.20

Acquisition_Additional_Informa

Acquisition - Additional Information (Detail) (USD $)	2 Months Ended	9 Months Ended	2 Months Ended	4 Months Ended	9 Months Ended	2 Months Ended	3 Months Ended	9 Months Ended	2 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2013	Sep. 30, 2013	Jun. 30, 2013	Apr. 30, 2013	Sep. 30, 2013	Jun. 30, 2013	Sep. 30, 2013	Sep. 30, 2013	Jun. 30, 2013	Jun. 30, 2013	Sep. 30, 2013
			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]	Noncontrolling Interest Retained by MPC [Member]	Noncontrolling Interest Retained by MPC [Member]	Noncontrolling Interest Retained by MPC [Member]
										MPC [Member]	MPC [Member]
Business Acquisition [Line Items]	'	'	'	'	'	'	'	'	'	'	'
Additional interest in Pipe Line Holdings	5.00%	'	'	'	'	'	'	'	'	'	5.00%
Purchase of additional interest in Pipe Line Holdings	$100	$100	'	'	'	'	'	'	'	'	'
Purchase of additional interest in Pipe Line Holdings	'	-100	'	'	'	'	'	'	53.6	'	-53.6
Purchase of additional interest in Pipe Line Holdings	'	'	'	'	'	$46.40	$0	$46.40	'	'	'
Percent of the net income related to assets contributed to MPLX LP	'	'	56.00%	51.00%	56.00%	'	'	'	'	44.00%	'

Acquisition_Summary_Changes_in

Acquisition - Summary Changes in MPLX LP's Ownership Interest in Pipe Line Holdings (Detail) (USD $)	3 Months Ended	9 Months Ended	2 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2013	Jun. 30, 2013	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2012	Sep. 30, 2012	Sep. 30, 2012
			General Partner [Member]	General Partner [Member]	General Partner [Member]	Predecessor [Member]	Predecessor [Member]	Predecessor [Member]	Predecessor [Member]
								General Partner [Member]	General Partner [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]	'	'	'	'	'	'	'	'	'
Net income attributable to MPLX LP	$21.50	$57.70	'	$0.50	$1.20	$36.90	$101.90	'	'
Transfer to noncontrolling interest retained by MPC:	'	'	'	'	'	'	'	'	'
Decrease in general partner-MPC equity for purchase of additional interest in Pipe Line Holdings	'	'	-46.4	0	-46.4	'	'	0	0
Change from net income attributable to MPLX LP and transfer to noncontrolling interest retained by MPC	$21.50	$11.30	'	'	'	$36.90	$101.90	'	'

Related_Party_Agreements_and_T2

Related Party Agreements and Transactions - Additional Information (Detail) (USD $)

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

0 Months Ended

1 Months Ended

In Millions, unless otherwise specified

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2012

Sep. 30, 2013

Oct. 02, 2012

Sep. 30, 2013

Jun. 21, 2011

Sep. 28, 2012

Sep. 30, 2013

Predecessor [Member]

MPC [Member]

Blanchard Pipe Line Company LLC [Member]

MPCIF [Member]

Centennial [Member]

Muskegon [Member]

General and Administrative Expenses [Member]

Predecessor [Member]

Commercial Agreements [Member]

Operating Agreements [Member]

Management Services Agreements [Member]

Employee Services Agreements [Member]

Operating Agreements [Member]

Revolving Credit Agreement [Member]

Predecessor [Member]

MPC [Member]

Crude Oil [Member]

Pipelines Products [Member]

Handling Crude Oil and Products [Member]

Storage Services Butane Cavern [Member]

Storage Services Tank Farms [Member]

Crude Oil and Product Pipeline [Member]

Wood River to Patoka System and Barge Dock [Member]

Minimum [Member]

Maximum [Member]

Agreement

Forecast [Member]

Agreement

Forecast [Member]

Revolving Credit Agreement [Member]

10-Year Transportation Services [Member]

Five-Year Transportation Services [Member]

10-Year Transportation Services [Member]

Five-Year Transportation Services [Member]

10-Year Storage Services Agreement [Member]

Three-Year Storage Services Agreement [Member]

Storage Services Agreements [Member]

Transportation Services Agreements [Member]

Agreement

Related Party Transaction [Line Items]

Equity method investments, ownership percentage

50.00%

60.00%

Number of agreements

Duration of agreements (in years)

