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

Fair value (as of the inception of the agreements) of a natural gas purchasing contract that is embedded in a processing agreement. The natural gas purchase is considered an embedded derivative; however, in accordance with GAAP, the fair value of the embedded derivative at inception is deemed to be allocable to the host contract and is not separately recorded as an embedded derivative.

No definition available.

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

Fair value as of the balance sheet date of a natural gas purchase agreement embedded in a gas processing agreement. The natural gas purchase agreement is an embedded derivative. This amount includes the value of the embedded purchase agreement at the inception of the arrangement. however, in accordance with GAAP, the fair value of the embedded derivative at inception is deemed to be allocable to the host contract and is not separately recorded as an embedded derivative.

No definition available.

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

Represents the term for which the counterparty has the option to renew a purchase agreement related to an embedded derivative.

No definition available.

Name:	mwe_EmbeddedDerivativeRenewalTerm
Namespace Prefix:	mwe_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Aggregate notional amount of natural gas price risk derivatives embedded in commodity contracts with notional amount expressed in dekatherms per day.

No definition available.

Name:	mwe_NonmonetaryNotionalAmountOfEmbeddedDerivativeInCommodityContract
Namespace Prefix:	mwe_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

Aggregate notional amount of crude oil price risk derivatives for instruments with notional amounts expressed in nonmonetary units.

No definition available.

Name:	mwe_NonmonetaryNotionalAmountOfPriceRiskDerivativesOfCrudeOil
Namespace Prefix:	mwe_
Data Type:	num:volumeItemType
Balance Type:	na
Period Type:	instant

Aggregate notional amount of natural gas price risk derivatives for instruments with notional amounts expressed in nonmonetary units.

No definition available.

Name:	mwe_NonmonetaryNotionalAmountOfPriceRiskDerivativesOfNaturalGas
Namespace Prefix:	mwe_
Data Type:	num:energyItemType
Balance Type:	na
Period Type:	instant

Aggregate notional amount of natural gas liquids price risk derivatives for instruments with notional amounts expressed in nonmonetary units.

No definition available.

Name:	mwe_NonmonetaryNotionalAmountOfPriceRiskDerivativesOfNaturalGasLiquids
Namespace Prefix:	mwe_
Data Type:	num:volumeItemType
Balance Type:	na
Period Type:	instant

- Details

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

Fair value as of the balance sheet date of the embedded derivative or group of embedded derivatives classified as an asset.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4B

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624163-113959

Name:	us-gaap_EmbeddedDerivativeFairValueOfEmbeddedDerivativeAsset
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Fair value as of the balance sheet date of the embedded derivative or group of embedded derivatives classified as a liability.