Environmental Developments Environmental Developments (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Property, Plant and Equipment [Line Items]
Net property, plant and equipment	$ 4,465		$ 4,516
Midwest Generation LLC
Property, Plant and Equipment [Line Items]
Net property, plant and equipment	2,034		2,078
Midwest Generation LLC | Joliet Station
Property, Plant and Equipment [Line Items]
Net property, plant and equipment	687	[1]
Midwest Generation LLC | Powerton Station
Property, Plant and Equipment [Line Items]
Net property, plant and equipment	810
Midwest Generation LLC | Will County Station
Property, Plant and Equipment [Line Items]
Net property, plant and equipment	471	[2]
Midwest Generation LLC | Fleet assets
Property, Plant and Equipment [Line Items]
Net property, plant and equipment	$ 66

[1] The Joliet Station is composed of Units 6, 7, and 8.

[2] The Will County Station is composed of Units 3 and 4. Midwest Generation permanently shut down Will County Station Units 1 and 2, totaling 299 MW of capacity, on December 29, 2010 in accordance with the CPS.