MPLX (MPLX) 10-K 27 Feb 15 2014 FY Annual report Financial data

Document_and_Entity_Informatio

Document and Entity Information (USD $)	12 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2014	Jun. 30, 2014	Feb. 13, 2015
Entity Information [Line Items]
Document Type	10-K
Amendment Flag	FALSE
Document Period End Date	31-Dec-14
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Trading Symbol	MPLX
Entity Registrant Name	MPLX LP
Entity Central Index Key	1552000
Current Fiscal Year End Date	-19
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Public Float		$1,300
Common Units [Member]
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding			43,341,098
Subordinated Units [Member]
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding			36,951,515
General Partner Units [Member]
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding			1,638,625

Consolidated_Statements_of_Inc

Consolidated Statements of Income (USD $)	2 Months Ended	12 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenues and other income:
Sales and other operating revenues		$69.20	$78.90
Sales to related parties		450.9	384.2
Loss on sale of assets		0	0
Other income		5.2	4.4
Other income - related parties		23	18.8
Total revenues and other income		548.3	486.3
Costs and expenses:
Cost of revenues (excludes items below)		145.3	135.9
Purchases from related parties		97.5	94.6
Depreciation		50.2	48.9
General and administrative expenses		64.8	53.7
Other taxes		7.2	6.2
Total costs and expenses		365	339.3
Income from operations		183.3	147
Related party interest and other financial income		0	0
Net interest and other financial costs		-5.3	-1.1
Income before income taxes		178	145.9
Provision (benefit) for income taxes		-0.1	-0.2
Net income		178.1	146.1	26.3
Less: Net income attributable to MPC-retained interest		56.8	68.2
Net income attributable to MPLX LP	13.1	121.3	77.9	13.1
Less: General partner’s interest in net income attributable to MPLX LP subsequent to initial public offering	0.2	5.9	1.7	0.2
Limited partners’ interest in net income attributable to MPLX LP		115.4	76.2	12.9
Weighted average limited partner units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic	75.4	75.9	75.4
Weighted Average Limited Partnership Units Outstanding, Diluted	75.4	75.9	75.4
Predecessor [Member]
Costs and expenses:
Net income				117.7
Net income attributable to MPLX LP				117.7
Predecessor and Successor [Member]
Revenues and other income:
Sales and other operating revenues				74.4
Sales to related parties				367.8
Loss on sale of assets				-0.3
Other income				6.9
Other income - related parties				13.1
Total revenues and other income				461.9
Costs and expenses:
Cost of revenues (excludes items below)				173.8
Purchases from related parties				44.4
Depreciation				39.4
General and administrative expenses				49.8
Other taxes				11.3
Total costs and expenses				318.7
Income from operations				143.2
Related party interest and other financial income				1.3
Net interest and other financial costs				-0.2
Income before income taxes				144.3
Provision (benefit) for income taxes				0.3
Net income				144
Less: Net income attributable to MPC-retained interest				13.2
Net income attributable to MPLX LP				130.8
MPC [Member]
Revenues and other income:
Sales to related parties		450.9	384.2
Other income - related parties		21.8	17.6
MPC [Member] | Predecessor and Successor [Member]
Revenues and other income:
Sales to related parties				367.8
Other income - related parties				12
Limited Partners Common Units [Member]
Costs and expenses:
Net income attributable to MPLX LP	6.5	57.9	38.8
Per Unit Data (See Note 6)
Basic (in USD per unit)	$0.18	$1.55	$1.05	$0.18
Diluted (in USD per unit)	$0.18	$1.55	$1.05	$0.18
Weighted average limited partner units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic	37	37.4	37	37
Weighted Average Limited Partnership Units Outstanding, Diluted	37	37.4	37	37
Cash distributions declared per limited partner common unit (in USD per unit)		$1.41	$1.17	$0.18
Limited Partners Subordinated Units [Member] | MPC [Member]
Costs and expenses:
Net income attributable to MPLX LP	$6.40	$55.40	$37.40
Per Unit Data (See Note 6)
Basic (in USD per unit)	$0.17	$1.50	$1.01
Diluted (in USD per unit)	$0.17	$1.50	$1.01
Basic and Diluted (in USD per unit)		$1.50	$1.01	$0.17
Weighted average limited partner units outstanding:
Weighted Average Limited Partnership Units Outstanding, Basic	37	37	37
Weighted Average Limited Partnership Units Outstanding, Diluted	37	37	37
Basic and Diluted Units (in units)		37	37	37

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Current assets:
Cash and cash equivalents	$27.30	$54.10
Receivables	10.2	12.2
Receivables from related parties	41	48.3
Materials and supplies inventories	11.7	11.6
Other current assets	7	8.9
Total current assets	97.2	135.1
Property, plant and equipment, net	1,008.60	966.6
Goodwill	104.7	104.7
Other noncurrent assets	4	2.1
Total assets	1,214.50	1,208.50
Current liabilities:
Accounts payable	42.2	30.5
Payables to related parties	20.2	12.8
Deferred revenue - related parties	30.5	34
Accrued taxes	5.2	4
Long-term debt due within one year	0.8	0.7
Other current liabilities	1.7	1.4
Total current liabilities	100.6	83.4
Long-term deferred revenue related parties	4	0
Long-term debt	644	9.8
Deferred credits and other liabilities	2.2	1.2
Total liabilities	750.8	94.4
Commitments and contingencies (see Note 18)
Equity
Total MPLX LP partners' capital	458.2	646.2
Noncontrolling interest retained by MPC	5.5	467.9
Total equity	463.7	1,114.10
Total liabilities and equity	1,214.50	1,208.50
MPC [Member]
Current assets:
Receivables from related parties	40.5	47.4
Current liabilities:
Payables to related parties	20.1	12.8
Limited Partners Common Units [Member] | MPC [Member]
Equity
Total MPLX LP partners' capital	261.1	57.4
Limited Partners Subordinated Units [Member] | MPC [Member]
Equity
Total MPLX LP partners' capital	217.5	209.3
General Partner [Member] | MPC [Member]
Equity
Total MPLX LP partners' capital	-659.4	-32.5
Public [Member] | Limited Partners Common Units [Member]
Equity
Total MPLX LP partners' capital	$639	$412

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical)	Dec. 31, 2014	Dec. 31, 2013
General partner units outstanding	1,638,625
Limited Partners Common Units [Member] | Public [Member]
Units issued	23,400,000	19,900,000
Units outstanding	23,400,000	19,900,000
MPC [Member] | Limited Partners Common Units [Member]
Units issued	20,000,000	17,100,000
Units outstanding	20,000,000	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	1,600,000	1,500,000
General partner units outstanding	1,600,000	1,500,000

Consolidated_Statements_of_Cas

Consolidated Statements of Cash Flows (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating activities:
Net income	$178.10	$146.10	$26.30
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	50.2	48.9
Deferred income taxes	0	0.1
Asset retirement expenditures	-1.6	-8.3
Net loss on disposal of assets	0	0
Changes in:
Current receivables	2	5.4
Materials and supplies inventories	0.9	1.3
Current accounts payable and accrued liabilities	0.5	-2.6
Receivables from / liabilities to related parties	15.2	18.9
All other, net	1.5	2.4
Net cash provided by operating activities	246.8	212.2
Investing activities:
Additions to property, plant and equipment	-78.6	-106.5
Disposal of assets	0	0.2
Investments - repayments of loans receivable from a related party	0	0
All other, net	3.5	-7.3
Net cash provided by (used in) investing activities	-75.1	-113.6
Financing activities:
Long-term debt - borrowings	1,160	0
Long-term debt - repayments	-525.9	-0.7
Debt issuance costs	-2.7	0
Net proceeds from equity offerings	230.1	0
Proceeds from initial public offering distributed to MPC	0		-202.7
Quarterly distributions to unitholders and general partner	-103.1	-77.8
Quarterly distributions to noncontrolling interest retained by MPC	-46.9	-82.7
Distributions related to purchase of additional interest in Pipe Line Holdings	-910	-100
Net cash used in financing activities	-198.5	-261.2
Net increase (decrease) in cash and cash equivalents	-26.8	-162.6
Cash and cash equivalents at beginning of period	54.1	216.7	0.1
Cash and cash equivalents at end of period	27.3	54.1	216.7
MPC [Member]
Financing activities:
Proceeds from initial public offering distributed to MPC		0
Distributions to MPC	0	0
Predecessor and Successor [Member]
Operating activities:
Net income			144
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation			39.4
Deferred income taxes			-0.1
Asset retirement expenditures			-9.2
Net loss on disposal of assets			0.3
Changes in:
Current receivables			-2
Materials and supplies inventories			-1
Current accounts payable and accrued liabilities			-8
Receivables from / liabilities to related parties			26.9
All other, net			0.3
Net cash provided by operating activities			190.6
Investing activities:
Additions to property, plant and equipment			-135.6
Disposal of assets			1.3
Investments - repayments of loans receivable from a related party			221.7
All other, net			0
Net cash provided by (used in) investing activities			87.4
Financing activities:
Long-term debt - borrowings			0
Long-term debt - repayments			-0.7
Debt issuance costs			-2.4
Net proceeds from equity offerings			407.1
Proceeds from initial public offering distributed to MPC			-202.7
Quarterly distributions to unitholders and general partner			0
Quarterly distributions to noncontrolling interest retained by MPC			0
Distributions related to purchase of additional interest in Pipe Line Holdings			0
Distributions to MPC			-262.7
Net cash used in financing activities			-61.4
Net increase (decrease) in cash and cash equivalents			216.6
Predecessor and Successor [Member] | MPC [Member]
Financing activities:
Distributions to MPC			-262.7
Predecessor [Member]
Operating activities:
Net income			117.7
Financing activities:
Cash and cash equivalents at beginning of period			$0.10

Consolidated_Statements_of_Equ

Consolidated Statements of Equity/Net Investment (USD $)	Total	MPC [Member]	Predecessor [Member]	Limited Partners Common Units [Member]	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]	Limited Partners Subordinated Units [Member]	General Partner [Member]	General Partner [Member]	Noncontrolling Interest [Member]	Noncontrolling Interest [Member]	Noncontrolling Interest [Member]	Net Investment [Member]
In Millions					Public [Member]	MPC [Member]	Predecessor [Member]	Predecessor [Member]		MPC [Member]	Predecessor [Member]	MPC [Member]	Predecessor [Member]		MPC [Member]	Predecessor [Member]	Predecessor [Member]
							Public [Member]	MPC [Member]			MPC [Member]		MPC [Member]			MPC [Member]
Beginning Balance at Dec. 31, 2011			$1,239.20				$0	$0			$0		$0			$0	$1,239.20
Net income	26.3		117.7		3.5	3				6.4		0.2			13.2		117.7
Contributions from (distributions to) MPC per consolidated statements of equity/net investment			-360.9														-360.9
Allocation of net investment to unitholders								192.4			361.5		13.5			428.6	-996
Net proceeds from the initial public offering	-407.1				-407.1
Proceeds from initial public offering distributed to MPC	-202.7			-105.4					-97.3
Allocation of prefunded capital expenditures to noncontrolling interest	0					-32.6				-61.3					93.9
Equity-based compensation	0.1				0.1
Ending Balance at Dec. 31, 2012	1,226.80				410.7	57.4				209.3		13.7			535.7
Purchase/contribution of additional interest in Pipe Line Holdings	-100											-46.4			-53.6
Net income	146.1				20.4	17.6				38.2		1.7			68.2
Contributions from (distributions to) MPC per consolidated statements of equity/net investment		0.3
Proceeds from initial public offering distributed to MPC		0
Quarterly distributions to unitholders and general partner	-77.8				-20.5	-17.6				-38.2		-1.5
Quarterly distributions to noncontrolling interest retained by MPC		-82.7													-82.7
Non-cash contribution from MPC	0.3	0.3													0.3
Equity-based compensation	1.4				1.4
Ending Balance at Dec. 31, 2013	1,114.10				412	57.4				209.3		-32.5			467.9
Purchase/contribution of additional interest in Pipe Line Holdings	-910					200						-637.6			-472.4
Equity offerings, net of issuance costs	230.1				221.3							8.8			0
Net income	178.1				31.1	26.6				57.7		5.9			56.8
Contributions from (distributions to) MPC per consolidated statements of equity/net investment		0.1
Proceeds from initial public offering distributed to MPC	0
Quarterly distributions to unitholders and general partner	-103.1				-26.7	-22.9				-49.5		-4
Quarterly distributions to noncontrolling interest retained by MPC		-46.9													-46.9
Non-cash contribution from MPC		0.1													0.1
Equity-based compensation	1.3				1.3
Ending Balance at Dec. 31, 2014	$463.70				$639	$261.10				$217.50		($659.40)			$5.50

Description_of_the_Business_an

Description of the Business and Basis of Presentation	12 Months Ended
	Dec. 31, 2014
Text Block [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 December 31, 2014, the Partnership’s assets consisted of a 99.5 percent indirect interest in a network of common carrier crude oil and product pipeline systems and associated storage assets in the Midwest and Gulf Coast regions of the United States. The Partnership also owns a 100 percent interest in a butane cavern in Neal, West Virginia with approximately one million barrels of natural gas liquids storage capacity.
	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 “Initial Offering”) of 19,895,000 common units, 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 99.5 percent general partner interest at December 31, 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, 69 percent as of March 1, 2014 and 99.5 percent as of December 1, 2014 as discussed in Note 4. Pipe Line Holdings owns 100 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.
	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 Initial Offering. Prior to the Initial Offering, MPLX LP Predecessor results also included MPL’s minority undivided joint interests in two crude oil pipeline systems that were not contributed to the Partnership at the Initial Offering. See Note 5.
	Net income attributable to MPLX LP for the period prior to the Initial Offering on October 31, 2012 included 100 percent of the net income related to the assets that were contributed to MPLX LP, while net income attributable to MPLX LP for the period in 2012 following the Initial Offering and the years ended December 31, 2014 and 2013 reflect only the general partner interest in Pipe Line Holdings. For the periods subsequent to the Initial Offering, we consolidated the results of operations of Pipe Line Holdings and then recorded a noncontrolling interest deduction for the limited partner interest in Pipe Line Holdings retained by MPC. The Neal, West Virginia butane cavern financial results are reflected only in the periods following the Initial Offering. Additional differences in revenues and expenses are attributable to changes in agreements and activities, as detailed in Note 5. Due to these factors, our results of operations subsequent to the Initial 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 Initial 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 Initial Offering, our financial position, results of operations and cash flows consist of consolidated MPLX LP activities and balances.
	The consolidated statements of income for periods prior to the Initial Offering 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 consolidated financial statements, including the assumptions regarding allocating general corporate expenses from MPC, are reasonable. However, these consolidated financial statements do not include all of the actual expenses that would have been incurred had we been a stand-alone publicly traded partnership during the periods presented prior to the Initial Offering. Actual costs that would have been incurred if we had been a stand-alone publicly traded partnership would depend on the level of incremental general and administrative expenses incurred and the cost of services provided by our general partner and its affiliates. Subsequent to the Initial Offering, MPC provides executive management services, certain operational and management services and certain general and administrative services to us pursuant to an omnibus agreement and two employee services agreements. See Note 5 for a description of these agreements.

Summary_of_Principal_Accountin

Summary of Principal Accounting Policies	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Summary of Principal Accounting Policies	Summary of Principal Accounting Policies
	Use of estimates – The preparation of financial statements in accordance with generally accepted accounting principles requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities as of the date of the consolidated financial statements and the reported amounts of revenues and expenses during the respective reporting periods.
	Principles applied in consolidation – These consolidated financial statements include the accounts of our majority-owned and controlled subsidiaries. All significant intercompany transactions and accounts have been eliminated. We consolidate Pipe Line Holdings, in which we own the 99.5 percent general partner interest and record a noncontrolling interest for the 0.5 percent limited partner interest retained by MPC as of December 31, 2014.
	Revenue recognition – Revenues are recognized for crude oil and product pipeline transportation based on the delivery of actual volumes transported at regulated tariff rates. When MPC ships volumes on our pipeline systems under a joint tariff with a third party, those revenues are recorded as sales and other operating revenues, and not as sales to related parties, because we receive payment from the third party. Revenues are recognized for crude oil and refined product storage as performed based on contractual rates. Operating fees received for operating pipeline systems are recognized as a component of other income in the period the service is performed.
	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. 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 quarters or eight quarters under the terms of the applicable transportation services agreement. The deficiency payments are initially recorded as deferred revenue - related parties. We recognize revenues for the deficiency payments at the earlier of when credits are used for volumes transported in excess of minimum 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. 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.
	Cash and cash equivalents – Cash and cash equivalents include cash on hand and on deposit and investments in highly liquid debt instruments with maturities generally of three months or less.
	Restricted cash - Restricted cash consists of cash advances to be used for the operation and maintenance of an operated pipeline system. At December 31, 2014 and 2013, the amount of restricted cash included in other current assets on the consolidated balance sheet was $3.8 million and $7.3 million, respectively.
	Receivables – Our receivables primarily consist of customer accounts receivable that are recorded at the invoiced amounts and do not bear interest. Account balances for these customer receivables are charged directly to bad debt expense when it becomes probable the receivable will not be collected.
	Materials and supplies inventories – Inventories consist of materials and supplies, and cost is determined primarily under the specific identification method.
	Imbalances – Within our pipelines and storage assets we experience volume gains and losses due to pressure and temperature changes, evaporation and variances in meter readings and other measurement methods. Until settled, positive crude oil imbalances are recorded as other current assets and negative crude oil imbalances are recorded as accounts payable. Positive and negative product imbalances are settled monthly by cash payments.
	Property, plant and equipment – Property, plant and equipment are recorded at cost and depreciated on a straight-line basis for groups of property having similar economic characteristics over the estimated useful lives. Such assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. If the sum of the expected undiscounted future cash flows from the use of the asset and its eventual disposition is less than the carrying amount of the asset, an impairment loss is recognized based on the fair value of the asset.
	When items of property, plant and equipment are sold or otherwise disposed of, any gains or losses are reported in the statement of income. Gains on the disposal of property, plant and equipment are recognized when earned, which is generally at the time of closing. If a loss on disposal is expected, such losses are recognized when the assets are classified as held for sale.
	Interest expense is capitalized for qualifying assets under construction. Capitalized interest costs are included in property, plant and equipment and are depreciated over the useful life of the related asset.
	Goodwill – Goodwill represents the excess of the purchase price over the estimated fair value of the net assets acquired in the acquisition of a business. Goodwill was allocated to the Predecessor from MPC based on the fair market value of the Predecessor’s net property, plant and equipment relative to the fair market value of MPC Pipeline Transportation reporting unit’s net property, plant and equipment as of June 30, 2005, the date on which the transaction was completed. Such goodwill is not amortized, but rather is tested for impairment annually and when events or changes in circumstances indicate that the fair value of a reporting unit with goodwill has been reduced below carrying value. The fair value of the reporting unit is determined and compared to the book value of the reporting unit. If the fair value of the reporting unit is less than the book value, including goodwill, the implied fair value of goodwill is calculated. The excess, if any, of the book value over the implied fair value of goodwill is charged to net income.
	Major maintenance activities – Costs for planned integrity management projects are expensed in the period incurred. These types of costs include in-line inspection services, contractor repair services, materials and supplies, equipment rentals and labor costs.
	Other taxes – Other taxes primarily include payroll taxes for periods prior to October 1, 2012 and real estate taxes.
	Environmental costs – Environmental expenditures are capitalized if the costs mitigate or prevent future contamination or if the costs improve environmental safety or efficiency of the existing assets. We recognize remediation costs and penalties when the responsibility to remediate is probable and the amount of associated costs can be reasonably estimated. The timing of remediation accruals coincides with completion of a feasibility study or the commitment to a formal plan of action. Remediation liabilities are accrued based on estimates of known environmental exposure. A receivable is recorded for environmental costs indemnified by MPC.
	Asset retirement obligations – The fair value of asset retirement obligations is recognized in the period in which the obligations are incurred if a reasonable estimate of fair value can be made. Asset retirement obligations are recorded as long-term liabilities with an offsetting asset retirement cost recorded as an increase to the associated property, plant and equipment. The amounts recorded for such obligations are based on the most probable current cost projections. Asset retirement obligations have not been recognized for our assets because the fair value cannot be reasonably estimated since the settlement dates of the obligations are indeterminate. Such obligations will be recognized in the period when sufficient information becomes available to estimate a range of potential settlement dates. The asset retirement obligations principally include hazardous material disposal and removal or dismantlement requirements associated with the closure of our pipeline system and storage assets.
	Income taxes – Following the Initial Offering, our operations are treated as a partnership for federal and state income tax purposes, with each partner being separately taxed on its share of the taxable income. Prior to the Initial Offering, the Predecessor’s taxable income was included in the consolidated U.S. federal income tax returns of MPC and in a number of consolidated state income tax returns. Therefore, we have excluded income taxes from these consolidated financial statements, except for certain state jurisdictions that tax partnerships.
	Employee benefit plans – The Partnership has no employees and effective October 1, 2012, we entered into two employee services agreements with MPC. For periods prior to October 1, 2012, employees of the Predecessor participated in the various employee benefit plans of MPC. These plans included a qualified, non-contributory defined benefit retirement plan, an employee savings plan, employee and retiree medical and life insurance plans, a dental plan and other such benefits. For the purposes of these consolidated financial statements, the Predecessor was considered to be participating in multiemployer benefit plans of MPC. As a participant in multiemployer benefit plans, the Predecessor recognized as expense in each period the required allocation from MPC, and it did not recognize any employee benefit plan liabilities. While the Predecessor was considered to participate in multiemployer plans of MPC, those benefit plans are not technically multiemployer plans. Therefore, we have not included the disclosures required for multiemployer plans.
	Equity-based compensation arrangements – The fair value of phantom unit awards granted to non-employee directors is based on the fair market value of MPLX LP common units on the date of grant. Our equity-based compensation expense is recognized at the time of grant for phantom units since they vest immediately and are not forfeitable.
	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 use a Monte Carlo valuation model to calculate a grant date fair value.
	Net income per limited partner unit - We use the two-class method when calculating the net income per unit applicable to limited partners, because we have more than one participating security. The classes of participating securities include common units, subordinated units, general partner units, certain equity-based compensation awards and incentive distribution rights. 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.
	Net investment – The net investment represented a net balance reflecting MPC’s initial investment in the Predecessor and subsequent adjustments resulting from the operations of the Predecessor and various transactions between the Predecessor and MPC. Transactions affecting the net investment include general, administrative and overhead allocations incurred by MPC that were allocated to the Predecessor. There were no terms of settlement or interest charges associated with the net investment balance.
	Comprehensive income – We have reported no comprehensive income due to the absence of items of other comprehensive income in the periods presented.

Accounting_Standards

Accounting Standards	12 Months Ended
	Dec. 31, 2014
Accounting Standards [Abstract]
New Accounting Pronouncements and Changes in Accounting Principles [Text Block]	Accounting Standards
	Not Yet Adopted
	In February 2015, the Financial Accounting Standards Board (“FASB”) issued an accounting standards update making targeted changes to the current consolidation guidance. The new standard changes the way certain decisions are made related to substantive rights, related parties, and decision making fees when applying the VIE consolidation model and eliminates certain guidance for limited partnerships and similar entities under the voting interest consolidation model. The update is effective for annual periods beginning after December 15, 2015. Early adoption is permitted. At this point, we have not determined the impact of the new standards update on our consolidated financial statements and related disclosures.
	In August 2014, the FASB issued an accounting standards update requiring management to assess an entity’s ability to continue as a going concern and to provide related footnote disclosures in certain circumstances. Management will be required to assess if there is substantial doubt about an entity’s ability to continue as a going concern within one year after the date that the financial statements are issued. Disclosures will be required if conditions give rise to substantial doubt and the type of disclosure will be determined based on whether management’s plans will be able to alleviate the substantial doubt. The accounting standards update will be effective for the first annual period ending after December 15, 2016, and for annual periods and interim periods thereafter with early application permitted.
	In May 2014, the FASB issued an accounting standards update for revenue recognition that is aligned with the International Accounting Standards Board’s revenue recognition standard issued on the same day. The guidance in the update states that revenue is recognized when a customer obtains control of a good or service. Recognition of the revenue will involve a multiple step approach including identifying the contract, identifying the separate performance obligations, determining the transaction price, allocating the price to the performance obligations and then recognizing the revenue as the obligations are satisfied. Additional disclosures will be required to provide adequate information to understand the nature, amount, timing and uncertainty of reported revenues and revenues expected to be recognized. The accounting standards update will be effective on a retrospective or modified retrospective basis for annual reporting periods beginning after December 15, 2016, and interim periods within those years, with no early adoption permitted. At this point, we have not determined the impact of the new standard on our consolidated financial statements.
	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.

