Environmental Developments (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Property, Plant and Equipment [Line Items]
Net property, plant and equipment	$ 3,934		$ 4,516
Midwest Generation LLC
Property, Plant and Equipment [Line Items]
Net property, plant and equipment	1,542		2,078
Midwest Generation LLC | Joliet Station
Property, Plant and Equipment [Line Items]
Net property, plant and equipment	674	[1]
Midwest Generation LLC | Powerton Station
Property, Plant and Equipment [Line Items]
Net property, plant and equipment	802
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.