Commitments and Contingencies (Other Commitments - Fuel Supply Contracts) (Details) (Midwest Generation LLC, Coal Supply Agreements, USD $) In Millions, unless otherwise specified	Sep. 30, 2013
Midwest Generation LLC \| Coal Supply Agreements
Long-term Purchase Commitment [Line Items]
Total commitments under existing agreements	$ 336
Purchase commitments, remainder of 2013	186
Purchase commitments, 2014	$ 150

Minimum amount of purchase arrangement maturing in the remainder of the fiscal year following the latest fiscal year ended. Includes, but is not limited to, recorded and unrecorded purchase obligations, long-term purchase commitments, and short-term purchase commitments.

No definition available.

Name:	eme_PurchaseObligationFutureMinimumPaymentsRemainderofFiscalYear
Namespace Prefix:	eme_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant