MPLX (MPLX) 10-K 28 Feb 14 2013 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, 2013	Jun. 28, 2013	Feb. 14, 2014	Feb. 14, 2014	Feb. 14, 2014
			Common Units [Member]	Subordinated Units [Member]	General Partner Units [Member]
Entity Information [Line Items]	'	'	'	'	'
Document Type	'10-K	'	'	'	'
Amendment Flag	'false	'	'	'	'
Document Period End Date	31-Dec-13	'	'	'	'
Document Fiscal Year Focus	'2013	'	'	'	'
Document Fiscal Period Focus	'FY	'	'	'	'
Trading Symbol	'MPLX	'	'	'	'
Entity Registrant Name	'MPLX LP	'	'	'	'
Entity Central Index Key	'0001552000	'	'	'	'
Current Fiscal Year End Date	'--12-31	'	'	'	'
Entity Well-known Seasoned Issuer	'Yes	'	'	'	'
Entity Current Reporting Status	'Yes	'	'	'	'
Entity Voluntary Filers	'No	'	'	'	'
Entity Filer Category	'Large Accelerated Filer	'	'	'	'
Entity Common Stock, Shares Outstanding	'	'	36,956,712	36,951,515	1,508,331
Entity Public Float	'	$715.90	'	'	'

Consolidated_Statements_of_Inc

Consolidated Statements of Income (USD $)

12 Months Ended

3 Months Ended

12 Months Ended

In Millions, except Per Share data, unless otherwise specified

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2011

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Predecessor [Member]

MPC [Member]

MPC [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]

Public [Member]

Public [Member]

Predecessor [Member]

Predecessor and Successor [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

Limited Partners Common Units [Member]

Limited Partners Common Units [Member]

Revenues and other income:

'

'

'

'

'

'

'

'

'

'

'

'

Sales and other operating revenues

$78.90

'

$62.10

'

'

'

'

'

'

'

'

'

Sales to related parties

384.2

'

334.8

334.8

367.8

'

'

'

'

'

'

'

Loss on sale of assets

0

'

0

'

'

'

'

'

'

'

'

'

Other income

4.4

'

4.3

'

'

'

'

'

'

'

'

'

Other income - related parties

18.8

'

9.4

6.4

12

'

'

'

'

'

'

'

Total revenues and other income

486.3

'

410.6

'

'

'

'

'

'

'

'

'

Costs and expenses:

'

'

'

'

'

'

'

'

'

'

'

'

Cost of revenues (excludes items below)

135.9

'

162.9

'

'

'

'

'

'

'

'

'

Purchases from related parties

94.6

'

29

'

'

'

'

'

'

'

'

'

Depreciation

48.9

'

36.3

'

'

'

'

'

'

'

'

'

General and administrative expenses

53.7

'

38.5

'

'

'

'

'

'

'

'

'

Other taxes

6.2

'

11.9

'

'

'

'

'

'

'

'

'

Total costs and expenses

339.3

'

278.6

'

'

'

'

'

'

'

'

'

Income from operations

147

'

132

'

'

'

'

'

'

'

'

'

Related party interest and other financial income

0

'

2.3

'

'

'

'

'

'

'

'

'

Net interest and other financial income (costs)

-1.1

'

-0.2

'

'

'

'

'

'

'

'

'

Income before income taxes

145.9

'

134.1

'

'

'

'

'

'

'

'

'

Provision (benefit) for income taxes

-0.2

'

0.1

'

'

'

'

'

'

'

'

'

Net income

146.1

26.3

134

'

'

'

'

'

'

'

'

'

Less: Net income attributable to MPC-retained interest

68.2

'

0

'

'

'

'

'

'

'

'

'

Net income attributable to MPLX LP

77.9

'

134

'

'

'

'

6.4

37.4

'

'

'

Less: General partnerbs interest in net income attributable to MPLX LP subsequent to initial public offering

1.7

0.2

'

'

'

'

'

'

'

'

'

'

Limited partnersb interest in net income attributable to MPLX LP

$76.20

$12.90

'

'

'

'

'

'

'

'

'

'

Net income attributable to MPLX LP per limited partner unit:

'

'

'

'

'

'

'

'

'

'

'

'

Basic (in USD per unit)

'

'

'

'

'

'

'

$0.17

$1.01

'

'

'

Diluted (in USD per unit)

'

'

'

'

'

'

'

$0.17

$1.01

'

'

'

Basic and Diluted (in USD per unit)

'

'

'

'

'

'

'

'

$1.01

$0.17

'

'

Weighted average limited partner units outstanding (basic and diluted):

'

'

'

'

'

'

'

'

'

'

'

'

Basic and Diluted Units (in units)

'

'

'

'

'

17.1

17.1

'

37

37

19.9

19.9

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Current assets:	'	'
Cash and cash equivalents	$54.10	$216.70
Receivables	12.2	17.6
Receivables from related parties	48.3	38
Materials and supplies inventories	11.6	8.9
Other current assets	8.9	2.6
Total current assets	135.1	283.8
Property, plant and equipment, net	966.6	910
Goodwill	104.7	104.7
Other noncurrent assets	2.1	2.8
Total assets	1,208.50	1,301.30
Current liabilities:	'	'
Accounts payable	30.5	39
Payables to related parties	12.8	13.4
Deferred revenue - related parties	34	4.2
Accrued taxes	4	3
Long-term debt due within one year	0.7	0.7
Other current liabilities	1.4	3
Total current liabilities	83.4	63.3
Long-term debt	9.8	10.6
Deferred credits and other liabilities	1.2	0.6
Total liabilities	94.4	74.5
Commitments and contingencies (see Note 18)	'	'
Equity	'	'
Total MPLX LP partners' capital	646.2	691.1
Noncontrolling interest retained by MPC	467.9	535.7
Total equity	1,114.10	1,226.80
Total liabilities and equity	1,208.50	1,301.30
MPC [Member]	'	'
Current assets:	'	'
Receivables from related parties	47.4	37.6
Current liabilities:	'	'
Payables to related parties	12.8	13.4
Deferred revenue - related parties	34	4.2
Limited Partners Common Units [Member] | MPC [Member]	'	'
Equity	'	'
Total MPLX LP partners' capital	57.4	57.4
Limited Partners Subordinated Units [Member] | MPC [Member]	'	'
Equity	'	'
Total MPLX LP partners' capital	209.3	209.3
General Partner [Member] | MPC [Member]	'	'
Equity	'	'
Total MPLX LP partners' capital	-32.5	13.7
Public [Member] | Limited Partners Common Units [Member]	'	'
Equity	'	'
Total MPLX LP partners' capital	$412	$410.70

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical)	Dec. 31, 2013	Dec. 31, 2012
General partner units outstanding	1,508,225	'
Limited Partners Common Units [Member] | Public [Member]	'	'
Units issued	19,900,000	19,900,000
Units outstanding	19,900,000	19,900,000
MPC [Member] | Limited Partners Common Units [Member]	'	'
Units issued	17,100,000	17,100,000
Units outstanding	17,100,000	17,100,000
MPC [Member] | Limited Partners Subordinated Units [Member]	'	'
Units issued	37,000,000	37,000,000
Units outstanding	37,000,000	37,000,000
MPC [Member] | General Partner [Member]	'	'
Units issued	1,500,000	1,500,000
General partner units outstanding	1,500,000	1,500,000

Consolidated_Statements_of_Cas

Consolidated Statements of Cash Flows (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
		Predecessor and Successor [Member]	Predecessor [Member]
Operating activities:	'	'	'
Net income	$146.10	$144	$134
Adjustments to reconcile net income to net cash provided by operating activities:	'	'	'
Depreciation	48.9	39.4	36.3
Deferred income taxes	0.1	-0.1	-0.2
Asset retirement expenditures	-8.3	-9.2	-2
Net loss on disposal of assets	0	0.3	0
Changes in:	'	'	'
Current receivables	5.4	-2	-1
Materials and supplies inventories	1.3	-1	-0.2
Current accounts payable and accrued liabilities	-2.6	-8	11.2
Receivables from / liabilities to related parties	18.9	26.9	1.5
All other, net	2.4	0.3	2.3
Net cash provided by operating activities	212.2	190.6	181.9
Investing activities:	'	'	'
Additions to property, plant and equipment	-106.5	-135.6	-49.8
Disposal of assets	0.2	1.3	0
Investments in related party debt securities - purchases	0	0	-260.6
Investments in related party debt securities - redemptions	0	0	311.7
Investments - loans to a related party	0	0	-220
Investments - repayments of loans receivable from a related party	0	221.7	0
All other, net	-7.3	0	0
Net cash provided by (used in) investing activities	-113.6	87.4	-218.7
Financing activities:	'	'	'
Long-term debt - repayments	-0.7	-0.7	-0.6
Debt issuance costs	0	-2.4	0
Net proceeds from initial public offering	0	407.1	0
Quarterly distributions to unitholders and general partner	-77.8	0	0
Quarterly distributions to noncontrolling interest retained by MPC	-82.7	0	0
Distributions related to purchase of additional interest in Pipe Line Holdings	-100	0	0
Contributions from (distributions to) MPC	'	-262.7	37.3
Net cash provided by (used in) financing activities	-261.2	-61.4	36.7
Net increase (decrease) in cash and cash equivalents	-162.6	216.6	-0.1
Cash and cash equivalents	54.1	216.7	0.1
Cash and cash equivalents at end of period	$54.10	$216.70	$0.10

Consolidated_Statements_of_Equ

Consolidated Statements of Equity/Net Investment (USD $)

Total

MPC [Member]

Predecessor [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 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]

Net Investment [Member]

In Millions

MPC [Member]

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, 2010

'

'

$1,068.20

'

'

'

$0

$0

'

$0

'

$0

'

'

$0

'

$1,068.20

Net income

'

'

134

'

'

'

'

'

'

'

'

'

'

'

'

'

134

Contributions from (distributions to) MPC per consolidated statements of equity/net investment

'

'

37

37

'

'

'

'

'

'

'

'

'

'

'

'

37

Proceeds from initial public offering distributed to MPC

'

'

0

0

'

'

'

'

'

'

'

'

'

'

'

'

'

Non-cash contribution from MPC

'

'

0

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Ending 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

'

'

0

'

'

'

'

192.4

'

361.5

'

13.5

'

'

428.6

'

-996

Proceeds from initial public offering distributed to MPC

407.1

-202.7

'

'

407.1

-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

'

0

'

Purchase 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

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

'

0

'

Beginning Balance at Apr. 28, 2013

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Purchase of additional interest in Pipe Line Holdings

'

'

'

'

'

'

'

'

'

'

'

'

53.6

'

'

'

'

Ending Balance at Dec. 31, 2013

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

$0

'

Description_of_the_Business_an

Description of the Business and Basis of Presentation	12 Months Ended
	Dec. 31, 2013
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, 2013, the Partnership’s assets consisted of a 56.0 percent indirect interest in a network of common carrier crude oil and product pipeline systems and associated storage assets in the Midwest and Gulf Coast regions of the United States. The Partnership also owns a 100 percent interest in a butane cavern in Neal, West Virginia with approximately 1.0 million barrels of storage capacity.
	On October 31, 2012, the Partnership completed its initial public offering (the “Offering”) of 19,895,000 common units (including 2,595,000 common units issued pursuant to the exercise of the underwriters’ over-allotment option), representing limited partner interests. Unless the context otherwise requires, references in this report to “MPLX LP,” the "Partnership,” “we,” “our,” “us,” or like terms used in the present tense or for periods starting on or after October 31, 2012, refer to MPLX LP and its subsidiaries, including MPLX Operations LLC (“MPLX Operations”) and MPLX Terminal and Storage LLC (“MPLX Terminal and Storage”), both wholly-owned subsidiaries, and MPLX Pipe Line Holdings LP (“Pipe Line Holdings”), of which MPLX LP owned a 56.0 percent general partner interest at December 31, 2013. 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. Prior to June 30, 2011, MPC was a wholly-owned subsidiary of Marathon Oil Corporation. Marathon Oil Corporation and all its subsidiaries and equity method investments not spun off with MPC are referred to as “Marathon Oil.” The Partnership was formed on March 27, 2012, as a Delaware limited partnership. See Note 4 for information regarding the closing of the Offering.
	MPLX LP Predecessor included the assets, liabilities and results of operations of certain crude oil and product pipeline systems and associated storage assets of MPC operated and held by MPL and ORPL prior to their contribution to the Partnership in connection with the Offering. Prior to the Offering, MPLX LP Predecessor results also included MPL’s minority undivided joint interests in two crude oil pipeline systems that were not contributed to the Partnership at the Offering. See Note 5.
	Net income attributable to MPLX LP for the period prior to the Offering on October 31, 2012 and the year ended December 31, 2011 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 Offering and the year ended December 31, 2013 reflect the 51.0 percent general partner interest in Pipe Line Holdings initially contributed to MPLX LP through April 30, 2013 and thereafter the 56.0 percent general partner interest. For the periods subsequent to the Offering through April 30, 2013, we consolidated the results of operations of Pipe Line Holdings and then recorded a 49.0 percent noncontrolling interest deduction for the limited partner interest in Pipe Line Holdings retained by MPC. On May 1, 2013, we acquired an additional 5.0 percent interest in Pipe Line Holdings from MPC for consideration of $100.0 million in cash. Following the acquisition, we continued to consolidate the results of operations of Pipe Line Holdings and then recorded a 44.0 percent noncontrolling interest deduction for the limited partner interest in Pipe Line Holdings retained by MPC. The change in the Pipe Line Holdings general partner ownership interest is discussed in Note 3. The Neal, West Virginia butane cavern financial results are reflected only in the periods following the 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 Offering are not comparable to our Predecessor’s historical results of operations.
	Our operations consist of one reportable segment.
	Basis of Presentation – Prior to the Offering on October 31, 2012, our financial position, results of operations and cash flows consisted of MPLX LP Predecessor, which represented a combined reporting entity. Subsequent to the Offering, our financial position, results of operations and cash flows consist of consolidated MPLX LP activities and balances.
	The consolidated statements of income for periods prior to the Offering included expense allocations for certain corporate functions historically performed by MPC and prior to June 30, 2011 by Marathon Oil, 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 and prior to June 30, 2011 from Marathon Oil, 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 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 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.
	Certain reclassifications of prior periods' data have been made to conform to current classifications. On the December 31, 2012 consolidated balance sheet, deferred revenue-related parties of $4.2 million has been reclassified from payables to related parties to a separate line.

Summary_of_Principal_Accountin

Summary of Principal Accounting Policies	12 Months Ended
	Dec. 31, 2013
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 56.0 percent general partner interest and record a noncontrolling interest for the 44.0 percent limited partner interest retained by MPC as of December 31, 2013.
	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.
	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, 2013, the amount of restricted cash included in other current assets on the consolidated balance sheet was $7.3 million.
	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 relative fair market value of the Predecessor’s net property, plant and equipment 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 is less than the book value, including goodwill, then the recorded goodwill is impaired to its implied fair value with a charge 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 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 Offering, the Predecessor’s taxable income was included in the consolidated U.S. federal income tax returns of MPC and of Marathon Oil prior to June 30, 2011, 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 have a grant date fair value of $0.74 per unit, as calculated using a Monte Carlo valuation model.
	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 and by Marathon Oil prior to June 30, 2011 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.

Acquisition

Acquisition	12 Months Ended
	Dec. 31, 2013
Business Combinations [Abstract]	'
Acquisition	'
	Acquisition
	On May 1, 2013, we acquired an additional 5.0 percent interest in Pipe Line Holdings from MPC for consideration of $100.0 million, which was funded with cash on hand. We recorded the additional 5.0 percent interest in Pipe Line Holdings at its historical carrying value of $53.6 million and the excess cash paid over historical carrying value of $46.4 million as a decrease to general partner equity. Prior to this transaction, the 5.0 percent interest was held by MPC and was reflected as part of the noncontrolling interest retained by MPC in our consolidated financial statements. The acquisition of the additional 5.0 percent interest was accounted for on a prospective basis as we acquired an additional interest in an already-consolidated entity. The cash flow impact is reflected in financing activities. Beginning May 1, 2013, our consolidated financial statements reflect the 56.0 percent general partner interest in Pipe Line Holdings owned by MPLX LP, while the 44.0 percent limited partner interest held by MPC is reflected as a noncontrolling interest. The terms of the acquisition were approved by the conflicts committee of the board of directors of our general partner, which is comprised entirely of independent directors.
	Changes in MPLX LP’s equity resulting from changes in its ownership interest in Pipe Line Holdings were as follows:
	(In millions)	2013			2012		2011
	Net income attributable to MPLX LP	$	77.9		$	130.8	$	134
	Transfer to noncontrolling interest retained by MPC:
	Decrease in general partner-MPC equity for purchase of additional interest in Pipe Line Holdings	(46.4		)	—		—
	Change from net income attributable to MPLX LP and transfer to noncontrolling interest retained by MPC	$	31.5		$	130.8	$	134

Initial_Public_Offering

Initial Public Offering	12 Months Ended
	Dec. 31, 2013
Equity [Abstract]	'
Initial Public Offering	'
	Initial Public Offering
	On October 26, 2012, the Partnership’s common units began trading on the New York Stock Exchange under the ticker symbol “MPLX.” On October 31, 2012, we closed our initial public offering of 19,895,000 common units at a price to the public of $22.00 per unit, which included a 2,595,000 common unit over-allotment option that was exercised in full by the underwriters.
	In exchange for MPC’s contribution of assets and liabilities to the Partnership, MPC received:
	•	17,056,515 common units and 36,951,515 subordinated units, representing an aggregate 71.6 percent limited partner interest in us;
	•	all of our incentive distribution rights;
	•	1,508,225 general partner units, representing a 2.0 percent general partner interest; and
	•	an aggregate cash distribution of $202.7 million.
	The Partnership received net proceeds of $407.1 million from the sale of 19,895,000 common units, after deducting underwriting discounts and commissions, structuring fees and offering expenses (the “Offering Costs”) of $30.6 million. The Partnership retained $191.6 million of these net proceeds to prefund certain estimated expansion capital expenditures, $10.4 million for working capital purposes and paid financing costs of $2.4 million before the cash distribution to MPC.
	Reconciliation of Cash Proceeds
	(In millions)
	Total proceeds from the Offering	$	437.7
	Less: Offering Costs	(30.6		)
	Net proceeds from the 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 Offering	$	202.7

Related_Party_Agreements_and_T

Related Party Agreements and Transactions	12 Months Ended
	Dec. 31, 2013
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.
	•	Marathon Oil until June 30, 2011.
	•	Centennial Pipeline LLC (“Centennial”), in which MPC has a 50.0 percent interest. Centennial owns a products pipeline and storage facility.
	•	Muskegon Pipeline LLC (“Muskegon”), in which MPC has a 60.0 percent interest. Muskegon owns a common carrier products pipeline.
	Commercial Agreements
	At the closing of the Offering, the Partnership entered into long-term, fee-based transportation services agreements with MPC. On October 1, 2012, MPL entered into long-term, fee-based storage services agreements with MPC. Under these agreements, we provide transportation and storage services to MPC, and MPC has committed to provide us with minimum quarterly throughput and storage volumes of crude oil and products and minimum storage volumes of butane. We believe the terms and conditions under these agreements, as well as our initial agreements with MPC described below, are generally no less favorable to either party than those that could have been negotiated with unaffiliated parties with respect to similar services.
	These commercial agreements with MPC include:
	•	three separate 10-year transportation services agreements and one five-year transportation services agreement under which MPC pays the Partnership fees for transporting crude oil on each of our crude oil pipeline systems;
	•	four separate 10-year transportation services agreements under which MPC pays the Partnership fees for transporting products on each of our product pipeline systems;
	•	a five-year transportation services agreement under which MPC pays the Partnership fees for handling crude oil and products at our Wood River, Illinois barge dock;
	•	a 10-year storage services agreement under which MPC pays the Partnership fees for providing storage services at our Neal, West Virginia butane cavern; and
	•	four separate three-year storage services agreements under which MPC pays the Partnership fees for providing storage services at our tank farms.
	All of our transportation services agreements for our crude oil and product pipeline systems (other than our Wood River, Illinois to Patoka, Illinois crude system) automatically renew for up to two additional five-year terms unless terminated by either party. The transportation services agreements for our Wood River to Patoka crude system and our barge dock automatically renew for up to four additional two-year terms unless terminated by either party. Our butane cavern storage services agreement does not automatically renew. Our storage services agreements for our tank farms automatically renew for additional one-year terms unless terminated by either party.
	Under our transportation services agreements, if MPC fails to transport its minimum throughput volumes during any quarter, then MPC will pay us a deficiency payment equal to the volume of the deficiency multiplied by the tariff rate then in effect. If the minimum capacity of the pipeline falls below the level of MPC’s commitment at any time or if capacity on the pipeline is required to be allocated among shippers because volume nominations exceed available capacity, depending on the cause of the reduction in capacity, MPC’s commitment may be reduced or MPC will receive a credit for its minimum volume commitment for that period. In addition to MPC’s minimum volume commitment, MPC is also responsible for any loading, handling, transfer and other charges with respect to volumes we transport for MPC. If we agree to make any capital expenditures at MPC’s request, MPC will reimburse us for, or we will have the right in certain circumstances, to file for an increased tariff rate to recover the actual cost of such capital expenditures. Our transportation services agreements include provisions that permit MPC to suspend, reduce or terminate its obligations under the applicable agreement if certain events occur. These events include MPC deciding to permanently or indefinitely suspend refining operations at one or more of its refineries for at least twelve consecutive months and certain force majeure events that would prevent us or MPC from performing required services under the applicable agreement.
	Under the storage services agreements, we are obligated to make available to MPC on a firm basis the available storage capacity at our tank farms and butane cavern, and MPC pays us a per-barrel fee for such storage capacity, regardless of whether MPC fully utilizes the available capacity. We may adjust the per-barrel fee by a percentage equal to an increase in the PPI in early 2014.
	Operating Agreements
	At the closing of the Offering, the Partnership entered into an operating services agreement with MPC under which we operate various pipeline systems owned by MPC. In addition, under existing operating services agreements that MPL had previously entered into with MPC and third parties, MPL continues to operate various pipeline systems owned by MPC and third parties. Under these operating services agreements, the Partnership receives an operating fee for operating the assets and is reimbursed for 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 that it acquired in conjunction with MPC’s acquisition of the Galveston Bay refinery. Under the operating agreement, we receive an initial annual fee of $1.0 million, subject to adjustment for inflation, and are reimbursed for specific costs associated with operating the pipeline systems. The term of the operating agreement is until December 31, 2014, and it is automatically extended from year to year thereafter unless terminated by either party at least three months prior to the end of the term.
	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. Under the agreement we receive an annual fee of initially $1.8 million, subject to adjustment for inflation, and 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 Offering, MPL entered into two management services agreements with MPC under which we provide certain management services to MPC with respect to certain of MPC’s retained pipeline assets. We receive fixed annual fees under the agreements for providing the required management services in the amount of $0.7 million. We may adjust annually for inflation and based on changes in the scope of management services provided.
	Omnibus Agreement
	Upon the closing of the Offering, the Partnership entered into an omnibus agreement with MPC that addresses our payment of a fixed annual fee to MPC for the provision of executive management services by certain executive officers of our general partner and our reimbursement of MPC for the provision of certain general and administrative services to us, as well as MPC’s indemnification of us for certain matters, including environmental, title and tax matters.
	Employee Services Agreements
	Effective October 1, 2012, the Partnership entered into two employee services agreements with MPC under which we reimburse MPC for the provision of certain operational and management services to us in support of our pipelines, barge dock, butane cavern and tank farms.
	Related Party Transactions
	We believe that transactions with related parties, other than certain transactions with MPC and Marathon Oil related to the provision of administrative services for periods prior to the Offering, were conducted on terms comparable to those with unrelated parties. See below for a description of transactions with MPC and Marathon Oil.
	Sales to related parties were as follows:
	(In millions)		2013			2012		2011
	MPC		$	384.2		$	367.8	$	334.8
	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)		2013			2012		2011
	MPC		$	17.6		$	12	$	6.4
	Marathon Oil		—			—		1.9
	Centennial		1.1			1		1
	Muskegon		0.1			0.1		0.1
	Total		$	18.8		$	13.1	$	9.4
	Subsequent to the Offering, MPC provides executive management services and certain general and administrative services to us under terms of the omnibus agreement. For periods prior to the Offering, MPC and, prior to June 30, 2011, Marathon Oil 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)		2013			2012		2011
	MPC		$	18		$	13.6	$	14.7
	Marathon Oil		—			—		0.5
	Total		$	18		$	13.6	$	15.2
	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 and Marathon Oil for periods prior to the Offering. These charges were as follows:
	(In millions)		2013			2012		2011
	MPC		$	31.4		$	22.6	$	19.2
	Marathon Oil		—			—		1.7
	Total		$	31.4		$	22.6	$	20.9
	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)		2013			2012		2011
	MPC		$	8		$	5.5	$	3.6
	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 and $0.6 million for 2012 and 2011, respectively.
	(In millions)		2013			2012		2011
	Purchases from related parties		$	76.6		$	30.8	$	13.8
	General and administrative expenses		15.7			23.1		14.6
	Total		$	92.3		$	53.9	$	28.4
	Receivables from related parties were as follows:
			December 31,
	(In millions)		2013			2012
	MPC		$	47.4		$	37.6
Centennial		0.6			0.3
Muskegon		0.3			0.1
Total		$	48.3		$	38
Prepaid assets with related parties included in other current assets on the consolidated balance sheets were as follows:
		December 31,
(In millions)		2013			2012
MPC		$	0.4		$	—
At December 31, 2013 and 2012, we also had long-term receivables of $0.2 million and $0.3 million from MPC, which were included in other noncurrent assets on the consolidated balance sheets, related to an indemnification provided under the omnibus agreement.
Payables to related parties were as follows:
		December 31,
(In millions)		2013			2012
MPC		$	12.8		$	13.4
Under our transportation services agreements, if MPC fails to transport its minimum throughput volumes during any quarter, then MPC will pay us a deficiency payment equal to the volume of the deficiency multiplied by the tariff rate then in effect. The deficiency amounts are recorded as deferred revenue-related parties. MPC may then apply the amount of any such deficiency payments as a credit for volumes transported on the applicable pipeline system in excess of its minimum volume commitment during the following four or eight quarters under the terms of the applicable transportation services agreement. We recognize revenues for the deficiency payments 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 2013, and the period October 31, 2012 through December 31, 2012, MPC did not transport its minimum committed volumes on certain pipeline systems. The resulting deficiencies were all related to transportation services agreements which have four quarter make-up periods. The deferred revenue-related parties associated with the minimum volume deficiencies were as follows:
	December 31,
(In millions)	2013			2012
Minimum volume deficiencies - MPC	$	34		$	4.2
To centralize cash management activities for MPC, MPC Investment Fund, Inc. (“MPCIF”), a wholly-owned subsidiary of MPC, was established and an agreement was executed on June 21, 2011, among MPCIF, MPL and ORPL. On a daily basis, we sent our excess cash to MPCIF as an advance or requested cash from MPCIF as a draw. Our net cash balance with MPCIF on the last day of each quarter was classified as loans receivable from related party or as loans payable to related party. The loan balance remained constant until the last day of the next quarter. Loans receivable earned interest at the three-month London Interbank Offered Rate (“LIBOR”) plus 10 basis points. Loans payable bore interest at the three-month LIBOR plus 50 basis points. At the end of each quarter, the net balance of the daily advances and draws and the accrued interest was rolled into the loan balance for the subsequent quarter. 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. At December 31, 2011, our loans receivable from MPCIF were $220.4 million. The agreement was terminated on September 28, 2012, in connection with the Offering.
In 2011, we had agreements with MOC Portfolio Delaware, Inc. (“PFD”), a subsidiary of Marathon Oil, to invest our excess cash. Such investments consisted of shares of PFD Redeemable Class A, Series 1 Preferred Stock (“PFD Preferred Stock”). We had the right to redeem all or any portion of the PFD Preferred Stock on any business day at $2,000 per share. Dividends on PFD Preferred Stock were declared and settled daily. All of our investments in PFD Preferred Stock were redeemed prior to the termination of these agreements on June 30, 2011.
Related party interest and other financial income were as follows:
(In millions)		2013			2012		2011
Dividend income:
PFD Preferred Stock		$	—		$	—	$	1.9
Interest income:
Loans receivable from MPCIF		—			1.3		0.4
Related party interest and other financial income		$	—		$	1.3	$	2.3
In 2012, we sold a newly constructed asset to MPC for its $1.3 million carrying value.
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 $0.3 million for 2013 and net non-cash distributions to MPC were $98.2 million and $0.3 million in 2012 and 2011. We recorded property, plant and equipment additions related to capitalized interest incurred by MPC on our behalf of $0.3 million, $0.7 million and less than $0.1 million in 2013, 2012 and 2011, 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 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 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, 2013
Earnings Per Share [Abstract]	'
Net Income Per Limited Partner Unit	'
	Net Income Per Limited Partner Unit
	Net income per unit applicable to common limited partner units and to subordinated limited partner units is computed by dividing the respective limited partners’ interest in net income attributable to MPLX LP by the weighted average number of common units and subordinated units outstanding. Because we have more than one class of participating securities, we use the two-class method when calculating the net income per unit applicable to limited partners. The classes of participating securities include common units, subordinated units, general partner units, certain equity-based compensation awards and incentive distribution rights.
	Net income per limited partner unit is only calculated for the periods subsequent to the 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, 2013 or 2012.
	(In millions)	2013				October 31, 2012 to December 31, 2012
	Net income attributable to MPLX LP subsequent to initial public offering	$	77.9			$	13.1
	Less: General partner’s distributions declared (including IDRs)(1)	1.9				0.3
	Limited partners’ distributions declared on common units(1)	43.2				6.5
	Limited partner’s distributions declared on subordinated units(1)	43.1				6.5
	Distributions in excess of net income attributable to MPLX LP	$	(10.3	)		$	(0.2	)
	(1)	On January 23, 2014, we announced the board of directors of our general partner had declared a quarterly cash distribution of $0.3125 per unit, totaling $23.7 million. This distribution was paid on February 14, 2014 to unitholders of record on February 4, 2014.
			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:
	Distributions declared (including IDRs)		$	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 per limited partner unit:
	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, 2013
Equity [Abstract]	'
Equity	'
	Equity
	Units Outstanding - We had 36,951,515 common units outstanding as of December 31, 2013. Of that number, 17,056,515 were owned by MPC, which also owned 36,951,515 subordinated units. The 2.0 percent general partner interest, represented by 1,508,225 general partner units, was owned by MPC.
	There were no changes to the number of units outstanding since the Offering, through December 31, 2013.
	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 2.0 percent general partner interest and assume that our general partner has contributed any additional capital necessary to maintain its 2.0 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)	2013			October 31, 2012 to December 31, 2012
	Net income attributable to MPLX subsequent to initial public offering	$	77.9		$	13.1
	Less: Income allocated to incentive distribution rights	0.1			—
	Net income attributable to MPLX LP available to general and limited partners	$	77.8		$	13.1
	General partner's 2.0% interest in net income attributable to MPLX LP	$	1.6		$	0.2
	Income allocated to incentive distribution rights	0.1			—
	General partner's interest in net income attributable to MPLX LP subsequent to initial public offering	$	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 23, 2014, we declared a quarterly cash distribution, based on the results of the fourth quarter of 2013, totaling $23.7 million, or $0.3125 per unit. This distribution was paid on February 14, 2014 to unitholders of record on February 4, 2014.
	The allocation of total quarterly cash distributions to general and limited partners is as follows for the year ended December 31, 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)	2013			October 31, 2012 to December 31, 2012
	General partner's distributions:
	General partner's distributions	$	1.8		$	0.3
	General partner's incentive distribution rights	0.1			—
	Total general partner's distributions	$	1.9		$	0.3
	Limited partners' distributions:
	Common unitholders	$	43.2		$	6.5
	Subordinated unitholders	43.1			6.5
	Total limited partners' distributions	86.3			13
	Total cash distributions declared	$	88.2		$	13.3

Major_Customer_and_Concentrati

Major Customer and Concentration of Credit Risk	12 Months Ended
	Dec. 31, 2013
Risks and Uncertainties [Abstract]	'
Major Customer and Concentration of Credit Risk	'
	Major Customer and Concentration of Credit Risk
	MPC accounted for 82.6 percent, 82.2 percent and 83.1 percent of our total revenues and other income for 2013, 2012 and 2011, 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, 2013
Other Income and Expenses [Abstract]	'
Other Items	'
	Other Items
	Net interest and other financial income (costs) were:
	(In millions)	2013			2012			2011
	Interest income	$	0.3		$	0.1		$	—
	Interest expense	(1.1		)	(0.8		)	(0.7		)
	Interest capitalized	0.9			0.7			0.5
	Other financial costs	(1.2		)	(0.2		)	—
	Net interest and other financial income (costs)	$	(1.1	)	$	(0.2	)	$	(0.2	)

Income_Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2013
Income Tax Disclosure [Abstract]	'
Income Taxes	'
	Income Taxes
	We are not a taxable entity for United States federal income tax purposes or for the majority of states that impose an income tax. Taxes on our net income generally are borne by our partners through the allocation of taxable income. Our income tax provision results from partnership activity in the states of Texas, Tennessee and Kentucky.
	Our income tax benefit was $0.2 million in 2013 and expense of $0.3 million and $0.1 million for 2012 and 2011. Our effective tax rate was (0.1) percent, 0.2 percent and 0.1 percent for 2013, 2012 and 2011.
	The following table summarizes the activity in unrecognized tax benefits for 2013 and 2012. There were no unrecognized tax benefits in 2011.
	(In millions)	2013			2012
	January 1 balance	$	0.2		$	—
	Additions (reductions) for tax positions of prior years	(0.2		)	0.2
	December 31 balance	$	—		$	0.2
	If the unrecognized tax benefits as of December 31, 2013 were recognized, less than $0.1 million would affect our effective income tax rate. There were no uncertain tax positions as of December 31, 2013 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 $0.1 million in 2013, and net expenses of $0.1 million in 2012 and less than $0.1 million in 2011. As of December 31, 2013 and 2012, less than $0.1 million and $0.1 million of interest and penalties were accrued related to income taxes. In addition, we have state tax year 2012 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, 2013 and 2012, we had no net deferred tax liability.

Property_Plant_and_Equipment

Property, Plant and Equipment	12 Months Ended
	Dec. 31, 2013
Property, Plant and Equipment [Abstract]	'
Property, Plant and Equipment	'
	Property, Plant and Equipment
		Estimated	December 31,
	(In millions)	Useful  Lives	2013		2012
	Land		$	5.3	$	5.3
	Pipelines and related assets	19 - 42 years	1,060.70		971.7
	Storage and delivery facilities	24 - 37 years	165.7		164.8
	Other	10 - 25 years	22.8		22.6
	Assets under construction		21.9		21.9
	Total		1,276.40		1,186.30
	Less accumulated depreciation		309.8		276.3
	Property, plant and equipment, net		$	966.6	$	910
	Property, plant and equipment includes gross assets acquired under capital leases of $24.9 million at December 31, 2013 and 2012, with related amounts in accumulated depreciation of $5.7 million and $5.0 million at December 31, 2013 and 2012.

Goodwill

Goodwill	12 Months Ended
	Dec. 31, 2013
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 changes in the carrying amount of goodwill for 2013 and 2012 were as follows:
	(In millions)		2013		2012
	Beginning balance		$	104.7	$	134.2
	Distribution to MPC (1)		—		(29.5		)
	Ending balance		$	104.7	$	104.7
	(1)	Associated with the minority undivided joint interests in two crude oil pipeline systems distributed on October 31, 2012, in conjunction with the Offering. See Notes 5 and 15.

Fair_Value_Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2013
Fair Value Disclosures [Abstract]	'
Fair Value Measurements	'
	Fair Value Measurements
	Fair Values – Recurring
	There were no assets accounted for at fair value on a recurring basis at December 31, 2013 and 2012.
	Fair Values – Reported
	Our primary financial instruments are trade receivables and payables. We believe the carrying values of our current assets and liabilities approximate fair value. Our fair value assessment incorporates a variety of considerations, including (1) the short-term duration of the instruments, (2) MPC’s investment-grade credit rating and (3) our historical incurrence of and expected future insignificance of bad debt expense, which includes an evaluation of counterparty credit risk.

Debt

Debt	12 Months Ended
	Dec. 31, 2013
Debt Disclosure [Abstract]	'
Debt	'
	Debt
	Our outstanding borrowings at December 31, 2013 and 2012 consisted of the following:
			December 31,
	(In millions)		2013		2012
	MPL - capital lease obligations due 2020		$	10.5	$	11.3
	Amounts due within one year		0.7		0.7
	Total long-term debt due after one year		$	9.8	$	10.6
	The following table shows five years of scheduled debt payments.
	(In millions)
	2014	$	0.7
	2015	0.8
	2016	0.9
	2017	0.9
	2018	1
	Revolving Credit Agreement
	On September 14, 2012, MPLX Operations, an affiliate of MPC and a wholly-owned subsidiary of the Partnership, as the borrower, and the Partnership, as the parent guarantor, entered into a five-year revolving credit agreement (“Credit Agreement”) with a syndicate of lenders. The Credit Agreement became effective following the Offering and has an initial borrowing capacity of $500.0 million. The Credit Agreement includes letter of credit issuing capacity of up to $250.0 million and swingline loan capacity of up to $50.0 million. There were no borrowings or letters of credit outstanding under the Credit Agreement at December 31, 2013 or 2012.
	As defined in the Credit Agreement, the financial covenant requires us to maintain a ratio of Consolidated Total Debt as of the end of each fiscal quarter to Consolidated EBITDA for the prior four fiscal quarters of not greater than 5.0 to 1.0 (or 5.5 to 1.0 during the six-month period following certain acquisitions). We were in compliance with this financial covenant at December 31, 2013 and 2012 as well as other covenants contained in the Credit Agreement.
	Borrowings under the Credit Agreement bear interest at the Adjusted LIBO Rate plus a margin, or the Alternate Base Rate plus a margin (as defined in the Credit Agreement). The Credit Agreement also provides for customary fees, including administrative agent fees, commitment fees ranging from 0.10 percent to 0.35 percent of the unused portion, depending on the Partnership’s ratio of Consolidated Total Debt to Consolidated EBITDA for the prior four fiscal quarters prior to the rating date, or the Partnership’s credit ratings subsequent to the rating date, fronting and issuance fees in respect to letters of credit and other fees.

Supplemental_Cash_Flow_Informa

Supplemental Cash Flow Information	12 Months Ended
	Dec. 31, 2013
Supplemental Cash Flow Elements [Abstract]	'
Supplemental Cash Flow Information	'
	Supplemental Cash Flow Information
	(In millions)	2013			2012			2011
	Net cash provided by operating activities included:
	Interest paid (net of amounts capitalized)	$	—		$	—		$	0.2
	Income taxes paid	0.1			0.2			0.3
	Non-cash investing and financing activities:
	Net transfers of property, plant and equipment to materials and	$	4		$	—		$	—
	supplies inventories
	Property, plant and equipment contributed by MPC	0.3			—			—
	Distribution to MPC in conjunction with the 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 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)	2013			2012			2011
	Additions to property, plant and equipment	$	106.5		$	135.6		$	49.8
	Plus: Increase (decrease) in capital accruals	(5.3		)	3.2			4.1
	Asset retirement expenditures	8.3			9.2			2
	Total capital expenditures	$	109.5		$	148		$	55.9
	The following is a reconciliation of contributions from (distributions to) MPC:
	(In millions)	2013			2012			2011
	Contributions from (distributions to) MPC per consolidated statements of cash flows	$	—		$	(262.7	)	$	37.3
	Net non-cash contributions from (distributions to) MPC	0.3			(98.2		)	(0.3		)
	Contributions from (distributions to) MPC per consolidated statements of equity/net investment	$	0.3		$	(360.9	)	$	37
	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, 2013
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, 2013 were less than $0.1 million.
	The fair values of phantom units are based on the fair value of MPLX LP common limited partner units on the grant date.
	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 75.0 percent in cash and 25.0 percent in MPLX LP common units. The performance units paying out in cash are accounted for as liability awards and recorded at fair value with a mark-to-market adjustment made each quarter. The performance units paying out in units are accounted for as equity awards and have a grant date fair value of $0.74 per unit, as calculated using a Monte Carlo valuation model.
	Outstanding Phantom Unit Awards
	The following is a summary of phantom unit award activity of MPLX LP common limited partner units in 2013:
			Phantom Units
			Number		Weighted		Aggregate Intrinsic Value (In millions)
			of Units		Average
					Fair Value
	Outstanding at December 31, 2012		1,951		$	26.69
	Granted		75,803		34.02
	Outstanding at December 31, 2013		77,754		33.84
	Vested and expected to vest at December 31, 2013		76,515		33.85		$	3.4
	Convertible at December 31, 2013		15,072		34.7		$	0.7
	The 15,072 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.
	As of December 31, 2013, unrecognized compensation cost related to phantom unit awards was $1.3 million, which is expected to be recognized over a weighted average period of 1.8 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 2013:
		Number of Units
	Outstanding at December 31, 2012	—
	Granted	436,917
	Settled	—
	Forfeited	—
	Outstanding at December 31, 2013	436,917
	As of December 31, 2013, unrecognized compensation cost related to equity-classified performance unit awards was $0.2 million, which is expected to be recognized over a weighted average period of 2 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:
	Expected annual distributions per unit	$	1.05
	Risk-free interest rate	0.35		%
	Look-back period	2.84 years
	Expected volatility	16.75		%
	Grant date fair value of performance units granted	$	0.74
	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 $1.4 million in 2013 and $0.1 million in 2012 for the period subsequent to the 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 2011 and for 2012 for the period prior to the transfer of the MPL employees on October 1, 2012, expense allocated to the Predecessor was $0.6 million and $0.9 million. For the period October 1, 2012 through December 31, 2012 and for 2013, $0.2 million and $1.0 million, respectively, of stock-based compensation expenses were charged to MPLX under our employee matters agreement with MPC.

Leases

Leases	12 Months Ended
	Dec. 31, 2013
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, 2013, 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
	2014	$	1.4	$	8.7
	2015	1.3		8.7
	2016	1.4		8.1
	2017	1.3		7.3
	2018	1.4		7.3
	Later years	6.6		10.9
	Total minimum lease payments	13.4		$	51
	Less imputed interest costs	2.9
	Present value of net minimum lease payments	$	10.5
	Operating lease rental expense was:
	(In millions)	2013		2012		2011
	Minimum rental	$	9.9	$	7	$	2.4

Commitments_and_Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2013
Commitments and Contingencies Disclosure [Abstract]	'
Commitments and Contingencies	'
	Commitments and Contingencies
	We are the subject of, or a party to, a number of pending or threatened legal actions, contingencies and commitments involving a variety of matters, including laws and regulations relating to the environment. Some of these matters are discussed below. For matters for which we have not recorded an accrued liability, we are unable to estimate a range of possible losses for the reasons discussed in more detail below. However, the ultimate resolution of some of these contingencies could, individually or in the aggregate, be material.
	Environmental matters – We are subject to federal, state and local laws and regulations relating to the environment. These laws generally provide for control of pollutants released into the environment and require responsible parties to undertake remediation of hazardous waste disposal sites. Penalties may be imposed for noncompliance.
	At December 31, 2013 and 2012, accrued liabilities for remediation totaled $1.1 million and $1.5 million, respectively. At December 31, 2013 and 2012, it is reasonably possible that an estimated loss existed of up to $0.4 million and $0.5 million in excess of the amount accrued for remediation. However, it is not presently possible to estimate the ultimate amount of all remediation costs that might be incurred or the penalties, if any, that may be imposed. Receivables from MPC for indemnification of environmental costs related to incidents occurring prior to the Offering were $0.3 million and $2.0 million at December 31, 2013 and 2012, respectively.
	Legal Proceedings – In 2003, the State of Illinois brought an action against the Premcor Refining Group, Inc. (“Premcor”) and Apex Refining Company (“Apex”) asserting claims for environmental cleanup related to the refinery owned by these entities in the Hartford/Wood River, Illinois area. In 2006, Premcor and Apex filed 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 $1.4 million as of December 31, 2013, which consist of leases of vehicles extending through 2018 that contain general lease indemnities and guaranteed residual values.
	Over the years, we have sold various assets in the normal course of our business. Certain of the related agreements contain performance and general guarantees, including guarantees regarding inaccuracies in representations, warranties, covenants and agreements, and environmental and general indemnifications that require us to perform upon the occurrence of a triggering event or condition. These guarantees and indemnifications are part of the normal course of selling assets. We are typically not able to calculate the maximum potential amount of future payments that could be made under such contractual provisions because of the variability inherent in the guarantees and indemnities. Most often, the nature of the guarantees and indemnities is such that there is no appropriate method for quantifying the exposure because the underlying triggering event has little or no past experience upon which a reasonable prediction of the outcome can be based.
	Contractual commitments – At December 31, 2013 our contractual commitments to acquire property, plant and equipment totaled $9.6 million. Our contractual commitments at December 31, 2013 were primarily related to a multi-year SCADA (supervisory control and data acquisition) system upgrade project.

Subsequent_Event

Subsequent Event	12 Months Ended
	Dec. 31, 2013
Subsequent Events [Abstract]	'
Subsequent Events	'
	Subsequent Event
	On February 27, 2014, we entered into an agreement to acquire an additional 13.0 percent interest in Pipe Line Holdings from MPC for consideration of $310.0 million. The transaction will be effective March 1, 2014. We intend to finance the transaction with $40.0 million cash on hand and by borrowing $270.0 million on our revolver. Following the acquisition, our ownership interest in Pipe Line Holdings will be 69.0 percent.

Condensed_Consolidating_Financ

Condensed Consolidating Financial Statement	12 Months Ended
	Dec. 31, 2013
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 are anticipated to be fully and unconditionally guaranteed by the Parent Guarantor.
	The following condensed consolidating financial information reflects the Partnership’s stand-alone accounts, the accounts of the Subsidiary Issuer, the combined accounts of the Non-Guarantor Subsidiaries, consolidating adjustments and the Partnership’s consolidated financial information. The condensed consolidating financial information should be read in conjunction with the Partnership’s accompanying consolidated financial statements and related notes. The Parent Guarantor's and the Subsidiary Issuer’s investment in and equity income from their consolidated subsidiaries are presented in accordance with the equity method of accounting in which the equity income from consolidated subsidiaries includes the results of operations of the Partnership assets for periods including and subsequent to the Offering. For periods prior to the Offering, Non-Guarantor Subsidiaries represent MPLX LP Predecessor.
	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 income (costs)	—			(2.3		)	1.2			—			(1.1		)
	Income before income taxes	77.9			83			153.3			(168.3		)	145.9
	Provision (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 income (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 Statements of Income
		Year Ended December 31, 2011
	(in millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
	Revenues and other income:
	Sales and other operating revenues	$	—		$	—		$	62.1		$	—		$	62.1
	Sales to related parties	—			—			334.8			—			334.8
	Other income	—			—			4.3			—			4.3
	Other income - related parties	—			—			9.4			—			9.4
Total revenues and other income	—			—			410.6			—			410.6
Costs and expenses:
Cost of revenues (excludes items below)	—			—			162.9			—			162.9
Purchases from related parties	—			—			29			—			29
Depreciation	—			—			36.3			—			36.3
General and administrative expenses	—			—			38.5			—			38.5
Other taxes	—			—			11.9			—			11.9
Total costs and expenses	—			—			278.6			—			278.6
Income from operations	—			—			132			—			132
Related party interest and other financial income	—			—			2.3			—			2.3
Net interest and other financial income (costs)	—			—			(0.2		)	—			(0.2		)
Income before income taxes	—			—			134.1			—			134.1
Provision for income taxes	—			—			0.1			—			0.1
Net income	$	—		$	—		$	134		$	—		$	134
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 Balance Sheets
	31-Dec-12
(in millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
Assets
Current assets:
Cash and cash equivalents	$	1		$	9.7		$	206		$	—		$	216.7
Receivables	—			—			17.6			—			17.6
Receivables from related parties	2.9			0.2			37.6			(2.7		)	38
Materials and supplies inventories	—			—			8.9			—			8.9
Other current assets	0.3			—			2.3			—			2.6
Total current assets	4.2			9.9			272.4			(2.7		)	283.8
Property, plant and equipment, net	—			—			910			—			910
Investment in subsidiaries	689.3			679.6			—			(1,368.9		)	—
Goodwill	—			—			104.7			—			104.7
Other noncurrent assets	—			2.3			0.5			—			2.8
Total assets	$	693.5		$	691.8		$	1,287.60		$	(1,371.6	)	$	1,301.30
Liabilities
Current liabilities:
Accounts payable	$	0.1		$	—		$	38.9		$	—		$	39
Payables to related parties	2.3			2.5			11.3			(2.7		)	13.4
Deferred revenue - related parties	—			—			4.2			—			4.2
Accrued taxes	—			—			3			—			3
Long-term debt due within one year	—			—			0.7			—			0.7
Other current liabilities	—			—			3			—			3
Total current liabilities	2.4			2.5			61.1			(2.7		)	63.3
Long-term debt	—			—			10.6			—			10.6
Deferred credits and other liabilities	—			—			0.6			—			0.6
Total liabilities	2.4			2.5			72.3			(2.7		)	74.5
Equity
MPLX LP partners’ capital	691.1			689.3			1,215.30			(1,904.6		)	691.1
Noncontrolling interest retained by MPC	—			—			—			535.7			535.7
Total equity	691.1			689.3			1,215.30			(1,368.9		)	1,226.80
Total liabilities and equity	$	693.5		$	691.8		$	1,287.60		$	(1,371.6	)	$	1,301.30
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
Consolidating Statements of Cash Flow
	Year Ended December 31, 2011
(in millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
Net cash provided by operating activities	$	—		$	—		$	181.9		$	—		$	181.9
Investing activities:
Additions to property, plant and equipment	—			—			(49.8		)	—			(49.8		)
Investments in related party debt securities - purchases	—			—			(260.6		)	—			(260.6		)
- redemptions	—			—			311.7			—			311.7
Investments - loans to a related party	—			—			(220.0		)	—			(220.0		)
Net cash used in investing activities	—			—			(218.7		)	—			(218.7		)
Financing activities:
Long-term debt - repayments	—			—			(0.6		)	—			(0.6		)
Contributions from MPC	—			—			37.3			—			37.3
Net cash provided by financing activities	—			—			36.7			—			36.7
Net decrease in cash and cash equivalents	—			—			(0.1		)	—			(0.1		)
Cash and cash equivalents at beginning of period	—			—			0.2			—			0.2
Cash and cash equivalents at end of period	$	—		$	—		$	0.1		$	—		$	0.1

Select_Quarterly_Financial_Dat

Select Quarterly Financial Data	12 Months Ended
	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]	'
Select Quarterly Financial Data	'
	Select Quarterly Financial Data (Unaudited)
		2013								2012
	(In millions, except per unit data)	1st Qtr.		2nd Qtr.		3rd Qtr.		4th Qtr.		1st Qtr.		2nd Qtr.		3rd Qtr.		4th Qtr.
	Revenues	$	109.9	$	116.1	$	118.2	$	118.9	$	97.5	$	105.6	$	116.2	$	122.9
	Income from operations	35.5		35.2		39.8		36.5		34.9		29.5		36.3		42.5
	Net income	35.3		34.8		39.2		36.8		35.2		29.8		36.9		42.1
	Net income attributable to MPLX LP	17.6		18.6		21.5		20.2		35.2		29.8		36.9		28.9
	Net income attributable to MPLX LP subsequent to initial public offering on October 31, 2012	17.6		18.6		21.5		20.2		—		—		—		13.1
	Net income attributable to MPLX LP subsequent to initial public offering per limited partner unit:
	Common - basic	$	0.26	$	0.26	$	0.29	$	0.27	$	—	$	—	$	—	$	0.18
	Common - diluted	0.26		0.26		0.29		0.27		—		—		—		0.18
	Subordinated	0.21		0.23		0.29		0.27		—		—		—		0.17
	Distributions declared per limited partner common unit	$	0.2725	$	0.285	$	0.2975	$	0.3125	$	—	$	—	$	—	$	0.1769
	Distributions declared:
	Limited partner units - Public	$	5.4	$	5.7	$	5.9	$	6.2	$	—	$	—	$	—	$	3.5
	Limited partner units - MPC	14.7		15.4		16.1		16.9		—		—		—		9.5
	General partner units - MPC	0.4		0.4		0.5		0.5		—		—		—		0.3
	Incentive distribution rights - MPC	—		—		—		0.1		—		—		—		—
	Total distributions declared	$	20.5	$	21.5	$	22.5	$	23.7	$	—	$	—	$	—	$	13.3

Summary_of_Principal_Accountin1

Summary of Principal Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2013
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 56.0 percent general partner interest and record a noncontrolling interest for the 44.0 percent limited partner interest retained by MPC as of December 31, 2013.
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.
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, 2013, the amount of restricted cash included in other current assets on the consolidated balance sheet was $7.3 million.
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 relative fair market value of the Predecessor’s net property, plant and equipment 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 is less than the book value, including goodwill, then the recorded goodwill is impaired to its implied fair value with a charge 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 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 Offering, the Predecessor’s taxable income was included in the consolidated U.S. federal income tax returns of MPC and of Marathon Oil prior to June 30, 2011, 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 have a grant date fair value of $0.74 per unit, as calculated using a Monte Carlo valuation model.
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 and by Marathon Oil prior to June 30, 2011 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, 2013
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)	2013			2012		2011
	Net income attributable to MPLX LP	$	77.9		$	130.8	$	134
	Transfer to noncontrolling interest retained by MPC:
	Decrease in general partner-MPC equity for purchase of additional interest in Pipe Line Holdings	(46.4		)	—		—
	Change from net income attributable to MPLX LP and transfer to noncontrolling interest retained by MPC	$	31.5		$	130.8	$	134

Initial_Public_Offering_Tables

Initial Public Offering (Tables)	12 Months Ended
	Dec. 31, 2013
Equity [Abstract]	'
Summary of Reconciliation of Proceeds from Initial Public Offering	'
	Reconciliation of Cash Proceeds
	(In millions)
	Total proceeds from the Offering	$	437.7
	Less: Offering Costs	(30.6		)
	Net proceeds from the 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 Offering	$	202.7

Related_Party_Agreements_and_T1

Related Party Agreements and Transactions (Tables)	12 Months Ended
	Dec. 31, 2013
Related Party Transactions [Abstract]	'
Sales to Related Parties	'
	Sales to related parties were as follows:
	(In millions)		2013			2012		2011
	MPC		$	384.2		$	367.8	$	334.8
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)		2013			2012		2011
	MPC		$	17.6		$	12	$	6.4
	Marathon Oil		—			—		1.9
	Centennial		1.1			1		1
	Muskegon		0.1			0.1		0.1
	Total		$	18.8		$	13.1	$	9.4
Summary of Charges for Services Included in Purchases from Related Parties	'
	Charges for services included in purchases from related parties primarily relate to services that support our operations and maintenance activities. These charges were as follows:
	(In millions)		2013			2012		2011
	MPC		$	18		$	13.6	$	14.7
	Marathon Oil		—			—		0.5
	Total		$	18		$	13.6	$	15.2
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 and Marathon Oil for periods prior to the Offering. These charges were as follows:
	(In millions)		2013			2012		2011
	MPC		$	31.4		$	22.6	$	19.2
	Marathon Oil		—			—		1.7
	Total		$	31.4		$	22.6	$	20.9
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)		2013			2012		2011
	MPC		$	8		$	5.5	$	3.6
Summary of Allocated Related Party Employee Benefit Costs by Income Statement Line Item	'
	(In millions)		2013			2012		2011
	Purchases from related parties		$	76.6		$	30.8	$	13.8
	General and administrative expenses		15.7			23.1		14.6
	Total		$	92.3		$	53.9	$	28.4
Receivables from Related Parties	'
	Receivables from related parties were as follows:
			December 31,
	(In millions)		2013			2012
	MPC		$	47.4		$	37.6
	Centennial		0.6			0.3
	Muskegon		0.3			0.1
	Total		$	48.3		$	38
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)		2013			2012
	MPC		$	0.4		$	—
Payables to Related Parties	'
	Payables to related parties were as follows:
			December 31,
	(In millions)		2013			2012
	MPC		$	12.8		$	13.4
Schedule Of Related Party Deferred Revenue Table	'
	The deferred revenue-related parties associated with the minimum volume deficiencies were as follows:
		December 31,
	(In millions)	2013			2012
	Minimum volume deficiencies - MPC	$	34		$	4.2
Related Party Net Interest and Other Financial Income	'
	Related party interest and other financial income were as follows:
	(In millions)		2013			2012		2011
	Dividend income:
	PFD Preferred Stock		$	—		$	—	$	1.9
	Interest income:
	Loans receivable from MPCIF		—			1.3		0.4
	Related party interest and other financial income		$	—		$	1.3	$	2.3

Net_Income_Per_Limited_Partner1

Net Income Per Limited Partner Unit (Tables)	12 Months Ended
	Dec. 31, 2013
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 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, 2013 or 2012.
	(In millions)	2013				October 31, 2012 to December 31, 2012
	Net income attributable to MPLX LP subsequent to initial public offering	$	77.9			$	13.1
	Less: General partner’s distributions declared (including IDRs)(1)	1.9				0.3
	Limited partners’ distributions declared on common units(1)	43.2				6.5
	Limited partner’s distributions declared on subordinated units(1)	43.1				6.5
	Distributions in excess of net income attributable to MPLX LP	$	(10.3	)		$	(0.2	)
	(1)	On January 23, 2014, we announced the board of directors of our general partner had declared a quarterly cash distribution of $0.3125 per unit, totaling $23.7 million. This distribution was paid on February 14, 2014 to unitholders of record on February 4, 2014.
Schedule of Basic and Diluted Earnings Per Unit	'
			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:
	Distributions declared (including IDRs)		$	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 per limited partner unit:
	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, 2013
Equity [Abstract]	'
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)	2013			October 31, 2012 to December 31, 2012
	Net income attributable to MPLX subsequent to initial public offering	$	77.9		$	13.1
	Less: Income allocated to incentive distribution rights	0.1			—
	Net income attributable to MPLX LP available to general and limited partners	$	77.8		$	13.1
	General partner's 2.0% interest in net income attributable to MPLX LP	$	1.6		$	0.2
	Income allocated to incentive distribution rights	0.1			—
	General partner's interest in net income attributable to MPLX LP subsequent to initial public offering	$	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, 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)	2013			October 31, 2012 to December 31, 2012
	General partner's distributions:
	General partner's distributions	$	1.8		$	0.3
	General partner's incentive distribution rights	0.1			—
	Total general partner's distributions	$	1.9		$	0.3
	Limited partners' distributions:
	Common unitholders	$	43.2		$	6.5
	Subordinated unitholders	43.1			6.5
	Total limited partners' distributions	86.3			13
	Total cash distributions declared	$	88.2		$	13.3

Other_Items_Tables

Other Items (Tables)	12 Months Ended
	Dec. 31, 2013
Other Income and Expenses [Abstract]	'
Summary of Interest and Other Financial Income (Costs)	'
	Net interest and other financial income (costs) were:
	(In millions)	2013			2012			2011
	Interest income	$	0.3		$	0.1		$	—
	Interest expense	(1.1		)	(0.8		)	(0.7		)
	Interest capitalized	0.9			0.7			0.5
	Other financial costs	(1.2		)	(0.2		)	—
	Net interest and other financial income (costs)	$	(1.1	)	$	(0.2	)	$	(0.2	)

Income_Taxes_Tables

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2013
Income Tax Disclosure [Abstract]	'
Schedule of Unrecognized Tax Benefits	'
	The following table summarizes the activity in unrecognized tax benefits for 2013 and 2012. There were no unrecognized tax benefits in 2011.
	(In millions)	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, 2013
Property, Plant and Equipment [Abstract]	'
Summary of Property, Plant and Equipment	'
		Estimated	December 31,
	(In millions)	Useful  Lives	2013		2012
	Land		$	5.3	$	5.3
	Pipelines and related assets	19 - 42 years	1,060.70		971.7
	Storage and delivery facilities	24 - 37 years	165.7		164.8
	Other	10 - 25 years	22.8		22.6
	Assets under construction		21.9		21.9
	Total		1,276.40		1,186.30
	Less accumulated depreciation		309.8		276.3
	Property, plant and equipment, net		$	966.6	$	910

Goodwill_Tables

Goodwill (Tables)	12 Months Ended
	Dec. 31, 2013
Goodwill and Intangible Assets Disclosure [Abstract]	'
Changes in Carrying Amount of Goodwill	'
	The changes in the carrying amount of goodwill for 2013 and 2012 were as follows:
	(In millions)		2013		2012
	Beginning balance		$	104.7	$	134.2
	Distribution to MPC (1)		—		(29.5		)
	Ending balance		$	104.7	$	104.7
	(1)	Associated with the minority undivided joint interests in two crude oil pipeline systems distributed on October 31, 2012, in conjunction with the Offering. See Notes 5 and 15.

Debt_Tables

Debt (Tables)	12 Months Ended
	Dec. 31, 2013
Debt Disclosure [Abstract]	'
Summary of Outstanding Borrowings	'
	Our outstanding borrowings at December 31, 2013 and 2012 consisted of the following:
			December 31,
	(In millions)		2013		2012
	MPL - capital lease obligations due 2020		$	10.5	$	11.3
	Amounts due within one year		0.7		0.7
	Total long-term debt due after one year		$	9.8	$	10.6
Schedule of Debt Payments	'
	The following table shows five years of scheduled debt payments.
	(In millions)
	2014	$	0.7
	2015	0.8
	2016	0.9
	2017	0.9
	2018	1

Supplemental_Cash_Flow_Informa1

Supplemental Cash Flow Information (Tables)	12 Months Ended
	Dec. 31, 2013
Supplemental Cash Flow Elements [Abstract]	'
Summary of Supplemental Cash Flow Information	'
	(In millions)	2013			2012			2011
	Net cash provided by operating activities included:
	Interest paid (net of amounts capitalized)	$	—		$	—		$	0.2
	Income taxes paid	0.1			0.2			0.3
	Non-cash investing and financing activities:
	Net transfers of property, plant and equipment to materials and	$	4		$	—		$	—
	supplies inventories
	Property, plant and equipment contributed by MPC	0.3			—			—
	Distribution to MPC in conjunction with the 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 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)	2013			2012			2011
	Additions to property, plant and equipment	$	106.5		$	135.6		$	49.8
	Plus: Increase (decrease) in capital accruals	(5.3		)	3.2			4.1
	Asset retirement expenditures	8.3			9.2			2
	Total capital expenditures	$	109.5		$	148		$	55.9
Summary of Reconciliation of Contributions from (Distributions to) Former Parent Company	'
	The following is a reconciliation of contributions from (distributions to) MPC:
	(In millions)	2013			2012			2011
	Contributions from (distributions to) MPC per consolidated statements of cash flows	$	—		$	(262.7	)	$	37.3
	Net non-cash contributions from (distributions to) MPC	0.3			(98.2		)	(0.3		)
	Contributions from (distributions to) MPC per consolidated statements of equity/net investment	$	0.3		$	(360.9	)	$	37

EquityBased_Compensation_Plan_

Equity-Based Compensation Plan (Tables)	12 Months Ended
	Dec. 31, 2013
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 2013:
			Phantom Units
			Number		Weighted		Aggregate Intrinsic Value (In millions)
			of Units		Average
					Fair Value
	Outstanding at December 31, 2012		1,951		$	26.69
	Granted		75,803		34.02
	Outstanding at December 31, 2013		77,754		33.84
	Vested and expected to vest at December 31, 2013		76,515		33.85		$	3.4
	Convertible at December 31, 2013		15,072		34.7		$	0.7
Performance Units [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 2013:
		Number of Units
	Outstanding at December 31, 2012	—
	Granted	436,917
	Settled	—
	Forfeited	—
	Outstanding at December 31, 2013	436,917
Schedule of Share-based Compensation, Performance Unit Awards, Valuation Assumptions [Table Text Block]	'
	Expected annual distributions per unit	$	1.05
	Risk-free interest rate	0.35		%
	Look-back period	2.84 years
	Expected volatility	16.75		%
	Grant date fair value of performance units granted	$	0.74

Leases_Tables

Leases (Tables)	12 Months Ended
	Dec. 31, 2013
Leases [Abstract]	'
Future Minimum Commitments	'
	Future minimum commitments as of December 31, 2013, 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
	2014	$	1.4	$	8.7
	2015	1.3		8.7
	2016	1.4		8.1
	2017	1.3		7.3
	2018	1.4		7.3
	Later years	6.6		10.9
	Total minimum lease payments	13.4		$	51
	Less imputed interest costs	2.9
	Present value of net minimum lease payments	$	10.5
Operating Lease Rental Expense	'
	Operating lease rental expense was:
	(In millions)	2013		2012		2011
	Minimum rental	$	9.9	$	7	$	2.4

Condensed_Consolidating_Financ1

Condensed Consolidating Financial Statement (Tables)	12 Months Ended
	Dec. 31, 2013
Condensed Financial Information of Parent Company Only Disclosure [Abstract]	'
Condensed Financial Statements	'
	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 income (costs)	—			(2.3		)	1.2			—			(1.1		)
	Income before income taxes	77.9			83			153.3			(168.3		)	145.9
	Provision (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 income (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 Statements of Income
		Year Ended December 31, 2011
	(in millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
	Revenues and other income:
	Sales and other operating revenues	$	—		$	—		$	62.1		$	—		$	62.1
	Sales to related parties	—			—			334.8			—			334.8
	Other income	—			—			4.3			—			4.3
	Other income - related parties	—			—			9.4			—			9.4
	Total revenues and other income	—			—			410.6			—			410.6
	Costs and expenses:
	Cost of revenues (excludes items below)	—			—			162.9			—			162.9
Purchases from related parties	—			—			29			—			29
Depreciation	—			—			36.3			—			36.3
General and administrative expenses	—			—			38.5			—			38.5
Other taxes	—			—			11.9			—			11.9
Total costs and expenses	—			—			278.6			—			278.6
Income from operations	—			—			132			—			132
Related party interest and other financial income	—			—			2.3			—			2.3
Net interest and other financial income (costs)	—			—			(0.2		)	—			(0.2		)
Income before income taxes	—			—			134.1			—			134.1
Provision for income taxes	—			—			0.1			—			0.1
Net income	$	—		$	—		$	134		$	—		$	134
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 Balance Sheets
	31-Dec-12
(in millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
Assets
Current assets:
Cash and cash equivalents	$	1		$	9.7		$	206		$	—		$	216.7
Receivables	—			—			17.6			—			17.6
Receivables from related parties	2.9			0.2			37.6			(2.7		)	38
Materials and supplies inventories	—			—			8.9			—			8.9
Other current assets	0.3			—			2.3			—			2.6
Total current assets	4.2			9.9			272.4			(2.7		)	283.8
Property, plant and equipment, net	—			—			910			—			910
Investment in subsidiaries	689.3			679.6			—			(1,368.9		)	—
Goodwill	—			—			104.7			—			104.7
Other noncurrent assets	—			2.3			0.5			—			2.8
Total assets	$	693.5		$	691.8		$	1,287.60		$	(1,371.6	)	$	1,301.30
Liabilities
Current liabilities:
Accounts payable	$	0.1		$	—		$	38.9		$	—		$	39
Payables to related parties	2.3			2.5			11.3			(2.7		)	13.4
Deferred revenue - related parties	—			—			4.2			—			4.2
Accrued taxes	—			—			3			—			3
Long-term debt due within one year	—			—			0.7			—			0.7
Other current liabilities	—			—			3			—			3
Total current liabilities	2.4			2.5			61.1			(2.7		)	63.3
Long-term debt	—			—			10.6			—			10.6
Deferred credits and other liabilities	—			—			0.6			—			0.6
Total liabilities	2.4			2.5			72.3			(2.7		)	74.5
Equity
MPLX LP partners’ capital	691.1			689.3			1,215.30			(1,904.6		)	691.1
Noncontrolling interest retained by MPC	—			—			—			535.7			535.7
Total equity	691.1			689.3			1,215.30			(1,368.9		)	1,226.80
Total liabilities and equity	$	693.5		$	691.8		$	1,287.60		$	(1,371.6	)	$	1,301.30
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
Consolidating Statements of Cash Flow
	Year Ended December 31, 2011
(in millions)	Parent Guarantor			Subsidiary Issuer			Non-Guarantor Subsidiaries			Consolidating Adjustments			Total
Net cash provided by operating activities	$	—		$	—		$	181.9		$	—		$	181.9
Investing activities:
Additions to property, plant and equipment	—			—			(49.8		)	—			(49.8		)
Investments in related party debt securities - purchases	—			—			(260.6		)	—			(260.6		)
- redemptions	—			—			311.7			—			311.7
Investments - loans to a related party	—			—			(220.0		)	—			(220.0		)
Net cash used in investing activities	—			—			(218.7		)	—			(218.7		)
Financing activities:
Long-term debt - repayments	—			—			(0.6		)	—			(0.6		)
Contributions from MPC	—			—			37.3			—			37.3
Net cash provided by financing activities	—			—			36.7			—			36.7
Net decrease in cash and cash equivalents	—			—			(0.1		)	—			(0.1		)
Cash and cash equivalents at beginning of period	—			—			0.2			—			0.2
Cash and cash equivalents at end of period	$	—		$	—		$	0.1		$	—		$	0.1

Select_Quarterly_Financial_Dat1

Select Quarterly Financial Data (Tables)	12 Months Ended
	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]	'
Summary of Quarterly Financial Data	'
		2013								2012
	(In millions, except per unit data)	1st Qtr.		2nd Qtr.		3rd Qtr.		4th Qtr.		1st Qtr.		2nd Qtr.		3rd Qtr.		4th Qtr.
	Revenues	$	109.9	$	116.1	$	118.2	$	118.9	$	97.5	$	105.6	$	116.2	$	122.9
	Income from operations	35.5		35.2		39.8		36.5		34.9		29.5		36.3		42.5
	Net income	35.3		34.8		39.2		36.8		35.2		29.8		36.9		42.1
	Net income attributable to MPLX LP	17.6		18.6		21.5		20.2		35.2		29.8		36.9		28.9
	Net income attributable to MPLX LP subsequent to initial public offering on October 31, 2012	17.6		18.6		21.5		20.2		—		—		—		13.1
	Net income attributable to MPLX LP subsequent to initial public offering per limited partner unit:
	Common - basic	$	0.26	$	0.26	$	0.29	$	0.27	$	—	$	—	$	—	$	0.18
	Common - diluted	0.26		0.26		0.29		0.27		—		—		—		0.18
	Subordinated	0.21		0.23		0.29		0.27		—		—		—		0.17
	Distributions declared per limited partner common unit	$	0.2725	$	0.285	$	0.2975	$	0.3125	$	—	$	—	$	—	$	0.1769
	Distributions declared:
	Limited partner units - Public	$	5.4	$	5.7	$	5.9	$	6.2	$	—	$	—	$	—	$	3.5
	Limited partner units - MPC	14.7		15.4		16.1		16.9		—		—		—		9.5
	General partner units - MPC	0.4		0.4		0.5		0.5		—		—		—		0.3
	Incentive distribution rights - MPC	—		—		—		0.1		—		—		—		—
	Total distributions declared	$	20.5	$	21.5	$	22.5	$	23.7	$	—	$	—	$	—	$	13.3

Description_of_the_Business_an1

Description of the Business and Basis of Presentation (Detail) (USD $)

8 Months Ended

12 Months Ended

0 Months Ended

0 Months Ended

1 Months Ended

4 Months Ended

8 Months Ended

12 Months Ended

4 Months Ended

8 Months Ended

12 Months Ended

In Millions, except Share data, unless otherwise specified

Dec. 31, 2013

Dec. 31, 2013

Oct. 31, 2012

Oct. 02, 2012

Oct. 31, 2012

Oct. 31, 2012

Oct. 31, 2012

Apr. 30, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2011

Dec. 31, 2013

Dec. 31, 2013

Apr. 30, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2012

bbl

MPC [Member]

MPC [Member]

Limited Partners Common Units [Member]

Limited Partners Common Units [Member]

Other Ownership Interest Common Units [Member]

MPLX Pipe Line Holdings LP [Member]

MPLX Pipe Line Holdings LP [Member]

MPLX Pipe Line Holdings LP [Member]

MPLX Pipe Line Holdings LP [Member]

Predecessor [Member]

Storage Services Butane Cavern [Member]

Crude Oil And Product Pipeline Systems [Member]

Retained Interest [Member]

Retained Interest [Member]

Noncontrolling Interest [Member]

Deferred Revenue Related Party [Member]

Segment

Pipeline

Employee Services Agreements [Member]

MPL and ORPL [Member]

MPC [Member]

MPC [Member]

MPC [Member]

Agreement

Description Of Business And Basis Of Presentation [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Ownership percentage

'

'

'

'

'

'

'

51.00%

56.00%

56.00%

100.00%

'

100.00%

56.00%

'

'

44.00%

'

Storage capacity (in number of barrels)

'

1,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Date for initial public offering completed

'

'October 31, 2012

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Common units for initial public offering

'

'

'

'

19,895,000

19,895,000

2,595,000

'

'

'

'

'

'

'

'

'

'

'

Date of partnership formation

'

27-Mar-12

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Number of pipeline systems

'

'

2

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Percentage of net income related to assets

'

'

'

'

'

'

'

'

'

'

'

100.00%

'

'

49.00%

44.00%

'

'

Percentage of partners interest acquired

5.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

5.00%

'

Payments to acquire businesses

$100

$100

'

'

'

'

'

'

'

'

'

$0

'

'

'

'

'

'

Number of segments

'

1

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Number of agreements

'

'

'

2

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Prior period reclassification adjustment

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

$4.20

Summary_of_Principal_Accountin2

Summary of Principal Accounting Policies (Detail) (USD $)	12 Months Ended			12 Months Ended		4 Months Ended	8 Months Ended	12 Months Ended	4 Months Ended	8 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Oct. 02, 2012	Dec. 31, 2012	Dec. 31, 2011	Apr. 30, 2013	Dec. 31, 2013	Dec. 31, 2013	Apr. 30, 2013	Dec. 31, 2013	Dec. 31, 2013
		Employees	MPC [Member]	Predecessor and Successor [Member]	Predecessor [Member]	MPLX Pipe Line Holdings LP [Member]	MPLX Pipe Line Holdings LP [Member]	MPLX Pipe Line Holdings LP [Member]	Retained Interest [Member]	Retained Interest [Member]	Officer [Member]
			Employee Services Agreements [Member]						MPC [Member]	MPC [Member]	Mplx 2012 Incentive Compensation Plan [Member]
			Agreement								Performance Shares [Member]
Summary of Principal Accounting Policies [Line Items]	'	'	'	'	'	'	'	'	'	'	'
Ownership percentage	'	'	'	'	'	51.00%	56.00%	56.00%	'	'	'
Percentage of net income related to assets	'	'	'	'	100.00%	'	'	'	49.00%	44.00%	'
Restricted cash	$7,300,000	'	'	'	'	'	'	'	'	'	'
Number of employees	'	0	'	'	'	'	'	'	'	'	'
Number of agreements	'	'	2	'	'	'	'	'	'	'	'
Grant date fair value of performance units granted	'	'	'	'	'	'	'	'	'	'	$0.74
Comprehensive income	$0	'	'	$0	$0	'	'	'	'	'	'

Acquisition_Narrative_Details

Acquisition (Narrative) (Details) (USD $)	4 Months Ended	8 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Apr. 30, 2013	Dec. 31, 2013	Dec. 31, 2013
Business Acquisition [Line Items]	'	'	'
Percentage of partners interest acquired	'	5.00%	'
Payments to acquire businesses	'	100	100
Purchase of additional interest in Pipe Line Holdings	'	'	-100
Noncontrolling Interest [Member]	'	'	'
Business Acquisition [Line Items]	'	'	'
Purchase of additional interest in Pipe Line Holdings	'	53.6	'
General Partner [Member]	'	'	'
Business Acquisition [Line Items]	'	'	'
Decrease in general partner equity	'	46.4	46.4
MPC [Member] | Noncontrolling Interest [Member]	'	'	'
Business Acquisition [Line Items]	'	'	'
Percentage of partners interest acquired	'	5.00%	'
Purchase of additional interest in Pipe Line Holdings	'	'	-53.6
Ownership percentage	'	44.00%	'
MPLX Pipe Line Holdings LP [Member]	'	'	'
Business Acquisition [Line Items]	'	'	'
Ownership percentage	51.00%	56.00%	56.00%
Pipe Line Holding [Member]	'	'	'
Business Acquisition [Line Items]	'	'	'
Effective date of acquisition	'	1-May-13	'

Acquisition_Summary_of_Changes

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

3 Months Ended

12 Months Ended

3 Months Ended

12 Months Ended

3 Months Ended

8 Months Ended

12 Months Ended

In Millions, unless otherwise specified

Dec. 31, 2013

Sep. 30, 2013

Jun. 30, 2013

Mar. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Sep. 30, 2012

Jun. 30, 2012

Mar. 31, 2012

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor [Member]

Predecessor [Member]

Predecessor [Member]

General Partner [Member]

General Partner [Member]

General Partner [Member]

General Partner [Member]

General Partner [Member]

Predecessor and Successor [Member]

Predecessor [Member]

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

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Net income attributable to MPLX LP

$20.20

$21.50

$18.60

$17.60

$13.10

$77.90

$28.90

$36.90

$29.80

$35.20

$130.80

$0

$117.70

$134

$0.20

'

$1.70

'

'

Transfer to noncontrolling interest retained by MPC:

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

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

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

-46.4

-46.4

0

0

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

'

'

'

'

'

$31.50

'

'

'

'

$130.80

'

'

$134

'

'

'

'

'

Initial_Public_Offering_Narrat

Initial Public Offering (Narrative) (Detail) (USD $)

0 Months Ended

12 Months Ended

0 Months Ended

1 Months Ended

0 Months Ended

1 Months Ended

0 Months Ended

12 Months Ended

0 Months Ended

In Millions, except Share data, unless otherwise specified

Oct. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Oct. 31, 2012

Oct. 31, 2012

Oct. 31, 2012

Oct. 31, 2012

Oct. 31, 2012

Oct. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Oct. 31, 2012

Oct. 31, 2012

Oct. 31, 2012

Oct. 31, 2012

Capital Expenditures [Member]

Working Capital [Member]

Other Ownership Interest Common Units [Member]

Limited Partners Common Units [Member]

Limited Partners Common Units [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

Limited Partners Common Units [Member]

Limited Partners Subordinated Units [Member]

Limited Partner [Member]

General Partner [Member]

Initial Public Offering [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Common units for initial public offering

'

'

'

'

'

2,595,000

19,895,000

19,895,000

'

'

'

'

'

'

'

Price of common unit (in USD per common unit)

$22

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Partners capital account units (in number of common units)

'

'

'

'

'

'

'

'

'

'

'

17,056,515

36,951,515

'

1,508,225

Aggregate Percentage of limited partner interest

'

'

'

'

'

'

'

'

'

'

'

'

'

71.60%

'

Aggregate percentage of general partner interest

'

2.00%

'

'

'

'

'

'

'

'

'

'

'

'

2.00%

Proceeds from initial public offering distributed to MPC

$407.10

$0

($407.10)

'

'

'

'

'

($202.70)

$0

$202.70

'

'

'

'

Offering Costs

30.6

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Cash contribution to Pipe Line Holdings

'

'

0

191.6

10.4

'

'

'

'

'

'

'

'

'

'

Financing Costs

$2.40

$0

'

'

'

'

'

'

'

'

'

'

'

'

'

Initial_Public_Offering_Summar

Initial Public Offering (Summary of Reconciliation of Proceeds from Initial Public Offering) (Detail) (USD $)	0 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Oct. 31, 2012	Dec. 31, 2013	Dec. 31, 2012
Initial Public Offering [Line Items]	'	'	'
Total proceeds from the Offering	$437.70	'	'
Less: Offering Costs	-30.6	'	'
Net proceeds from the Offering	407.1	0	-407.1
Less: Revolving credit facility origination fees	-2.4	0	'
Cash contribution to Pipe Line Holdings	'	'	0
MPC [Member]	'	'	'
Initial Public Offering [Line Items]	'	'	'
Net proceeds from the Offering	-202.7	0	202.7
Working Capital [Member]	'	'	'
Initial Public Offering [Line Items]	'	'	'
Cash contribution to Pipe Line Holdings	-10.4	'	'
Capital Expenditures [Member]	'	'	'
Initial Public Offering [Line Items]	'	'	'
Cash contribution to Pipe Line Holdings	($191.60)	'	'

Related_Party_Agreements_and_T2

Related Party Agreements and Transactions (Narrative) (Detail) (USD $)

12 Months Ended

3 Months Ended

12 Months Ended

9 Months Ended

12 Months Ended

0 Months Ended

1 Months Ended

12 Months Ended

12 Months Ended

12 Months Ended

9 Months Ended

12 Months Ended

9 Months Ended

12 Months Ended

1 Months Ended

12 Months Ended

0 Months Ended

12 Months Ended

In Millions, except Per Share data, unless otherwise specified

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2012

Dec. 31, 2013

Sep. 30, 2012

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2013

Oct. 31, 2012

Oct. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2011

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Sep. 30, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Sep. 30, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2011

Sep. 28, 2012

Dec. 31, 2013

Jun. 30, 2011

Dec. 31, 2011

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Predecessor and Successor [Member]

Predecessor [Member]

General and administrative expenses [Member]

General and administrative expenses [Member]

General and administrative expenses [Member]

General and administrative expenses [Member]

General and administrative expenses [Member]

Ten Year Storage Services Agreement [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

Blanchard Pipe Line Company Limited Liability Company [Member]

MPCIF [Member]

MPCIF [Member]

MPCIF [Member]

MPCIF [Member]

PFD [Member]

PFD [Member]

Capline System [Member]

Capline System [Member]

Centennial [Member]

Muskegon [Member]

Predecessor [Member]

Predecessor [Member]

Predecessor [Member]

Commercial Agreements [Member]

Pipeline

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor [Member]

Operating Agreements [Member]

Operating Agreements [Member]

Management Services Agreements [Member]

Employee Services Agreements [Member]

Ten Year Transportation Services Agreement [Member]

Ten Year Transportation Services Agreement [Member]

Five Year Transportation Services Agreement [Member]

Five Year Transportation Services Agreement [Member]

Five Year Transportation Services Agreement [Member]

Ten Year Storage Services Agreement [Member]

Three Year Storage Services Agreement [Member]

Transportation Services Agreements [Member]

Transportation Services Agreements [Member]

Transportation Services Agreements [Member]

Storage Services Agreements [Member]

Capitalized Interest [Member]

Capitalized Interest [Member]

Capitalized Interest [Member]

Operating Agreements [Member]

Predecessor [Member]

Predecessor [Member]

Predecessor [Member]

Predecessor [Member]

Predecessor [Member]

Predecessor [Member]

Operating Agreements [Member]

MPC [Member]

MPC [Member]

MPC [Member]

Storage Services Butane Cavern [Member]

Minimum [Member]

Maximum [Member]

Agreement

Agreement

Commercial Agreements [Member]

Commercial Agreements [Member]

Commercial Agreements [Member]

Commercial Agreements [Member]

Commercial Agreements [Member]

Commercial Agreements [Member]

Commercial Agreements [Member]

Commercial Agreements [Member]

Commercial Agreements [Member]

Commercial Agreements [Member]

Commercial Agreements [Member]

Maximum [Member]

Revolving Credit Agreement [Member]

Revolving Credit Agreement [Member]

mi

Operating Agreements [Member]

Agreement

Crude oil [Member]

Pipelines products [Member]

Crude oil [Member]

Crude oil [Member]

Handling Crude Oil And Products [Member]

Storage Services Butane Cavern [Member]

Storage services tank farms [Member]

Crude oil and product pipeline [Member]

Crude oil and product pipeline [Member]

Wood River To Patoka System And Barge Dock [Member]

Storage services tank farms [Member]

in

Agreement

Agreement

Agreement

Agreement

Agreement

Agreement

Agreement

Related Party Transaction [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Equity method investment, ownership percentage

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

32.60%

50.00%

60.00%

Commercial Agreements

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Number of agreements

'

'

'

'

'

'

'

'

1

'

'

'

'

'

'

'

'

'

2

2

3

4

'

1

1

'

4

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Duration of agreements (in years)

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'1 year

'5 years

'

'

'10 years

'10 years

'5 years

'

'5 years

'10 years

'3 years

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Number of renewals

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

2

4

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Renewal term agreement

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'5 years

'

'2 years

'1 year

'

'

'

'

'

'

'

'

'

'

'5 years

'

'

'

Operating Agreements

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Sales to related parties

$384.20

$367.80

$334.80

'

'

'

'

'

'

'

'

$384.20

'

$367.80

'

$334.80

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

$1

'

'

'

'

'

'

$1.80

'

'

'

Pipeline Length

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

635

'

'

'

Pipeline Diameter

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

40

'

'

'

Management Services Agreements

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Fees received for providing management services

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

0.7

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Related Party Transactions

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Pension expenses

'

'

'

'

'

'

9.5

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Stock-based compensation plans expenses

'

'

'

0.2

1

0.9

1.1

0.6

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Long-term receivables

'

'

'

'

'

'

'

'

'

'

'

0.2

0.3

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Line of credit facility initiation date for MPCIF

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

21-Jun-11

'

'

'

'

'

'

Basis spread on variable rate

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

0.10%

'

'

'

'

'

'

'

Description of variable rate basis

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'three-month LIBOR

'

'

'

'

'

'

'

Basis spread on variable rate

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

0.50%

'

'

'

'

'

'

'

Description of variable rate basis

' at the Adjusted LIBO Rate plus a margin, or the Alternate Base Rate plus a margin (as defined in the Credit Agreement)

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'three-month LIBOR

'

'

'

'

'

'

'

Loans receivable from related party

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

220.4

'

'

'

'

'

'

'

'

Expiration date

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

28-Sep-12

'

'

'

'

'

'

'

'

'

Redemption price of Preferred Stock

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

$2,000

'

'

'

'

Agreement termination date

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

30-Jun-11

'

'

'

'

'

Carrying value of newly constructed sold asset

'

'

'

'

'

'

'

'

'

'

'

'

'

1.3

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Non-cash Distributions to MPC

'

'

'

'

'

'

'

'

'

'

'

'

'

98.2

0.3

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Non-cash contribution from MPC

0.3

0

0

'

'

'

'

'

'

'

'

0.3

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

0.3

0.7

0.1

'

'

'

'

'

'

'

'

'

'

'

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

'

'

'

'

'

'

'

'

'

$3.70

$3.70

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

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, 2013	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2011
		MPC [Member]	Predecessor and Successor [Member]	Predecessor and Successor [Member]	Predecessor [Member]	Predecessor [Member]
				MPC [Member]		MPC [Member]
Related Party Transaction [Line Items]	'	'	'	'	'	'
Sales to related parties	$384.20	$384.20	$367.80	$367.80	$334.80	$334.80

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, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2011

Dec. 31, 2011

Dec. 31, 2011

Dec. 31, 2011

MPC [Member]

Marathon Oil [Member]

Centennial [Member]

Muskegon [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor [Member]

Predecessor [Member]

Predecessor [Member]

Predecessor [Member]

Predecessor [Member]

MPC [Member]

Marathon Oil [Member]

Centennial [Member]

Muskegon [Member]

MPC [Member]

Marathon Oil [Member]

Centennial [Member]

Muskegon [Member]

Related Party Transaction [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Fees received for operating pipelines for related parties

$18.80

$17.60

$0

$1.10

$0.10

$13.10

$12

$0

$1

$0.10

$9.40

$6.40

$1.90

$1

$0.10

Related_Party_Agreements_and_T5

Related Party Agreements and Transactions (Summary of Charges for Services Included in Purchases from Related Parties) (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2013	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2011	Dec. 31, 2011
		MPC [Member]	Marathon Oil [Member]	Predecessor and Successor [Member]	Predecessor and Successor [Member]	Predecessor and Successor [Member]	Predecessor [Member]	Predecessor [Member]	Predecessor [Member]
					MPC [Member]	Marathon Oil [Member]		MPC [Member]	Marathon Oil [Member]
Related Party Transaction [Line Items]	'	'	'	'	'	'	'	'	'
Charges for services included in purchases from related party	$18	$18	$0	$13.60	$13.60	$0	$15.20	$14.70	$0.50

Related_Party_Agreements_and_T6

Related Party Agreements and Transactions (Summary of Related Party Charges for Services Included in General and Administrative Expenses) (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2013	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2011	Dec. 31, 2011
		MPC [Member]	Marathon Oil [Member]	Predecessor and Successor [Member]	Predecessor and Successor [Member]	Predecessor and Successor [Member]	Predecessor [Member]	Predecessor [Member]	Predecessor [Member]
					MPC [Member]	Marathon Oil [Member]		MPC [Member]	Marathon Oil [Member]
Related Party Transaction [Line Items]	'	'	'	'	'	'	'	'	'
Charges for services included in general and administrative expenses	$31.40	$31.40	$0	$22.60	$22.60	$0	$20.90	$19.20	$1.70

Related_Party_Agreements_and_T7

Related Party Agreements and Transactions (Summary of Related Party Costs Added to Property, Plant and Equipment) (Detail) (MPC [Member], Asset under construction [Member], USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
		Predecessor and Successor [Member]	Predecessor [Member]
Property, Plant and Equipment [Line Items]	'	'	'
Costs added to property, plant and equipment	$8	$5.50	$3.60

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, 2013	Dec. 31, 2013	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2011	Dec. 31, 2011
		Purchases From Related Parties [Member]	General and administrative expenses [Member]	Predecessor and Successor [Member]	Predecessor and Successor [Member]	Predecessor and Successor [Member]	Predecessor [Member]	Predecessor [Member]	Predecessor [Member]
					Purchases From Related Parties [Member]	General and administrative expenses [Member]		Purchases From Related Parties [Member]	General and administrative expenses [Member]
Related Party Transaction [Line Items]	'	'	'	'	'	'	'	'	'
Expenses incurred under employee services agreements	$92.30	$76.60	$15.70	$53.90	$30.80	$23.10	$28.40	$13.80	$14.60

Related_Party_Agreements_and_T9

Related Party Agreements and Transactions (Receivables from Related Parties) (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Related Party Transaction [Line Items]	'	'
Receivables from related parties	$48.30	$38
MPC [Member]	'	'
Related Party Transaction [Line Items]	'	'
Receivables from related parties	47.4	37.6
Centennial [Member]	'	'
Related Party Transaction [Line Items]	'	'
Receivables from related parties	0.6	0.3
Muskegon [Member]	'	'
Related Party Transaction [Line Items]	'	'
Receivables from related parties	$0.30	$0.10

Recovered_Sheet1

Related Party Agreements and Transactions (Prepaid Assets with Related Parties) (Details) (MPC [Member], USD $)	Dec. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
MPC [Member]	'	'
Related Party Transaction [Line Items]	'	'
Prepaid assets	$0.40	$0

Recovered_Sheet2

Related Party Agreements and Transactions (Payables to Related Parties) (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Related Party Transaction [Line Items]	'	'
Payables to related parties	$12.80	$13.40
MPC [Member]	'	'
Related Party Transaction [Line Items]	'	'
Payables to related parties	$12.80	$13.40

Recovered_Sheet3

Related Party Agreements and Transactions (Summary of Deferred Revenue - Related Parties) (Details) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Related Party Transaction [Line Items]	'	'
Deferred revenue - related parties	$34	$4.20
MPC [Member]	'	'
Related Party Transaction [Line Items]	'	'
Deferred revenue - related parties	$34	$4.20

Recovered_Sheet4

Related Party Agreements and Transactions (Related Party Net Interest and Other Financial Income) (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2013	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2011	Dec. 31, 2011
		Marathon Oil [Member]	MPCIF [Member]	Predecessor and Successor [Member]	Predecessor and Successor [Member]	Predecessor and Successor [Member]	Predecessor [Member]	Predecessor [Member]	Predecessor [Member]
					Marathon Oil [Member]	MPCIF [Member]		Marathon Oil [Member]	MPCIF [Member]
Interest income:	'	'	'	'	'	'	'	'	'
Related party interest and other financial income	$0	$0	$0	$1.30	$0	$1.30	$2.30	$1.90	$0.40

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

2 Months Ended

3 Months Ended

12 Months Ended

3 Months Ended

12 Months Ended

3 Months Ended

12 Months Ended

3 Months Ended

12 Months Ended

3 Months Ended

In Millions, except Share data, unless otherwise specified

Dec. 31, 2012

Dec. 31, 2013

Sep. 30, 2013

Jun. 30, 2013

Mar. 31, 2013

Dec. 31, 2012

Mar. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Sep. 30, 2013

Jun. 30, 2013

Mar. 31, 2013

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Successor [Member]

Successor [Member]

Successor [Member]

Successor [Member]

General Partner [Member]

General Partner [Member]

General Partner [Member]

Limited Partners Common Units [Member]

Limited Partners Common Units [Member]

Limited Partners Common Units [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

Successor [Member]

Successor [Member]

Limited Partners Subordinated Units [Member]

Limited Partners Subordinated Units [Member]

Limited Partners Subordinated Units [Member]

Successor [Member]

Net Income Per Share [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Potentially dilutive securities

'

'

'

'

'

'

0

0

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Net income attributable to MPLX LP subsequent to initial public offering

'

$20.20

$21.50

$18.60

$17.60

$13.10

'

$77.90

$13.10

$13.10

$77.90

$13.10

$0.20

$1.70

'

$6.50

$38.80

'

'

'

'

'

'

$6.40

$37.40

'

Less: distribution declared

13.3

'

'

'

'

13.3

'

88.2

'

'

'

'

0.3

1.9

[1]

0.3

[1]

6.5

43.2

[1]

6.5

[1]

-16.9

-16.1

-15.4

-14.7

-9.5

6.5

43.1

[1]

6.5

[1]

Distributions in excess of net income attributable to MPLX LP

'

'

'

'

'

($0.20)

'

($10.30)

'

($0.20)

'

'

($0.10)

($0.20)

'

$0

($4.40)

'

'

'

'

'

'

($0.10)

($5.70)

'

[1]

On JanuaryB 23, 2014, we announced the board of directors of our general partner had declared a quarterly cash distribution of $0.3125 per unit, totaling $23.7 million. This distribution was paid on FebruaryB 14, 2014 to unitholders of record on FebruaryB 4, 2014.

Net_Income_Per_Limited_Partner3

Net Income Per Limited Partner Unit (Schedule of Distributions by Partner by Class Parenthetical) (Detail) (USD $)	2 Months Ended	3 Months Ended	12 Months Ended	0 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2013	Feb. 14, 2014	Feb. 04, 2014	Jan. 23, 2014
				Subsequent Event [Member]	Subsequent Event [Member]	Subsequent Event [Member]
Net Income Per Share [Line Items]	'	'	'	'	'	'
Distribution made to limited partner, declaration date	'	'	'	'	'	23-Jan-14
Cash distributions declared per limited partner common unit (in USD per unit)	'	'	'	'	'	$0.31
Cash distributions declared	($13.30)	($13.30)	($88.20)	'	'	$23.70
Distribution date	'	'	'	14-Feb-14	'	'
Date of record	'	'	'	'	4-Feb-14	'

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

3 Months Ended

12 Months Ended

3 Months Ended

12 Months Ended

3 Months Ended

12 Months Ended

In Millions, except Per Share data, unless otherwise specified

Dec. 31, 2012

Dec. 31, 2013

Sep. 30, 2013

Jun. 30, 2013

Mar. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2013

Sep. 30, 2013

Jun. 30, 2013

Mar. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Sep. 30, 2013

Jun. 30, 2013

Mar. 31, 2013

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2013

General Partner [Member]

General Partner [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 Common Units [Member]

Limited Partners Common Units [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

Limited Partners Subordinated Units [Member]

Limited Partners Subordinated Units [Member]

Schedule Of Earnings Per Share Basic And Diluted [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Cash distributions declared

$13.30

'

'

'

'

$13.30

$88.20

$0.30

$1.90

[1]

'

'

'

'

$6.50

$43.20

[1]

'

($16.90)

($16.10)

($15.40)

($14.70)

($9.50)

$6.50

$43.10

[1]

Earnings:

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Distributions in excess of net income attributable to MPLX LP

'

'

'

'

'

-0.2

-10.3

-0.1

-0.2

'

'

'

'

0

-4.4

'

'

'

'

'

'

-0.1

-5.7

Net income attributable to MPLX LP

'

$20.20

$21.50

$18.60

$17.60

$13.10

$77.90

$0.20

$1.70

'

'

'

'

$6.50

$38.80

'

'

'

'

'

'

$6.40

$37.40

Weighted average units outstanding:

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Basic (shares)

'

'

'

'

'

75.4

75.4

1.4

1.4

'

'

'

'

37

37

'

'

'

'

'

'

37

37

Diluted (shares)

'

'

'

'

'

75.4

75.4

1.4

1.4

'

'

'

'

37

37

'

'

'

'

'

'

37

37

Net income attributable to MPLX LP subsequent to initial public offering per limited partner unit:

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Basic and diluted (in USD per unit)

'

'

'

'

'

'

'

'

'

$0.27

$0.29

$0.26

$0.26

$0.18

$1.05

$0.18

'

'

'

'

'

$0.17

$1.01

Diluted (in USD per unit)

'

'

'

'

'

'

'

'

'

$0.27

$0.29

$0.26

$0.26

$0.18

$1.05

$0.18

'

'

'

'

'

$0.17

$1.01

[1]

On JanuaryB 23, 2014, we announced the board of directors of our general partner had declared a quarterly cash distribution of $0.3125 per unit, totaling $23.7 million. This distribution was paid on FebruaryB 14, 2014 to unitholders of record on FebruaryB 4, 2014.

Equity_Narrative_Details

Equity (Narrative) (Details) (USD $)

2 Months Ended

3 Months Ended

12 Months Ended

3 Months Ended

0 Months Ended

In Millions, except Share data, unless otherwise specified

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Sep. 30, 2013

Jun. 30, 2013

Mar. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Feb. 14, 2014

Feb. 04, 2014

Jan. 23, 2014

Limited Partners Common Units [Member]

Subordinated Units [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

Subsequent Event [Member]

Subsequent Event [Member]

Subsequent Event [Member]

Limited Partners Common Units [Member]

Stockholders Equity Note [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Common units outstanding

'

'

'

36,951,515

'

'

'

'

'

'

17,056,515

'

'

'

Other ownership interests, units outstanding

'

'

'

'

36,951,515

'

'

'

'

'

'

'

'

'

Aggregate percentage of general partner interest

'

'

2.00%

'

'

'

'

'

'

'

'

'

'

'

General partner units outstanding

'

'

1,508,225

'

'

'

'

'

'

'

'

'

'

'

Distribution made to limited partner, declaration date

'

'

'

'

'

'

'

'

'

'

'

'

'

23-Jan-14

Cash distributions declared

($13.30)

($13.30)

($88.20)

'

'

$16.90

$16.10

$15.40

$14.70

$9.50

'

'

'

$23.70

Cash distributions declared per limited partner common unit (in USD per unit)

'

'

'

'

'

'

'

'

'

'

'

'

'

$0.31

Distribution date

'

'

'

'

'

'

'

'

'

'

'

14-Feb-14

'

'

Date of record

'

'

'

'

'

'

'

'

'

'

'

'

4-Feb-14

'

Equity_Incentive_Distribution_

Equity (Incentive Distribution Rights) (Details) (USD $)	12 Months Ended
	Dec. 31, 2013
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, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Dec. 31, 2013	Dec. 31, 2012
Net Income Allocation table [Line Items]	'	'	'	'	'	'	'	'
Net income attributable to MPLX LP	'	$20.20	$21.50	$18.60	$17.60	$13.10	$77.90	'
Income allocated to incentive distribution rights	0	'	'	'	'	'	0.1	'
Net income attributable to MPLX LP available to general and limited partners	13.1	'	'	'	'	'	77.8	'
General partner's 2.0% interest in net income attributable to MPLX LP	0.2	'	'	'	'	'	1.6	'
Less: General partnerbs interest in net income attributable to MPLX LP subsequent to initial public offering	0.2	'	'	'	'	'	1.7	0.2
Aggregate percentage of general partner interest	'	'	'	'	'	'	2.00%	'
Successor [Member]	'	'	'	'	'	'	'	'
Net Income Allocation table [Line Items]	'	'	'	'	'	'	'	'
Net income attributable to MPLX LP	$13.10	'	'	'	'	$13.10	$77.90	$13.10

Equity_Cash_Distributions_Deta

Equity (Cash Distributions) (Details) (USD $)	2 Months Ended	3 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2013
Incentive Distribution Made to Managing Member or General Partner [Line Items]	'	'	'
General partner's distributions	$0.30	'	$1.80
General partner's incentive distribution rights	0	'	0.1
Total general partner's distributions	0.3	'	1.9
Limited partners' distributions	13	'	86.3
Cash distributions declared	13.3	13.3	88.2
Limited Partners Common Units [Member]	'	'	'
Incentive Distribution Made to Managing Member or General Partner [Line Items]	'	'	'
Limited partners' distributions	6.5	'	43.2
Subordinated Units [Member]	'	'	'
Incentive Distribution Made to Managing Member or General Partner [Line Items]	'	'	'
Limited partners' distributions	$6.50	'	$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, 2013	Dec. 31, 2012	Dec. 31, 2011
		Predecessor and Successor [Member]	Predecessor [Member]
Concentration Risk [Line Items]	'	'	'
Concentration risk, percentage	82.60%	82.20%	83.10%

Other_Items_Detail

Other Items (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
		Predecessor and Successor [Member]	Predecessor [Member]
Other Items Net Interest And Other Financial Income [Line Items]	'	'	'
Interest income	$0.30	$0.10	$0
Interest expense	-1.1	-0.8	-0.7
Interest capitalized	0.9	0.7	0.5
Other financial costs	-1.2	-0.2	0
Net interest and other financial income (costs)	($1.10)	($0.20)	($0.20)

Income_Taxes_Narrative_Detail

Income Taxes (Narrative) (Detail) (USD $)	12 Months Ended				12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2011	Dec. 31, 2011
			Maximum [Member]	Maximum [Member]	Predecessor and Successor [Member]	Predecessor and Successor [Member]	Predecessor [Member]	Predecessor [Member]
								Maximum [Member]
Income Taxes [Line Items]	'	'	'	'	'	'	'	'
Income tax expense	($200,000)	'	'	'	$300,000	$100,000	$100,000	'
Income tax rate	-0.10%	'	'	'	0.20%	'	0.10%	'
Unrecognized tax benefits	0	200,000	'	'	'	'	0	'
Unrecognized tax benefits affecting effective income tax rate	'	'	100,000	'	'	'	'	'
Uncertain tax positions	0	'	'	'	'	'	'	'
Interest and penalties, net expenses	-100,000	'	'	'	100,000	'	'	100,000
Accrual of interest and penalties	'	'	100,000	100,000	'	'	'	'
Net deferred tax liability	$0	$0	'	'	'	'	'	'

Income_Taxes_Schedule_of_Unrec

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

Property_Plant_and_Equipment_N

Property Plant and Equipment (Narrative) (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Interest Cost Capitalized And Depreciation And Amortization Expenses For Property, Plant and Equipment [Line Items]	'	'
Accumulated depreciation	$309.80	$276.30
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	$5.70	$5

Property_Plant_and_Equipment_S

Property, Plant and Equipment (Summary of Property, Plant and Equipment) (Detail) (USD $)

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

In Millions, unless otherwise specified

Land [Member]

Land [Member]

Pipelines and related assets [Member]

Pipelines and related assets [Member]

Storage and delivery facilities [Member]

Storage and delivery facilities [Member]

Other [Member]

Other [Member]

Asset under construction [Member]

Asset under construction [Member]

Minimum [Member]

Minimum [Member]

Minimum [Member]

Maximum [Member]

Maximum [Member]

Maximum [Member]

Pipelines and related assets [Member]

Storage and delivery facilities [Member]

Other [Member]

Pipelines and related assets [Member]

Storage and delivery facilities [Member]

Other [Member]

Property, Plant and Equipment [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Estimated Useful Lives

'

'

'

'

'

'

'

'

'

'

'

'

'19 years

'24 years

'10 years

'42 years

'37 years

'25 years

Property plant and equipment, gross

$1,276.40

$1,186.30

$5.30

$5.30

$1,060.70

$971.70

$165.70

$164.80

$22.80

$22.60

$21.90

$21.90

'

'

'

'

'

'

Less accumulated depreciation

309.8

276.3

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Property, plant and equipment, net

$966.60

$910

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Goodwill_Narrative_Detail

Goodwill (Narrative) (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
		Predecessor and Successor [Member]
Goodwill [Line Items]	'	'
Goodwill impairment loss	$0	$0

Goodwill_Changes_in_Carrying_A

Goodwill (Changes in Carrying Amount of Goodwill) (Detail) (USD $)	12 Months Ended					0 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011	Oct. 31, 2012
			Predecessor and Successor [Member]		Predecessor [Member]	MPC [Member]
						Pipeline
Goodwill [Roll Forward]	'		'		'	'
Beginning balance	$104.70		'		$134.20	'
Distribution to MPC	0	[1]	-29.5	[1]	'	'
Ending balance	$104.70		'		$134.20	'
Number of pipeline systems	'		'		'	2
[1]	Associated with the minority undivided joint interests in two crude oil pipeline systems distributed on OctoberB 31, 2012, in conjunction with the Offering. See Notes 5 and 15.

Fair_Value_Measurements_Detail

Fair Value Measurements (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012
Fair Value Disclosures [Abstract]	'	'
Assets fair value	$0	$0

Debt_Summary_of_Outstanding_Bo

Debt (Summary of Outstanding Borrowings) (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Proforma Debt Instrument [Line Items]	'	'
Amounts due within one year	$0.70	$0.70
Total long-term debt due after one year	9.8	10.6
Marathon Pipe Line Llc [Member]	'	'
Proforma Debt Instrument [Line Items]	'	'
MPL - capital lease obligations due 2020	$10.50	$11.30

Debt_Schedule_of_Debt_Payments

Debt (Schedule of Debt Payments) (Detail) (USD $)	Dec. 31, 2013
In Millions, unless otherwise specified
Debt Disclosure [Abstract]	'
2014	$0.70
2015	0.8
2016	0.9
2017	0.9
2018	$1

Debt_Narrative_Detail

Debt (Narrative) (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Proforma Debt Instrument [Line Items]	'	'
Description of variable rate basis	' at the Adjusted LIBO Rate plus a margin, or the Alternate Base Rate plus a margin (as defined in the Credit Agreement)	'
MPLX Operations LLC [Member] | Revolving Credit Agreement [Member] | Payment Guarantee [Member]	'	'
Proforma Debt Instrument [Line Items]	'	'
Line of credit facility initiation date	14-Sep-12	'
Term of senior unsecured revolving credit agreement (in years)	'5 years	'
Borrowings outstanding	$0	$0
MPLX Operations LLC [Member] | Revolving Credit Agreement [Member] | Payment Guarantee [Member] | Maximum [Member]	'	'
Proforma Debt Instrument [Line Items]	'	'
Line of credit facility current 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	'
Commitment fees ranging under credit agreement (in percentage)	0.35%	'
MPLX Operations LLC [Member] | Revolving Credit Agreement [Member] | Payment Guarantee [Member] | Minimum [Member]	'	'
Proforma Debt Instrument [Line Items]	'	'
Commitment fees ranging under credit agreement (in percentage)	0.10%	'
MPLX Operations LLC [Member] | Letter of credit [Member] | Payment Guarantee [Member] | Maximum [Member]	'	'
Proforma Debt Instrument [Line Items]	'	'
Line of credit facility current borrowing capacity	250,000,000	'
MPLX Operations LLC [Member] | Swingline Loan [Member] | Payment Guarantee [Member] | Maximum [Member]	'	'
Proforma Debt Instrument [Line Items]	'	'
Line of credit facility current borrowing capacity	$50,000,000	'

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, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2011

Dec. 31, 2011

Dec. 31, 2011

Dec. 31, 2011

Dec. 31, 2011

Dec. 31, 2011

Dec. 31, 2011

Dec. 31, 2011

Dec. 31, 2011

Dec. 31, 2011

MPC [Member]

Receivables from related parties [Member]

Materials & supplies inventories [Member]

Property plant and equipment [Member]

Goodwill [Member]

Other noncurrent assets [Member]

Accounts payable [Member]

Accrued taxes [Member]

Receivables from related parties [Member]

Payables to related parties [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor [Member]

Predecessor [Member]

Predecessor [Member]

Predecessor [Member]

Predecessor [Member]

Predecessor [Member]

Predecessor [Member]

Predecessor [Member]

Predecessor [Member]

Predecessor [Member]

Predecessor [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

Receivables from related parties [Member]

Materials & supplies inventories [Member]

Property plant and equipment [Member]

Goodwill [Member]

Other noncurrent assets [Member]

Accounts payable [Member]

Accrued taxes [Member]

Receivables from related parties [Member]

Payables to related parties [Member]

MPC [Member]

Receivables from related parties [Member]

Materials & supplies inventories [Member]

Property plant and equipment [Member]

Goodwill [Member]

Other noncurrent assets [Member]

Accounts payable [Member]

Accrued taxes [Member]

Receivables from related parties [Member]

Payables to related parties [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

Net cash provided by operating activities included:

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Interest paid (net of amounts capitalized)

$0

'

'

'

'

'

'

'

'

'

'

$0

'

'

'

'

'

'

'

'

'

'

$0.20

'

'

'

'

'

'

'

'

'

'

Income taxes paid

0.1

'

'

'

'

'

'

'

'

'

'

0.2

'

'

'

'

'

'

'

'

'

'

0.3

'

'

'

'

'

'

'

'

'

'

Net transfers of property, plant and equipment to materials and supplies inventories

4

'

'

'

'

'

'

'

'

'

'

0

'

'

'

'

'

'

'

'

'

'

0

'

'

'

'

'

'

'

'

'

'

Property, plant and equipment contributed by MPC

0.3

0.3

'

'

'

'

'

'

'

'

'

0

'

'

'

'

'

'

'

'

'

'

0

'

'

'

'

'

'

'

'

'

'

Non-cash investing and financing activities:

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Distribution to MPC in conjunction with the Offering

'

0.3

0

0

0

0

0

0

0

'

'

'

-98.2

-5.7

-0.3

-187.7

-29.5

-2.2

-0.3

-1

'

'

'

-0.3

0

0

0

0

0

0

0

'

'

Contribution from MPC in conjunction with the Offering

'

'

'

'

$0

'

$0

'

$0

$0

$0

'

'

'

'

$121.40

'

$0.30

'

$0.10

$7.40

$3.90

'

'

'

'

$0

'

$0

'

$0

$0

$0

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, 2013	Dec. 31, 2012	Dec. 31, 2011
		Predecessor and Successor [Member]	Predecessor [Member]
Supplemental Cash Flow Information [Line Items]	'	'	'
Additions to property, plant and equipment	$106.50	$135.60	$49.80
Plus: Increase (decrease) in capital accruals	-5.3	3.2	4.1
Asset retirement expenditures	8.3	9.2	2
Total capital expenditures	$109.50	$148	$55.90

Supplemental_Cash_Flow_Informa4

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, 2013	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2011
	MPC [Member]	Predecessor and Successor [Member]	Predecessor [Member]	Predecessor [Member]	Predecessor [Member]
		MPC [Member]			MPC [Member]
Supplemental Cash Flow Information [Line Items]	'	'	'	'	'
Contributions from (distributions to) MPC per consolidated statements of cash flows	$0	($262.70)	'	'	$37.30
Net non-cash contributions from (distributions to) MPC	0.3	-98.2	'	'	-0.3
Contributions from (distributions to) MPC per consolidated statements of equity/net investment	$0.30	($360.90)	($360.90)	$37	$37

EquityBased_Compensation_Plan_1

Equity-Based Compensation Plan (Narrative) (Detail) (USD $)

12 Months Ended

9 Months Ended

12 Months Ended

3 Months Ended

12 Months Ended

9 Months Ended

12 Months Ended

In Millions, except Share data, unless otherwise specified

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Sep. 30, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Sep. 30, 2012

Dec. 31, 2012

Dec. 31, 2011

Phantom Units [Member]

Performance Units [Member]

Mplx 2012 Incentive Compensation Plan [Member]

Mplx 2012 Incentive Compensation Plan [Member]

Maximum [Member]

Officer [Member]

Officer [Member]

Officer [Member]

General and administrative expenses [Member]

General and administrative expenses [Member]

General and administrative expenses [Member]

General and administrative expenses [Member]

General and administrative expenses [Member]

Mplx 2012 Incentive Compensation Plan [Member]

Mplx 2012 Incentive Compensation Plan [Member]

Mplx 2012 Incentive Compensation Plan [Member]

Maximum [Member]

Predecessor [Member]

Predecessor [Member]

Predecessor [Member]

Phantom Units [Member]

Performance Units [Member]

Mplx 2012 Incentive Compensation Plan [Member]

Period

Phantom Units [Member]

Description of the Plan

'

'

'

'

'

'

'

'

'

'

'

'

'

Common units available for delivery under the plan

'

'

'

'

2,750,000

'

'

'

'

'

'

'

'

Unit-based awards under the Plan

'

'

'

'

'

'

'

'

'

'

'

'

'

Award requisite service period

'

'

'

'

'

'3 years

'36 months

'

'

'

'

'

'

Accrued distributions

'

'

'

'

'

'

'

$0.10

'

'

'

'

'

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

'

$0.74

'

'

'

'

$0.74

'

'

'

'

'

'

Outstanding Phantom Unit Awards

'

'

'

'

'

'

'

'

'

'

'

'

'

Convertible at December 31, 2013

15,072

'

'

'

'

'

'

'

'

'

'

'

'

Unrecognized compensation cost

1.3

0.2

'

'

'

'

'

'

'

'

'

'

'

Cost not yet recognized, period for recognition

'1 year 9 months 18 days

'2 years

'

'

'

'

'

'

'

'

'

'

'

Compensation Expense

'

'

'

'

'

'

'

'

'

'

'

'

'

Total compensation expense

'

'

1.4

0.1

'

'

'

'

'

'

'

'

'

Stock-based compensation plans expenses

'

'

'

'

'

'

'

'

$0.20

$1

$0.90

$1.10

$0.60

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, 2013
Phantom Units [Member]	'
Number of Units	'
Outstanding at December 31, 2012	1,951
Granted	75,803
Outstanding at December 31, 2013	77,754
Vested and expected to vest at December 31, 2013	76,515
Convertible at December 31, 2013	15,072
Weighted Average Fair Value	'
Outstanding at December 31, 2012 (usd per share)	$26.69
Granted (usd per share)	$34.02
Outstanding at December 31, 2013 (usd per share)	$33.84
Vested and expected to vest at December 31, 2013 (usd per share)	$33.85
Convertible at December 31, 2013 (usd per share)	$34.70
Aggregate Intrinsic Value	'
Vested and expected to vest at December 31, 2013	$3.40
Convertible at December 31, 2013	$0.70

EquityBased_Compensation_Plan_3

Equity-Based Compensation Plan (Summary of Performance Unit Award Activity) (Details) (Performance Units [Member])	12 Months Ended
	Dec. 31, 2013
Performance Units [Member]	'
Number of Units	'
Outstanding at December 31, 2012	0
Granted	436,917
Settled	0
Forfeited	0
Outstanding at December 31, 2013	436,917

EquityBased_Compensation_Plan_4

Equity-Based Compensation Plan (Assumptions) (Details) (Performance Units [Member], USD $)	12 Months Ended
	Dec. 31, 2013
Performance Units [Member]	'
Equity-Based Compensation [Line Items]	'
Expected annual distributions per unit	$1.05
Risk-free interest rate	0.35%
Look-back period	'2 years 10 months 1 day
Expected volatility	16.75%
Grant date fair value of performance units granted	$0.74

Leases_Future_Minimum_Commitme

Leases (Future Minimum Commitments) (Detail) (USD $)	Dec. 31, 2013
In Millions, unless otherwise specified
Capital Lease Obligations	'
2014	$1.40
2015	1.3
2016	1.4
2017	1.3
2018	1.4
Later years	6.6
Total minimum lease payments	13.4
Less imputed interest costs	2.9
Present value of net minimum lease payments	10.5
Operating Lease Obligations	'
2014	8.7
2015	8.7
2016	8.1
2017	7.3
2018	7.3
Later years	10.9
Total minimum lease payments	$51

Leases_Operating_Lease_Rental_

Leases (Operating Lease Rental Expense) (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
		Predecessor and Successor [Member]	Predecessor [Member]
Schedule Of Future Minimum Sublease Rentals [Line Items]	'	'	'
Minimum rental	$9.90	$7	$2.40

Commitments_and_Contingencies_

Commitments and Contingencies (Detail) (USD $)	12 Months Ended		12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2013	Dec. 31, 2012
			Predecessor and Successor [Member]	Environmental Loss Contingency [Member]	Environmental Loss Contingency [Member]
				MPC [Member]	MPC [Member]
Commitments And Contingencies [Line Items]	'	'	'	'	'
Accrued liabilities for remediation	$1.10	$1.50	'	'	'
Loss exposure in excess of accrual	0.4	'	0.5	'	'
Receivable from MPC for indemnification of environmental costs	'	'	'	0.3	2
Maximum potential undiscounted payments	1.4	'	'	'	'
Contractual commitments to acquire property, plant and equipment	$9.60	'	'	'	'

Subsequent_Event_Details

Subsequent Event (Details) (USD $)	8 Months Ended	12 Months Ended	0 Months Ended	8 Months Ended	0 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2013	Feb. 26, 2014	Dec. 31, 2013	Mar. 01, 2014	Mar. 01, 2014	Mar. 01, 2014
			Subsequent Event [Member]	Pipe Line Holding [Member]	Pipe Line Holding [Member]	Cash [Member]	Revolving Credit Agreement [Member]
					Subsequent Event [Member]	Pipe Line Holding [Member]	Pipe Line Holding [Member]
						Subsequent Event [Member]	Subsequent Event [Member]
Subsequent Event [Line Items]	'	'	'	'	'	'	'
Subsequent event date	'	'	27-Feb-14	'	'	'	'
Percentage of partners interest acquired	5.00%	'	'	'	13.00%	'	'
Payments to acquire businesses	$100	$100	'	'	$310	$40	$270
Effective date of acquisition	'	'	'	1-May-13	1-Mar-14	'	'
Ownership percentage	'	'	'	'	69.00%	'	'

Condensed_Consolidating_Financ2

Condensed Consolidating Financial Statements (Condensed Income Statement) (Detail) (USD $)

3 Months Ended

12 Months Ended

3 Months Ended

12 Months Ended

2 Months Ended

3 Months Ended

12 Months Ended

In Millions, unless otherwise specified

Dec. 31, 2013

Sep. 30, 2013

Jun. 30, 2013

Mar. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2012

Sep. 30, 2012

Jun. 30, 2012

Mar. 31, 2012

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Parent Guarantor [Member]

Subsidiary Issuer [Member]

Non-Guarantor Subsidiaries [Member]

Consolidating Eliminations [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor [Member]

Predecessor [Member]

Predecessor [Member]

Predecessor [Member]

Predecessor [Member]

Predecessor [Member]

Predecessor [Member]

Predecessor [Member]

Successor [Member]

Successor [Member]

Successor [Member]

Successor [Member]

Successor [Member]

Successor [Member]

Successor [Member]

Successor [Member]

Parent Guarantor [Member]

Subsidiary Issuer [Member]

Non-Guarantor Subsidiaries [Member]

Consolidating Eliminations [Member]

Parent Guarantor [Member]

Subsidiary Issuer [Member]

Non-Guarantor Subsidiaries [Member]

Non-Guarantor Subsidiaries [Member]

Consolidating Eliminations [Member]

Parent Guarantor [Member]

Subsidiary Issuer [Member]

Non-Guarantor Subsidiaries [Member]

Consolidating Eliminations [Member]

Revenues and other income:

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Sales and other operating revenues

'

'

'

'

'

$78.90

'

$0

$0

$78.90

$0

'

'

'

'

$74.40

'

$0

$0

$74.40

$0

'

'

$62.10

'

'

'

$62.10

'

'

'

'

'

'

'

'

'

Sales to related parties

'

'

'

'

'

384.2

'

0

0

384.2

0

'

'

'

'

367.8

'

0

0

367.8

0

'

'

334.8

'

'

'

334.8

'

'

'

'

'

'

'

'

'

Loss on sale of assets

'

'

'

'

'

0

'

'

'

'

'

'

'

'

'

-0.3

'

0

0

-0.3

0

'

'

0

'

'

'

'

'

'

'

'

'

'

'

'

'

Other income

'

'

'

'

'

4.4

'

0

0

4.4

0

'

'

'

'

6.9

'

0

0

6.9

0

'

'

4.3

'

'

'

4.3

'

'

'

'

'

'

'

'

'

Other income - related parties

'

'

'

'

'

18.8

'

0

0

18.8

0

'

'

'

'

13.1

'

0

0

13.1

0

'

'

9.4

'

'

'

9.4

'

'

'

'

'

'

'

'

'

Equity in earnings of subsidiaries

'

'

'

'

'

0

'

83

85.3

0

-168.3

'

'

'

'

0

'

14

14.2

0

-28.2

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Total revenues and other income

'

'

'

'

'

486.3

'

83

85.3

486.3

-168.3

'