Acquisition

Acquisition	12 Months Ended
	Dec. 31, 2014
Business Combinations [Abstract]
Acquisition	Acquisition
	Effective December 1, 2014, we purchased a 22.875 percent interest in Pipe Line Holdings from subsidiaries of MPC for consideration of $600.0 million, which was financed through borrowings under our bank revolving credit facility, as discussed in Note 14. In addition, we accepted a contribution of 7.625 percent of outstanding partnership interests of Pipe Line Holdings from subsidiaries of MPC in exchange for the issuance of equity valued at $200.0 million, as discussed in Note 7. We recorded the combined 30.5 percent interest at its historical carrying value of $334.8 million and the excess cash paid and equity contributed over historical carrying value of $465.2 million as a decrease to general partner equity. Prior to this transaction, the 30.5 percent interest was held by MPC and was reflected as part of the noncontrolling interest retained by MPC in our consolidated financial statements. Beginning December 1, 2014, our consolidated financial statements reflect the 99.5 percent general partner interest in Pipe Line Holdings owned by MPLX LP, while the 0.5 percent limited partner interest held by MPC is reflected as a noncontrolling interest.
	On March 1, 2014, we acquired a 13 percent interest in Pipe Line Holdings from MPC for consideration of $310.0 million, which was funded with $40.0 million of cash on hand and $270.0 million of borrowings on our bank revolving credit facility. We recorded the 13 percent interest in Pipe Line Holdings at its historical carrying value of $137.5 million and the excess cash paid over historical carrying value of $172.5 million as a decrease to general partner equity.
	In addition, 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 were as follows:
	(In millions)	2014			2013			2012
	Net income attributable to MPLX LP	$	121.3		$	77.9		$	130.8
	Transfer to noncontrolling interest retained by MPC:
	Decrease in general partner-MPC equity for purchases of additional interest in Pipe Line Holdings	(637.7		)	(46.4		)	—
	Change from net income attributable to MPLX LP and transfer to noncontrolling interest retained by MPC	$	(516.4	)	$	31.5		$	130.8

Related_Party_Agreements_and_T

Related Party Agreements and Transactions	12 Months Ended
	Dec. 31, 2014
Related Party Transactions [Abstract]
Related Party Agreements and Transactions	Related Party Agreements and Transactions
	Our related parties included:
	•	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 percent interest. Centennial owns a products pipeline and storage facility.
	•	Muskegon Pipeline LLC (“Muskegon”), in which MPC has a 60 percent interest. Muskegon owns a common carrier products pipeline.
	Commercial Agreements
	At the closing of the Initial 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 volumes on crude oil and products systems and minimum storage volumes of crude oil, products and 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. We may adjust the per-barrel fee by a percentage equal to an increase in the PPI in early 2015.
	Operating Agreements
	At the closing of the Initial 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 all 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 in length 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 Pipe Line”), a wholly-owned subsidiary of MPC, under which we operate various pipeline systems in Texas owned by Blanchard Pipe Line. We received $1.1 million in fees under this agreement in 2014. This agreement is subject to adjustment for inflation, and in addition, we are reimbursed for specific costs associated with operating the pipeline systems. The term of this agreement is through December 31, 2015, 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.
	Effective October 1, 2013, MPL entered into an operating and maintenance agreement with the owners of the Capline pipeline system. The Capline system is a 635 mile, 40-inch crude oil pipeline running from St. James, Louisiana to Patoka, Illinois. MPC owns a 32.6 percent undivided joint interest in the Capline system. We received $3.6 million in fees under this agreement in 2014. This agreement is subject to adjustment for inflation, and in addition, we are reimbursed for specific costs associated with operating the pipeline system. The initial term of this agreement is until August 31, 2018, and it is automatically extended for successive five year terms thereafter unless terminated by either party at least ten months prior to the end of the term.
	Management Services Agreements
	Prior to the closing of the Initial 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 received $0.8 million in fees under these agreements in 2014. We may adjust annually for inflation and based on changes in the scope of management services provided.
	Omnibus Agreement
	Upon the closing of the Initial 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.
	Loan Agreement
	On March 31, 2014, Pipe Line Holdings entered into a credit agreement with MPL Investment LLC, a subsidiary of MPC, providing for a $50 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 London Interbank Offered Rate (“LIBOR”) plus 1.375 percent. As of December 31, 2014, there were no borrowings outstanding under this facility.
	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 Initial Offering, were conducted on terms comparable to those with unrelated parties.
	Sales to related parties were as follows:
	(In millions)		2014			2013		2012
	MPC		$	450.9		$	384.2	$	367.8
	Related party sales to MPC consisted 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:
	(In millions)		2014			2013		2012
	MPC		$	21.8		$	17.6	$	12
	Centennial		1.1			1.1		1
	Muskegon		0.1			0.1		0.1
	Total		$	23		$	18.8	$	13.1
	Subsequent to the Initial 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 Initial 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:
	(In millions)		2014			2013		2012
	MPC		$	24.1		$	18	$	13.6
	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 Initial Offering. These charges were as follows:
	(In millions)		2014			2013		2012
	MPC		$	30.7		$	31.4	$	22.6
	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:
	(In millions)		2014			2013		2012
	MPC		$	7.5		$	8	$	5.5
	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. Our allocated share of benefit plan expenses recorded in general and administrative expenses for 2012 included $9.5 million of pension expenses related to lump sum payments made by MPC for periods prior to the October 1, 2012 employee transfer date. 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 $1.1 million for 2012.
	(In millions)		2014			2013		2012
	Purchases from related parties		$	73.4		$	76.6	$	30.8
	General and administrative expenses		23.6			15.7		23.1
	Total		$	97		$	92.3	$	53.9
	Receivables from related parties were as follows:
			December 31,
	(In millions)		2014			2013
	MPC		$	40.5		$	47.4
	Centennial		0.3			0.6
	Muskegon		0.2			0.3
	Total		$	41		$	48.3
	Prepaid assets with related parties included in other current assets on the consolidated balance sheets were as follows:
			December 31,
(In millions)		2014			2013
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:
	December 31,
(In millions)	2014			2013
MPC	$	—		$	0.2
Payables to related parties were as follows:
		December 31,
(In millions)		2014			2013
MPC		$	20.1		$	12.8
Centennial		0.1			—
Total		$	20.2		$	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 at the earlier of 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 2014 and 2013, MPC did not transport its minimum committed volumes on certain pipeline systems. In addition, capital projects we are undertaking at the request of MPC are reimbursed in cash and recognized in income over the remaining term of the applicable transportation services agreements. The deferred revenue-related parties associated with the minimum volume deficiencies and project reimbursements were as follows:
	December 31,
(In millions)	2014			2013
Minimum volume deficiencies - MPC	$	29.9		$	34
Project reimbursements - MPC	4.6			—
Total	$	34.5		$	34
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 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. The agreement was scheduled to terminate on January 1, 2020, however at any time during the agreement, a loan from MPCIF could be repaid or a demand for repayment could be made for a loan to MPCIF. We could terminate our participation at any time during the agreement. The agreement was terminated on September 28, 2012, in connection with the Initial Offering. Related party interest and other financial income were $1.3 million for 2012.
Certain asset transfers between us and MPC and certain expenses, such as stock-based compensation, incurred by MPC on our behalf have been recorded as non-cash capital contributions or distributions. The non-cash contributions from MPC were $0.1 million and $0.3 million for 2014 and 2013, respectively, and net non-cash distributions to MPC were $98.2 million in 2012. We recorded property, plant and equipment additions related to capitalized interest incurred by MPC on our behalf of $0.1 million, $0.3 million and $0.7 million in 2014, 2013 and 2012, which were reflected as contributions from MPC.
Also included in the 2012 net non-cash distribution were several transactions that occurred on October 31, 2012, in conjunction with the Initial Offering. MPL made a $224.1 million distribution to MPC of its minority undivided joint interests in two crude oil pipeline systems. The revenues, expenses, assets and liabilities attributable to these two pipeline systems were included in the consolidated financial statements for periods prior to the Initial Offering. MPC made a $121.4 million contribution to us of its Neal, West Virginia butane cavern. MPC also contributed net assets of $3.7 million to us related to the balancing of working capital and the indemnification of environmental liabilities.

Net_Income_Per_Limited_Partner

Net Income Per Limited Partner Unit	12 Months Ended
	Dec. 31, 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.
	Net income per limited partner unit is only calculated for the periods subsequent to the Initial Offering as no units were outstanding prior to October 31, 2012. Diluted net income per limited partner unit is the same as basic net income per limited partner unit as there were no potentially dilutive common or subordinated units outstanding as of December 31, 2014, 2013 or 2012.
	(In millions)	2014				2013				October 31, 2012 to December 31, 2012
	Net income attributable to MPLX LP subsequent to initial public offering	$	121.3			$	77.9			$	13.1
	Less: General partner’s distributions declared (including IDRs)(1)	5.8				1.9				0.3
	Limited partners’ distributions declared on common units(1)	54.5				43.2				6.5
	Limited partner’s distributions declared on subordinated	52.1				43.1				6.5
	units(1)
	Distributions less than (in excess of) net income attributable to MPLX LP	$	8.9			$	(10.3	)		$	(0.2	)
	(1)	On January 20, 2015, we announced the board of directors of our general partner had declared a quarterly cash distribution of $0.3825 per unit, totaling $33.0 million. This distribution was paid on February 13, 2015 to unitholders of record on February 3, 2015.
			2014
	(In millions, except per-unit data)		General				Limited Partners’				Limited			Total
			Partner				Common Units				Partner’s
											Subordinated
											Units
	Basic and diluted net income attributable to MPLX LP per unit:
	Net income attributable to MPLX LP:
	Distributions declared (including IDRs)		$	5.8			$	54.5			$	52.1		$	112.4
	Distributions less than net income attributable to MPLX LP		2.2				3.4				3.3			8.9
	Net income attributable to MPLX LP		$	8			$	57.9			$	55.4		$	121.3
	Weighted average units outstanding:
	Basic		1.5				37.4				37			75.9
	Diluted		1.5				37.4				37			75.9
	Net income attributable to MPLX LP per limited partner unit:
	Basic						$	1.55			$	1.5
	Diluted						$	1.55			$	1.5
			2013
	(In millions, except per-unit data)		General				Limited Partners’				Limited			Total
			Partner				Common Units				Partner’s
											Subordinated
											Units
	Basic and diluted net income attributable to MPLX LP per unit:
	Net income attributable to MPLX LP:
	Distribution declared		$	1.9			$	43.2			$	43.1		$	88.2
	Distributions in excess of net income attributable to MPLX LP		(0.2		)		(4.4		)		(5.7		)	(10.3		)
	Net income attributable to MPLX LP		$	1.7			$	38.8			$	37.4		$	77.9
	Weighted average units outstanding:
	Basic		1.4				37				37			75.4
	Diluted		1.4				37				37			75.4
	Net income attributable to MPLX LP:
	Basic						$	1.05			$	1.01
	Diluted						$	1.05			$	1.01
			October 31, 2012 to December 31, 2012
	(In millions, except per-unit data)		General				Limited Partners’				Limited			Total
			Partner				Common Units				Partner’s
											Subordinated
											Units
	Basic and diluted net income attributable to MPLX LP per unit:
	Net income attributable to MPLX LP subsequent to initial public offering:
	Distribution declared		$	0.3			$	6.5			$	6.5		$	13.3
	Distributions in excess of net income attributable to MPLX LP subsequent to initial public offering		(0.1		)		—				(0.1		)	(0.2		)
	Net income attributable to MPLX LP subsequent to initial public offering		$	0.2			$	6.5			$	6.4		$	13.1
	Weighted average units outstanding:
	Basic		1.4				37				37			75.4
	Diluted		1.4				37				37			75.4
	Net income attributable to MPLX LP subsequent to initial public offering per limited partner unit:
	Basic						$	0.18			$	0.17
	Diluted						$	0.18			$	0.17

Equity

Equity	12 Months Ended
	Dec. 31, 2014
Equity [Abstract]
Equity	Equity
	Units Outstanding - We had 43,341,098 common units outstanding as of December 31, 2014. Of that number, 19,980,619 were owned by MPC, which also owned 36,951,515 subordinated units. The two percent general partner interest, represented by 1,638,625 general partner units, was owned by MPC.
	The table below summarizes the changes in the number of units outstanding through December 31, 2014:
	(In units)	Common			Subordinated			General Partner			Total
	Units issued in initial public offering	36,951,515			36,951,515			1,508,225			75,411,255
	Balance at December 31, 2012	36,951,515			36,951,515			1,508,225			75,411,255
	Balance at December 31, 2013	36,951,515			36,951,515			1,508,225			75,411,255
	Unit-based compensation awards	15,479			—			316			15,795
	Contribution of interest in Pipe Line Holdings(1)(3)	2,924,104			—			59,676			2,983,780
	December 2014 equity offering(2)(3)	3,450,000			—			70,408			3,520,408
	Balance at December 31, 2014	43,341,098			36,951,515			1,638,625			81,931,238
	(1) Effective December 1, 2014, as discussed in Note 4, we accepted a contribution of 7.625 percent of outstanding partnership interests of Pipe Line Holdings from subsidiaries of MPC in exchange for the issuance of equity valued at $200.0 million. The equity consideration consists of 2,924,104 MPLX common units and was calculated by dividing $200.0 million by the average closing price for MPLX common units for the ten trading days preceding December 1, 2014, which was $68.397.
	(2) On December 8, 2014, we closed an equity offering of 3,450,000 common units representing limited partner interests, at a public offering price of $66.68 per unit. We used the net proceeds of $221.3 million to repay borrowings under our revolving credit facility and for general partnership purposes.
	(3) As a result of the contribution mentioned above and December 2014 equity offering, MPLX GP contributed $8.8 million in exchange for 130,084 general partner units to maintain its two percent general partnership interest.
	Issuance of Additional Securities - Our partnership agreement authorizes us to issue an unlimited number of additional partnership securities for the consideration and on the terms and conditions determined by our general partner without the approval of the unitholders.
	Incentive Distribution Rights - The following table illustrates the percentage allocations of available cash from operating surplus between the common and subordinated unitholders and our general partner based on the specified target distribution levels. The amounts set forth under “Marginal percentage interest in distributions” are the percentage interests of our general partner and common and subordinated unitholders in any available cash from operating surplus we distribute up to and including the corresponding amount in the column “Total quarterly distribution per unit target amount.” The percentage interests shown for our common and subordinated unitholders and our general partner for the minimum quarterly distribution are also applicable to quarterly distribution amounts that are less than the minimum quarterly distribution. The percentage interests set forth below for our general partner include its two percent general partner interest and assume that our general partner has contributed any additional capital necessary to maintain its two percent general partner interest, our general partner has not transferred its incentive distribution rights and that there are no arrearages on common units.
									Marginal percentage interest
									in distributions
			Total quarterly distribution						Unitholders			General Partner
			per unit target amount
	Minimum quarterly distribution		$	0.2625					98	%		2	%
	First target distribution		above $0.2625				up to $0.301875		98	%		2	%
	Second target distribution		above $0.301875				up to $0.328125		85	%		15	%
	Third target distribution		above $0.328125				up to $0.393750		75	%		25	%
	Thereafter		above $0.393750						50	%		50	%
	Net Income Allocation - The following table presents the allocation of the general partner’s interest in net income attributable to MPLX LP:
	(In millions)	2014				2013				October 31, 2012 to December 31, 2012
	Net income attributable to MPLX LP	$	121.3			$	77.9			$	13.1
	Less: General partner's incentive distribution rights	3.6				0.1				—
	Net income attributable to MPLX LP available to general and limited partners	$	117.7			$	77.8			$	13.1
	General partner’s two percent interest in net income attributable to	$	2.3			$	1.6			$	0.2
	MPLX LP
	General partner's incentive distribution rights	3.6				0.1				—
	General partner's interest in net income attributable to MPLX LP	$	5.9			$	1.7			$	0.2
	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 January 20, 2015, we declared a quarterly cash distribution, based on the results of the fourth quarter of 2014, totaling $33.0 million, or $0.3825 per unit. This distribution was paid on February 13, 2015 to unitholders of record on February 3, 2015.
	The allocation of total quarterly cash distributions to general and limited partners is as follows for the year ended December 31, 2014, 2013 and the period October 31, 2012 to December 31, 2012. 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.
	(In millions)	2014				2013				October 31, 2012 to December 31, 2012
	General partner's distributions:
	General partner's distributions	$	2.2			$	1.8			$	0.3
	General partner's incentive distribution rights	3.6				0.1				—
	Total general partner's distributions	$	5.8			$	1.9			$	0.3
	Limited partners' distributions:
	Common unitholders	$	54.5			$	43.2			$	6.5
	Subordinated unitholders	52.1				43.1				6.5
	Total limited partners' distributions	106.6				86.3				13
	Total cash distributions declared	$	112.4			$	88.2			$	13.3

Major_Customer_and_Concentrati

Major Customer and Concentration of Credit Risk	12 Months Ended
	Dec. 31, 2014
Risks and Uncertainties [Abstract]
Major Customer and Concentration of Credit Risk	Major Customer and Concentration of Credit Risk
	MPC accounted for 86 percent, 83 percent and 82 percent of our total revenues and other income for 2014, 2013 and 2012, excluding revenues attributable to volumes shipped by MPC under joint tariffs with third parties, which are treated as third-party revenue for accounting purposes. We provide crude oil and product pipeline transportation and storage services to MPC and operate pipelines on behalf of MPC.
	We have a concentration of trade receivables due from customers in the same industry, MPC, integrated oil companies, independent refining companies and other pipeline companies. These concentrations of customers may impact our overall exposure to credit risk as they may be similarly affected by changes in economic, regulatory and other factors. We manage our exposure to credit risk through credit analysis, credit limit approvals and monitoring procedures, and for certain transactions, we may request letters of credit, prepayments or guarantees.

Other_Items

Other Items	12 Months Ended
	Dec. 31, 2014
Other Income and Expenses [Abstract]
Other Items	Other Items
	Net interest and other financial costs were:
	(In millions)	2014			2013			2012
	Interest income	$	—		$	(0.3	)	$	(0.1	)
	Interest expense	5			1.1			0.8
	Interest capitalized	(0.9		)	(0.9		)	(0.7		)
	Other financial costs	1.2			1.2			0.2
	Net interest and other financial costs	$	5.3		$	1.1		$	0.2

Income_Taxes

Income Taxes	12 Months Ended
	Dec. 31, 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 Texas, Tennessee and Kentucky.
	Our income tax benefit was $0.1 million in 2014 and $0.2 million for 2013 and expense of $0.3 million for 2012. Our effective tax rate was (0.1) percent for 2014 and 2013 and 0.2 percent for 2012.
	The following table summarizes the activity in unrecognized tax benefits:
	(In millions)	2014		2013			2012
	January 1 balance	$	—	$	0.2		$	—
	Additions (reductions) for tax positions of prior years	—		(0.2		)	0.2
	December 31 balance	$	—	$	—		$	0.2
	As there were no unrecognized tax benefits as of December 31, 2014, there would be no affect on our effective income tax rate if they were recognized. There were no uncertain tax positions as of December 31, 2014 for which it is reasonably possible that the amount of unrecognized tax benefits would significantly increase or decrease during the next twelve months.
	Any interest and penalties related to income taxes were recorded as a part of the provision for income taxes. Such interest and penalties were net benefits of less than $0.1 million and $0.1 million in 2014 and 2013, respectively, and net expenses of $0.1 million in 2012. As of December 31, 2014 and 2013, less than $0.1 million of interest and penalties were accrued related to income taxes. In addition, we have state tax years 2012 - 2013 open to examination.
	In taxable jurisdictions, we recorded deferred income taxes on all temporary differences between the book basis and the tax basis of assets and liabilities. At December 31, 2014 and 2013, our net deferred tax liability was $0.1 million.

Property_Plant_and_Equipment

Property, Plant and Equipment	12 Months Ended
	Dec. 31, 2014
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment	Property, Plant and Equipment
		Estimated	December 31,
	(In millions)	Useful Lives	2014		2013
	Land		$	5.5	$	5.3
	Pipelines and related assets	19 - 42 years	1,081.40		1,060.70
	Storage and delivery facilities	24 - 37 years	165.8		165.7
	Other	10 - 25 years	23.7		22.8
	Assets under construction		85.3		21.9
	Total		1,361.70		1,276.40
	Less accumulated depreciation		353.1		309.8
	Property, plant and equipment, net		$	1,008.60	$	966.6
	Property, plant and equipment includes gross assets acquired under capital leases of $24.9 million at December 31, 2014 and 2013, with related amounts in accumulated depreciation of $6.5 million and $5.7 million at December 31, 2014 and 2013.

Goodwill

Goodwill	12 Months Ended
	Dec. 31, 2014
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill	Goodwill
	Goodwill is tested for impairment on an annual basis and when events or changes in circumstances indicate the fair value of our reporting unit has been reduced below carrying value. We have performed our annual impairment tests, and no impairment in the carrying value of goodwill has been identified during the periods presented.
	The were no changes in the carrying amount of goodwill for 2014 and 2013.

Fair_Value_Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 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 December 31, 2014 and 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 December 31, 2014 and 2013.
		December 31,
		2014				2013
	(In millions)	Fair Value		Carrying Value		Fair Value		Carrying Value
	Long-term debt	$	635.6	$	635	$	—	$	—

Debt

Debt	12 Months Ended
	Dec. 31, 2014
Debt Disclosure [Abstract]
Debt	Debt
	Our outstanding borrowings at December 31, 2014 and 2013 consisted of the following:
			December 31,
	(In millions)		2014		2013
	MPLX Operations - bank revolving credit agreement due 2017		$	—	$	—
	MPLX - bank revolving credit facility due 2019		385		—
	MPLX - term loan facility due 2019		250		—
	MPL - capital lease obligations due 2020		9.8		10.5
	Total		644.8		10.5
	Amounts due within one year		0.8		0.7
	Total long-term debt due after one year		$	644	$	9.8
	The following table shows five years of scheduled debt payments.
	(In millions)
	2015	$	0.8
	2016	0.9
	2017	0.9
	2018	1
	2019	636
	Credit Agreements
	On November 20, 2014, MPLX entered into a credit agreement with a syndicate of lenders (“MPLX Credit Agreement”) which provides for a five-year, $1 billion bank revolving credit facility and a $250 million term loan facility. The maturity on both facilities is November 20, 2019.
	The bank revolving credit facility includes letter of credit issuing capacity of up to $250 million and swingline capacity of up to $100 million. The borrowing capacity under the MPLX Credit Agreement may be increased by up to an additional $500 million, subject to certain conditions, including the consent of lenders whose commitments would increase. In addition, the maturity date may be extended up to two additional one-year periods subject to the approval of lenders holding the majority of the commitments then outstanding, provided that the commitments of any non-consenting lenders will be terminated on the original maturity date.
	The term loan facility was drawn in full on November 20, 2014. The maturity date for the term loan facility may be extended for up to two additional one-year periods subject to the consent of the lenders holding a majority of the outstanding term loan borrowings, provided that the portion of the term loan borrowings held by any non-consenting lenders will continue to be due and payable on the original maturity date. The borrowings under this facility during 2014 were at an average interest rate of 1.412 percent.
	Borrowings under the MPLX Credit Agreement bear interest at either the Adjusted LIBOR or the Alternate Base Rate (as defined in the MPLX Credit Agreement) plus a specified margin. We are charged various fees and expenses in connection with the agreement, including administrative agent fees, commitment fees on the unused portion of the revolving credit facility and fees with respect to issued and outstanding letters of credit. The applicable interest rates and certain of the fees fluctuate based on the credit ratings in effect from time to time on our long-term debt.
	The MPLX Credit Agreement includes certain representations and warranties, affirmative and restrictive covenants and events of default that we consider to be usual and customary for an agreement of this type. This agreement includes a financial covenant that requires us to maintain a ratio of Consolidated Total Debt as of the end of each fiscal quarter to Consolidated EBITDA (both as defined in the MPLX Credit Agreement) for the prior four fiscal quarters of no greater than 5.0 to 1.0 (or 5.5 to 1.0 for up to two fiscal quarters following certain acquisitions.) Consolidated EBITDA is subject to adjustments for certain acquisitions completed and capital projects undertaken during the relevant period. Other covenants restrict us from incurring debt, creating liens on our assets and entering into transactions with affiliates. As of December 31, 2014, we were in compliance with the covenants contained in the MPLX Credit Agreement.
	In connection with the entering into the above mentioned MPLX Credit Agreement, we terminated our previously existing $500 million five-year MPLX Operations bank revolving credit agreement, dated as of September 14, 2012. During 2014, we borrowed $280.0 million under this agreement, at an average interest rate of 1.535 percent, per annum, and repaid all of these borrowings.
	During 2014, we borrowed $630.0 million under the new bank revolving credit facility, at an average interest rate of 1.402 percent, per annum, and repaid $245.0 million of these borrowings. At December 31, 2014, we had $385.0 million of borrowings and no letters of credit outstanding under this facility, resulting in total unused loan availability of $615.0 million, or 61.5 percent of the borrowing capacity.
	On March 31, 2014, Pipe Line Holdings entered into a credit agreement with MPL Investment LLC, a subsidiary of MPC. As of December 31, 2014, there were no borrowings outstanding under this facility. A description of this agreement is discussed in detail in Note 5.