'10 years

'5 years

'10 years

'5 years

'10 years

'3 years

'1 year

'5 years

Number of Renewals

1,000,000

Renewal Term Agreement

'1 year

'5 years

'2 years

Annual fees received from Blanchard

$97.80

$284.20

$96.60

$265.80

$97.80

$284.20

$96.60

$265.80

Fees received for providing management services

0.7

Stock-based compensation plans expense

0.2

0.8

Long-term receivables from related parties

$0.30

Line of credit facility initiation date for MPCIF

21-Jun-11

Loans Receivable, Basis Spread on Variable Rate

0.10%

Loans Receivable, Description of Variable Rate Basis

'three-month LIBOR

Debt Instrument, Basis Spread on Variable Rate

0.50%

Debt Instrument, Description of Variable Rate Basis

'three-month LIBOR

Related_Party_Agreements_and_T3

Related Party Agreements and Transactions - Sales to Related Parties (Detail) (USD $)	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2012	Sep. 30, 2012	Sep. 30, 2012
			MPC [Member]	MPC [Member]	Predecessor [Member]	Predecessor [Member]	Predecessor [Member]	Predecessor [Member]
							MPC [Member]	MPC [Member]
Related Party Transaction [Line Items]	'	'	'	'	'	'	'	'
Sales to related parties	$97.80	$284.20	$97.80	$284.20	$96.60	$265.80	$96.60	$265.80

Related_Party_Agreements_and_T4

Related Party Agreements and Transactions - Summary of Fees Received for Operating Pipelines for Related Parties Included in Other Income - Related Parties (Detail) (USD $)

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

In Millions, unless otherwise specified

Sep. 30, 2013

Sep. 30, 2012

MPC [Member]

Centennial [Member]

Muskegon [Member]

Predecessor [Member]

MPC [Member]

Centennial [Member]

Muskegon [Member]

Related Party Transaction [Line Items]

Fees received for operating pipelines for related parties

$4.60

$13.40

$4.30

$12.50

$0.30

$0.80

$0.10

$3.20

$9.60

$8.80

$0.10

$0.70

$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	3 Months Ended	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2012
	Sep. 30, 2013	Sep. 30, 2013	Predecessor [Member]	Predecessor [Member]
Related Party Transaction [Line Items]	'	'	'	'
Charges for services included in purchases from related party	$4.20	$13.30	$4.40	$11.30

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	3 Months Ended	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2012
	Sep. 30, 2013	Sep. 30, 2013	Predecessor [Member]	Predecessor [Member]
Related Party Transaction [Line Items]	'	'	'	'
Charges for services included in general and administrative expenses	$7.70	$23.50	$5.90	$17.40

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	3 Months Ended	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2012
	Sep. 30, 2013	Sep. 30, 2013	Predecessor [Member]	Predecessor [Member]
Property, Plant and Equipment [Line Items]	'	'	'	'
Costs added to property, plant and equipment	$2.40	$5.90	$1.40	$4

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

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

In Millions, unless otherwise specified

Sep. 30, 2013

Sep. 30, 2012

Purchases from Related Parties [Member]

General and Administrative Expenses [Member]

Predecessor [Member]

Purchases from Related Parties [Member]

General and Administrative Expenses [Member]

Related Party Transaction [Line Items]

Expenses incurred under employee services agreements

$23.10

$68.10

$19.60

$56.10

$3.50

$12

$8.30

$29.50

$3.30

$10

$19.50

Related_Party_Agreements_and_T9

Related Party Agreements and Transactions - Receivables from Related Parties (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Related Party Transaction [Line Items]	'	'
Current receivables from related parties	$40.50	$38
MPC [Member]	'	'
Related Party Transaction [Line Items]	'	'
Current receivables from related parties	39	37.6
Centennial [Member]	'	'
Related Party Transaction [Line Items]	'	'
Current receivables from related parties	1	0.3
Muskegon [Member]	'	'
Related Party Transaction [Line Items]	'	'
Current receivables from related parties	$0.50	$0.10

Recovered_Sheet1

Related Party Agreements and Transactions - Payables to Related Parties (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Related Party Transaction [Line Items]	'	'
Payables to related parties	$9.90	$13.40
MPC [Member]	'	'
Related Party Transaction [Line Items]	'	'
Payables to related parties	9.8	13.4
Centennial [Member]	'	'
Related Party Transaction [Line Items]	'	'
Payables to related parties	$0.10	$0

Recovered_Sheet2

Related Party Agreements and Transactions - Summary of Deferred Revenue - Related Parties (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Related Party Transaction [Line Items]	'	'
Deferred revenue - related parties	$27.10	$4.20
MPC [Member]	'	'
Related Party Transaction [Line Items]	'	'
Deferred revenue - related parties	$27.10	$4.20

Recovered_Sheet3

Related Party Agreements and Transactions - Related Party Net Interest and Other Financial Income (Detail) (USD $)	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2012	Sep. 30, 2012	Sep. 30, 2012
			MPCIF [Member]	MPCIF [Member]	Predecessor [Member]	Predecessor [Member]	Predecessor [Member]	Predecessor [Member]
							MPCIF [Member]	MPCIF [Member]
Related Party Transaction [Line Items]	'	'	'	'	'	'	'	'
Interest income on loans receivable from MPCIF	$0	$0	$0	$0	$0.50	$1.30	$0.50	$1.30

Net_Income_Per_Limited_Partner2

Net Income Per Limited Partner Unit - Schedule of Net Income Per Limited Partner Unit (Detail) (USD $)

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

1 Months Ended

In Millions, except Per Share data, unless otherwise specified

Sep. 30, 2013

Oct. 31, 2013

General Partner [Member]

Limited Partners Common Units [Member]

Limited Partners Subordinated Units [Member]

Subsequent Event [Member]

MPC [Member]

Net Income Per Share [Line Items]

Distribution Made to Limited Partner, Date of Record

4-Nov-13

Distribution Made to Limited Partner, Declaration Date

22-Oct-13

Cash distributions declared per limited partner common unit

$0.30

$0.86

$0.30

Net income attributable to MPLX LP

$21.50

$57.70

$0.50

$1.20

$10.50

$29.10

$10.50

$27.40

Less: Distribution declared

22.5

64.5

0.5

[1]

1.3

[1]

31.6

[1]

31.6

[1]

22.5

Distribution Made to Limited Partner, Distribution Date

14-Nov-13

Distributions in excess of net income attributable to MPLX LP

($1)

($6.80)

($0.10)

($0.50)

($2.50)

($0.50)

($4.20)

[1]

On OctoberB 22, 2013, we announced the board of directors of our general partner had declared a quarterly cash distribution of $0.2975 per unit, totaling $22.5 million. This distribution will be paid on NovemberB 14, 2013, to unitholders of record on NovemberB 4, 2013.

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, 2013		Sep. 30, 2013
Net income attributable to MPLX LP:	'		'
Distributions declared	$22.50		$64.50
Distributions in excess of net income attributable to MPLX LP	-1		-6.8
Net income attributable to MPLX LP	21.5		57.7
General Partner [Member]	'		'
Net income attributable to MPLX LP:	'		'
Distributions declared	0.5	[1]	1.3	[1]
Distributions in excess of net income attributable to MPLX LP	0		-0.1
Net income attributable to MPLX LP	0.5		1.2
Limited Partners Common Units [Member]	'		'
Net income attributable to MPLX LP:	'		'
Distributions declared	11	[1]	31.6	[1]
Distributions in excess of net income attributable to MPLX LP	-0.5		-2.5
Net income attributable to MPLX LP	10.5		29.1
Weighted average units outstanding:	'		'
Weighted Average Limited Partnership Units Outstanding, Basic	37		37
Weighted Average Limited Partnership Units Outstanding, Diluted	37		37
Net income attributable to MPLX LP per limited partner unit:	'		'
Basic (in USD per unit)	$0.29		$0.79
Diluted (in USD per unit)	$0.29		$0.79
MPC [Member] \| Limited Partners Common Units [Member]	'		'
Net income attributable to MPLX LP per limited partner unit:	'		'
Basic (in USD per unit)	$0.29		$0.79
Diluted (in USD per unit)	$0.29		$0.79
MPC [Member] \| Limited Partners Subordinated Units [Member]	'		'
Net income attributable to MPLX LP:	'		'
Distributions declared	11	[1]	31.6	[1]
Distributions in excess of net income attributable to MPLX LP	-0.5		-4.2
Net income attributable to MPLX LP	$10.50		$27.40
Weighted average units outstanding:	'		'
Weighted Average Limited Partnership Units Outstanding, Basic	37		37
Weighted Average Limited Partnership Units Outstanding, Diluted	37		37
Net income attributable to MPLX LP per limited partner unit:	'		'
Basic (in USD per unit)	$0.29		$0.74
Diluted (in USD per unit)	$0.29		$0.74

[1]	On OctoberB 22, 2013, we announced the board of directors of our general partner had declared a quarterly cash distribution of $0.2975 per unit, totaling $22.5 million. This distribution will be paid on NovemberB 14, 2013, to unitholders of record on NovemberB 4, 2013.

Other_Items_Summary_of_Interes

Other Items - Summary of Interest and Other Financial (Costs) (Detail) (USD $)	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2012
	Sep. 30, 2013	Sep. 30, 2013	Predecessor [Member]	Predecessor [Member]
Other Items Net Interest And Other Financial Income [Line Items]	'	'	'	'
Interest income	$0	$0.30	$0	$0
Interest expense	-0.2	-0.8	-0.2	-0.5
Interest capitalized	0.2	0.6	0.2	0.5
Other financial costs	-0.2	-0.8	0	0
Interest and other financial income (costs)	($0.20)	($0.70)	$0	$0

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Sep. 30, 2012
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Dec. 31, 2012	Predecessor [Member]	Predecessor [Member]
Income Taxes [Line Items]	'	'	'	'	'	'
Income tax expense	$0.40	($0.10)	$0.50	'	($0.10)	$0.10
Effective tax rate	1.00%	'	0.50%	'	-0.30%	0.10%
Unrecognized tax benefits	$0.20	'	$0.20	$0.20	'	'

Property_Plant_and_Equipment_P

Property, Plant and Equipment - Property, Plant and Equipment (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Property, Plant and Equipment [Line Items]	'	'
Gross property, plant and equipment	$1,261.30	$1,186.30
Less accumulated depreciation	305.1	276.3
Net property, plant and equipment	956.2	910
Land [Member]	'	'
Property, Plant and Equipment [Line Items]	'	'
Gross property, plant and equipment	4.6	4.6
Pipelines and Related Assets [Member]	'	'
Property, Plant and Equipment [Line Items]	'	'
Gross property, plant and equipment	1,020.50	971.8
Storage and Delivery Facilities [Member]	'	'
Property, Plant and Equipment [Line Items]	'	'
Gross property, plant and equipment	165	165.3
Other [Member]	'	'
Property, Plant and Equipment [Line Items]	'	'
Gross property, plant and equipment	22.8	22.7
Assets Under Construction [Member]	'	'
Property, Plant and Equipment [Line Items]	'	'
Gross property, plant and equipment	$48.40	$21.90

Debt_Summary_of_Outstanding_Bo

Debt - Summary of Outstanding Borrowings (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Debt Instrument [Line Items]	'	'
Total	$10.70	$11.30
Amounts due within one year	0.7	0.7
Total long-term debt due after one year	10	10.6
MPL [Member]	'	'
Debt Instrument [Line Items]	'	'
MPL - capital lease obligation due 2020	10.7	11.3
Revolving Credit Agreement [Member]	'	'
Debt Instrument [Line Items]	'	'
MPLX Operations - $500.0 million revolving credit agreement due 2017	0	0
Revolving Credit Agreement [Member] \| MPLX Operations LLC [Member]	'	'
Debt Instrument [Line Items]	'	'
MPLX Operations - $500.0 million revolving credit agreement due 2017	$0	$0

Debt_Summary_of_Outstanding_Bo1

Debt - Summary of Outstanding Borrowings (Parenthetical) (Detail) (MPLX Operations LLC [Member], Revolving Credit Agreement [Member], USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
MPLX Operations LLC [Member] \| Revolving Credit Agreement [Member]	'	'
Debt Instrument [Line Items]	'	'
Revolving credit agreement due 2017	$500	$500

Debt_Additional_Information_De

Debt - Additional Information (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Revolving Credit Agreement [Member]	'	'
Debt Instrument [Line Items]	'	'
Borrowings outstanding	$0	$0
Letters of Credit [Member]	'	'
Debt Instrument [Line Items]	'	'
Borrowings outstanding	$0	$0