'

'

'

461.9

'

14

14.2

461.9

-28.2

'

'

410.6

'

'

'

410.6

'

'

'

'

'

'

'

'

'

Costs and expenses:

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Cost of revenues (excludes items below)

'

'

'

'

'

135.9

'

0

0

135.9

0

'

'

'

'

173.8

'

0

0

173.8

0

'

'

162.9

'

'

'

162.9

'

'

'

'

'

'

'

'

'

Purchases from related parties

'

'

'

'

'

94.6

'

0

0

94.6

0

'

'

'

'

44.4

'

0

0

44.4

0

'

'

29

'

'

'

29

'

'

'

'

'

'

'

'

'

Depreciation

'

'

'

'

'

48.9

'

0

0

48.9

0

'

'

'

'

39.4

'

0

0

39.4

0

'

'

36.3

'

'

'

36.3

'

'

'

'

'

'

'

'

'

General and administrative expenses

'

'

'

'

'

53.7

'

5

0

48.7

0

'

'

'

'

49.8

'

0.9

0

48.9

0

'

'

38.5

'

'

'

38.5

'

'

'

'

'

'

'

'

'

Other taxes

'

'

'

'

'

6.2

'

0.1

0

6.1

0

'

'

'

'

11.3

'

0

0

11.3

0

'

'

11.9

'

'

'

11.9

'

'

'

'

'

'

'

'

'

Total costs and expenses

'

'

'

'

'

339.3

'

5.1

0

334.2

0

'

'

'

'

318.7

'

0.9

0

317.8

0

'

'

278.6

'

'

'

278.6

'

'

'

'

'

'

'

'

'

Income from operations

36.5

39.8

35.2

35.5

'

147

'

77.9

85.3

152.1

-168.3

42.5

36.3

29.5

34.9

143.2

'

13.1

14.2

144.1

-28.2

'

'

132

'

'

'

132

'

'

'

'

'

'

'

'

'

Related party interest and other financial income

'

'

'

'

'

0

'

'

'

'

'

'

'

'

'

1.3

'

0

0

1.3

0

'

'

2.3

'

'

'

2.3

'

'

'

'

'

'

'

'

'

Net interest and other financial income (costs)

'

'

'

'

'

-1.1

'

0

-2.3

1.2

0

'

'

'

'

-0.2

'

0

-0.2

0

0

'

'

-0.2

'

'

'

-0.2

'

'

'

'

'

'

'

'

'

Income before income taxes

'

'

'

'

'

145.9

'

77.9

83

153.3

-168.3

'

'

'

'

144.3

'

13.1

14

145.4

-28.2

'

'

134.1

'

'

'

134.1

'

'

'

'

'

'

'

'

'

Provision (benefit) for income taxes

'

'

'

'

'

-0.2

'

0

0

-0.2

0

'

'

'

'

0.3

0.1

0

0

0.3

0

'

'

0.1

'

'

'

0.1

'

'

'

'

'

'

'

'

'

Net income

36.8

39.2

34.8

35.3

'

146.1

26.3

77.9

83

153.5

-168.3

42.1

36.9

29.8

35.2

144

'

13.1

14

145.1

-28.2

'

117.7

134

'

'

'

134

'

'

'

'

'

'

'

'

'

Less: Net income attributable to MPC-retained interest

'

'

'

'

'

68.2

'

0

0

0

68.2

'

'

'

'

13.2

'

0

0

0

13.2

'

'

0

'

'

'

'

'

'

'

'

'

'

'

'

'

Net income attributable to MPLX LP

$20.20

$21.50

$18.60

$17.60

$13.10

$77.90

'

$77.90

$83

$153.50

($236.50)

$28.90

$36.90

$29.80

$35.20

$130.80

'

$13.10

$14

$145.10

($41.40)

$0

$117.70

$134

$0

$0

$117.70

'

$0

$13.10

$13.10

$77.90

$13.10

$13.10

$14

$27.40

($41.40)

Condensed_Consolidating_Financ3

Condensed Consolidating Financial Statements (Condensed Balance Sheet) (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Current assets:	'	'
Cash and cash equivalents	$54.10	$216.70
Receivables	12.2	17.6
Receivables from related parties	48.3	38
Materials and supplies inventories	11.6	8.9
Other current assets	8.9	2.6
Total current assets	135.1	283.8
Property, plant and equipment, net	966.6	910
Investment in subsidiaries	0	0
Goodwill	104.7	104.7
Other noncurrent assets	2.1	2.8
Total assets	1,208.50	1,301.30
Current liabilities:	'	'
Accounts payable	30.5	39
Payables to related parties	12.8	13.4
Deferred revenue - related parties	34	4.2
Accrued taxes	4	3
Long-term debt due within one year	0.7	0.7
Other current liabilities	1.4	3
Total current liabilities	83.4	63.3
Long-term debt	9.8	10.6
Deferred credits and other liabilities	1.2	0.6
Total liabilities	94.4	74.5
Equity [Abstract]	'	'
MPLX LP partners' capital	646.2	691.1
Noncontrolling interest retained by MPC	467.9	535.7
Total equity	1,114.10	1,226.80
Total liabilities and equity	1,208.50	1,301.30
Parent Guarantor [Member]	'	'
Current assets:	'	'
Cash and cash equivalents	0	1
Receivables	0.4	0
Receivables from related parties	0	2.9
Materials and supplies inventories	0	0
Other current assets	0.3	0.3
Total current assets	0.7	4.2
Property, plant and equipment, net	0	0
Investment in subsidiaries	647.1	689.3
Goodwill	0	0
Other noncurrent assets	0	0
Total assets	647.8	693.5
Current liabilities:	'	'
Accounts payable	0.2	0.1
Payables to related parties	0.7	2.3
Deferred revenue - related parties	0	0
Accrued taxes	0.3	0
Long-term debt due within one year	0	0
Other current liabilities	0	0
Total current liabilities	1.2	2.4
Long-term debt	0	0
Deferred credits and other liabilities	0.4	0
Total liabilities	1.6	2.4
Equity [Abstract]	'	'
MPLX LP partners' capital	646.2	691.1
Noncontrolling interest retained by MPC	0	0
Total equity	646.2	691.1
Total liabilities and equity	647.8	693.5
Subsidiary Issuer [Member]	'	'
Current assets:	'	'
Cash and cash equivalents	7.2	9.7
Receivables	0	0
Receivables from related parties	0	0.2
Materials and supplies inventories	0	0
Other current assets	0	0
Total current assets	7.2	9.9
Property, plant and equipment, net	0	0
Investment in subsidiaries	721.6	679.6
Goodwill	0	0
Other noncurrent assets	1.8	2.3
Total assets	730.6	691.8
Current liabilities:	'	'
Accounts payable	0.3	0
Payables to related parties	83.2	2.5
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	83.5	2.5
Long-term debt	0	0
Deferred credits and other liabilities	0	0
Total liabilities	83.5	2.5
Equity [Abstract]	'	'
MPLX LP partners' capital	647.1	689.3
Noncontrolling interest retained by MPC	0	0
Total equity	647.1	689.3
Total liabilities and equity	730.6	691.8
Non-Guarantor Subsidiaries [Member]	'	'
Current assets:	'	'
Cash and cash equivalents	46.9	206
Receivables	11.8	17.6
Receivables from related parties	132.2	37.6
Materials and supplies inventories	11.6	8.9
Other current assets	8.6	2.3
Total current assets	211.1	272.4
Property, plant and equipment, net	966.6	910
Investment in subsidiaries	0	0
Goodwill	104.7	104.7
Other noncurrent assets	0.3	0.5
Total assets	1,282.70	1,287.60
Current liabilities:	'	'
Accounts payable	30	38.9
Payables to related parties	12.8	11.3
Deferred revenue - related parties	34	4.2
Accrued taxes	3.7	3
Long-term debt due within one year	0.7	0.7
Other current liabilities	1.4	3
Total current liabilities	82.6	61.1
Long-term debt	9.8	10.6
Deferred credits and other liabilities	0.8	0.6
Total liabilities	93.2	72.3
Equity [Abstract]	'	'
MPLX LP partners' capital	1,189.50	1,215.30
Noncontrolling interest retained by MPC	0	0
Total equity	1,189.50	1,215.30
Total liabilities and equity	1,282.70	1,287.60
Consolidating Eliminations [Member]	'	'
Current assets:	'	'
Cash and cash equivalents	0	0
Receivables	0	0
Receivables from related parties	-83.9	-2.7
Materials and supplies inventories	0	0
Other current assets	0	0
Total current assets	-83.9	-2.7
Property, plant and equipment, net	0	0
Investment in subsidiaries	-1,368.70	-1,368.90
Goodwill	0	0
Other noncurrent assets	0	0
Total assets	-1,452.60	-1,371.60
Current liabilities:	'	'
Accounts payable	0	0
Payables to related parties	-83.9	-2.7
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	-83.9	-2.7
Long-term debt	0	0
Deferred credits and other liabilities	0	0
Total liabilities	-83.9	-2.7
Equity [Abstract]	'	'
MPLX LP partners' capital	-1,836.60	-1,904.60
Noncontrolling interest retained by MPC	467.9	535.7
Total equity	-1,368.70	-1,368.90
Total liabilities and equity	($1,452.60)	($1,371.60)

Condensed_Consolidating_Financ4

Condensed Consolidating Financial Statements (Condensed Cash Flow Statement) (Details) (USD $)

12 Months Ended

In Millions, unless otherwise specified

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2011

Dec. 31, 2011

Parent Guarantor [Member]

Subsidiary Issuer [Member]

Non-Guarantor Subsidiaries [Member]

Consolidating Eliminations [Member]

MPC [Member]

MPC [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor [Member]

Predecessor [Member]

Predecessor [Member]

Parent Guarantor [Member]

Subsidiary Issuer [Member]

Non-Guarantor Subsidiaries [Member]

Consolidating Eliminations [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

Non-Guarantor Subsidiaries [Member]

MPC [Member]

Parent Guarantor [Member]

Subsidiary Issuer [Member]

Non-Guarantor Subsidiaries [Member]

Consolidating Eliminations [Member]

Condensed Cash Flow Statements, Captions [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Net cash provided by (used in) operating activities

$212.20

'

$76.80

$102.50

$208.80

($175.90)

'

'

$190.60

($1.40)

$1.70

$190.30

$0

'

'

'

'

'

$181.90

$181.90

'

Investing activities:

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Additions to property, plant and equipment

-106.5

'

'

-0.2

-106.3

'

'

'

-135.6

'

'

-135.6

'

'

'

'

'

'

-49.8

-49.8

'

Investment in Pipe Line Holdings

'

'

'

-100

'

100

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Distributions from (contributions to) subsidiaries

'

'

'

'

'

'

'

'

0

-202

10.4

191.6

0

'

'

'

'

'

'

'

'

Loans to affiliates

'

'

'

'

-100

100

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Repayments of loans from affiliates

'

'

'

'

26

-26

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Disposal of assets

0.2

'

'

'

0.2

'

'

'

1.3

0

0

1.3

0

'

'

'

'

'

0

'

'

Investments in related party debt securities - purchases

0

'

'

'

'

'

'

'

0

'

'

'

'

'

'

'

'

'

-260.6

-260.6

'

Investments in related party debt securities - redemptions

0

'

'

'

'

'

'

'

0

'

'

'

'

'

'

'

'

'

311.7

311.7

'

Investments - loans to a related party

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

-220

-220

'

Investments - repayments of loans receivable from a related party

0

'

'

'

'

'

'

'

221.7

0

0

221.7

0

'

'

'

'

'

0

'

'

All other, net

-7.3

'

'

'

-7.3

'

'

'

0

'

'

'

'

'

'

'

'

'

0

'

'

Net cash provided by (used in) investing activities

-113.6

'

'

-100.2

-187.4

174

'

'

87.4

-202

10.4

279

0

'

'

'

'

'

-218.7

-218.7

'

Financing activities:

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Proceeds from borrowings from affiliates

'

'

'

100

'

-100

0

'

'

'

'

'

'

262.7

'

'

'

'

'

'

-37.3

Payments on borrowings from affiliates

'

'

'

-26

'

26

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Long-term debt - repayments

-0.7

'

'

'

-0.7

'

'

'

-0.7

0

0

-0.7

0

'

'

'

'

'

-0.6

-0.6

'

Debt issuance costs

0

'

'

'

'

'

'

'

-2.4

'

-2.4

'

'

'

'

'

'

'

0

'

'

Net proceeds from initial public offering

0

-407.1

'

'

'

'

0

202.7

407.1

407.1

0

0

0

-202.7

-202.7

0

0

0

0

'

0

Quarterly distributions to unitholders and general partner

-77.8

'

-77.8

-78.8

-97.1

175.9

'

'

0

'

'

'

'

'

'

'

'

'

0

'

'

Quarterly distributions to noncontrolling interest retained by MPC

-82.7

'

'

'

-82.7

'

'

'

0

'

'

'

'

'

'

'

'

'

0

'

'

Distributions related to purchase of additional interest in Pipe Line Holdings

-100

'

'

'

'

-100

'

'

0

'

'

'

'

'

'

'

'

'

0

'

'

Contributions from (distributions to) MPC

'

'

'

'

'

'

0

'

-262.7

0

0

-262.7

0

-262.7

'

'

'

'

37.3

37.3

37.3

Net cash provided by (used in) financing activities

-261.2

'

-77.8

-4.8

-180.5

1.9

'

'

-61.4

204.4

-2.4

-263.4

0

'

'

'

'

'

36.7

36.7

'

Net increase (decrease) in cash and cash equivalents

-162.6

'

-1

-2.5

-159.1

0

'

'

216.6

1

9.7

205.9

0

'

'

'

'

'

-0.1

-0.1

'

Cash and cash equivalents at end of period

$54.10

$216.70

$0

$7.20

$46.90

$0

'

'

$216.70

'

'

'

'

'

'

'

'

'

$0.10

$0.10

'

Select_Quarterly_Financial_Dat2

Select Quarterly Financial Data (Detail) (USD $)

2 Months Ended

3 Months Ended

12 Months Ended

3 Months Ended

12 Months Ended

3 Months Ended

12 Months Ended

3 Months Ended

12 Months Ended

In Millions, except Per Share data, unless otherwise specified

Dec. 31, 2012

Dec. 31, 2013

Sep. 30, 2013

Jun. 30, 2013

Mar. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Sep. 30, 2013

Jun. 30, 2013

Mar. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Sep. 30, 2013

Jun. 30, 2013

Mar. 31, 2013

Dec. 31, 2012

Dec. 31, 2012

Sep. 30, 2012

Jun. 30, 2012

Mar. 31, 2012

Dec. 31, 2012

Dec. 31, 2013

Sep. 30, 2013

Jun. 30, 2013

Mar. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Sep. 30, 2013

Jun. 30, 2013

Mar. 31, 2013

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

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 Common Units [Member]

Limited Partners Common Units [Member]

Limited Partners Subordinated Units [Member]

Limited Partners Subordinated Units [Member]

Limited Partners Subordinated Units [Member]

Limited Partners Subordinated Units [Member]

Limited Partners Subordinated Units [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Predecessor and Successor [Member]

Public [Member]

Public [Member]

Public [Member]

Public [Member]

Public [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

MPC [Member]

Limited Partners Subordinated Units [Member]

Limited Partners Subordinated Units [Member]

Limited Partners Subordinated Units [Member]

Quarterly Financial Data [Line Items]

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Revenues

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$118.90

$118.20

$116.10

$109.90

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$486.30

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$122.90

$116.20

$105.60

$97.50

$461.90

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Income from operations

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36.5

39.8

35.2

35.5

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147

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42.5

36.3

29.5

34.9

143.2

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Net income

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36.8

39.2

34.8

35.3

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146.1

26.3

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42.1

36.9

29.8

35.2

144

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Net income attributable to MPLX LP

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20.2

21.5

18.6

17.6

13.1

77.9

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6.5

38.8

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28.9

36.9

29.8

35.2

130.8

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6.4

37.4

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Net income attributable to MPLX LP subsequent to initial public offering on October 31, 2012

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20.2

21.5

18.6

17.6

13.1

77.9

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6.5

38.8

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28.9

36.9

29.8

35.2

130.8

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6.4

37.4

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Net income attributable to MPLX LP subsequent to initial public offering per limited partner unit:

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Basic (in USD per unit)

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$0.27

$0.29

$0.26

$0.26

$0.18

$1.05

$0.18

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$0.17

$1.01

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Diluted (in USD per unit)

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$0.27

$0.29

$0.26

$0.26

$0.18

$1.05

$0.18

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$0.17

$1.01

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Basic and Diluted (in USD per unit)

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$0.27

$0.29

$0.23

$0.21

$0.17

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$1.01

$0.17

Cash distributions declared per limited partner common unit (in USD per unit)

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$0.31

$0.30

$0.28

$0.27

$0.18

$1.17

$0.18

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Distributions declared: [Abstract]

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Less: distribution declared

-13.3

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-13.3

-88.2

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-6.5

-43.2

[1]

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6.2

5.9

5.7

5.4

3.5

16.9

16.1

15.4

14.7

9.5

-6.5

-43.1

[1]

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General partner units

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0.5

0.5

0.4

0.4

0.3

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Incentive distribution rights

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0.1

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0.1

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Total distributions declared

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$23.70

$22.50

$21.50

$20.50

$13.30

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[1]

On JanuaryB 23, 2014, we announced the board of directors of our general partner had declared a quarterly cash distribution of $0.3125 per unit, totaling $23.7 million. This distribution was paid on FebruaryB 14, 2014 to unitholders of record on FebruaryB 4, 2014.