Supplemental_Cash_Flow_Informa

Supplemental Cash Flow Information	12 Months Ended
	Dec. 31, 2014
Supplemental Cash Flow Elements [Abstract]
Supplemental Cash Flow Information	Supplemental Cash Flow Information
	(In millions)		2014			2013			2012
	Net cash provided by operating activities included:
	Interest paid (net of amounts capitalized)		$	2.9		$	—		$	—
	Income taxes paid (refunded)		(0.3		)	0.1			0.2
	Non-cash investing and financing activities:
	Net transfers of property, plant and equipment to materials and		$	1		$	4		$	—
	supplies inventories
	Property, plant and equipment contributed by MPC		0.1			0.3			—
	Contribution - common units issued		200			—			—
	Distribution to MPC in conjunction with the Initial Offering
	Receivables from related parties		—			—			(5.7		)
	Materials and supplies inventories		—			—			(0.3		)
	Property, plant and equipment		—			—			(187.7		)
	Goodwill		—			—			(29.5		)
	Other noncurrent assets		—			—			(2.2		)
	Accounts payable		—			—			(0.3		)
	Accrued taxes		—			—			(1.0		)
	Contribution from MPC in conjunction with the Initial Offering
	Receivables from related parties		$	—		$	—		$	7.4
	Property, plant and equipment		—			—			121.4
	Other noncurrent assets		—			—			0.3
	Payables to related parties		—			—			3.9
	Accrued taxes		—			—			0.1
	The consolidated statements of cash flows exclude changes to the consolidated balance sheets that did not affect cash. The following is a reconciliation of additions to property, plant and equipment to total capital expenditures:
	(In millions)		2014			2013			2012
	Additions to property, plant and equipment		$	78.6		$	106.5		$	135.6
	Plus: Increase (decrease) in capital accruals		12.9			(5.3		)	3.2
	Asset retirement expenditures		1.6			8.3			9.2
	Total capital expenditures		$	93.1		$	109.5		$	148
	The following is a reconciliation of cash proceeds from the Partnership’s initial public offering, which closed on October 31, 2012:
	(In millions)
	Total proceeds from the Initial Offering	$	437.7
	Less: Offering Costs	(30.6		)
	Net proceeds from the Initial Offering	407.1
	Less: Revolving credit facility origination fees	(2.4		)
	Cash retained by MPLX LP	(10.4		)
	Cash contribution to Pipe Line Holdings	(191.6		)
	Net proceeds distributed to MPC from the Initial Offering	$	202.7
	The following is a reconciliation of contributions from (distributions to) MPC:
	(In millions)		2014			2013			2012
	Contributions from (distributions to) MPC per consolidated statements of cash flows		$	—		$	—		$	(262.7	)
	Net non-cash contributions from (distributions to) MPC		0.1			0.3			(98.2		)
	Contributions from (distributions to) MPC per consolidated statements of equity/net investment		$	0.1		$	0.3		$	(360.9	)
	See Note 5 for information regarding net non-cash contributions from (distributions to) MPC.

EquityBased_Compensation_Plan

Equity-Based Compensation Plan	12 Months Ended
	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Equity-Based Compensation Plan	Equity-Based Compensation Plan
	Description of the Plan
	The MPLX LP 2012 Incentive Compensation Plan (“MPLX 2012 Plan”) authorizes the MPLX GP LLC board of directors (the “Board”) to grant unit options, unit appreciation rights, restricted units and phantom units, distribution equivalent rights, unit awards, profits interest units, performance units and other unit-based awards to our or any of our affiliates’ employees, officers and directors, including directors and officers of MPC. No more than 2.75 million MPLX LP common limited partner units may be delivered under the MPLX 2012 Plan. Units delivered pursuant to an award granted under the MPLX 2012 Plan may be funded through acquisition on the open market, from the partnership or from an affiliate of the partnership, as determined by the Board.
	Unit-based awards under the Plan
	We expense all unit-based payments to employees and non-employee directors based on the grant date fair value of the awards over the requisite service period, adjusted for estimated forfeitures.
	Phantom Units – We grant phantom units under the MPLX 2012 Plan to non-employee directors of MPLX LP’s general partner and MPC. Awards to non-employee directors are 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.
	We grant phantom units 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 are 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 will not have the right to vote such units and cash distributions declared will be accrued and paid upon vesting. The accrued distributions at December 31, 2014 were $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.
	Performance Units - We grant performance units 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 month requisite service period. These performance units will pay out 75 percent in cash and 25 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 $1.16 per unit for 2014 and $0.77 per unit for 2013, as calculated using a Monte Carlo valuation model.
	Outstanding Phantom Unit Awards
	The following is a summary of phantom unit award activity of MPLX LP common limited partner units in 2014:
			Phantom Units
			Number			Weighted			Aggregate Intrinsic Value (In millions)
			of Units			Average
						Fair Value
	Outstanding at December 31, 2013		77,754			$	33.84
	Granted		51,904			49.56
	Settled		(27,360	)		34.45
	Forfeited		(1,529	)		40.91
	Outstanding at December 31, 2014		100,769			41.66
	Vested and expected to vest at December 31, 2014		99,566			41.62			$	7.3
	Convertible at December 31, 2014		25,892			42.21			$	1.9
	The 25,892 convertible units are held by our non-employee directors and are non-forfeitable and issuable upon the director’s departure from our board of directors.
	The following is a summary of the values related to phantom units held by officers and non-employee directors:
		Phantom Units
		Intrinsic Value of Units Issued During the Period (in millions)				Weighted Average Grant Date Fair Value of Units Granted During the Period
	2014	$	1.3			$	49.56
	2013	—				—
	2012	—				—
	As of December 31, 2014, unrecognized compensation cost related to phantom unit awards was $2.1 million, which is expected to be recognized over a weighted average period of 1.6 years.
	Outstanding Performance Unit Awards
	The following is a summary of activity of performance unit awards paying out in MPLX LP common limited partner units in 2014:
		Performance Units
		Number of Units			Weighted
					Average
					Fair Value
	Outstanding at December 31, 2013	436,917			$	0.77
	Granted	500,507			1.16
	Forfeited	(13,281	)		1.05
	Outstanding at December 31, 2014	924,143			0.98
	As of December 31, 2014, unrecognized compensation cost related to equity-classified performance unit awards was $0.5 million, which is expected to be recognized over a weighted average period of 1.5 years.
	Performance units paying out in units have a grant date fair value calculated using a Monte Carlo valuation model, which requires the input of subjective assumptions. The following table provides a summary of these assumptions:
		2014				2013
	Risk-free interest rate	0.63		%		0.35		%
	Look-back period	2.84 years				2.84 years
	Expected volatility	17.17		%		16.75		%
	Grant date fair value of performance units granted	$	1.16			$	0.77
	The assumption for expected volatility of our unit price reflects the average historical volatility for a selected group of peer publicly traded partnerships. The look-back period reflects the remaining performance period at the grant date. The risk-free interest rate for the remaining performance period as of the grant date is based on the U.S. Treasury yield curve in effect at the time of the grant.
	Total Unit-Based Compensation Expense
	Total unit-based compensation expense for awards settling in MPLX LP limited partner common units was $2.7 million in 2014, $1.4 million in 2013 and $0.1 million in 2012 for the period subsequent to the Initial Offering.
	MPC’s Stock-based Compensation
	Certain MPL employees supporting the Predecessor’s operations were historically granted long-term incentive compensation awards under MPC’s stock-based compensation programs, which primarily consist of stock options, restricted common stock, and performance units. The Predecessor is allocated expenses for these stock-based compensation costs and these costs are included in general and administrative expenses. For the period in 2012 prior to the transfer of the MPL employees on October 1, 2012, expense allocated to the Predecessor were $0.9 million. Stock-based compensation expenses charged to MPLX under our employee services agreement with MPC were $1.0 million for 2014 and 2013 and $0.2 million for the period October 1, 2012 through December 31, 2012.

Leases

Leases	12 Months Ended
	Dec. 31, 2014
Leases [Abstract]
Leases	Leases
	We lease a pipeline, vehicles, building space, pipeline equipment and land under long-term operating leases. Most of these leases include renewal options. We also lease certain pipelines under a capital lease that has a fixed price purchase option in 2020. Future minimum commitments as of December 31, 2014, for capital lease obligations and for operating lease obligations having initial or remaining non-cancelable lease terms in excess of one year are as follows:
	(In millions)	Capital		Operating
		Lease		Lease
		Obligations		Obligations
	2015	$	1.4	$	9.6
	2016	1.4		9.5
	2017	1.4		8.7
	2018	1.4		8.4
	2019	1.4		3.7
	Later years	5.1		7.7
	Total minimum lease payments	12.1		$	47.6
	Less imputed interest costs	2.3
	Present value of net minimum lease payments	$	9.8
	Operating lease rental expense was:
	(In millions)	2014		2013		2012
	Minimum rental expense	$	10.4	$	9.9	$	7

Commitments_and_Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 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 December 31, 2014 and 2013, accrued liabilities for remediation totaled $0.6 million and $1.1 million, respectively. At December 31, 2014 and 2013, it is reasonably possible that an estimated loss existed of up to $0.3 million and $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 the penalties, if any, that may be imposed. There were no receivables from MPC for indemnification of environmental costs related to incidents occurring prior to the Initial Offering at December 31, 2014 and $0.3 million at December 31, 2013.
	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 December 31, 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 December 31, 2014 our contractual commitments to acquire property, plant and equipment totaled $25.0 million. Our contractual commitments at December 31, 2014 were primarily related to the Cornerstone Pipeline project, a butane cavern build in Robinson, Illinois, a tank expansion project in Patoka, Illinois, a capacity expansion project on the Patoka to Robinson system and other pipeline related projects.

Subsequent_Event

Subsequent Event	12 Months Ended
	Dec. 31, 2014
Subsequent Events [Abstract]
Subsequent Events	Subsequent Event
	On February 12, 2015, we completed an initial underwritten public offering of $500.0 million aggregate principal amount of four percent unsecured senior notes due February 15, 2025 (the “Senior Notes”). The Senior Notes were offered at a price to the public of 99.64 percent of par. The net proceeds of this offering were used to repay the amounts outstanding under the MPLX Credit Agreement, as well as for general partnership purposes.

Condensed_Consolidating_Financ

Condensed Consolidating Financial Statement	12 Months Ended
	Dec. 31, 2014
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Condensed Consolidating Financial Statements	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 would be fully and unconditionally guaranteed by the Parent Guarantor. The Subsidiary Issuer has not previously issued any debt securities and we do not expect the Subsidiary Issuer to issue any debt securities in the foreseeable future. We anticipate that any future debt securities issued by the Partnership would be issued by the Parent Guarantor and would not be guaranteed by the Subsidiary Issuer or any other subsidiaries of the Partnership.
	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 for periods including and subsequent to the Initial Offering. For periods prior to the Initial Offering, Non-Guarantor Subsidiaries represent MPLX LP Predecessor.
	Consolidating Statements of Income
		Year Ended December 31, 2014
	(in millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
	Revenues and other income:
	Sales and other operating revenues	$	—		$	—		$	69.2		$	—		$	69.2
	Sales to related parties	—			—			450.9			—			450.9
	Other income	—			—			5.2			—			5.2
	Other income - related parties	—			—			23			—			23
	Equity in earnings of subsidiaries	130.6			136.1			—			(266.7		)	—
	Total revenues and other income	130.6			136.1			548.3			(266.7		)	548.3
	Costs and expenses:
	Cost of revenues (excludes items below)	—			—			145.3			—			145.3
	Purchases from related parties	—			—			97.5			—			97.5
	Depreciation	—			—			50.2			—			50.2
	General and administrative expenses	7.6			—			57.2			—			64.8
	Other taxes	0.1			—			7.1			—			7.2
	Total costs and expenses	7.7			—			357.3			—			365
	Income from operations	122.9			136.1			191			(266.7		)	183.3
	Net interest and other financial costs (income)	1.6			5.5			(1.8		)	—			5.3
	Income before income taxes	121.3			130.6			192.8			(266.7		)	178
	Benefit for income taxes	—			—			(0.1		)	—			(0.1		)
	Net income	121.3			130.6			192.9			(266.7		)	178.1
	Less: Net income attributable to MPC-retained interest	—			—			—			56.8			56.8
	Net income attributable to MPLX LP	$	121.3		$	130.6		$	192.9		$	(323.5	)	$	121.3
	Consolidating Statements of Income
		Year Ended December 31, 2013
	(in millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
	Revenues and other income:
	Sales and other operating revenues	$	—		$	—		$	78.9		$	—		$	78.9
	Sales to related parties	—			—			384.2			—			384.2
	Other income	—			—			4.4			—			4.4
	Other income - related parties	—			—			18.8			—			18.8
	Equity in earnings of subsidiaries	83			85.3			—			(168.3		)	—
	Total revenues and other income	83			85.3			486.3			(168.3		)	486.3
	Costs and expenses:
	Cost of revenues (excludes items below)	—			—			135.9			—			135.9
	Purchases from related parties	—			—			94.6			—			94.6
	Depreciation	—			—			48.9			—			48.9
	General and administrative expenses	5			—			48.7			—			53.7
	Other taxes	0.1			—			6.1			—			6.2
	Total costs and expenses	5.1			—			334.2			—			339.3
	Income from operations	77.9			85.3			152.1			(168.3		)	147
	Net interest and other financial costs (income)	—			2.3			(1.2		)	—			1.1
	Income before income taxes	77.9			83			153.3			(168.3		)	145.9
	Benefit for income taxes	—			—			(0.2		)	—			(0.2		)
	Net income	77.9			83			153.5			(168.3		)	146.1
	Less: Net income attributable to MPC-retained interest	—			—			—			68.2			68.2
	Net income attributable to MPLX LP	$	77.9		$	83		$	153.5		$	(236.5	)	$	77.9
	Consolidating Statements of Income
		Year Ended December 31, 2012
	(in millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
	Revenues and other income:
	Sales and other operating revenues	$	—		$	—		$	74.4		$	—		$	74.4
	Sales to related parties	—			—			367.8			—			367.8
	Loss on sale of assets	—			—			(0.3		)	—			(0.3		)
	Other income	—			—			6.9			—			6.9
	Other income - related parties	—			—			13.1			—			13.1
	Equity in earnings of subsidiaries	14			14.2			—			(28.2		)	—
	Total revenues and other income	14			14.2			461.9			(28.2		)	461.9
	Costs and expenses:
	Cost of revenues (excludes items below)	—			—			173.8			—			173.8
Purchases from related parties	—			—			44.4			—			44.4
Depreciation	—			—			39.4			—			39.4
General and administrative expenses	0.9			—			48.9			—			49.8
Other taxes	—			—			11.3			—			11.3
Total costs and expenses	0.9			—			317.8			—			318.7
Income from operations	13.1			14.2			144.1			(28.2		)	143.2
Related party interest and other financial income	—			—			1.3			—			1.3
Net interest and other financial costs	—			0.2			—			—			0.2
Income before income taxes	13.1			14			145.4			(28.2		)	144.3
Provision for income taxes	—			—			0.3			—			0.3
Net income	13.1			14			145.1			(28.2		)	144
Less: Net income attributable to MPC-retained interest	—			—			—			13.2			13.2
Net income attributable to MPLX LP	13.1			14			145.1			(41.4		)	130.8
Less: Predecessor income prior to initial public offering on October 31, 2012	—			—			117.7			—			117.7
Net income attributable to MPLX LP subsequent to initial public offering	$	13.1		$	14		$	27.4		$	(41.4	)	$	13.1
Consolidating Balance Sheets
	31-Dec-14
(in millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
Assets
Current assets:
Cash and cash equivalents	$	1.2		$	0.5		$	25.6		$	—		$	27.3
Receivables	0.1			—			10.1			—			10.2
Receivables from related parties	—			5.9			184.8			(149.7		)	41
Materials and supplies inventories	—			—			11.7			—			11.7
Other current assets	0.4			—			6.6			—			7
Total current assets	1.7			6.4			238.8			(149.7		)	97.2
Property, plant and equipment, net	—			—			1,008.60			—			1,008.60
Investment in subsidiaries	1,098.50			1,234.70			—			(2,333.2		)	—
Goodwill	—			—			104.7			—			104.7
Other noncurrent assets	2.7			1.3			—			—			4
Total assets	$	1,102.90		$	1,242.40		$	1,352.10		$	(2,482.9	)	$	1,214.50
Liabilities
Current liabilities:
Accounts payable	$	1		$	—		$	41.2		$	—		$	42.2
Payables to related parties	6.3			143.9			19.7			(149.7		)	20.2
Deferred revenue - related parties	—			—			30.5			—			30.5
Accrued taxes	0.1			—			5.1			—			5.2
Long-term debt due within one year	—			—			0.8			—			0.8
Other current liabilities	0.6			—			1.1			—			1.7
Total current liabilities	8			143.9			98.4			(149.7		)	100.6
Long-term deferred revenue - related parties	—			—			4			—			4
Long-term debt	635			—			9			—			644
Deferred credits and other liabilities	1.7			—			0.5			—			2.2
Total liabilities	644.7			143.9			111.9			(149.7		)	750.8
Equity
MPLX LP partners’ capital	458.2			1,098.50			1,240.20			(2,338.7		)	458.2
Noncontrolling interest retained by MPC	—			—			—			5.5			5.5
Total equity	458.2			1,098.50			1,240.20			(2,333.2		)	463.7
Total liabilities and equity	$	1,102.90		$	1,242.40		$	1,352.10		$	(2,482.9	)	$	1,214.50
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
Consolidating Statements of Cash Flow
	Year Ended December 31, 2014
(in millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
Net cash provided by (used in) operating activities	$	106.9		$	94		$	248.1		$	(202.2	)	$	246.8
Investing activities:
Additions to property, plant and equipment	—			(0.3		)	(78.3		)	—			(78.6		)
Investment in Pipe Line Holdings	—			(910.0		)	—			910			—
Investment in MPLX Operations	(865.0		)	865			—			—			—
Loans to affiliates	—			—			(51.0		)	51			—
All other, net	—			—			3.5			—			3.5
Net cash provided by (used in) investing activities	(865.0		)	(45.3		)	(125.8		)	961			(75.1		)
Financing activities:
Proceeds from borrowings from affiliates	—			51			—			(51.0		)	—
Long-term debt - borrowings	880			280			—			—			1,160.00
- repayments	(245.0		)	(280.0		)	(0.9		)	—			(525.9		)
Debt issuance costs	(2.7		)	—			—			—			(2.7		)
Net proceeds from equity offerings	230.1			—			—			—			230.1
Quarterly distributions	(103.1		)	(106.4		)	(95.8		)	202.2			(103.1		)
Quarterly distributions to noncontrolling interest retained by MPC	—			—			(46.9		)	—			(46.9		)
Distributions related to purchase of additional interest in Pipe Line Holdings	—			—			—			(910.0		)	(910.0		)
Net cash provided by (used in) financing activities	759.3			(55.4		)	(143.6		)	(758.8		)	(198.5		)
Net decrease in cash and cash equivalents	1.2			(6.7		)	(21.3		)	—			(26.8		)
Cash and cash equivalents at beginning of period	—			7.2			46.9			—			54.1
Cash and cash equivalents at end of period	$	1.2		$	0.5		$	25.6		$	—		$	27.3
Consolidating Statements of Cash Flow
	Year Ended December 31, 2013
(in millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
Net cash provided by (used in) operating activities	$	76.8		$	102.5		$	208.8		$	(175.9	)	$	212.2
Investing activities:
Additions to property, plant and equipment	—			(0.2		)	(106.3		)	—			(106.5		)
Investment in Pipe Line Holdings	—			(100.0		)	—			100			—
Loans to affiliates	—			—			(100.0		)	100			—
Repayments of loans from affiliates	—			—			26			(26.0		)	—
Disposal of assets	—			—			0.2			—			0.2
All other, net	—			—			(7.3		)	—			(7.3		)
Net cash provided by (used in) investing activities	—			(100.2		)	(187.4		)	174			(113.6		)
Financing activities:
Proceeds from borrowings from affiliates	—			100			—			(100.0		)	—
Payments on borrowings from affiliates	—			(26.0		)	—			26			—
Long-term debt - repayments	—			—			(0.7		)	—			(0.7		)
Quarterly distributions	(77.8		)	(78.8		)	(97.1		)	175.9			(77.8		)
Quarterly distributions to noncontrolling interest retained by MPC	—			—			(82.7		)	—			(82.7		)
Distributions related to purchase of additional interest in Pipe Line Holdings	—			—			—			(100.0		)	(100.0		)
Net cash provided by (used in) financing activities	(77.8		)	(4.8		)	(180.5		)	1.9			(261.2		)
Net decrease in cash and cash equivalents	(1.0		)	(2.5		)	(159.1		)	—			(162.6		)
Cash and cash equivalents at beginning of period	1			9.7			206			—			216.7
Cash and cash equivalents at end of period	$	—		$	7.2		$	46.9		$	—		$	54.1
Consolidating Statements of Cash Flow
	Year Ended December 31, 2012
(in millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
Net cash provided by (used in) operating activities	$	(1.4	)	$	1.7		$	190.3		$	—		$	190.6
Investing activities:
Additions to property, plant and equipment	—			—			(135.6		)	—			(135.6		)
Distributions from (contributions to) subsidiaries	(202.0		)	10.4			191.6			—			—
Disposal of assets	—			—			1.3			—			1.3
Investments - repayments of loans receivable from a related party	—			—			221.7			—			221.7
Net cash provided by (used in) investing activities	(202.0		)	10.4			279			—			87.4
Financing activities:
Long-term debt - repayments	—			—			(0.7		)	—			(0.7		)
Debt issuance costs	—			(2.4		)	—			—			(2.4		)
Net proceeds from initial public offering	407.1			—			—			—			407.1
Proceeds from initial public offering distributed to MPC	(202.7		)	—			—			—			(202.7		)
Distributions to MPC	—			—			(262.7		)	—			(262.7		)
Net cash provided by (used in) financing activities	204.4			(2.4		)	(263.4		)	—			(61.4		)
Net increase in cash and cash equivalents	1			9.7			205.9			—			216.6
Cash and cash equivalents at beginning of period	—			—			0.1			—			0.1
Cash and cash equivalents at end of period	$	1		$	9.7		$	206		$	—		$	216.7

Select_Quarterly_Financial_Dat

Select Quarterly Financial Data	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Select Quarterly Financial Data	Select Quarterly Financial Data (Unaudited)
		2014								2013
	(In millions, except per unit data)	1st Qtr.		2nd Qtr.		3rd Qtr.		4th Qtr.		1st Qtr.		2nd Qtr.		3rd Qtr.		4th Qtr.
	Revenues	$	130.7	$	126.8	$	130.7	$	131.9	$	109.9	$	116.1	$	118.2	$	118.9
	Income from operations	56.3		44.3		44.2		38.5		35.5		35.2		39.8		36.5
	Net income	55.7		42.9		43.1		36.4		35.3		34.8		39.2		36.8
	Net income attributable to MPLX LP	34.2		28.8		29.1		29.2		17.6		18.6		21.5		20.2
	Net income attributable to MPLX LP per limited partner unit:
	Common - basic	$	0.41	$	0.37	$	0.37	$	0.38	$	0.26	$	0.26	$	0.29	$	0.27
	Common - diluted	0.41		0.37		0.37		0.38		0.26		0.26		0.29		0.27
	Subordinated - basic and diluted	0.41		0.37		0.37		0.33		0.21		0.23		0.29		0.27
	Distributions declared per limited partner common unit	$	0.3275	$	0.3425	$	0.3575	$	0.3825	$	0.2725	$	0.285	$	0.2975	$	0.3125
	Distributions declared:
	Limited partner units - Public	$	6.5	$	6.8	$	7.2	$	8.9	$	5.4	$	5.7	$	5.9	$	6.2
	Limited partner units - MPC	17.7		18.6		19.2		21.7		14.7		15.4		16.1		16.9
	General partner units - MPC	0.5		0.5		0.5		0.7		0.4		0.4		0.5		0.5
	Incentive distribution rights - MPC	0.3		0.6		1		1.7		—		—		—		0.1
	Total distributions declared	$	25	$	26.5	$	27.9	$	33	$	20.5	$	21.5	$	22.5	$	23.7

