Subsequent Events (Details) (Utica Shale Joint Venture)	Dec. 31, 2011
Utica Shale Joint Venture
Subsequent events
Percentage of ownership interest held in joint venture	60.00%

Represents the percentage of ownership interest held in joint venture.

No definition available.

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