Segment Information (Details 4) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Segment information
Management fee revenues unconsolidated affiliate	$ 5.4	$ 2.1	$ 11.5	$ 2.7
Southwest Segment
Segment information
Revenue deferral adjustment	0.2	0.2	0.1	0.4
Northeast Segment
Segment information
Revenue deferral adjustment	$ 0.9	$ 1.5	$ 0.1	$ 3.4

The difference between the revenue recognized on the consolidated financial statements and the recognized for segment purposes. Certain contracts in which more consideration is received in the initial years than in later years and GAAP requires the entity to recognize the revenue evenly over the term of the contract by deferring revenue in the initial years. The chief operating decision maker evaluates segment performance and makes decisions based on the actual consideration received and therefore the segment revenues do not reflect the revenue deferral.

No definition available.

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