Summary_of_Principal_Accountin1

Summary of Principal Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Use of estimates	Use of estimates – The preparation of financial statements in accordance with generally accepted accounting principles requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities as of the date of the consolidated financial statements and the reported amounts of revenues and expenses during the respective reporting periods.
Principles applied in consolidation	Principles applied in consolidation – These consolidated financial statements include the accounts of our majority-owned and controlled subsidiaries. All significant intercompany transactions and accounts have been eliminated. We consolidate Pipe Line Holdings, in which we own the 99.5 percent general partner interest and record a noncontrolling interest for the 0.5 percent limited partner interest retained by MPC as of December 31, 2014.
Revenue recognition	Revenue recognition – Revenues are recognized for crude oil and product pipeline transportation based on the delivery of actual volumes transported at regulated tariff rates. When MPC ships volumes on our pipeline systems under a joint tariff with a third party, those revenues are recorded as sales and other operating revenues, and not as sales to related parties, because we receive payment from the third party. Revenues are recognized for crude oil and refined product storage as performed based on contractual rates. Operating fees received for operating pipeline systems are recognized as a component of other income in the period the service is performed.
	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. 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 quarters or eight quarters under the terms of the applicable transportation services agreement. The deficiency payments are initially recorded as deferred revenue - related parties. We recognize revenues for the deficiency payments at the earlier of when credits are used for volumes transported in excess of minimum 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. 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.
Cash and cash equivalents	Cash and cash equivalents – Cash and cash equivalents include cash on hand and on deposit and investments in highly liquid debt instruments with maturities generally of three months or less.
Restricted cash	Restricted cash - Restricted cash consists of cash advances to be used for the operation and maintenance of an operated pipeline system. At December 31, 2014 and 2013, the amount of restricted cash included in other current assets on the consolidated balance sheet was $3.8 million and $7.3 million, respectively.
Receivables	Receivables – Our receivables primarily consist of customer accounts receivable that are recorded at the invoiced amounts and do not bear interest. Account balances for these customer receivables are charged directly to bad debt expense when it becomes probable the receivable will not be collected.
Materials and supplies inventories	Materials and supplies inventories – Inventories consist of materials and supplies, and cost is determined primarily under the specific identification method.
Imbalances	Imbalances – Within our pipelines and storage assets we experience volume gains and losses due to pressure and temperature changes, evaporation and variances in meter readings and other measurement methods. Until settled, positive crude oil imbalances are recorded as other current assets and negative crude oil imbalances are recorded as accounts payable. Positive and negative product imbalances are settled monthly by cash payments.
Property, plant and equipment	Property, plant and equipment – Property, plant and equipment are recorded at cost and depreciated on a straight-line basis for groups of property having similar economic characteristics over the estimated useful lives. Such assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. If the sum of the expected undiscounted future cash flows from the use of the asset and its eventual disposition is less than the carrying amount of the asset, an impairment loss is recognized based on the fair value of the asset.
	When items of property, plant and equipment are sold or otherwise disposed of, any gains or losses are reported in the statement of income. Gains on the disposal of property, plant and equipment are recognized when earned, which is generally at the time of closing. If a loss on disposal is expected, such losses are recognized when the assets are classified as held for sale.
	Interest expense is capitalized for qualifying assets under construction. Capitalized interest costs are included in property, plant and equipment and are depreciated over the useful life of the related asset.
Goodwill	Goodwill – Goodwill represents the excess of the purchase price over the estimated fair value of the net assets acquired in the acquisition of a business. Goodwill was allocated to the Predecessor from MPC based on the fair market value of the Predecessor’s net property, plant and equipment relative to the fair market value of MPC Pipeline Transportation reporting unit’s net property, plant and equipment as of June 30, 2005, the date on which the transaction was completed. Such goodwill is not amortized, but rather is tested for impairment annually and when events or changes in circumstances indicate that the fair value of a reporting unit with goodwill has been reduced below carrying value. The fair value of the reporting unit is determined and compared to the book value of the reporting unit. If the fair value of the reporting unit is less than the book value, including goodwill, the implied fair value of goodwill is calculated. The excess, if any, of the book value over the implied fair value of goodwill is charged to net income.
Major maintenance activities	Major maintenance activities – Costs for planned integrity management projects are expensed in the period incurred. These types of costs include in-line inspection services, contractor repair services, materials and supplies, equipment rentals and labor costs.
Other taxes	Other taxes – Other taxes primarily include payroll taxes for periods prior to October 1, 2012 and real estate taxes.
Environmental costs	Environmental costs – Environmental expenditures are capitalized if the costs mitigate or prevent future contamination or if the costs improve environmental safety or efficiency of the existing assets. We recognize remediation costs and penalties when the responsibility to remediate is probable and the amount of associated costs can be reasonably estimated. The timing of remediation accruals coincides with completion of a feasibility study or the commitment to a formal plan of action. Remediation liabilities are accrued based on estimates of known environmental exposure. A receivable is recorded for environmental costs indemnified by MPC.
Asset retirement obligations	Asset retirement obligations – The fair value of asset retirement obligations is recognized in the period in which the obligations are incurred if a reasonable estimate of fair value can be made. Asset retirement obligations are recorded as long-term liabilities with an offsetting asset retirement cost recorded as an increase to the associated property, plant and equipment. The amounts recorded for such obligations are based on the most probable current cost projections. Asset retirement obligations have not been recognized for our assets because the fair value cannot be reasonably estimated since the settlement dates of the obligations are indeterminate. Such obligations will be recognized in the period when sufficient information becomes available to estimate a range of potential settlement dates. The asset retirement obligations principally include hazardous material disposal and removal or dismantlement requirements associated with the closure of our pipeline system and storage assets.
Income taxes	Income taxes – Following the Initial Offering, our operations are treated as a partnership for federal and state income tax purposes, with each partner being separately taxed on its share of the taxable income. Prior to the Initial Offering, the Predecessor’s taxable income was included in the consolidated U.S. federal income tax returns of MPC and in a number of consolidated state income tax returns. Therefore, we have excluded income taxes from these consolidated financial statements, except for certain state jurisdictions that tax partnerships.
Employee benefit plans	Employee benefit plans – The Partnership has no employees and effective October 1, 2012, we entered into two employee services agreements with MPC. For periods prior to October 1, 2012, employees of the Predecessor participated in the various employee benefit plans of MPC. These plans included a qualified, non-contributory defined benefit retirement plan, an employee savings plan, employee and retiree medical and life insurance plans, a dental plan and other such benefits. For the purposes of these consolidated financial statements, the Predecessor was considered to be participating in multiemployer benefit plans of MPC. As a participant in multiemployer benefit plans, the Predecessor recognized as expense in each period the required allocation from MPC, and it did not recognize any employee benefit plan liabilities. While the Predecessor was considered to participate in multiemployer plans of MPC, those benefit plans are not technically multiemployer plans. Therefore, we have not included the disclosures required for multiemployer plans.
Equity-based compensation arrangements	Equity-based compensation arrangements – The fair value of phantom unit awards granted to non-employee directors is based on the fair market value of MPLX LP common units on the date of grant. Our equity-based compensation expense is recognized at the time of grant for phantom units since they vest immediately and are not forfeitable.
	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 use a Monte Carlo valuation model to calculate a grant date fair value.
Net income per limited partner unit	Net income per limited partner unit - We use the two-class method when calculating the net income per unit applicable to limited partners, because we have more than one participating security. The classes of participating securities include common units, subordinated units, general partner units, certain equity-based compensation awards and incentive distribution rights. 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.
Net investment	Net investment – The net investment represented a net balance reflecting MPC’s initial investment in the Predecessor and subsequent adjustments resulting from the operations of the Predecessor and various transactions between the Predecessor and MPC. Transactions affecting the net investment include general, administrative and overhead allocations incurred by MPC that were allocated to the Predecessor. There were no terms of settlement or interest charges associated with the net investment balance.
Comprehensive income	Comprehensive income – We have reported no comprehensive income due to the absence of items of other comprehensive income in the periods presented.

Acquisition_Tables

Acquisition (Tables)	12 Months Ended
	Dec. 31, 2014
Business Combinations [Abstract]
Effects of Changes From Changes in Ownership Interest	Changes in MPLX LP’s equity resulting from changes in its ownership interest in Pipe Line Holdings were as follows:
	(In millions)	2014			2013			2012
	Net income attributable to MPLX LP	$	121.3		$	77.9		$	130.8
	Transfer to noncontrolling interest retained by MPC:
	Decrease in general partner-MPC equity for purchases of additional interest in Pipe Line Holdings	(637.7		)	(46.4		)	—
	Change from net income attributable to MPLX LP and transfer to noncontrolling interest retained by MPC	$	(516.4	)	$	31.5		$	130.8

Related_Party_Agreements_and_T1

Related Party Agreements and Transactions (Tables)	12 Months Ended
	Dec. 31, 2014
Related Party Transactions [Abstract]
Sales to Related Parties	Sales to related parties were as follows:
	(In millions)		2014			2013		2012
	MPC		$	450.9		$	384.2	$	367.8
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:
	(In millions)		2014			2013		2012
	MPC		$	21.8		$	17.6	$	12
	Centennial		1.1			1.1		1
	Muskegon		0.1			0.1		0.1
	Total		$	23		$	18.8	$	13.1
Summary of Charges for Services Included in Purchases from Related Parties	Charges for services included in purchases from related parties primarily relate to services that support our operations and maintenance activities. These charges were as follows:
	(In millions)		2014			2013		2012
	MPC		$	24.1		$	18	$	13.6
Summary of Related Party Charges for Services Included in General and Administrative Expenses	Charges for services included in general and administrative expenses primarily relate to services that support our executive management, accounting and human resources activities, and allocations of corporate overhead costs from MPC for periods prior to the Initial Offering. These charges were as follows:
	(In millions)		2014			2013		2012
	MPC		$	30.7		$	31.4	$	22.6
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:
	(In millions)		2014			2013		2012
	MPC		$	7.5		$	8	$	5.5
Summary of Allocated Related Party Employee Benefit Costs by Income Statement Line Item
	(In millions)		2014			2013		2012
	Purchases from related parties		$	73.4		$	76.6	$	30.8
	General and administrative expenses		23.6			15.7		23.1
	Total		$	97		$	92.3	$	53.9
Receivables from Related Parties	Receivables from related parties were as follows:
			December 31,
	(In millions)		2014			2013
	MPC		$	40.5		$	47.4
	Centennial		0.3			0.6
	Muskegon		0.2			0.3
	Total		$	41		$	48.3
Schedule Of Prepaid Assets With Related Parties	Prepaid assets with related parties included in other current assets on the consolidated balance sheets were as follows:
			December 31,
	(In millions)		2014			2013
	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:
		December 31,
	(In millions)	2014			2013
	MPC	$	—		$	0.2
Payables to Related Parties	Payables to related parties were as follows:
			December 31,
	(In millions)		2014			2013
	MPC		$	20.1		$	12.8
	Centennial		0.1			—
	Total		$	20.2		$	12.8
Schedule Of Related Party Deferred Revenue Table	The deferred revenue-related parties associated with the minimum volume deficiencies and project reimbursements were as follows:
		December 31,
	(In millions)	2014			2013
	Minimum volume deficiencies - MPC	$	29.9		$	34
	Project reimbursements - MPC	4.6			—
	Total	$	34.5		$	34

Net_Income_Per_Limited_Partner1

Net Income Per Limited Partner Unit (Tables)	12 Months Ended
	Dec. 31, 2014
Earnings Per Share [Abstract]
Schedule of Net Income Per Limited Partner Unit	Net income per limited partner unit is only calculated for the periods subsequent to the Initial Offering as no units were outstanding prior to October 31, 2012. Diluted net income per limited partner unit is the same as basic net income per limited partner unit as there were no potentially dilutive common or subordinated units outstanding as of December 31, 2014, 2013 or 2012.
	(In millions)	2014				2013				October 31, 2012 to December 31, 2012
	Net income attributable to MPLX LP subsequent to initial public offering	$	121.3			$	77.9			$	13.1
	Less: General partner’s distributions declared (including IDRs)(1)	5.8				1.9				0.3
	Limited partners’ distributions declared on common units(1)	54.5				43.2				6.5
	Limited partner’s distributions declared on subordinated	52.1				43.1				6.5
	units(1)
	Distributions less than (in excess of) net income attributable to MPLX LP	$	8.9			$	(10.3	)		$	(0.2	)
	(1)	On January 20, 2015, we announced the board of directors of our general partner had declared a quarterly cash distribution of $0.3825 per unit, totaling $33.0 million. This distribution was paid on February 13, 2015 to unitholders of record on February 3, 2015.
Schedule of Basic and Diluted Earnings Per Unit
			2014
	(In millions, except per-unit data)		General				Limited Partners’				Limited			Total
			Partner				Common Units				Partner’s
											Subordinated
											Units
	Basic and diluted net income attributable to MPLX LP per unit:
	Net income attributable to MPLX LP:
	Distributions declared (including IDRs)		$	5.8			$	54.5			$	52.1		$	112.4
	Distributions less than net income attributable to MPLX LP		2.2				3.4				3.3			8.9
	Net income attributable to MPLX LP		$	8			$	57.9			$	55.4		$	121.3
	Weighted average units outstanding:
	Basic		1.5				37.4				37			75.9
	Diluted		1.5				37.4				37			75.9
	Net income attributable to MPLX LP per limited partner unit:
	Basic						$	1.55			$	1.5
	Diluted						$	1.55			$	1.5
			2013
	(In millions, except per-unit data)		General				Limited Partners’				Limited			Total
			Partner				Common Units				Partner’s
											Subordinated
											Units
	Basic and diluted net income attributable to MPLX LP per unit:
	Net income attributable to MPLX LP:
	Distribution declared		$	1.9			$	43.2			$	43.1		$	88.2
	Distributions in excess of net income attributable to MPLX LP		(0.2		)		(4.4		)		(5.7		)	(10.3		)
	Net income attributable to MPLX LP		$	1.7			$	38.8			$	37.4		$	77.9
	Weighted average units outstanding:
	Basic		1.4				37				37			75.4
	Diluted		1.4				37				37			75.4
	Net income attributable to MPLX LP:
	Basic						$	1.05			$	1.01
	Diluted						$	1.05			$	1.01
			October 31, 2012 to December 31, 2012
	(In millions, except per-unit data)		General				Limited Partners’				Limited			Total
			Partner				Common Units				Partner’s
											Subordinated
											Units
	Basic and diluted net income attributable to MPLX LP per unit:
	Net income attributable to MPLX LP subsequent to initial public offering:
	Distribution declared		$	0.3			$	6.5			$	6.5		$	13.3
	Distributions in excess of net income attributable to MPLX LP subsequent to initial public offering		(0.1		)		—				(0.1		)	(0.2		)
	Net income attributable to MPLX LP subsequent to initial public offering		$	0.2			$	6.5			$	6.4		$	13.1
	Weighted average units outstanding:
	Basic		1.4				37				37			75.4
	Diluted		1.4				37				37			75.4
	Net income attributable to MPLX LP subsequent to initial public offering per limited partner unit:
	Basic						$	0.18			$	0.17
	Diluted						$	0.18			$	0.17

Equity_Tables

Equity (Tables)	12 Months Ended
	Dec. 31, 2014
Equity [Abstract]
Schedule of Stockholders Equity	The table below summarizes the changes in the number of units outstanding through December 31, 2014:
	(In units)	Common			Subordinated			General Partner			Total
	Units issued in initial public offering	36,951,515			36,951,515			1,508,225			75,411,255
	Balance at December 31, 2012	36,951,515			36,951,515			1,508,225			75,411,255
	Balance at December 31, 2013	36,951,515			36,951,515			1,508,225			75,411,255
	Unit-based compensation awards	15,479			—			316			15,795
	Contribution of interest in Pipe Line Holdings(1)(3)	2,924,104			—			59,676			2,983,780
	December 2014 equity offering(2)(3)	3,450,000			—			70,408			3,520,408
	Balance at December 31, 2014	43,341,098			36,951,515			1,638,625			81,931,238
	(1) Effective December 1, 2014, as discussed in Note 4, we accepted a contribution of 7.625 percent of outstanding partnership interests of Pipe Line Holdings from subsidiaries of MPC in exchange for the issuance of equity valued at $200.0 million. The equity consideration consists of 2,924,104 MPLX common units and was calculated by dividing $200.0 million by the average closing price for MPLX common units for the ten trading days preceding December 1, 2014, which was $68.397.
	(2) On December 8, 2014, we closed an equity offering of 3,450,000 common units representing limited partner interests, at a public offering price of $66.68 per unit. We used the net proceeds of $221.3 million to repay borrowings under our revolving credit facility and for general partnership purposes.
	(3) As a result of the contribution mentioned above and December 2014 equity offering, MPLX GP contributed $8.8 million in exchange for 130,084 general partner units to maintain its two percent general partnership interest.
Schedule of Incentive Distributions Made to Managing Members or General Partners by Distribution
									Marginal percentage interest
									in distributions
			Total quarterly distribution						Unitholders			General Partner
			per unit target amount
	Minimum quarterly distribution		$	0.2625					98	%		2	%
	First target distribution		above $0.2625				up to $0.301875		98	%		2	%
	Second target distribution		above $0.301875				up to $0.328125		85	%		15	%
	Third target distribution		above $0.328125				up to $0.393750		75	%		25	%
	Thereafter		above $0.393750						50	%		50	%
Schedule Of Calculation Of Net Income Applicable to Partners	Net Income Allocation - The following table presents the allocation of the general partner’s interest in net income attributable to MPLX LP:
	(In millions)	2014				2013				October 31, 2012 to December 31, 2012
	Net income attributable to MPLX LP	$	121.3			$	77.9			$	13.1
	Less: General partner's incentive distribution rights	3.6				0.1				—
	Net income attributable to MPLX LP available to general and limited partners	$	117.7			$	77.8			$	13.1
	General partner’s two percent interest in net income attributable to	$	2.3			$	1.6			$	0.2
	MPLX LP
	General partner's incentive distribution rights	3.6				0.1				—
	General partner's interest in net income attributable to MPLX LP	$	5.9			$	1.7			$	0.2
Distributions Made to Limited Partner, by Distribution	The allocation of total quarterly cash distributions to general and limited partners is as follows for the year ended December 31, 2014, 2013 and the period October 31, 2012 to December 31, 2012. 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.
	(In millions)	2014				2013				October 31, 2012 to December 31, 2012
	General partner's distributions:
	General partner's distributions	$	2.2			$	1.8			$	0.3
	General partner's incentive distribution rights	3.6				0.1				—
	Total general partner's distributions	$	5.8			$	1.9			$	0.3
	Limited partners' distributions:
	Common unitholders	$	54.5			$	43.2			$	6.5
	Subordinated unitholders	52.1				43.1				6.5
	Total limited partners' distributions	106.6				86.3				13
	Total cash distributions declared	$	112.4			$	88.2			$	13.3

Other_Items_Tables

Other Items (Tables)	12 Months Ended
	Dec. 31, 2014
Other Income and Expenses [Abstract]
Summary of Interest and Other Financial Income (Costs)	Net interest and other financial costs were:
	(In millions)	2014			2013			2012
	Interest income	$	—		$	(0.3	)	$	(0.1	)
	Interest expense	5			1.1			0.8
	Interest capitalized	(0.9		)	(0.9		)	(0.7		)
	Other financial costs	1.2			1.2			0.2
	Net interest and other financial costs	$	5.3		$	1.1		$	0.2

Income_Taxes_Tables

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Schedule of Unrecognized Tax Benefits	The following table summarizes the activity in unrecognized tax benefits:
	(In millions)	2014		2013			2012
	January 1 balance	$	—	$	0.2		$	—
	Additions (reductions) for tax positions of prior years	—		(0.2		)	0.2
	December 31 balance	$	—	$	—		$	0.2

Property_Plant_and_Equipment_T

Property, Plant and Equipment (Tables)	12 Months Ended
	Dec. 31, 2014
Property, Plant and Equipment [Abstract]
Summary of Property, Plant and Equipment
		Estimated	December 31,
	(In millions)	Useful Lives	2014		2013
	Land		$	5.5	$	5.3
	Pipelines and related assets	19 - 42 years	1,081.40		1,060.70
	Storage and delivery facilities	24 - 37 years	165.8		165.7
	Other	10 - 25 years	23.7		22.8
	Assets under construction		85.3		21.9
	Total		1,361.70		1,276.40
	Less accumulated depreciation		353.1		309.8
	Property, plant and equipment, net		$	1,008.60	$	966.6

Fair_Value_Measurements_Fair_V

Fair Value Measurements Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Fair Value, by Balance Sheet Grouping	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 December 31, 2014 and 2013.
		December 31,
		2014				2013
	(In millions)	Fair Value		Carrying Value		Fair Value		Carrying Value
	Long-term debt	$	635.6	$	635	$	—	$	—

Debt_Tables

Debt (Tables)	12 Months Ended
	Dec. 31, 2014
Debt Disclosure [Abstract]
Summary of Outstanding Borrowings	Our outstanding borrowings at December 31, 2014 and 2013 consisted of the following:
			December 31,
	(In millions)		2014		2013
	MPLX Operations - bank revolving credit agreement due 2017		$	—	$	—
	MPLX - bank revolving credit facility due 2019		385		—
	MPLX - term loan facility due 2019		250		—
	MPL - capital lease obligations due 2020		9.8		10.5
	Total		644.8		10.5
	Amounts due within one year		0.8		0.7
	Total long-term debt due after one year		$	644	$	9.8
Schedule of Debt Payments	The following table shows five years of scheduled debt payments.
	(In millions)
	2015	$	0.8
	2016	0.9
	2017	0.9
	2018	1
	2019	636

Supplemental_Cash_Flow_Informa1

Supplemental Cash Flow Information (Tables)	12 Months Ended
	Dec. 31, 2014
Supplemental Cash Flow Elements [Abstract]
Summary of Supplemental Cash Flow Information
	(In millions)		2014			2013			2012
	Net cash provided by operating activities included:
	Interest paid (net of amounts capitalized)		$	2.9		$	—		$	—
	Income taxes paid (refunded)		(0.3		)	0.1			0.2
	Non-cash investing and financing activities:
	Net transfers of property, plant and equipment to materials and		$	1		$	4		$	—
	supplies inventories
	Property, plant and equipment contributed by MPC		0.1			0.3			—
	Contribution - common units issued		200			—			—
	Distribution to MPC in conjunction with the Initial Offering
	Receivables from related parties		—			—			(5.7		)
	Materials and supplies inventories		—			—			(0.3		)
	Property, plant and equipment		—			—			(187.7		)
	Goodwill		—			—			(29.5		)
	Other noncurrent assets		—			—			(2.2		)
	Accounts payable		—			—			(0.3		)
	Accrued taxes		—			—			(1.0		)
	Contribution from MPC in conjunction with the Initial Offering
	Receivables from related parties		$	—		$	—		$	7.4
	Property, plant and equipment		—			—			121.4
	Other noncurrent assets		—			—			0.3
	Payables to related parties		—			—			3.9
	Accrued taxes		—			—			0.1
Summary of Reconciliation of Additions to Property, Plant and Equipment to Total Capital Expenditures	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:
	(In millions)		2014			2013			2012
	Additions to property, plant and equipment		$	78.6		$	106.5		$	135.6
	Plus: Increase (decrease) in capital accruals		12.9			(5.3		)	3.2
	Asset retirement expenditures		1.6			8.3			9.2
	Total capital expenditures		$	93.1		$	109.5		$	148
Schedule of Sale of Stock by Subsidiary or Equity Method Investee Disclosure	The following is a reconciliation of cash proceeds from the Partnership’s initial public offering, which closed on October 31, 2012:
	(In millions)
	Total proceeds from the Initial Offering	$	437.7
	Less: Offering Costs	(30.6		)
	Net proceeds from the Initial Offering	407.1
	Less: Revolving credit facility origination fees	(2.4		)
	Cash retained by MPLX LP	(10.4		)
	Cash contribution to Pipe Line Holdings	(191.6		)
	Net proceeds distributed to MPC from the Initial Offering	$	202.7
Summary of Reconciliation of Contributions from (Distributions to) Former Parent Company	The following is a reconciliation of contributions from (distributions to) MPC:
	(In millions)		2014			2013			2012
	Contributions from (distributions to) MPC per consolidated statements of cash flows		$	—		$	—		$	(262.7	)
	Net non-cash contributions from (distributions to) MPC		0.1			0.3			(98.2		)
	Contributions from (distributions to) MPC per consolidated statements of equity/net investment		$	0.1		$	0.3		$	(360.9	)

