Commitments and Contingencies (Contingencies) (Details) (USD $)	12 Months Ended			1 Months Ended		12 Months Ended		1 Months Ended	12 Months Ended	1 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Jan. 31, 2012 Midwest Generation LLC claims	Oct. 31, 2011 Midwest Generation LLC claims	Dec. 31, 2012 Midwest Generation LLC sites	Dec. 31, 2011 Midwest Generation LLC	Aug. 31, 2009 Midwest Generation LLC Violation of certain operating permit requirements sites	Dec. 31, 2012 Midwest Generation LLC Violation of certain operating permit requirements claims	Jan. 31, 2011 Homer City owners residents	Feb. 29, 2012 Homer City Alleged violations of emissions standards and limitations under the CAA and the Pennsylvania Air Pollution Control Act	Jan. 31, 2011 Homer City Violations of the PSD and Title V provisions of the CAA
Contingencies
Number of coal-fired electric generating stations (in sites)								6
Penalties sought, per violation, per day, minimum end of range								$ 25,000				$ 32,500
Penalties sought, per violation, per day, maximum end of range								37,500				37,500
Number of fully dismissed claims in the complaint filed by the US EPA and the State of Illinois (in claims)									9
Number of claims in the complaint filed by the US EPA and the State of Illinois (in claims)									10
Number of complaints filed asserting claims of nuisance, negligence, trespass, and strict liability (in claims)				2
Similar number of complaints previously filed asserting claims of nuisance, negligence, trespass, and strict liability that were dismissed					2
Number of plaintiffs (in residents)										2
Number of prior owners of facilities (in owners)										2
Notice of intent to sue, notice time period											60 days
Accrued liability for environmental remediation						9,000,000
Identified sites for environmental remediation (in number of sites)						4
Receivable related to insurance claims	3,000,000	3,000,000				3,000,000
Gain related to business interruption insurance coverage		5,000,000	2,000,000				2,000,000
Proceeds from settlement of insurance claims	$ 2,000,000	$ 16,000,000				$ 2,000,000	$ 11,000,000