Commitments and Contingencies (Sale-Leaseback Commitments) (Details) (Powerton-Joliet Stations, Sale-Leaseback, Midwest Generation LLC, USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012
Powerton-Joliet Stations, Sale-Leaseback | