This element represents the actual amount of the VIE's capital that the non-controlling interest owners have funded.

No definition available.

Name:	mwe_ActualCapitalContributedByNoncontrollingOwners
Namespace Prefix:	mwe_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The actual amount of the VIE's capital that has been funded by the reporting entity to date.

+ References

No definition available.

+ Details

Name:	mwe_ActualCapitalContributionToDate
Namespace Prefix:	mwe_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Once the non-controlling interest owners have reached their required minimum capital contribution, this represents the aggregate amount of capital contributed to the VIE from all members that must be achieved before the non-controlling interest owners have the option to contribute additional capital.

+ References

No definition available.

+ Details

Name:	mwe_AggregateContributionsToVIEThreshold
Namespace Prefix:	mwe_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the distributions received by the investee from its subsidiary.

+ References

No definition available.

+ Details

Name:	mwe_DistributionReceivedFromSubsidiary
Namespace Prefix:	mwe_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The maximum percentage of additional capital funding of the VIE the non-controlling interest owners have the right, but not obligation, to make after the aggregate contribution threshold from all members has been reached and until they reach their threshold ownership percentage for pro rata contributions.

+ References

No definition available.

+ Details

Name:	mwe_MaximumPercentageOfAdditionalNoncontrollingCapitalContributionBeforeThresholdOwnership
Namespace Prefix:	mwe_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the aggregate amount of capital contributed to the VIE from all members to date.

+ References

No definition available.

+ Details

Name:	mwe_MembersContributionToVieThreshold
Namespace Prefix:	mwe_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

This element represents the amount of the VIE's capital requirements that the non-controlling interest owners are obligated to fund under the joint venture agreement.

+ References

No definition available.

+ Details

Name:	mwe_MinimumCapitalContributedByNoncontrollingOwners
Namespace Prefix:	mwe_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the special non-cash allocation of income to non-controlling interest during the period.

+ References

No definition available.

+ Details

Name:	mwe_NoncontrollingOwnersAccrualOfPreferenceAmount
Namespace Prefix:	mwe_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The minimum capital contributions (since inception) required of the non-controlling interest in order for it to receive a quarterly special allocation of income. The special allocation of income will be based only on amounts contributed in excess of this threshold and ends after the fifth year of the joint venture.

+ References

No definition available.

+ Details

Name:	mwe_NoncontrollingOwnersPreferenceThreshold
Namespace Prefix:	mwe_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the reporting entity's threshold percentage of the aggregate investment balances in the investment, above which the de facto agent relationship between the members qualifying the investment as a VIE will no longer exist.

+ References

No definition available.

+ Details

Name:	mwe_PercentageInvestmentBalanceForEndOfDeFactoAgentRelationship
Namespace Prefix:	mwe_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the reporting entity's threshold percentage of the aggregate investment balances in the VIE, above which the cash available for distribution is to be distributed in accordance with each member's investment balance.

+ References

No definition available.

+ Details

Name:	mwe_PercentageInvestmentBalanceTargetForDistributionOfAvailableCash
Namespace Prefix:	mwe_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the percentage of cash available for distribution that the reporting entity will receive from the VIE for all periods prior to the earlier of five years from the start of the joint venture and the date at which the reporting entity owns a specified percentage of the aggregate investment balance in the VIE. After such date, the cash available for distribution will be distributed in accordance with each member's investment balance.

+ References

No definition available.

+ Details

Name:	mwe_PercentageOfCashGenerationReceived
Namespace Prefix:	mwe_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the maximum percentage to which the reporting entity can increase its ownership interest in the VIE, unless the non-controlling interest owners elect not to maintain their ownership interest.

+ References

No definition available.

+ Details

Name:	mwe_PercentageOfOwnershipInterestInJointVentureMaximum
Namespace Prefix:	mwe_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the percentage of capital funding required of the reporting entity after the non-controlling interest owners have reached their required minimum capital contribution and until the aggregate contribution threshold has been reached.

+ References

No definition available.