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, 2013	Sep. 30, 2012
Non-cash investing and financing activities:	'	'
Net transfers of property, plant and equipment to materials and supplies inventories	$3.80	'
Non-Cash Contribution from MPC	0.2	0
MPC [Member]	'	'
Net cash provided by operating activities included:	'	'
Income taxes paid through MPC	0	'
Predecessor [Member]	'	'
Non-cash investing and financing activities:	'	'
Net transfers of property, plant and equipment to materials and supplies inventories	'	0
Predecessor [Member] \| MPC [Member]	'	'
Net cash provided by operating activities included:	'	'
Income taxes paid through MPC	'	$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, 2013	Sep. 30, 2012
	Sep. 30, 2013	Predecessor [Member]
Supplemental Cash Flow Information [Line Items]	'	'
Additions to property, plant and equipment	$81	$109.20
Plus: Increase in capital accruals	1.6	9
Asset retirement expenditures	3.4	2.6
Total capital expenditures	$86	$120.80

EquityBased_Compensation_Plan_1

Equity-Based Compensation Plan - Additional Information (Detail) (USD $)	9 Months Ended
In Millions, except Share data, unless otherwise specified	Sep. 30, 2013
Phantom Units [Member]	'
Equity Transactions And Share Based Compensation [Line Items]	'
Incentive Compensation Plan, grant (in units)	72,411
MPLX LP 2012 Incentive Compensation Plan [Member] \| Performance Units [Member] \| MPLX Officers [Member]	'
Equity Transactions And Share Based Compensation [Line Items]	'
Incentive Compensation Plan, grant (in units)	1,747,670
Requisite service period (in number of months)	'36 months
Measurement periods (in number of measurement periods)	4
Performance units paid in cash (in percentage)	75.00%
Performance units paid in common units (in percentage)	25.00%
Performance units grant date fair value (in USD per unit)	0.74
MPLX LP 2012 Incentive Compensation Plan [Member] \| Phantom Units [Member] \| MPLX Officers [Member]	'
Equity Transactions And Share Based Compensation [Line Items]	'
Incentive Compensation Plan, grant (in units)	62,682
Requisite service period (in number of months)	'3 years
MPLX LP 2012 Incentive Compensation Plan [Member] \| Phantom Units [Member] \| MPLX Officers [Member] \| Maximum [Member]	'
Equity Transactions And Share Based Compensation [Line Items]	'
Accrued distributions	0.1
MPLX LP 2012 Incentive Compensation Plan [Member] \| Phantom Units [Member] \| Non-employee Directors [Member]	'
Equity Transactions And Share Based Compensation [Line Items]	'
Incentive Compensation Plan, grant (in units)	9,729

EquityBased_Compensation_Plan_2

Equity-Based Compensation Plan - Summary of Phantom Unit Award Activity (Detail) (Phantom Units [Member], USD $)	9 Months Ended
	Sep. 30, 2013
Phantom Units [Member]	'
Number of Units	'
Number of Units, Outstanding beginning	1,951
Number of Units, Granted	72,411
Number of Units, Outstanding ending	74,362
Weighted Average Fair Value	'
Weighted Average Fair Value, Outstanding beginning (in USD per unit)	$26.69
Weighted Average Fair Value, Granted (in USD per unit)	$33.35
Weighted Average Fair Value, Outstanding ending (in USD per unit)	$33.18

EquityBased_Compensation_Plan_3

Equity-Based Compensation Plan - Summary of Performance Unit Award Activity (Detail) (Performance Units [Member])	9 Months Ended
	Sep. 30, 2013
Performance Units [Member]	'
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]	'
Number of Units, Outstanding beginning	0
Number of Units, Granted	436,917
Number of Units, Outstanding ending	436,917

Commitments_and_Contingencies_

Commitments and Contingencies - Additional Information (Detail) (USD $)	9 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Commitments And Contingencies [Line Items]	'	'
Accrued liabilities for remediation	$1.50	$1.50
Maximum potential undiscounted payments	1.4	1.5
Contractual commitments to acquire property, plant and equipment	13.3	'
Maximum [Member]	'	'
Commitments And Contingencies [Line Items]	'	'
Site contingency, loss exposure in excess of accrual	0.4	0.5
Environmental Loss Contingency [Member] \| MPC [Member]	'	'
Commitments And Contingencies [Line Items]	'	'
Receivables from MPC for indemnification of environmental costs	$0.70	$2