EquityBased_Compensation_Plan_

Equity-Based Compensation Plan (Tables)	12 Months Ended
	Dec. 31, 2014
Phantom Units [Member]
Equity Transactions And Share Based Compensation [Line Items]
Summary of Phantom Unit Award Activity	The following is a summary of phantom unit award activity of MPLX LP common limited partner units in 2014:
			Phantom Units
			Number			Weighted			Aggregate Intrinsic Value (In millions)
			of Units			Average
						Fair Value
	Outstanding at December 31, 2013		77,754			$	33.84
	Granted		51,904			49.56
	Settled		(27,360	)		34.45
	Forfeited		(1,529	)		40.91
	Outstanding at December 31, 2014		100,769			41.66
	Vested and expected to vest at December 31, 2014		99,566			41.62			$	7.3
	Convertible at December 31, 2014		25,892			42.21			$	1.9
Schedule of Share-based Compensation Arrangement by Share-based Payment Award, Restricted Stock Units, Vested and Expected to Vest	The following is a summary of the values related to phantom units held by officers and non-employee directors:
		Phantom Units
		Intrinsic Value of Units Issued During the Period (in millions)				Weighted Average Grant Date Fair Value of Units Granted During the Period
	2014	$	1.3			$	49.56
	2013	—				—
	2012	—				—
Performance Shares [Member]
Equity Transactions And Share Based Compensation [Line Items]
Summary of Phantom Unit Award Activity	The following is a summary of activity of performance unit awards paying out in MPLX LP common limited partner units in 2014:
		Performance Units
		Number of Units			Weighted
					Average
					Fair Value
	Outstanding at December 31, 2013	436,917			$	0.77
	Granted	500,507			1.16
	Forfeited	(13,281	)		1.05
	Outstanding at December 31, 2014	924,143			0.98
Schedule of Share-based Compensation, Performance Unit Awards, Valuation Assumptions
		2014				2013
	Risk-free interest rate	0.63		%		0.35		%
	Look-back period	2.84 years				2.84 years
	Expected volatility	17.17		%		16.75		%
	Grant date fair value of performance units granted	$	1.16			$	0.77

Leases_Tables

Leases (Tables)	12 Months Ended
	Dec. 31, 2014
Leases [Abstract]
Future Minimum Commitments	Future minimum commitments as of December 31, 2014, for capital lease obligations and for operating lease obligations having initial or remaining non-cancelable lease terms in excess of one year are as follows:
	(In millions)	Capital		Operating
		Lease		Lease
		Obligations		Obligations
	2015	$	1.4	$	9.6
	2016	1.4		9.5
	2017	1.4		8.7
	2018	1.4		8.4
	2019	1.4		3.7
	Later years	5.1		7.7
	Total minimum lease payments	12.1		$	47.6
	Less imputed interest costs	2.3
	Present value of net minimum lease payments	$	9.8
Operating Lease Rental Expense	Operating lease rental expense was:
	(In millions)	2014		2013		2012
	Minimum rental expense	$	10.4	$	9.9	$	7

Condensed_Consolidating_Financ1

Condensed Consolidating Financial Statement (Tables)	12 Months Ended
	Dec. 31, 2014
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Condensed Financial Statements
	Consolidating Statements of Income
		Year Ended December 31, 2014
	(in millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
	Revenues and other income:
	Sales and other operating revenues	$	—		$	—		$	69.2		$	—		$	69.2
	Sales to related parties	—			—			450.9			—			450.9
	Other income	—			—			5.2			—			5.2
	Other income - related parties	—			—			23			—			23
	Equity in earnings of subsidiaries	130.6			136.1			—			(266.7		)	—
	Total revenues and other income	130.6			136.1			548.3			(266.7		)	548.3
	Costs and expenses:
	Cost of revenues (excludes items below)	—			—			145.3			—			145.3
	Purchases from related parties	—			—			97.5			—			97.5
	Depreciation	—			—			50.2			—			50.2
	General and administrative expenses	7.6			—			57.2			—			64.8
	Other taxes	0.1			—			7.1			—			7.2
	Total costs and expenses	7.7			—			357.3			—			365
	Income from operations	122.9			136.1			191			(266.7		)	183.3
	Net interest and other financial costs (income)	1.6			5.5			(1.8		)	—			5.3
	Income before income taxes	121.3			130.6			192.8			(266.7		)	178
	Benefit for income taxes	—			—			(0.1		)	—			(0.1		)
	Net income	121.3			130.6			192.9			(266.7		)	178.1
	Less: Net income attributable to MPC-retained interest	—			—			—			56.8			56.8
	Net income attributable to MPLX LP	$	121.3		$	130.6		$	192.9		$	(323.5	)	$	121.3
	Consolidating Statements of Income
		Year Ended December 31, 2013
	(in millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
	Revenues and other income:
	Sales and other operating revenues	$	—		$	—		$	78.9		$	—		$	78.9
	Sales to related parties	—			—			384.2			—			384.2
	Other income	—			—			4.4			—			4.4
	Other income - related parties	—			—			18.8			—			18.8
	Equity in earnings of subsidiaries	83			85.3			—			(168.3		)	—
	Total revenues and other income	83			85.3			486.3			(168.3		)	486.3
	Costs and expenses:
	Cost of revenues (excludes items below)	—			—			135.9			—			135.9
	Purchases from related parties	—			—			94.6			—			94.6
	Depreciation	—			—			48.9			—			48.9
	General and administrative expenses	5			—			48.7			—			53.7
	Other taxes	0.1			—			6.1			—			6.2
	Total costs and expenses	5.1			—			334.2			—			339.3
	Income from operations	77.9			85.3			152.1			(168.3		)	147
	Net interest and other financial costs (income)	—			2.3			(1.2		)	—			1.1
	Income before income taxes	77.9			83			153.3			(168.3		)	145.9
	Benefit for income taxes	—			—			(0.2		)	—			(0.2		)
	Net income	77.9			83			153.5			(168.3		)	146.1
	Less: Net income attributable to MPC-retained interest	—			—			—			68.2			68.2
	Net income attributable to MPLX LP	$	77.9		$	83		$	153.5		$	(236.5	)	$	77.9
	Consolidating Statements of Income
		Year Ended December 31, 2012
	(in millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
	Revenues and other income:
	Sales and other operating revenues	$	—		$	—		$	74.4		$	—		$	74.4
	Sales to related parties	—			—			367.8			—			367.8
	Loss on sale of assets	—			—			(0.3		)	—			(0.3		)
	Other income	—			—			6.9			—			6.9
	Other income - related parties	—			—			13.1			—			13.1
	Equity in earnings of subsidiaries	14			14.2			—			(28.2		)	—
	Total revenues and other income	14			14.2			461.9			(28.2		)	461.9
	Costs and expenses:
	Cost of revenues (excludes items below)	—			—			173.8			—			173.8
	Purchases from related parties	—			—			44.4			—			44.4
	Depreciation	—			—			39.4			—			39.4
General and administrative expenses	0.9			—			48.9			—			49.8
Other taxes	—			—			11.3			—			11.3
Total costs and expenses	0.9			—			317.8			—			318.7
Income from operations	13.1			14.2			144.1			(28.2		)	143.2
Related party interest and other financial income	—			—			1.3			—			1.3
Net interest and other financial costs	—			0.2			—			—			0.2
Income before income taxes	13.1			14			145.4			(28.2		)	144.3
Provision for income taxes	—			—			0.3			—			0.3
Net income	13.1			14			145.1			(28.2		)	144
Less: Net income attributable to MPC-retained interest	—			—			—			13.2			13.2
Net income attributable to MPLX LP	13.1			14			145.1			(41.4		)	130.8
Less: Predecessor income prior to initial public offering on October 31, 2012	—			—			117.7			—			117.7
Net income attributable to MPLX LP subsequent to initial public offering	$	13.1		$	14		$	27.4		$	(41.4	)	$	13.1
Consolidating Balance Sheets
	31-Dec-14
(in millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
Assets
Current assets:
Cash and cash equivalents	$	1.2		$	0.5		$	25.6		$	—		$	27.3
Receivables	0.1			—			10.1			—			10.2
Receivables from related parties	—			5.9			184.8			(149.7		)	41
Materials and supplies inventories	—			—			11.7			—			11.7
Other current assets	0.4			—			6.6			—			7
Total current assets	1.7			6.4			238.8			(149.7		)	97.2
Property, plant and equipment, net	—			—			1,008.60			—			1,008.60
Investment in subsidiaries	1,098.50			1,234.70			—			(2,333.2		)	—
Goodwill	—			—			104.7			—			104.7
Other noncurrent assets	2.7			1.3			—			—			4
Total assets	$	1,102.90		$	1,242.40		$	1,352.10		$	(2,482.9	)	$	1,214.50
Liabilities
Current liabilities:
Accounts payable	$	1		$	—		$	41.2		$	—		$	42.2
Payables to related parties	6.3			143.9			19.7			(149.7		)	20.2
Deferred revenue - related parties	—			—			30.5			—			30.5
Accrued taxes	0.1			—			5.1			—			5.2
Long-term debt due within one year	—			—			0.8			—			0.8
Other current liabilities	0.6			—			1.1			—			1.7
Total current liabilities	8			143.9			98.4			(149.7		)	100.6
Long-term deferred revenue - related parties	—			—			4			—			4
Long-term debt	635			—			9			—			644
Deferred credits and other liabilities	1.7			—			0.5			—			2.2
Total liabilities	644.7			143.9			111.9			(149.7		)	750.8
Equity
MPLX LP partners’ capital	458.2			1,098.50			1,240.20			(2,338.7		)	458.2
Noncontrolling interest retained by MPC	—			—			—			5.5			5.5
Total equity	458.2			1,098.50			1,240.20			(2,333.2		)	463.7
Total liabilities and equity	$	1,102.90		$	1,242.40		$	1,352.10		$	(2,482.9	)	$	1,214.50
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
Consolidating Statements of Cash Flow
	Year Ended December 31, 2014
(in millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
Net cash provided by (used in) operating activities	$	106.9		$	94		$	248.1		$	(202.2	)	$	246.8
Investing activities:
Additions to property, plant and equipment	—			(0.3		)	(78.3		)	—			(78.6		)
Investment in Pipe Line Holdings	—			(910.0		)	—			910			—
Investment in MPLX Operations	(865.0		)	865			—			—			—
Loans to affiliates	—			—			(51.0		)	51			—
All other, net	—			—			3.5			—			3.5
Net cash provided by (used in) investing activities	(865.0		)	(45.3		)	(125.8		)	961			(75.1		)
Financing activities:
Proceeds from borrowings from affiliates	—			51			—			(51.0		)	—
Long-term debt - borrowings	880			280			—			—			1,160.00
- repayments	(245.0		)	(280.0		)	(0.9		)	—			(525.9		)
Debt issuance costs	(2.7		)	—			—			—			(2.7		)
Net proceeds from equity offerings	230.1			—			—			—			230.1
Quarterly distributions	(103.1		)	(106.4		)	(95.8		)	202.2			(103.1		)
Quarterly distributions to noncontrolling interest retained by MPC	—			—			(46.9		)	—			(46.9		)
Distributions related to purchase of additional interest in Pipe Line Holdings	—			—			—			(910.0		)	(910.0		)
Net cash provided by (used in) financing activities	759.3			(55.4		)	(143.6		)	(758.8		)	(198.5		)
Net decrease in cash and cash equivalents	1.2			(6.7		)	(21.3		)	—			(26.8		)
Cash and cash equivalents at beginning of period	—			7.2			46.9			—			54.1
Cash and cash equivalents at end of period	$	1.2		$	0.5		$	25.6		$	—		$	27.3
Consolidating Statements of Cash Flow
	Year Ended December 31, 2013
(in millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
Net cash provided by (used in) operating activities	$	76.8		$	102.5		$	208.8		$	(175.9	)	$	212.2
Investing activities:
Additions to property, plant and equipment	—			(0.2		)	(106.3		)	—			(106.5		)
Investment in Pipe Line Holdings	—			(100.0		)	—			100			—
Loans to affiliates	—			—			(100.0		)	100			—
Repayments of loans from affiliates	—			—			26			(26.0		)	—
Disposal of assets	—			—			0.2			—			0.2
All other, net	—			—			(7.3		)	—			(7.3		)
Net cash provided by (used in) investing activities	—			(100.2		)	(187.4		)	174			(113.6		)
Financing activities:
Proceeds from borrowings from affiliates	—			100			—			(100.0		)	—
Payments on borrowings from affiliates	—			(26.0		)	—			26			—
Long-term debt - repayments	—			—			(0.7		)	—			(0.7		)
Quarterly distributions	(77.8		)	(78.8		)	(97.1		)	175.9			(77.8		)
Quarterly distributions to noncontrolling interest retained by MPC	—			—			(82.7		)	—			(82.7		)
Distributions related to purchase of additional interest in Pipe Line Holdings	—			—			—			(100.0		)	(100.0		)
Net cash provided by (used in) financing activities	(77.8		)	(4.8		)	(180.5		)	1.9			(261.2		)
Net decrease in cash and cash equivalents	(1.0		)	(2.5		)	(159.1		)	—			(162.6		)
Cash and cash equivalents at beginning of period	1			9.7			206			—			216.7
Cash and cash equivalents at end of period	$	—		$	7.2		$	46.9		$	—		$	54.1
Consolidating Statements of Cash Flow
	Year Ended December 31, 2012
(in millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
Net cash provided by (used in) operating activities	$	(1.4	)	$	1.7		$	190.3		$	—		$	190.6
Investing activities:
Additions to property, plant and equipment	—			—			(135.6		)	—			(135.6		)
Distributions from (contributions to) subsidiaries	(202.0		)	10.4			191.6			—			—
Disposal of assets	—			—			1.3			—			1.3
Investments - repayments of loans receivable from a related party	—			—			221.7			—			221.7
Net cash provided by (used in) investing activities	(202.0		)	10.4			279			—			87.4
Financing activities:
Long-term debt - repayments	—			—			(0.7		)	—			(0.7		)
Debt issuance costs	—			(2.4		)	—			—			(2.4		)
Net proceeds from initial public offering	407.1			—			—			—			407.1
Proceeds from initial public offering distributed to MPC	(202.7		)	—			—			—			(202.7		)
Distributions to MPC	—			—			(262.7		)	—			(262.7		)
Net cash provided by (used in) financing activities	204.4			(2.4		)	(263.4		)	—			(61.4		)
Net increase in cash and cash equivalents	1			9.7			205.9			—			216.6
Cash and cash equivalents at beginning of period	—			—			0.1			—			0.1
Cash and cash equivalents at end of period	$	1		$	9.7		$	206		$	—		$	216.7

Select_Quarterly_Financial_Dat1

Select Quarterly Financial Data (Tables)	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Summary of Quarterly Financial Data
		2014								2013
	(In millions, except per unit data)	1st Qtr.		2nd Qtr.		3rd Qtr.		4th Qtr.		1st Qtr.		2nd Qtr.		3rd Qtr.		4th Qtr.
	Revenues	$	130.7	$	126.8	$	130.7	$	131.9	$	109.9	$	116.1	$	118.2	$	118.9
	Income from operations	56.3		44.3		44.2		38.5		35.5		35.2		39.8		36.5
	Net income	55.7		42.9		43.1		36.4		35.3		34.8		39.2		36.8
	Net income attributable to MPLX LP	34.2		28.8		29.1		29.2		17.6		18.6		21.5		20.2
	Net income attributable to MPLX LP per limited partner unit:
	Common - basic	$	0.41	$	0.37	$	0.37	$	0.38	$	0.26	$	0.26	$	0.29	$	0.27
	Common - diluted	0.41		0.37		0.37		0.38		0.26		0.26		0.29		0.27
	Subordinated - basic and diluted	0.41		0.37		0.37		0.33		0.21		0.23		0.29		0.27
	Distributions declared per limited partner common unit	$	0.3275	$	0.3425	$	0.3575	$	0.3825	$	0.2725	$	0.285	$	0.2975	$	0.3125
	Distributions declared:
	Limited partner units - Public	$	6.5	$	6.8	$	7.2	$	8.9	$	5.4	$	5.7	$	5.9	$	6.2
	Limited partner units - MPC	17.7		18.6		19.2		21.7		14.7		15.4		16.1		16.9
	General partner units - MPC	0.5		0.5		0.5		0.7		0.4		0.4		0.5		0.5
	Incentive distribution rights - MPC	0.3		0.6		1		1.7		—		—		—		0.1
	Total distributions declared	$	25	$	26.5	$	27.9	$	33	$	20.5	$	21.5	$	22.5	$	23.7

Description_of_the_Business_an1

Description of the Business and Basis of Presentation (Detail)	0 Months Ended		12 Months Ended	0 Months Ended		1 Months Ended	4 Months Ended	9 Months Ended	10 Months Ended	12 Months Ended
	Oct. 31, 2012	Mar. 27, 2012	Dec. 31, 2014	Oct. 31, 2012	Dec. 02, 2014	Dec. 31, 2014	Apr. 30, 2013	Nov. 30, 2014	Feb. 28, 2014	Dec. 31, 2012	Oct. 01, 2012
			bbl	Pipeline							Agreement
			Segment
Description Of Business And Basis Of Presentation [Line Items]
Storage capacity (in number of barrels)			1,000,000
Date of partnership formation		27-Mar-12
Date for initial public offering completed	31-Oct-12
Units issued in initial public offering	75,411,255
Number of segments			1
MPC [Member]
Description Of Business And Basis Of Presentation [Line Items]
Number of pipeline systems				2
MPC [Member] | Employee Services Agreements [Member]
Description Of Business And Basis Of Presentation [Line Items]
Number of agreements			2			2					2
Limited Partners Common Units [Member]
Description Of Business And Basis Of Presentation [Line Items]
Units issued in initial public offering	36,951,515
MPLX Pipe Line Holdings LP [Member]
Description Of Business And Basis Of Presentation [Line Items]
Ownership percentage					99.50%	99.50%	51.00%	69.00%	56.00%
MPLX Pipe Line Holdings LP [Member] | MPL and ORPL [Member]
Description Of Business And Basis Of Presentation [Line Items]
Ownership percentage			100.00%
Predecessor [Member]
Description Of Business And Basis Of Presentation [Line Items]
Percentage of net income related to assets										100.00%
Crude Oil And Product Pipeline Systems [Member]
Description Of Business And Basis Of Presentation [Line Items]
Ownership percentage						99.50%
Storage Services Butane Cavern [Member]
Description Of Business And Basis Of Presentation [Line Items]
Ownership percentage			100.00%
Public [Member] | Limited Partners Common Units [Member]
Description Of Business And Basis Of Presentation [Line Items]
Units issued in initial public offering	19,895,000

Summary_of_Principal_Accountin2

Summary of Principal Accounting Policies (Detail) (USD $)	12 Months Ended		0 Months Ended	1 Months Ended	4 Months Ended	9 Months Ended	10 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 02, 2014	Dec. 31, 2014	Apr. 30, 2013	Nov. 30, 2014	Feb. 28, 2014	Oct. 01, 2012
	Employees							Agreement
Summary of Principal Accounting Policies [Line Items]
Restricted cash	$3,800,000	$7,300,000		3,800,000
Number of employees	0			0
Comprehensive income	$0	$0
MPC [Member] | Employee Services Agreements [Member]
Summary of Principal Accounting Policies [Line Items]
Number of agreements	2			2				2
MPLX Pipe Line Holdings LP [Member]
Summary of Principal Accounting Policies [Line Items]
Ownership percentage			99.50%	99.50%	51.00%	69.00%	56.00%
Retained Interest [Member] | MPC [Member]
Summary of Principal Accounting Policies [Line Items]
Percentage of net income related to assets				0.50%

Acquisition_Narrative_Details

Acquisition (Narrative) (Details) (USD $)	0 Months Ended			12 Months Ended		1 Months Ended	4 Months Ended	9 Months Ended	10 Months Ended
In Millions, unless otherwise specified	Dec. 02, 2014	Mar. 02, 2014	2-May-13	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2014	Apr. 30, 2013	Nov. 30, 2014	Feb. 28, 2014	Dec. 01, 2014
Business Acquisition [Line Items]
Percentage of partners interest acquired	22.88%	13.00%	5.00%
Payments to acquire businesses	$600	$310	$100	$910	$100
Percentage Of Partners Interest Contributed	7.63%
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned										200
Percentage of Partners Interest Purchased and Contributed	30.50%
Purchase/contribution of additional interest in Pipe Line Holdings				-910	-100
Noncontrolling Interest [Member]
Business Acquisition [Line Items]
Purchase/contribution of additional interest in Pipe Line Holdings	334.8	137.5	53.6
General Partner [Member]
Business Acquisition [Line Items]
Decrease in general partner equity	465.2	172.5	46.4	637.7	46.4
MPC [Member] | Noncontrolling Interest [Member]
Business Acquisition [Line Items]
Purchase/contribution of additional interest in Pipe Line Holdings				-472.4	-53.6
Ownership percentage	0.50%
MPLX Pipe Line Holdings LP [Member]
Business Acquisition [Line Items]
Ownership percentage	99.50%					99.50%	51.00%	69.00%	56.00%
Pipe Line Holding [Member]
Business Acquisition [Line Items]
Effective date of acquisition	1-Dec-14	1-Mar-14	1-May-13
Cash [Member] | Pipe Line Holding [Member]
Business Acquisition [Line Items]
Payments to acquire businesses		40
Revolving Credit Facility [Member] | Pipe Line Holding [Member]
Business Acquisition [Line Items]
Payments to acquire businesses		$270

Acquisition_Summary_of_Changes

Acquisition (Summary of Changes in MPLX LP's Ownership Interest in Pipe Line Holdings) (Details) (USD $)

2 Months Ended

3 Months Ended

12 Months Ended

0 Months Ended

In Millions, unless otherwise specified

Dec. 31, 2012

Dec. 31, 2014

Sep. 30, 2014

Jun. 30, 2014

Mar. 31, 2014

Dec. 31, 2013

Sep. 30, 2013

Jun. 30, 2013

Mar. 31, 2013

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Dec. 02, 2014

Mar. 02, 2014

2-May-13

Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]

Net income attributable to MPLX LP

$13.10

$29.20

$29.10

$28.80

$34.20

$20.20

$21.50

$18.60

$17.60

$121.30

$77.90

$13.10

Transfer to noncontrolling interest retained by MPC:

Change from net income attributable to MPLX LP and transfer to noncontrolling interest retained by MPC

-516.4

31.5

Predecessor and Successor [Member]

Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]

Net income attributable to MPLX LP

130.8

Transfer to noncontrolling interest retained by MPC:

Change from net income attributable to MPLX LP and transfer to noncontrolling interest retained by MPC

130.8

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

-637.7

-46.4

-465.2

-172.5

-46.4

General Partner [Member] | Predecessor and Successor [Member]

Transfer to noncontrolling interest retained by MPC:

Decrease in general partner-MPC equity for purchases of additional interest in Pipe Line Holdings

$0

Related_Party_Agreements_and_T2

Related Party Agreements and Transactions (Narrative) (Detail) (USD $)	12 Months Ended		0 Months Ended	12 Months Ended	2 Months Ended	9 Months Ended	0 Months Ended	1 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Mar. 31, 2014	Dec. 31, 2012	Dec. 31, 2012	Sep. 30, 2012	Oct. 31, 2012	Oct. 31, 2012	Sep. 28, 2012
							Pipeline
Operating Agreements
Sales to related parties	$450.90	$384.20
Related Party Transactions
Related party interest and other financial income	0	0
Non-cash contribution from MPC		0.3
Related Party Revolving Credit Agreement [Member]
Loan Agreement [Abstract]
Line of credit facility initiation date for MPCIF			31-Mar-14
Line of Credit Facility, Current Borrowing Capacity	50
Expiration date	31-Mar-19
Description of variable rate basis			one-month term London Interbank Offered Rate (bLIBORb) plus 1.375 percent
Long-term Line of Credit	0
Predecessor and Successor [Member]
Operating Agreements
Sales to related parties				367.8
Related Party Transactions
Related party interest and other financial income				1.3
Non-cash contribution from MPC				0
General and administrative expenses [Member]
Related Party Transactions
Stock-based compensation plans expenses	1	1			0.2
General and administrative expenses [Member] | Predecessor and Successor [Member]
Related Party Transactions
Stock-based compensation plans expenses						1.1
General and administrative expenses [Member] | Predecessor [Member]
Related Party Transactions
Pension expenses						9.5
Stock-based compensation plans expenses						0.9
Five Year Transportation Services Agreement [Member] | Commercial Agreements [Member] | Handling Crude Oil And Products [Member]
Commercial Agreements
Number of agreements	1
Ten Year Storage Services Agreement [Member] | Commercial Agreements [Member] | Storage Services Butane Cavern [Member]
Commercial Agreements
Number of agreements	1
MPC [Member]
Operating Agreements
Sales to related parties	450.9	384.2
Related Party Transactions
Other Significant Noncash Transaction Value Of Consideration Given Or Received	-0.1	-0.3
Non-cash contribution from MPC	0.1	0.3
Minority interest distributions limited partnerships								224.1
Number of pipeline systems							2
Contribution from subsidiary								121.4
Contribution of net assets from former parent related to balancing of working capital and indemnification of environmental liabilities							3.7	3.7
MPC [Member] | Predecessor and Successor [Member]
Operating Agreements
Sales to related parties				367.8
Related Party Transactions
Other Significant Noncash Transaction Value Of Consideration Given Or Received				98.2
MPC [Member] | Operating Agreements [Member] | Minimum [Member]
Commercial Agreements
Duration of agreements (in years)	1 year
MPC [Member] | Operating Agreements [Member] | Maximum [Member]
Commercial Agreements
Duration of agreements (in years)	5 years
MPC [Member] | Management Services Agreements [Member]
Commercial Agreements
Number of agreements	2
Management Services Agreements
Fees received for providing management services	0.8
MPC [Member] | Employee Services Agreements [Member]
Commercial Agreements
Number of agreements	2
MPC [Member] | Ten Year Transportation Services Agreement [Member] | Commercial Agreements [Member] | Crude oil [Member]
Commercial Agreements
Number of agreements	3
Duration of agreements (in years)	10 years
MPC [Member] | Ten Year Transportation Services Agreement [Member] | Commercial Agreements [Member] | Pipelines products [Member]
Commercial Agreements
Number of agreements	4
Duration of agreements (in years)	10 years
MPC [Member] | Five Year Transportation Services Agreement [Member] | Commercial Agreements [Member] | Crude oil [Member]
Commercial Agreements
Number of agreements	1
Duration of agreements (in years)	5 years
MPC [Member] | Five Year Transportation Services Agreement [Member] | Commercial Agreements [Member] | Handling Crude Oil And Products [Member]
Commercial Agreements
Duration of agreements (in years)	5 years
MPC [Member] | Ten Year Storage Services Agreement [Member] | Commercial Agreements [Member] | Storage Services Butane Cavern [Member]
Commercial Agreements
Duration of agreements (in years)	10 years
MPC [Member] | Three Year Storage Services Agreement [Member] | Commercial Agreements [Member] | Storage services tank farms [Member]
Commercial Agreements
Number of agreements	4
Duration of agreements (in years)	3 years
MPC [Member] | Transportation Services Agreements [Member] | Commercial Agreements [Member] | Crude oil and product pipeline [Member]
Commercial Agreements
Number of renewals	2
Renewal term agreement	5 years
MPC [Member] | Transportation Services Agreements [Member] | Commercial Agreements [Member] | Wood River To Patoka System And Barge Dock [Member]
Commercial Agreements
Number of renewals	4
Renewal term agreement	2 years
MPC [Member] | Storage Services Agreements [Member] | Commercial Agreements [Member] | Storage services tank farms [Member]
Commercial Agreements
Renewal term agreement	1 year
MPC [Member] | Capitalized Interest [Member]
Related Party Transactions
Non-cash contribution from MPC	0.1	0.3		0.7
Blanchard Pipe Line Company Limited Liability Company [Member] | Operating Agreements [Member]
Operating Agreements
Sales to related parties	1.1
MPCIF [Member] | Predecessor and Successor [Member]
Related Party Transactions
Related party interest and other financial income				1.3
MPCIF [Member] | Predecessor [Member]
Loan Agreement [Abstract]
Expiration date	28-Sep-12
MPCIF [Member] | Predecessor [Member] | Revolving Credit Facility [Member]
Loan Agreement [Abstract]
Line of credit facility initiation date for MPCIF	21-Jun-11
Description of variable rate basis									three-month LIBOR
Related Party Transactions
Basis spread on variable rate									0.10%
Description of variable rate basis									three-month LIBOR
Basis spread on variable rate									0.50%
Capline System [Member] | Operating Agreements [Member]
Commercial Agreements
Renewal term agreement	5 years
Operating Agreements
Sales to related parties	$3.60
Pipeline Length	635
Pipeline Diameter	40
Centennial Pipeline Llc [Member] | MPC [Member]
Related Party Transaction [Line Items]
Equity method investment, ownership percentage	50.00%
Muskegon [Member] | MPC [Member]
Related Party Transaction [Line Items]
Equity method investment, ownership percentage	60.00%
Capline System [Member] | MPC [Member] | Operating Agreements [Member]
Related Party Transaction [Line Items]
Equity method investment, ownership percentage	32.60%

Related_Party_Agreements_and_T3

Related Party Agreements and Transactions (Sales to Related Parties) (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Related Party Transaction [Line Items]
Sales to related parties	$450.90	$384.20
MPC [Member]
Related Party Transaction [Line Items]
Sales to related parties	450.9	384.2
Predecessor and Successor [Member]
Related Party Transaction [Line Items]
Sales to related parties			367.8
Predecessor and Successor [Member] | MPC [Member]
Related Party Transaction [Line Items]
Sales to related parties			$367.80

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 $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Related Party Transaction [Line Items]
Fees received for operating pipelines for related parties	$23	$18.80
MPC [Member]
Related Party Transaction [Line Items]
Fees received for operating pipelines for related parties	21.8	17.6
Centennial Pipeline Llc [Member]
Related Party Transaction [Line Items]
Fees received for operating pipelines for related parties	1.1	1.1
Muskegon [Member]
Related Party Transaction [Line Items]
Fees received for operating pipelines for related parties	0.1	0.1
Predecessor and Successor [Member]
Related Party Transaction [Line Items]
Fees received for operating pipelines for related parties			13.1
Predecessor and Successor [Member] | MPC [Member]
Related Party Transaction [Line Items]
Fees received for operating pipelines for related parties			12
Predecessor and Successor [Member] | Centennial Pipeline Llc [Member]
Related Party Transaction [Line Items]
Fees received for operating pipelines for related parties			1
Predecessor and Successor [Member] | Muskegon [Member]
Related Party Transaction [Line Items]
Fees received for operating pipelines for related parties			$0.10

Related_Party_Agreements_and_T5

Related Party Agreements and Transactions (Summary of Charges for Services Included in Purchases from Related Parties) (Detail) (MPC [Member], USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Related Party Transaction [Line Items]
Charges for services included in purchases from related party	$24.10	$18
Predecessor and Successor [Member]
Related Party Transaction [Line Items]
Charges for services included in purchases from related party			$13.60

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 $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Related Party Transaction [Line Items]
Charges for services included in general and administrative expenses	$30.70	$31.40
Predecessor and Successor [Member]
Related Party Transaction [Line Items]
Charges for services included in general and administrative expenses			$22.60

Related_Party_Agreements_and_T7

Related Party Agreements and Transactions (Summary of Related Party Costs Added to Property, Plant and Equipment) (Detail) (MPC [Member], Asset under construction [Member], USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Property, Plant and Equipment [Line Items]
Costs added to property, plant and equipment	$7.50	$8
Predecessor and Successor [Member]
Property, Plant and Equipment [Line Items]
Costs added to property, plant and equipment			$5.50

Related_Party_Agreements_and_T8

Related Party Agreements and Transactions (Summary of Allocated Related Party Employee Benefit Costs by Income Statement Line Item) (Detail) (MPC [Member], USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	$97	$92.30
Purchases From Related Parties [Member]
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	73.4	76.6
General and administrative expenses [Member]
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements	23.6	15.7
Predecessor and Successor [Member]
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements			53.9
Predecessor and Successor [Member] | Purchases From Related Parties [Member]
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements			30.8
Predecessor and Successor [Member] | General and administrative expenses [Member]
Related Party Transaction [Line Items]
Expenses incurred under employee services agreements			$23.10

Related_Party_Agreements_and_T9

Related Party Agreements and Transactions (Receivables from Related Parties) (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Related Party Transaction [Line Items]
Receivables from related parties	$41	$48.30
MPC [Member]
Related Party Transaction [Line Items]
Receivables from related parties	40.5	47.4
Centennial Pipeline Llc [Member]
Related Party Transaction [Line Items]
Receivables from related parties	0.3	0.6
Muskegon [Member]
Related Party Transaction [Line Items]
Receivables from related parties	$0.20	$0.30

Recovered_Sheet1

Related Party Agreements and Transactions (Prepaid Assets with Related Parties) (Details) (MPC [Member], USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
MPC [Member]
Related Party Transaction [Line Items]
Prepaid assets	$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 $)	Dec. 31, 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 $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Related Party Transaction [Line Items]
Payables to related parties	$20.20	$12.80
MPC [Member]
Related Party Transaction [Line Items]
Payables to related parties	20.1	12.8
Centennial Pipeline Llc [Member]
Related Party Transaction [Line Items]
Payables to related parties	$0.10	$0

Recovered_Sheet4

Related Party Agreements and Transactions (Deferred Revenue from Related Parties) (Details) (MPC [Member], USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Related Party Transaction [Line Items]
Deferred Revenue Related Parties	$34.50	$34
Minimum Committed Volume Contracts [Member]
Related Party Transaction [Line Items]
Deferred Revenue Related Parties	29.9	34
Reimbursable Projects [Member]
Related Party Transaction [Line Items]
Deferred Revenue Related Parties	$4.60	$0

Net_Income_Per_Limited_Partner2

Net Income Per Limited Partner Unit (Schedule of Distributions by Partner Class) (Detail) (USD $)	2 Months Ended		3 Months Ended									12 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Net Income Per Share [Line Items]
Potentially dilutive securities											0	0
Net income attributable to MPLX LP subsequent to initial public offering	$13.10		$29.20	$29.10	$28.80	$34.20	$20.20	$21.50	$18.60	$17.60		$121.30		$77.90		$13.10
Less: distribution declared	-13.3											-112.4		-88.2
Distributions less than (in excess of) net income attributable to MPLX LP	-0.2											8.9		-10.3
Limited Partners Common Units [Member]
Net Income Per Share [Line Items]
Net income attributable to MPLX LP subsequent to initial public offering	6.5											57.9		38.8
Less: distribution declared	-6.5	[1]										-54.5	[1]	-43.2	[1]
Distributions less than (in excess of) net income attributable to MPLX LP	0											3.4		-4.4
MPC [Member]
Net Income Per Share [Line Items]
Less: distribution declared			-21.7	-19.2	-18.6	-17.7	-16.9	-16.1	-15.4	-14.7
MPC [Member] | General Partner [Member]
Net Income Per Share [Line Items]
Net income attributable to MPLX LP subsequent to initial public offering	0.2											8		1.7
Less: distribution declared	-0.3	[1]										-5.8	[1]	-1.9	[1]
Distributions less than (in excess of) net income attributable to MPLX LP	-0.1											2.2		-0.2
MPC [Member] | Limited Partners Subordinated Units [Member]
Net Income Per Share [Line Items]
Net income attributable to MPLX LP subsequent to initial public offering	6.4											55.4		37.4
Less: distribution declared	-6.5	[1]										-52.1	[1]	-43.1	[1]
Distributions less than (in excess of) net income attributable to MPLX LP	($0.10)											$3.30		($5.70)
[1]	On JanuaryB 20, 2015, we announced the board of directors of our general partner had declared a quarterly cash distribution of $0.3825 per unit, totaling $33.0 million. This distribution was paid on FebruaryB 13, 2015 to unitholders of record on FebruaryB 3, 2015.

Net_Income_Per_Limited_Partner3

Net Income Per Limited Partner Unit (Schedule of Distributions by Partner by Class Footnote) (Detail) (USD $)	2 Months Ended	12 Months Ended		0 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Feb. 13, 2015	Feb. 03, 2015	Jan. 20, 2015
Net Income Per Share [Line Items]
Cash distributions declared	$13.30	$112.40	$88.20
Subsequent Event [Member]
Net Income Per Share [Line Items]
Distribution made to limited partner, declaration date						20-Jan-15
Cash distributions declared per limited partner common unit (in USD per unit)						$0.38
Cash distributions declared						$33
Distribution date				13-Feb-15
Date of record					3-Feb-15

Net_Income_Per_Limited_Partner4

Net Income Per Limited Partner Unit (Basic and Diluted Earnings Per Unit) (Detail) (USD $)	2 Months Ended		3 Months Ended								12 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Net income attributable to MPLX LP:
Cash distributions declared	$13.30										$112.40		$88.20
Distributions less than (in excess of) net income attributable to MPLX LP	-0.2										8.9		-10.3
Net income attributable to MPLX LP	13.1		29.2	29.1	28.8	34.2	20.2	21.5	18.6	17.6	121.3		77.9		13.1
Weighted average units outstanding:
Basic (shares)	75.4										75.9		75.4
Diluted (shares)	75.4										75.9		75.4
Limited Partners Common Units [Member]
Net income attributable to MPLX LP:
Cash distributions declared	6.5	[1]									54.5	[1]	43.2	[1]
Distributions less than (in excess of) net income attributable to MPLX LP	0										3.4		-4.4
Net income attributable to MPLX LP	6.5										57.9		38.8
Weighted average units outstanding:
Basic (shares)	37										37.4		37		37
Diluted (shares)	37										37.4		37		37
Net income attributable to MPLX LP subsequent to initial public offering per limited partner unit:
Basic (in USD per unit)	$0.18		$0.38	$0.37	$0.37	$0.41	$0.27	$0.29	$0.26	$0.26	$1.55		$1.05		$0.18
Diluted (in USD per unit)	$0.18		$0.38	$0.37	$0.37	$0.41	$0.27	$0.29	$0.26	$0.26	$1.55		$1.05		$0.18
MPC [Member]
Net income attributable to MPLX LP:
Cash distributions declared			21.7	19.2	18.6	17.7	16.9	16.1	15.4	14.7
MPC [Member] | General Partner [Member]
Net income attributable to MPLX LP:
Cash distributions declared	0.3	[1]									5.8	[1]	1.9	[1]
Distributions less than (in excess of) net income attributable to MPLX LP	-0.1										2.2		-0.2
Net income attributable to MPLX LP	0.2										8		1.7
Weighted average units outstanding:
Basic (shares)	1.4										1.5		1.4
Diluted (shares)	1.4										1.5		1.4
MPC [Member] | Limited Partners Subordinated Units [Member]
Net income attributable to MPLX LP:
Cash distributions declared	6.5	[1]									52.1	[1]	43.1	[1]
Distributions less than (in excess of) net income attributable to MPLX LP	-0.1										3.3		-5.7
Net income attributable to MPLX LP	$6.40										$55.40		$37.40
Weighted average units outstanding:
Basic (shares)	37										37		37
Diluted (shares)	37										37		37
Net income attributable to MPLX LP subsequent to initial public offering per limited partner unit:
Basic (in USD per unit)	$0.17										$1.50		$1.01
Diluted (in USD per unit)	$0.17										$1.50		$1.01
[1]	On JanuaryB 20, 2015, we announced the board of directors of our general partner had declared a quarterly cash distribution of $0.3825 per unit, totaling $33.0 million. This distribution was paid on FebruaryB 13, 2015 to unitholders of record on FebruaryB 3, 2015.

Equity_Narrative_Details

Equity (Narrative) (Details) (USD $)	2 Months Ended		12 Months Ended				3 Months Ended								0 Months Ended			12 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Feb. 13, 2015	Feb. 03, 2015	Jan. 20, 2015	Dec. 31, 2012
Stockholders Equity Note [Line Items]
Aggregate percentage of general partner interest			2.00%
General partner units outstanding			1,638,625				1,638,625
Cash distributions declared	$13.30		$112.40		$88.20
Limited Partners Common Units [Member]
Stockholders Equity Note [Line Items]
Common units outstanding			43,341,098				43,341,098
Subordinated Units [Member]
Stockholders Equity Note [Line Items]
Other ownership interests, units outstanding			36,951,515				36,951,515
MPC [Member]
Stockholders Equity Note [Line Items]
Cash distributions declared							21.7	19.2	18.6	17.7	16.9	16.1	15.4	14.7
MPC [Member] | Limited Partners Common Units [Member]
Stockholders Equity Note [Line Items]
Common units outstanding			19,980,619				19,980,619
Subsequent Event [Member]
Stockholders Equity Note [Line Items]
Distribution made to limited partner, declaration date																	20-Jan-15
Cash distributions declared																	33
Cash distributions declared per limited partner common unit (in USD per unit)																	$0.38
Distribution date															13-Feb-15
Date of record																3-Feb-15
Limited Partners Common Units [Member]
Stockholders Equity Note [Line Items]
Cash distributions declared	$6.50	[1]	$54.50	[1]	$43.20	[1]
Cash distributions declared per limited partner common unit (in USD per unit)			$1.41		$1.17		$0.38	$0.36	$0.34	$0.33	$0.31	$0.30	$0.28	$0.27				$0.18
Limited Partners Common Units [Member] | MPC [Member]
Stockholders Equity Note [Line Items]
Common units outstanding			20,000,000		17,100,000		20,000,000				17,100,000
[1]	On JanuaryB 20, 2015, we announced the board of directors of our general partner had declared a quarterly cash distribution of $0.3825 per unit, totaling $33.0 million. This distribution was paid on FebruaryB 13, 2015 to unitholders of record on FebruaryB 3, 2015.

Equity_Changes_in_Partners_Cap

Equity (Changes in Partners Capital, Unit Rollforward) (Details)	0 Months Ended	12 Months Ended		1 Months Ended
	Oct. 31, 2012	Dec. 31, 2014		Dec. 31, 2014	Dec. 31, 2012	Dec. 31, 2013
Stockholders Equity [Line Items]
Units issued in initial public offering	75,411,255
Balance at December 31, 2012		75,411,255			75,411,255
Balance at December 31, 2013		81,931,238		81,931,238	75,411,255
Unit-based compensation awards		15,795
December 2014 equity offering(2)(3)		3,520,408	[1],[2]
Balance at December 31, 2014		81,931,238		81,931,238	75,411,255
Limited Partners Common Units [Member]
Stockholders Equity [Line Items]
Units issued in initial public offering	36,951,515
Balance at December 31, 2012		36,951,515			36,951,515
Balance at December 31, 2013		43,341,098		43,341,098	36,951,515
Unit-based compensation awards		15,479
December 2014 equity offering(2)(3)		3,450,000	[1],[2]
Balance at December 31, 2014		43,341,098		43,341,098	36,951,515
Limited Partners Subordinated Units [Member]
Stockholders Equity [Line Items]
Units issued in initial public offering	36,951,515
Balance at December 31, 2012					36,951,515	36,951,515
Balance at December 31, 2013		36,951,515		36,951,515	36,951,515	36,951,515
Balance at December 31, 2014		36,951,515		36,951,515	36,951,515	36,951,515
General Partner Units [Member]
Stockholders Equity [Line Items]
Units issued in initial public offering	1,508,225
Balance at December 31, 2012		1,508,225			1,508,225
Balance at December 31, 2013		1,638,625		1,638,625	1,508,225
Unit-based compensation awards		316
December 2014 equity offering(2)(3)		70,408	[1],[2]	130,084
Balance at December 31, 2014		1,638,625		1,638,625	1,508,225
Pipe Line Holding [Member]
Stockholders Equity [Line Items]
Contribution of interest in Pipe Line Holdings(1)(3)		2,983,780	[1],[3]
Pipe Line Holding [Member] | Limited Partners Common Units [Member]
Stockholders Equity [Line Items]
Contribution of interest in Pipe Line Holdings(1)(3)		2,924,104	[1],[3]
Pipe Line Holding [Member] | General Partner Units [Member]
Stockholders Equity [Line Items]
Contribution of interest in Pipe Line Holdings(1)(3)		59,676	[1],[3]
[1]	As a result of the contribution mentioned above and December 2014 equity offering, MPLX GP contributed $8.8 million in exchange for 130,084 general partner units to maintain its two percent general partnership interest.
[2]	On DecemberB 8, 2014, we closed an equity offering of 3,450,000 common units representing limited partner interests, at a public offering price of $66.68 per unit. We used the net proceeds of $221.3 million to repay borrowings under our revolving credit facility and for general partnership purposes.
[3]	Effective DecemberB 1, 2014, as discussed in Note 4, we accepted a contribution of 7.625 percent of outstanding partnership interests of Pipe Line Holdings from subsidiaries of MPC in exchange for the issuance of equity valued at $200.0 million. The equity consideration consists of 2,924,104 MPLX common units and was calculated by dividing $200.0 million by the average closing price for MPLX common units for the ten trading days preceding December 1, 2014, which was $68.397.

Equity_Changes_in_Partners_Cap1

Equity (Changes in Partners Capital Footnotes) (Details) (USD $)	0 Months Ended		12 Months Ended		0 Months Ended		1 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 08, 2014	Dec. 02, 2014	Dec. 31, 2014		Mar. 02, 2014	2-May-13	Dec. 31, 2014	Dec. 01, 2014
Stockholders Equity Note [Line Items]
Percentage Of Partners Interest Contributed		7.63%
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned								$200
December 2014 equity offering(2)(3)			3,520,408	[1],[2]
Shares Issued, Price Per Share	$66.68
Partners' Capital Account, Public Sale of Units Net of Offering Costs	221.3
Equity offerings, net of issuance costs			230.1
Aggregate percentage of general partner interest			2.00%
Pipe Line Holding [Member]
Stockholders Equity Note [Line Items]
Effective date of acquisition		1-Dec-14			1-Mar-14	1-May-13
Contribution of interest in Pipe Line Holdings(1)(3)			2,983,780	[1],[3]
Limited Partners Common Units [Member]
Stockholders Equity Note [Line Items]
December 2014 equity offering(2)(3)			3,450,000	[1],[2]
Limited Partners Common Units [Member] | Pipe Line Holding [Member]
Stockholders Equity Note [Line Items]
Contribution of interest in Pipe Line Holdings(1)(3)			2,924,104	[1],[3]
Shares Issued, Price Per Share								$68.40
General Partner Units [Member]
Stockholders Equity Note [Line Items]
December 2014 equity offering(2)(3)			70,408	[1],[2]			130,084
Equity offerings, net of issuance costs							$8.80
General Partner Units [Member] | Pipe Line Holding [Member]
Stockholders Equity Note [Line Items]
Contribution of interest in Pipe Line Holdings(1)(3)			59,676	[1],[3]
[1]	As a result of the contribution mentioned above and December 2014 equity offering, MPLX GP contributed $8.8 million in exchange for 130,084 general partner units to maintain its two percent general partnership interest.
[2]	On DecemberB 8, 2014, we closed an equity offering of 3,450,000 common units representing limited partner interests, at a public offering price of $66.68 per unit. We used the net proceeds of $221.3 million to repay borrowings under our revolving credit facility and for general partnership purposes.
[3]	Effective DecemberB 1, 2014, as discussed in Note 4, we accepted a contribution of 7.625 percent of outstanding partnership interests of Pipe Line Holdings from subsidiaries of MPC in exchange for the issuance of equity valued at $200.0 million. The equity consideration consists of 2,924,104 MPLX common units and was calculated by dividing $200.0 million by the average closing price for MPLX common units for the ten trading days preceding December 1, 2014, which was $68.397.

Equity_Incentive_Distribution_

Equity (Incentive Distribution Rights) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014
Minimum Quarterly Distribution [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Total quarterly distribution per unit target amount	$0.26
Minimum Quarterly Distribution [Member] | Limited Partner [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Marginal percentage interest in distributions	98.00%
Minimum Quarterly Distribution [Member] | General Partner [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Marginal percentage interest in distributions	2.00%
First Target Distribution [Member] | Limited Partner [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Marginal percentage interest in distributions	98.00%
First Target Distribution [Member] | General Partner [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Marginal percentage interest in distributions	2.00%
Second Target Distribution [Member] | Limited Partner [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Marginal percentage interest in distributions	85.00%
Second Target Distribution [Member] | General Partner [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Marginal percentage interest in distributions	15.00%
Third Target Distribution [Member] | Limited Partner [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Marginal percentage interest in distributions	75.00%
Third Target Distribution [Member] | General Partner [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Marginal percentage interest in distributions	25.00%
Thereafter | Limited Partner [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Marginal percentage interest in distributions	50.00%
Thereafter | General Partner [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Marginal percentage interest in distributions	50.00%
Minimum [Member] | First Target Distribution [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Total quarterly distribution per unit target amount	$0.26
Minimum [Member] | Second Target Distribution [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Total quarterly distribution per unit target amount	$0.30
Minimum [Member] | Third Target Distribution [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Total quarterly distribution per unit target amount	$0.33
Minimum [Member] | Thereafter
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Total quarterly distribution per unit target amount	$0.39
Maximum [Member] | First Target Distribution [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Total quarterly distribution per unit target amount	$0.30
Maximum [Member] | Second Target Distribution [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Total quarterly distribution per unit target amount	$0.33
Maximum [Member] | Third Target Distribution [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Total quarterly distribution per unit target amount	$0.39

Equity_Net_Income_Allocation_D

Equity (Net Income Allocation) (Details) (USD $)	2 Months Ended	3 Months Ended								12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Equity [Abstract]
Net income attributable to MPLX LP	$13.10	$29.20	$29.10	$28.80	$34.20	$20.20	$21.50	$18.60	$17.60	$121.30	$77.90	$13.10
General partner's incentive distribution rights	0									3.6	0.1
Net income attributable to MPLX LP available to general and limited partners	13.1									117.7	77.8
General partnerbs two percent interest in net income attributable to MPLX LP	0.2									2.3	1.6
Less: General partnerbs interest in net income attributable to MPLX LP subsequent to initial public offering	$0.20									$5.90	$1.70	$0.20
Aggregate percentage of general partner interest										2.00%

Equity_Cash_Distributions_Deta

Equity (Cash Distributions) (Details) (USD $)	2 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2014	Dec. 31, 2013
Incentive Distribution Made to Managing Member or General Partner [Line Items]
General partner's distributions	$0.30	$2.20	$1.80
General partner's incentive distribution rights	0	3.6	0.1
Total general partner's distributions	0.3	5.8	1.9
Limited partners' distributions	13	106.6	86.3
Cash distributions declared	13.3	112.4	88.2
Limited Partners Common Units [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Limited partners' distributions	6.5	54.5	43.2
Subordinated Units [Member]
Incentive Distribution Made to Managing Member or General Partner [Line Items]
Limited partners' distributions	$6.50	$52.10	$43.10

Major_Customer_and_Concentrati1

Major Customer and Concentration of Credit Risk (Details) (MPC [Member], Revenues And Other Income [Member], Customer Concentration Risk [Member])	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Concentration Risk [Line Items]
Concentration risk, percentage	86.00%	83.00%
Predecessor and Successor [Member]
Concentration Risk [Line Items]
Concentration risk, percentage			82.00%

Other_Items_Detail

Other Items (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Other Items Net Interest And Other Financial Income [Line Items]
Interest income	$0	($0.30)
Interest expense	-5	-1.1
Interest capitalized	-0.9	-0.9
Other financial costs	-1.2	-1.2
Net interest and other financial income (costs)	5.3	1.1
Predecessor and Successor [Member]
Other Items Net Interest And Other Financial Income [Line Items]
Interest income			-0.1
Interest expense			-0.8
Interest capitalized			-0.7
Other financial costs			-0.2
Net interest and other financial income (costs)			$0.20

Income_Taxes_Narrative_Detail

Income Taxes (Narrative) (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Taxes [Line Items]
Income tax expense (benefit)	($100,000)	($200,000)
Income tax rate	-0.10%	-0.10%
Unrecognized Tax Benefits	0	0	200,000
Unrecognized tax benefits affecting effective income tax rate	0
Uncertain tax positions	0
Interest and penalties, net expenses (benefits)		-100,000
Net deferred tax liability	100,000	-100,000
Maximum [Member]
Income Taxes [Line Items]
Interest and penalties, net expenses (benefits)	-100,000
Accrual of interest and penalties	100,000	100,000
Predecessor and Successor [Member]
Income Taxes [Line Items]
Income tax expense (benefit)			300,000
Income tax rate			0.20%
Interest and penalties, net expenses (benefits)			$100,000

Income_Taxes_Schedule_of_Unrec

Income Taxes (Schedule of Unrecognized Tax Benefits) (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
January 1 balance	$0	$0.20
Additions (reductions) for tax positions of prior years	0	-0.2
December 31 balance	0	0
Predecessor [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
January 1 balance				0
December 31 balance				0
Predecessor and Successor [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Additions (reductions) for tax positions of prior years			$0.20

Property_Plant_and_Equipment_S

Property, Plant and Equipment (Summary of Property, Plant and Equipment) (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	1,361.70	$1,276.40
Less accumulated depreciation	353.1	309.8
Property, plant and equipment, net	1,008.60	966.6
Land [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	5.5	5.3
Pipelines and related assets [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	1,081.40	1,060.70
Storage and delivery facilities [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	165.8	165.7
Other [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	23.7	22.8
Asset under construction [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment, gross	85.3	$21.90
Minimum [Member] | Pipelines and related assets [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	19 years
Minimum [Member] | Storage and delivery facilities [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	24 years
Minimum [Member] | Other [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	10 years
Maximum [Member] | Pipelines and related assets [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	42 years
Maximum [Member] | Storage and delivery facilities [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	37 years
Maximum [Member] | Other [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	25 years

Property_Plant_and_Equipment_N

Property Plant and Equipment (Narrative) (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Interest Cost Capitalized And Depreciation And Amortization Expenses For Property, Plant and Equipment [Line Items]
Accumulated depreciation	$353.10	$309.80
Assets Held under Capital Leases [Member]
Interest Cost Capitalized And Depreciation And Amortization Expenses For Property, Plant and Equipment [Line Items]
Capital leases	24.9	24.9
Accumulated depreciation	$6.50	$5.70

Goodwill_Narrative_Detail

Goodwill (Narrative) (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill impairment loss	$0
Goodwill, Other Changes	$0	$0

Fair_Value_Measurements_Detail

Fair Value Measurements (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Fair Value Disclosures [Abstract]
Assets fair value	$0	$0

Fair_Value_Measurements_Fair_V1

Fair Value Measurements Fair Value Measurements - Reported (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	$635.60	$0
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	$635	$0

Debt_Summary_of_Outstanding_Bo

Debt (Summary of Outstanding Borrowings) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Debt Instrument [Line Items]
Debt and Capital Lease Obligations	$644.80	$10.50
Amounts due within one year	0.8	0.7
Total long-term debt due after one year	644	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	0	0
Mplx Operations Llc [Member] | Line of Credit [Member] | MPLX Revolver [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	0	0
Mplx Lp [Member]
Debt Instrument [Line Items]
Amounts due within one year	0	0
Total long-term debt due after one year	635	0
Mplx Lp [Member] | Line of Credit [Member] | MPLX Revolver [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	385	0
Mplx Lp [Member] | Unsecured Debt [Member] | MPLX Term Loan [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	250	0
Marathon Pipe Line Llc [Member] | Capital Lease Obligations [Member]
Debt Instrument [Line Items]
MPL - capital lease obligations due 2020	$9.80	$10.50

Debt_Summary_of_Outstanding_Bo1

Debt (Summary of Outstanding Borrowings Table Due Dates) (Details)	12 Months Ended
	Dec. 31, 2014
Mplx Operations Llc [Member] | Line of Credit [Member] | MPLX Revolver [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Expiration Date	31-Oct-17
Mplx Lp [Member] | MPLX Revolver [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Expiration Date	20-Nov-19
Mplx Lp [Member] | MPLX Term Loan [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Expiration Date	20-Nov-19
Marathon Pipe Line Llc [Member]
Debt Instrument [Line Items]
Capital Lease Due Date Year	2020

Debt_Schedule_of_Debt_Payments

Debt (Schedule of Debt Payments) (Detail) (USD $)	Dec. 31, 2014
In Millions, unless otherwise specified
Debt Disclosure [Abstract]
2015	$0.80
2016	0.9
2017	0.9
2018	1
2019	$636

Debt_Narrative_Detail

Debt (Narrative) (Detail) (USD $)	0 Months Ended	12 Months Ended	0 Months Ended	1 Months Ended	11 Months Ended
	Mar. 31, 2014	Dec. 31, 2014	Nov. 21, 2014	Dec. 31, 2014	Nov. 19, 2014	Nov. 20, 2014	Dec. 31, 2013
Related Party Revolving Credit Agreement [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity		$50,000,000		$50,000,000
Line of Credit Facility, Expiration Date		31-Mar-19
Description of variable rate basis	one-month term London Interbank Offered Rate (bLIBORb) plus 1.375 percent
Line of Credit Facility, Initiation Date	31-Mar-14
Borrowings outstanding		0		0
MPLX Term Loan [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Debt Instrument, Issuance Date			20-Nov-14
Debt Instrument, Term		5 years
Line of Credit Facility, Maximum Borrowing Capacity						250,000,000
Line of Credit Facility, Expiration Date		20-Nov-19
Number Of Renewal Periods						2
Line Of Credit Facility Duration Of Renewal Period			1 year
Debt Instrument, Interest Rate, Effective Percentage		1.41%		1.41%
Description of variable rate basis				at either the Adjusted LIBOR or the Alternate Base Rate (as defined in the MPLX Credit Agreement) plus a specified margin
MPLX Term Loan [Member] | Mplx Lp [Member] | Maximum [Member]
Debt Instrument [Line Items]
Number of prior quarterly reporting periods used in determining compliance with covenant of ratio of consolidated net debt to consolidated EBITDA						4
Covenant of ratio of consolidated net debt to consolidated EBITDA (in ratio)						5
Covenant of ratio of consolidated net debt to consolidated EBITDA following certain acquisitions (in ratio)						5.5
MPLX Revolver [Member] | Mplx Operations Llc [Member]
Debt Instrument [Line Items]
Term of senior unsecured revolving credit agreement (in years)		5 years
MPLX Revolver [Member] | Mplx Operations Llc [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage		1.54%		1.54%
Line of Credit Facility, Initiation Date		14-Sep-12
Proceeds from Long-term Lines of Credit					280,000,000
MPLX Revolver [Member] | Mplx Operations Llc [Member] | Maximum [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity		500,000,000		500,000,000
MPLX Revolver [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Debt Instrument, Issuance Date			20-Nov-14
Debt Instrument, Term		5 years
Line of Credit Facility, Expiration Date		20-Nov-19
Number Of Renewal Periods						2
Line Of Credit Facility Duration Of Renewal Period			1 year
Debt Instrument, Interest Rate, Effective Percentage		1.40%		1.40%
Description of variable rate basis				Adjusted LIBOR or the Alternate Base Rate (as defined in the MPLX Credit Agreement) plus a specified margin
Proceeds from Long-term Lines of Credit				630,000,000
Repayments of Long-term Lines of Credit				245,000,000
Letters of Credit Outstanding, Amount		0		0
Line of Credit Facility, Remaining Borrowing Capacity		615,000,000		615,000,000
Line of Credit Facility, Remaining Borrowing Capacity, Percentage		61.50%		61.50%
MPLX Revolver [Member] | Mplx Lp [Member] | Maximum [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity		1,000,000,000		1,000,000,000
Line Of Credit Facility Additional Borrowing Capacity						500,000,000
Number of prior quarterly reporting periods used in determining compliance with covenant of ratio of consolidated net debt to consolidated EBITDA						4
Covenant of ratio of consolidated net debt to consolidated EBITDA (in ratio)						5
Covenant of ratio of consolidated net debt to consolidated EBITDA following certain acquisitions (in ratio)						5.5
MPLX Revolver [Member] | Mplx Lp [Member] | Letter of credit [Member] | Maximum [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity		250,000,000		250,000,000
MPLX Revolver [Member] | Mplx Lp [Member] | Swingline Loan [Member] | Maximum [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity		100,000,000		100,000,000
Line of Credit [Member] | MPLX Revolver [Member] | Mplx Operations Llc [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Expiration Date		31-Oct-17
Long-term Debt, Gross		0		0			0
Line of Credit [Member] | MPLX Revolver [Member] | Mplx Lp [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		$385,000,000		$385,000,000			$0

Supplemental_Cash_Flow_Informa2

Supplemental Cash Flow Information (Summary of Supplemental Cash Flow Information) (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Net cash provided by operating activities included:
Interest paid (net of amounts capitalized)	$2.90	$0
Income taxes paid (refunded)	-0.3	0.1
Non-cash investing and financing activities:
Net transfers of property, plant and equipment to materials and supplies inventories	1	4
Property, plant and equipment contributed by MPC		0.3
Fair Value Of Assets Contributed	200
MPC [Member]
Non-cash investing and financing activities:
Property, plant and equipment contributed by MPC	0.1	0.3
Receivables from related parties [Member] | MPC [Member]
Non-cash investing and financing activities:
Distribution to MPC in conjunction with the Initial Offering	0	0
Materials & supplies inventories [Member] | MPC [Member]
Non-cash investing and financing activities:
Distribution to MPC in conjunction with the Initial Offering	0	0
Property plant and equipment [Member] | MPC [Member]
Non-cash investing and financing activities:
Distribution to MPC in conjunction with the Initial Offering	0	0
Contribution from MPC in conjunction with the Initial Offering	0	0
Goodwill [Member] | MPC [Member]
Non-cash investing and financing activities:
Distribution to MPC in conjunction with the Initial Offering	0	0
Other noncurrent assets [Member] | MPC [Member]
Non-cash investing and financing activities:
Distribution to MPC in conjunction with the Initial Offering	0	0
Contribution from MPC in conjunction with the Initial Offering	0	0
Accounts payable [Member] | MPC [Member]
Non-cash investing and financing activities:
Distribution to MPC in conjunction with the Initial Offering	0	0
Accrued taxes [Member] | MPC [Member]
Non-cash investing and financing activities:
Distribution to MPC in conjunction with the Initial Offering	0	0
Contribution from MPC in conjunction with the Initial Offering	0	0
Receivables from related parties [Member] | MPC [Member]
Non-cash investing and financing activities:
Contribution from MPC in conjunction with the Initial Offering	0	0
Payables to related parties [Member] | MPC [Member]
Non-cash investing and financing activities:
Contribution from MPC in conjunction with the Initial Offering	0	0
Predecessor and Successor [Member]
Net cash provided by operating activities included:
Interest paid (net of amounts capitalized)			0
Income taxes paid (refunded)			0.2
Non-cash investing and financing activities:
Net transfers of property, plant and equipment to materials and supplies inventories			0
Property, plant and equipment contributed by MPC			0
Predecessor and Successor [Member] | Receivables from related parties [Member] | MPC [Member]
Non-cash investing and financing activities:
Distribution to MPC in conjunction with the Initial Offering			-5.7
Predecessor and Successor [Member] | Materials & supplies inventories [Member] | MPC [Member]
Non-cash investing and financing activities:
Distribution to MPC in conjunction with the Initial Offering			-0.3
Predecessor and Successor [Member] | Property plant and equipment [Member] | MPC [Member]
Non-cash investing and financing activities:
Distribution to MPC in conjunction with the Initial Offering			-187.7
Contribution from MPC in conjunction with the Initial Offering			121.4
Predecessor and Successor [Member] | Goodwill [Member] | MPC [Member]
Non-cash investing and financing activities:
Distribution to MPC in conjunction with the Initial Offering			-29.5
Predecessor and Successor [Member] | Other noncurrent assets [Member] | MPC [Member]
Non-cash investing and financing activities:
Distribution to MPC in conjunction with the Initial Offering			-2.2
Contribution from MPC in conjunction with the Initial Offering			0.3
Predecessor and Successor [Member] | Accounts payable [Member] | MPC [Member]
Non-cash investing and financing activities:
Distribution to MPC in conjunction with the Initial Offering			-0.3
Predecessor and Successor [Member] | Accrued taxes [Member] | MPC [Member]
Non-cash investing and financing activities:
Distribution to MPC in conjunction with the Initial Offering			-1
Contribution from MPC in conjunction with the Initial Offering			0.1
Predecessor and Successor [Member] | Receivables from related parties [Member] | MPC [Member]
Non-cash investing and financing activities:
Contribution from MPC in conjunction with the Initial Offering			7.4
Predecessor and Successor [Member] | Payables to related parties [Member] | MPC [Member]
Non-cash investing and financing activities:
Contribution from MPC in conjunction with the Initial Offering			3.9
Capitalized Interest [Member] | MPC [Member]
Non-cash investing and financing activities:
Property, plant and equipment contributed by MPC	$0.10	$0.30	$0.70

Supplemental_Cash_Flow_Informa3

Supplemental Cash Flow Information (Summary of Reconciliation of Additions to Property, Plant and Equipment to Total Capital Expenditures) (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Supplemental Cash Flow Information [Line Items]
Additions to property, plant and equipment	$78.60	$106.50
Plus: Increase (decrease) in capital accruals	12.9	-5.3
Asset retirement expenditures	1.6	8.3
Total capital expenditures	93.1	109.5
Predecessor and Successor [Member]
Supplemental Cash Flow Information [Line Items]
Additions to property, plant and equipment			135.6
Plus: Increase (decrease) in capital accruals			3.2
Asset retirement expenditures			9.2
Total capital expenditures			$148

Supplemental_Cash_Flow_Informa4

Supplemental Cash Flow Information Supplemental Cash Flow Information (Summary of Reconciliation of Cash Proceeds from IPO) (Details) (USD $)	0 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Oct. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Supplemental Cash Flow Information [Line Items]
Total proceeds from the Initial Offering	$437.70
Less: Offering Costs	-30.6
Net proceeds from the Initial Offering	407.1			407.1
Debt issuance costs	-2.4	-2.7	0
Cash Retained From Net Proceeds From Issuance Of Initial Public Offering				0
Proceeds from initial public offering distributed to MPC	202.7	0		202.7
MPC [Member]
Supplemental Cash Flow Information [Line Items]
Proceeds from initial public offering distributed to MPC			0
Working Capital [Member]
Supplemental Cash Flow Information [Line Items]
Cash Retained From Net Proceeds From Issuance Of Initial Public Offering	-10.4
Capital Expenditures [Member]
Supplemental Cash Flow Information [Line Items]
Cash Retained From Net Proceeds From Issuance Of Initial Public Offering	($191.60)

Supplemental_Cash_Flow_Informa5

Supplemental Cash Flow Information (Summary of Reconciliation of Contributions from (Distributions to) Former Parent Company) (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Supplemental Cash Flow Information [Line Items]
Contributions from (distributions to) MPC per consolidated statements of cash flows	$0	$0
MPC [Member]
Supplemental Cash Flow Information [Line Items]
Contributions from (distributions to) MPC per consolidated statements of cash flows	0	0
Other Significant Noncash Transaction Value Of Consideration Given Or Received	-0.1	-0.3
Contributions from (distributions to) MPC per consolidated statements of equity/net investment	0.1	0.3
Predecessor and Successor [Member] | MPC [Member]
Supplemental Cash Flow Information [Line Items]
Contributions from (distributions to) MPC per consolidated statements of cash flows			-262.7
Other Significant Noncash Transaction Value Of Consideration Given Or Received			98.2
Contributions from (distributions to) MPC per consolidated statements of equity/net investment			($360.90)

EquityBased_Compensation_Plan_1

Equity-Based Compensation Plan (Narrative) (Detail) (USD $)	12 Months Ended			2 Months Ended		9 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2012	Sep. 30, 2012
Phantom Units [Member]
Unit-based awards under the Plan
Grant date fair value of performance units granted	$49.56	$0	$0
Outstanding Phantom Unit Awards
Convertible at December 31, 2014	25,892
Unrecognized compensation cost	$2.10
Cost not yet recognized, period for recognition	1 year 7 months
Performance Shares [Member]
Unit-based awards under the Plan
Grant date fair value of performance units granted	$1.16	$0.77
Outstanding Phantom Unit Awards
Unrecognized compensation cost	0.5
Cost not yet recognized, period for recognition	1 year 6 months
Mplx 2012 Incentive Compensation Plan [Member]
Compensation Expense
Total compensation expense	2.7	1.4		0.1
Maximum [Member] | Mplx 2012 Incentive Compensation Plan [Member]
Description of the Plan
Common units available for delivery under the plan	2,750,000
Officer [Member] | Mplx 2012 Incentive Compensation Plan [Member] | Phantom Units [Member]
Unit-based awards under the Plan
Award requisite service period	3 years
Officer [Member] | Mplx 2012 Incentive Compensation Plan [Member] | Performance Shares [Member]
Unit-based awards under the Plan
Award requisite service period	36 months
Measurement period	4
Award percentage paid out in cash	75.00%
Award percentage paid out in stock	25.00%
Grant date fair value of performance units granted	$1.16	$0.77
Officer [Member] | Maximum [Member] | Mplx 2012 Incentive Compensation Plan [Member] | Phantom Units [Member]
Unit-based awards under the Plan
Accrued distributions	0.1
General and administrative expenses [Member]
Compensation Expense
Stock-based compensation plans expenses	1	1			0.2
General and administrative expenses [Member] | Predecessor [Member]
Compensation Expense
Stock-based compensation plans expenses						$0.90

EquityBased_Compensation_Plan_2

Equity-Based Compensation Plan (Summary of Phantom Unit Award Activity) (Detail) (Phantom Units [Member], USD $)	12 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Phantom Units [Member]
Number of Units
Outstanding at December 31, 2013	77,754
Granted	51,904
Settled	27,360
Forfeited	-1,529
Outstanding at December 31, 2014	100,769	77,754
Vested and expected to vest at December 31, 2014	99,566
Convertible at December 31, 2014	25,892
Weighted Average Fair Value
Outstanding at December 31, 2013 (usd per share)	$33.84
Granted (usd per share)	$49.56	$0	$0
Settled (usd per share)	$34.45
Forfeited (usd per share)	$40.91
Outstanding at December 31, 2014 (usd per share)	$41.66	$33.84
Vested and expected to vest at December 31, 2014 (usd per share)	$41.62
Convertible at December 31, 2014 (usd per share)	$42.21
Aggregate Intrinsic Value
Vested and expected to vest at December 31, 2014	$7.30
Convertible at December 31, 2014	$1.90

EquityBased_Compensation_Plan_3

Equity-Based Compensation Plan (Summary of Values Related To Vested and Unvested Restricted Stock Awards) (Details) (Phantom Units [Member], USD $)	12 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Phantom Units [Member]
Equity-Based Compensation [Line Items]
Grant date fair value of performance units granted	$49.56	$0	$0
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Vested In Period Total Intrinsic Value 1	$1.30	$0	$0

EquityBased_Compensation_Plan_4

Equity-Based Compensation Plan (Summary of Performance Unit Award Activity) (Details) (Performance Shares [Member], USD $)	12 Months Ended
	Dec. 31, 2014
Performance Shares [Member]
Number of Units
Outstanding at December 31, 2013	436,917
Granted	500,507
Forfeited	-13,281
Outstanding at December 31, 2014	924,143
Weighted Average Fair Value
Outstanding at December 31, 2013 (usd per share)	$0.77
Granted (usd per share)	$1.16
Forfeited (usd per share)	$1.05
Outstanding at December 31, 2014 (usd per share)	$0.98

EquityBased_Compensation_Plan_5

Equity-Based Compensation Plan (Assumptions) (Details) (Performance Shares [Member], USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Performance Shares [Member]
Equity-Based Compensation [Line Items]
Risk-free interest rate	0.63%	0.35%
Look-back period	2 years 10 months 3 days	2 years 10 months 3 days
Expected volatility	17.17%	16.75%
Grant date fair value of performance units granted	$1.16	$0.77

Leases_Future_Minimum_Commitme

Leases (Future Minimum Commitments) (Detail) (USD $)	Dec. 31, 2014
In Millions, unless otherwise specified
Capital Lease Obligations
2015	$1.40
2016	1.4
2017	1.4
2018	1.4
2019	1.4
Later years	5.1
Total minimum lease payments	12.1
Less imputed interest costs	2.3
Present value of net minimum lease payments	9.8
Operating Lease Obligations
2015	9.6
2016	9.5
2017	8.7
2018	8.4
2019	3.7
Later years	7.7
Total minimum lease payments	$47.60

Leases_Operating_Lease_Rental_

Leases (Operating Lease Rental Expense) (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule Of Future Minimum Sublease Rentals [Line Items]
Minimum rental expense	$10.40	$9.90
Predecessor and Successor [Member]
Schedule Of Future Minimum Sublease Rentals [Line Items]
Minimum rental expense			$7

Commitments_and_Contingencies_

Commitments and Contingencies (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Commitments And Contingencies [Line Items]
Accrued liabilities for remediation	$0.60	$1.10
Maximum potential undiscounted payments	1.7
Contractual commitments to acquire property, plant and equipment	25
Environmental Loss Contingency [Member] | MPC [Member]
Commitments And Contingencies [Line Items]
Receivable from MPC for indemnification of environmental costs	0	0.3
Maximum [Member]
Commitments And Contingencies [Line Items]
Loss exposure in excess of accrual	$0.30	$0.40

Subsequent_Event_Details

Subsequent Event (Details) (Subsequent Event [Member], USD $)	0 Months Ended
In Millions, unless otherwise specified	Feb. 12, 2015
Subsequent Event [Line Items]
Subsequent event date	12-Feb-15
Mplx Lp [Member] | Senior Notes Due Two Thousand Twenty Five [Member]
Subsequent Event [Line Items]
Debt Instrument, Face Amount	500
Debt Instrument, Interest Rate, Stated Percentage	4.00%
Debt Instrument, Maturity Date	15-Feb-25
Notes At Price	99.64%

Condensed_Consolidating_Financ2

Condensed Consolidating Financial Statements (Condensed Income Statement) (Detail) (USD $)	2 Months Ended	3 Months Ended								12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenues and other income:
Sales and other operating revenues										$69.20	$78.90
Sales to related parties										450.9	384.2
Loss on sale of assets										0	0
Other income										5.2	4.4
Other income - related parties										23	18.8
Equity in earnings of subsidiaries										0	0
Total revenues and other income										548.3	486.3
Costs and expenses:
Cost of revenues (excludes items below)										145.3	135.9
Purchases from related parties										97.5	94.6
Depreciation										50.2	48.9
General and administrative expenses										64.8	53.7
Other taxes										7.2	6.2
Total costs and expenses										365	339.3
Income from operations		38.5	44.2	44.3	56.3	36.5	39.8	35.2	35.5	183.3	147
Related party interest and other financial income										0	0
Net interest and other financial costs (income)										5.3	1.1
Income before income taxes										178	145.9
Provision (benefit) for income taxes										-0.1	-0.2
Net income		36.4	43.1	42.9	55.7	36.8	39.2	34.8	35.3	178.1	146.1	26.3
Less: Net income attributable to MPC-retained interest										56.8	68.2
Net income attributable to MPLX LP	13.1	29.2	29.1	28.8	34.2	20.2	21.5	18.6	17.6	121.3	77.9	13.1
Parent Guarantor [Member]
Revenues and other income:
Sales and other operating revenues										0	0
Sales to related parties										0	0
Other income										0	0
Other income - related parties										0	0
Equity in earnings of subsidiaries										130.6	83
Total revenues and other income										130.6	83
Costs and expenses:
Cost of revenues (excludes items below)										0	0
Purchases from related parties										0	0
Depreciation										0	0
General and administrative expenses										7.6	5
Other taxes										0.1	0.1
Total costs and expenses										7.7	5.1
Income from operations										122.9	77.9
Net interest and other financial costs (income)										1.6	0
Income before income taxes										121.3	77.9
Provision (benefit) for income taxes										0	0
Net income										121.3	77.9
Less: Net income attributable to MPC-retained interest										0	0
Net income attributable to MPLX LP										121.3	77.9	13.1
Subsidiary Issuer [Member]
Revenues and other income:
Sales and other operating revenues										0	0
Sales to related parties										0	0
Other income										0	0
Other income - related parties										0	0
Equity in earnings of subsidiaries										136.1	85.3
Total revenues and other income										136.1	85.3
Costs and expenses:
Cost of revenues (excludes items below)										0	0
Purchases from related parties										0	0
Depreciation										0	0
General and administrative expenses										0	0
Other taxes										0	0
Total costs and expenses										0	0
Income from operations										136.1	85.3
Net interest and other financial costs (income)										5.5	2.3
Income before income taxes										130.6	83
Provision (benefit) for income taxes										0	0
Net income										130.6	83
Less: Net income attributable to MPC-retained interest										0	0
Net income attributable to MPLX LP										130.6	83	14
Non-Guarantor Subsidiaries [Member]
Revenues and other income:
Sales and other operating revenues										69.2	78.9
Sales to related parties										450.9	384.2
Other income										5.2	4.4
Other income - related parties										23	18.8
Equity in earnings of subsidiaries										0	0
Total revenues and other income										548.3	486.3
Costs and expenses:
Cost of revenues (excludes items below)										145.3	135.9
Purchases from related parties										97.5	94.6
Depreciation										50.2	48.9
General and administrative expenses										57.2	48.7
Other taxes										7.1	6.1
Total costs and expenses										357.3	334.2
Income from operations										191	152.1
Net interest and other financial costs (income)										-1.8	-1.2
Income before income taxes										192.8	153.3
Provision (benefit) for income taxes										-0.1	-0.2
Net income										192.9	153.5
Less: Net income attributable to MPC-retained interest										0	0
Net income attributable to MPLX LP										192.9	153.5	27.4
Consolidating Eliminations [Member]
Revenues and other income:
Sales and other operating revenues										0	0
Sales to related parties										0	0
Other income										0	0
Other income - related parties										0	0
Equity in earnings of subsidiaries										-266.7	-168.3
Total revenues and other income										-266.7	-168.3
Costs and expenses:
Cost of revenues (excludes items below)										0	0
Purchases from related parties										0	0
Depreciation										0	0
General and administrative expenses										0	0
Other taxes										0	0
Total costs and expenses										0	0
Income from operations										-266.7	-168.3
Net interest and other financial costs (income)										0	0
Income before income taxes										-266.7	-168.3
Provision (benefit) for income taxes										0	0
Net income										-266.7	-168.3
Less: Net income attributable to MPC-retained interest										56.8	68.2
Net income attributable to MPLX LP										-323.5	-236.5	-41.4
Predecessor and Successor [Member]
Revenues and other income:
Sales and other operating revenues												74.4
Sales to related parties												367.8
Loss on sale of assets												-0.3
Other income												6.9
Other income - related parties												13.1
Equity in earnings of subsidiaries												0
Total revenues and other income												461.9
Costs and expenses:
Cost of revenues (excludes items below)												173.8
Purchases from related parties												44.4
Depreciation												39.4
General and administrative expenses												49.8
Other taxes												11.3
Total costs and expenses												318.7
Income from operations												143.2
Related party interest and other financial income												1.3
Net interest and other financial costs (income)												0.2
Income before income taxes												144.3
Provision (benefit) for income taxes												0.3
Net income												144
Less: Net income attributable to MPC-retained interest												13.2
Net income attributable to MPLX LP												130.8
Predecessor and Successor [Member] | Parent Guarantor [Member]
Revenues and other income:
Sales and other operating revenues												0
Sales to related parties												0
Loss on sale of assets												0
Other income												0
Other income - related parties												0
Equity in earnings of subsidiaries												14
Total revenues and other income												14
Costs and expenses:
Cost of revenues (excludes items below)												0
Purchases from related parties												0
Depreciation												0
General and administrative expenses												0.9
Other taxes												0
Total costs and expenses												0.9
Income from operations												13.1
Related party interest and other financial income												0
Net interest and other financial costs (income)												0
Income before income taxes												13.1
Provision (benefit) for income taxes												0
Net income												13.1
Less: Net income attributable to MPC-retained interest												0
Net income attributable to MPLX LP												13.1
Predecessor and Successor [Member] | Subsidiary Issuer [Member]
Revenues and other income:
Sales and other operating revenues												0
Sales to related parties												0
Loss on sale of assets												0
Other income												0
Other income - related parties												0
Equity in earnings of subsidiaries												14.2
Total revenues and other income												14.2
Costs and expenses:
Cost of revenues (excludes items below)												0
Purchases from related parties												0
Depreciation												0
General and administrative expenses												0
Other taxes												0
Total costs and expenses												0
Income from operations												14.2
Related party interest and other financial income												0
Net interest and other financial costs (income)												0.2
Income before income taxes												14
Provision (benefit) for income taxes												0
Net income												14
Less: Net income attributable to MPC-retained interest												0
Net income attributable to MPLX LP												14
Predecessor and Successor [Member] | Non-Guarantor Subsidiaries [Member]
Revenues and other income:
Sales and other operating revenues												74.4
Sales to related parties												367.8
Loss on sale of assets												-0.3
Other income												6.9
Other income - related parties												13.1
Equity in earnings of subsidiaries												0
Total revenues and other income												461.9
Costs and expenses:
Cost of revenues (excludes items below)												173.8
Purchases from related parties												44.4
Depreciation												39.4
General and administrative expenses												48.9
Other taxes												11.3
Total costs and expenses												317.8
Income from operations												144.1
Related party interest and other financial income												1.3
Net interest and other financial costs (income)												0
Income before income taxes												145.4
Provision (benefit) for income taxes												0.3
Net income												145.1
Less: Net income attributable to MPC-retained interest												0
Net income attributable to MPLX LP												145.1
Predecessor and Successor [Member] | Consolidating Eliminations [Member]
Revenues and other income:
Sales and other operating revenues												0
Sales to related parties												0
Loss on sale of assets												0
Other income												0
Other income - related parties												0
Equity in earnings of subsidiaries												-28.2
Total revenues and other income												-28.2
Costs and expenses:
Cost of revenues (excludes items below)												0
Purchases from related parties												0
Depreciation												0
General and administrative expenses												0
Other taxes												0
Total costs and expenses												0
Income from operations												-28.2
Related party interest and other financial income												0
Net interest and other financial costs (income)												0
Income before income taxes												-28.2
Provision (benefit) for income taxes												0
Net income												-28.2
Less: Net income attributable to MPC-retained interest												13.2
Net income attributable to MPLX LP												-41.4
Predecessor [Member]
Costs and expenses:
Net income												117.7
Net income attributable to MPLX LP												117.7
Predecessor [Member] | Parent Guarantor [Member]
Costs and expenses:
Net income attributable to MPLX LP												0
Predecessor [Member] | Subsidiary Issuer [Member]
Costs and expenses:
Net income attributable to MPLX LP												0
Predecessor [Member] | Non-Guarantor Subsidiaries [Member]
Costs and expenses:
Net income attributable to MPLX LP												117.7
Predecessor [Member] | Consolidating Eliminations [Member]
Costs and expenses:
Net income attributable to MPLX LP												$0

Condensed_Consolidating_Financ3

Condensed Consolidating Financial Statements (Condensed Balance Sheet) (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
In Millions, unless otherwise specified
Current assets:
Cash and cash equivalents	$27.30	$54.10	$216.70	$0.10
Receivables	10.2	12.2
Receivables from related parties	41	48.3
Materials and supplies inventories	11.7	11.6
Other current assets	7	8.9
Total current assets	97.2	135.1
Property, plant and equipment, net	1,008.60	966.6
Investment in subsidiaries	0	0
Goodwill	104.7	104.7
Other noncurrent assets	4	2.1
Total assets	1,214.50	1,208.50
Current liabilities:
Accounts payable	42.2	30.5
Payables to related parties	20.2	12.8
Deferred revenue - related parties	30.5	34
Accrued taxes	5.2	4
Long-term debt due within one year	0.8	0.7
Other current liabilities	1.7	1.4
Total current liabilities	100.6	83.4
Long-term deferred revenue related parties	4	0
Long-term debt	644	9.8
Deferred credits and other liabilities	2.2	1.2
Total liabilities	750.8	94.4
Equity [Abstract]
MPLX LP partners' capital	458.2	646.2
Noncontrolling interest retained by MPC	5.5	467.9
Total equity	463.7	1,114.10
Total liabilities and equity	1,214.50	1,208.50
Parent Guarantor [Member]
Current assets:
Cash and cash equivalents	1.2	0	1	0
Receivables	0.1	0.4
Receivables from related parties	0	0
Materials and supplies inventories	0	0
Other current assets	0.4	0.3
Total current assets	1.7	0.7
Property, plant and equipment, net	0	0
Investment in subsidiaries	1,098.50	647.1
Goodwill	0	0
Other noncurrent assets	2.7	0
Total assets	1,102.90	647.8
Current liabilities:
Accounts payable	1	0.2
Payables to related parties	6.3	0.7
Deferred revenue - related parties	0	0
Accrued taxes	0.1	0.3
Long-term debt due within one year	0	0
Other current liabilities	0.6	0
Total current liabilities	8	1.2
Long-term deferred revenue related parties	0
Long-term debt	635	0
Deferred credits and other liabilities	1.7	0.4
Total liabilities	644.7	1.6
Equity [Abstract]
MPLX LP partners' capital	458.2	646.2
Noncontrolling interest retained by MPC	0	0
Total equity	458.2	646.2
Total liabilities and equity	1,102.90	647.8
Subsidiary Issuer [Member]
Current assets:
Cash and cash equivalents	0.5	7.2	9.7	0
Receivables	0	0
Receivables from related parties	5.9	0
Materials and supplies inventories	0	0
Other current assets	0	0
Total current assets	6.4	7.2
Property, plant and equipment, net	0	0
Investment in subsidiaries	1,234.70	721.6
Goodwill	0	0
Other noncurrent assets	1.3	1.8
Total assets	1,242.40	730.6
Current liabilities:
Accounts payable	0	0.3
Payables to related parties	143.9	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	0
Total current liabilities	143.9	83.5
Long-term deferred revenue related parties	0
Long-term debt	0	0
Deferred credits and other liabilities	0	0
Total liabilities	143.9	83.5
Equity [Abstract]
MPLX LP partners' capital	1,098.50	647.1
Noncontrolling interest retained by MPC	0	0
Total equity	1,098.50	647.1
Total liabilities and equity	1,242.40	730.6
Non-Guarantor Subsidiaries [Member]
Current assets:
Cash and cash equivalents	25.6	46.9	206	0.1
Receivables	10.1	11.8
Receivables from related parties	184.8	132.2
Materials and supplies inventories	11.7	11.6
Other current assets	6.6	8.6
Total current assets	238.8	211.1
Property, plant and equipment, net	1,008.60	966.6
Investment in subsidiaries	0	0
Goodwill	104.7	104.7
Other noncurrent assets	0	0.3
Total assets	1,352.10	1,282.70
Current liabilities:
Accounts payable	41.2	30
Payables to related parties	19.7	12.8
Deferred revenue - related parties	30.5	34
Accrued taxes	5.1	3.7
Long-term debt due within one year	0.8	0.7
Other current liabilities	1.1	1.4
Total current liabilities	98.4	82.6
Long-term deferred revenue related parties	4
Long-term debt	9	9.8
Deferred credits and other liabilities	0.5	0.8
Total liabilities	111.9	93.2
Equity [Abstract]
MPLX LP partners' capital	1,240.20	1,189.50
Noncontrolling interest retained by MPC	0	0
Total equity	1,240.20	1,189.50
Total liabilities and equity	1,352.10	1,282.70
Consolidating Eliminations [Member]
Current assets:
Cash and cash equivalents	0	0	0
Receivables	0	0
Receivables from related parties	-149.7	-83.9
Materials and supplies inventories	0	0
Other current assets	0	0
Total current assets	-149.7	-83.9
Property, plant and equipment, net	0	0
Investment in subsidiaries	-2,333.20	-1,368.70
Goodwill	0	0
Other noncurrent assets	0	0
Total assets	-2,482.90	-1,452.60
Current liabilities:
Accounts payable	0	0
Payables to related parties	-149.7	-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	-149.7	-83.9
Long-term deferred revenue related parties	0
Long-term debt	0	0
Deferred credits and other liabilities	0	0
Total liabilities	-149.7	-83.9
Equity [Abstract]
MPLX LP partners' capital	-2,338.70	-1,836.60
Noncontrolling interest retained by MPC	5.5	467.9
Total equity	-2,333.20	-1,368.70
Total liabilities and equity	($2,482.90)	($1,452.60)

Condensed_Consolidating_Financ4

Condensed Consolidating Financial Statements (Condensed Cash Flow Statement) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	$246.80	$212.20
Investing activities:
Additions to property, plant and equipment	-78.6	-106.5
Disposal of assets	0	0.2
All other, net	3.5	-7.3
Net cash provided by (used in) investing activities	-75.1	-113.6
Financing activities:
Proceeds from borrowings from affiliates	0	0
Payments on borrowings from affiliates		0
Long-term debt - borrowings	1,160	0
Long-term debt - repayments	-525.9	-0.7
Debt issuance costs	-2.7	0
Net proceeds from equity offerings	230.1	0
Proceeds from initial public offering distributed to MPC	0		-202.7
Quarterly distributions	-103.1	-77.8
Quarterly distributions to noncontrolling interest retained by MPC	-46.9	-82.7
Distributions related to purchase of additional interest in Pipe Line Holdings	-910	-100
Net cash provided by (used in) financing activities	-198.5	-261.2
Net increase (decrease) in cash and cash equivalents	-26.8	-162.6
Cash and cash equivalents at beginning of period	54.1	216.7	0.1
Cash and cash equivalents at end of period	27.3	54.1	216.7
Parent Guarantor [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	106.9	76.8
Investing activities:
Payments of Capital Distribution	865
Net cash provided by (used in) investing activities	-865	0
Financing activities:
Long-term debt - borrowings	880
Long-term debt - repayments	-245
Debt issuance costs	-2.7
Net proceeds from equity offerings	230.1
Quarterly distributions	-103.1	-77.8
Net cash provided by (used in) financing activities	759.3	-77.8
Net increase (decrease) in cash and cash equivalents	1.2	-1
Cash and cash equivalents at beginning of period	0	1
Cash and cash equivalents at end of period	1.2	0
Subsidiary Issuer [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	94	102.5
Investing activities:
Additions to property, plant and equipment	-0.3	-0.2
Investment in Pipe Line Holdings	-910	-100
Payments of Capital Distribution	-865
Net cash provided by (used in) investing activities	-45.3	-100.2
Financing activities:
Proceeds from borrowings from affiliates	51	100
Payments on borrowings from affiliates		-26
Long-term debt - borrowings	280
Long-term debt - repayments	-280
Quarterly distributions	-106.4	-78.8
Net cash provided by (used in) financing activities	-55.4	-4.8
Net increase (decrease) in cash and cash equivalents	-6.7	-2.5
Cash and cash equivalents at beginning of period	7.2	9.7
Cash and cash equivalents at end of period	0.5	7.2
Non-Guarantor Subsidiaries [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	248.1	208.8
Investing activities:
Additions to property, plant and equipment	-78.3	-106.3
Investment in Pipe Line Holdings	0	0
Payments of Capital Distribution	0
Loans to affiliates	-51	-100
Proceeds from Related Party Debt		26
Disposal of assets		0.2
All other, net	3.5	-7.3
Net cash provided by (used in) investing activities	-125.8	-187.4
Financing activities:
Long-term debt - repayments	-0.9	-0.7
Quarterly distributions	-95.8	-97.1
Quarterly distributions to noncontrolling interest retained by MPC	-46.9	-82.7
Net cash provided by (used in) financing activities	-143.6	-180.5
Net increase (decrease) in cash and cash equivalents	-21.3	-159.1
Cash and cash equivalents at beginning of period	46.9	206
Cash and cash equivalents at end of period	25.6	46.9
Consolidating Eliminations [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	-202.2	-175.9
Investing activities:
Investment in Pipe Line Holdings	910	100
Loans to affiliates	51	100
Proceeds from Related Party Debt		-26
Net cash provided by (used in) investing activities	961	174
Financing activities:
Proceeds from borrowings from affiliates	-51	-100
Payments on borrowings from affiliates		26
Quarterly distributions	202.2	175.9
Distributions related to purchase of additional interest in Pipe Line Holdings	-910	-100
Net cash provided by (used in) financing activities	-758.8	1.9
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
MPC [Member]
Financing activities:
Proceeds from borrowings from affiliates	0	0
Proceeds from initial public offering distributed to MPC		0
Distributions to MPC	0	0
Predecessor and Successor [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities			190.6
Investing activities:
Additions to property, plant and equipment			-135.6
Repayments of loans from affiliates		0	0
Disposal of assets			1.3
Investments - repayments of loans receivable from a related party			221.7
All other, net			0
Net cash provided by (used in) investing activities			87.4
Financing activities:
Long-term debt - borrowings			0
Long-term debt - repayments			-0.7
Debt issuance costs			-2.4
Net proceeds from equity offerings			407.1
Proceeds from initial public offering distributed to MPC			-202.7
Quarterly distributions			0
Quarterly distributions to noncontrolling interest retained by MPC			0
Distributions related to purchase of additional interest in Pipe Line Holdings			0
Distributions to MPC			-262.7
Net cash provided by (used in) financing activities			-61.4
Net increase (decrease) in cash and cash equivalents			216.6
Predecessor and Successor [Member] | Parent Guarantor [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities			-1.4
Investing activities:
Repayments of loans from affiliates		0	-202
Disposal of assets		0
Net cash provided by (used in) investing activities			-202
Financing activities:
Net proceeds from equity offerings			407.1
Proceeds from initial public offering distributed to MPC			-202.7
Net cash provided by (used in) financing activities			204.4
Net increase (decrease) in cash and cash equivalents			1
Predecessor and Successor [Member] | Subsidiary Issuer [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities			1.7
Investing activities:
Repayments of loans from affiliates		0	10.4
Disposal of assets		0
Net cash provided by (used in) investing activities			10.4
Financing activities:
Debt issuance costs			-2.4
Net cash provided by (used in) financing activities			-2.4
Net increase (decrease) in cash and cash equivalents			9.7
Predecessor and Successor [Member] | Non-Guarantor Subsidiaries [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities			190.3
Investing activities:
Additions to property, plant and equipment			-135.6
Repayments of loans from affiliates			191.6
Disposal of assets			1.3
Investments - repayments of loans receivable from a related party			221.7
Net cash provided by (used in) investing activities			279
Financing activities:
Long-term debt - repayments			-0.7
Distributions to MPC			-262.7
Net cash provided by (used in) financing activities			-263.4
Net increase (decrease) in cash and cash equivalents			205.9
Predecessor and Successor [Member] | Consolidating Eliminations [Member]
Investing activities:
Disposal of assets		0
Predecessor and Successor [Member] | MPC [Member]
Financing activities:
Proceeds from borrowings from affiliates			262.7
Distributions to MPC			($262.70)

Select_Quarterly_Financial_Dat2

Select Quarterly Financial Data (Detail) (USD $)	2 Months Ended		3 Months Ended								12 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Quarterly Financial Data [Line Items]
Revenues			$131.90	$130.70	$126.80	$130.70	$118.90	$118.20	$116.10	$109.90
Income from operations			38.5	44.2	44.3	56.3	36.5	39.8	35.2	35.5	183.3		147
Net income			36.4	43.1	42.9	55.7	36.8	39.2	34.8	35.3	178.1		146.1		26.3
Net income attributable to MPLX LP	13.1		29.2	29.1	28.8	34.2	20.2	21.5	18.6	17.6	121.3		77.9		13.1
Distributions declared: [Abstract]
Limited partner units	13.3										112.4		88.2
Incentive distribution rights	0										3.6		0.1
Total distributions declared			33	27.9	26.5	25	23.7	22.5	21.5	20.5
Limited Partners Common Units [Member]
Quarterly Financial Data [Line Items]
Net income attributable to MPLX LP	6.5										57.9		38.8
Net income attributable to MPLX LP subsequent to initial public offering per limited partner unit:
Basic (in USD per unit)	$0.18		$0.38	$0.37	$0.37	$0.41	$0.27	$0.29	$0.26	$0.26	$1.55		$1.05		$0.18
Diluted (in USD per unit)	$0.18		$0.38	$0.37	$0.37	$0.41	$0.27	$0.29	$0.26	$0.26	$1.55		$1.05		$0.18
Cash distributions declared per limited partner common unit (in USD per unit)			$0.38	$0.36	$0.34	$0.33	$0.31	$0.30	$0.28	$0.27	$1.41		$1.17		$0.18
Distributions declared: [Abstract]
Limited partner units	6.5	[1]									54.5	[1]	43.2	[1]
Limited Partners Subordinated Units [Member]
Net income attributable to MPLX LP subsequent to initial public offering per limited partner unit:
Basic and Diluted (in USD per unit)			$0.33	$0.37	$0.37	$0.41	$0.27	$0.29	$0.23	$0.21
Public [Member]
Distributions declared: [Abstract]
Limited partner units			8.9	7.2	6.8	6.5	6.2	5.9	5.7	5.4
MPC [Member]
Distributions declared: [Abstract]
Limited partner units			21.7	19.2	18.6	17.7	16.9	16.1	15.4	14.7
General partner units			0.7	0.5	0.5	0.5	0.5	0.5	0.4	0.4
Incentive distribution rights			1.7	1	0.6	0.3	0.1	0	0	0
MPC [Member] | Limited Partners Subordinated Units [Member]
Quarterly Financial Data [Line Items]
Net income attributable to MPLX LP	6.4										55.4		37.4
Net income attributable to MPLX LP subsequent to initial public offering per limited partner unit:
Basic (in USD per unit)	$0.17										$1.50		$1.01
Diluted (in USD per unit)	$0.17										$1.50		$1.01
Basic and Diluted (in USD per unit)											$1.50		$1.01		$0.17
Distributions declared: [Abstract]
Limited partner units	$6.50	[1]									$52.10	[1]	$43.10	[1]
[1]	On JanuaryB 20, 2015, we announced the board of directors of our general partner had declared a quarterly cash distribution of $0.3825 per unit, totaling $33.0 million. This distribution was paid on FebruaryB 13, 2015 to unitholders of record on FebruaryB 3, 2015.