+ Details

Name:	mwe_PercentageOfRequiredCapitalContributionByReportingEntityAfterNCI
Namespace Prefix:	mwe_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the threshold percentage of ownership interest in the VIE by the non-controlling interest owners, which upon reaching they have the option to maintain through pro rata capital contributions.

+ References

No definition available.

+ Details

Name:	mwe_ThresholdPercentageOfNoncontrollingOwnershipInterestInJointVenture
Namespace Prefix:	mwe_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.

+ References

No definition available.

+ Details

Name:	us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The cash inflow from the capital received in cash from a partner in a partnership during the period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 14

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3255-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Financing Activities

-URI http://asc.fasb.org/extlink&oid=6513228

+ Details

Name:	us-gaap_ProceedsFromPartnershipContribution
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of revenue, fees and commissions earned from transactions between (a) a parent company and its subsidiaries; (b) subsidiaries of a common parent; (c) an entity and trusts for the benefit of employees, for example, but not limited to, pension and profit-sharing trusts that are managed by or under the trusteeship of the entity's management; (d) an entity and its principal, owners, management, or members of their immediate families; and (e) affiliates.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.1(e))

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Related Parties

-URI http://asc.fasb.org/extlink&oid=16382449

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 946

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.6-07.1(c))

-URI http://asc.fasb.org/extlink&oid=6488393&loc=d3e606610-122999

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Article 5

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 07

-Paragraph b

-Subparagraph 1

-Article 6

+ Details

Name:	us-gaap_RevenueFromRelatedParties
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_VariableInterestEntityLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of the Variable Interest Entity's (VIE) voting interest owned by (or beneficial interest in) the reporting entity (directly or indirectly).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 810

-SubTopic 10

-Section 50

-Paragraph 5A

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=28200181&loc=SL6759159-111685

+ Details

Name:	us-gaap_VariableInterestEntityOwnershipPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=mwe_MarkwestUticaEmgMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=mwe_OhioGatheringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CounterpartyNameAxis=mwe_SummitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=mwe_OhioGatheringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Variable Interest Entity (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
Variable Interest Entity (Details) - USD ($) $ in Millions	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014	Nov. 30, 2014	Jan. 31, 2014	Feb. 28, 2013
MarkWest Utica EMG
Variable Interest Entities
Distributions received from Ohio Gathering			$ 336.1
Ohio Gathering
Variable Interest Entities
Operational service revenue	$ 4.2	$ 1.0	$ 8.9	$ 1.0
Ohio Gathering \| Summit
Variable Interest Entities
Percentage of ownership interest held by non-controlling interest	40.00%		40.00%				1.00%
Total Variable Interest Entities \| MarkWest Utica EMG
Variable Interest Entities
Noncontrolling interest owners total funding commitment								$ 950.0
Percentage of capital required by reporting entity after non-controlling interest minimum contribution								100.00%
Aggregate contributions to VIE threshold								$ 2,000.0
Aggregate capital contributed by all members						$ 2,000.0
Maximum percentage of ownership interest in joint venture	70.00%		70.00%
Threshold percentage for non-controlling owners to make pro rata contributions	30.00%		30.00%
Maximum non-controlling percentage of capital contributions until pro rata ownership threshold is reached			10.00%
Noncontrolling interest owners actual contribution	$ 989.9		$ 989.9
Contribution by the Partnership	$ 1,379.6		$ 1,379.6
Percentage of ownership interest held by non-controlling interest	45.00%		45.00%
Noncontrolling interest owners capital contribution preference threshold								$ 500.0
Accrual of preference amount to EMG Utica's investment balance during the period	$ 10.9	$ 9.1	$ 21.2	$ 17.9
Investment balance as a percent of aggregate investment balances			55.00%
Percentage of available cash to be received by the Partnership	60.00%		60.00%
Percentage of aggregate investment balances for proportionate distribution of cash	60.00%		60.00%
Percentage of aggregate investment balances for end of de facto agent relationship			60.00%
Ohio Gathering \| Summit
Variable Interest Entities
Cash investment received					$ 341.1