v3.3.0.814

Equity-Based Compensation Plan - Additional Information (Detail) - Performance Shares [Member] - MPLX LP 2012 Incentive Compensation Plan [Member]	9 Months Ended
	Sep. 30, 2015 $ / shares
Equity Transactions And Share Based Compensation [Line Items]
Performance units grant date fair value (in USD per unit)	$ 1.03
Officer [Member]
Equity Transactions And Share Based Compensation [Line Items]
Share Based Compensation Arrangement By Share Based Payment Award Percentage Paid Out In Cash	75.00%
Share Based Compensation Arrangement By Share Based Payment Award Percentage Paid Out In Stock	25.00%

X

- Definition

Equity Transactions And Share Based Compensation [Line Items]

+ References

No definition available.

+ Details

Name:	mplx_EquityTransactionsAndShareBasedCompensationLineItems
Namespace Prefix:	mplx_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Share Based Compensation Arrangement By Share Based Payment Award Percentage Paid Out In Cash

+ References

No definition available.

+ Details

Name:	mplx_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentagePaidOutInCash
Namespace Prefix:	mplx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Share Based Compensation Arrangement By Share Based Payment Award Percentage Paid Out In Stock

+ References

No definition available.

+ Details

Name:	mplx_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentagePaidOutInStock
Namespace Prefix:	mplx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef



-Publisher FASB



-Name Accounting Standards Codification



-Topic 718



-SubTopic 10



-Section 50



-Paragraph 2



-Subparagraph (c)(2)(iii)(1)



-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_PerformanceSharesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_PlanNameAxis=mplx_MplxTwoThousandTwelveIncentiveCompensationPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_TitleOfIndividualAxis=us-gaap_OfficerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: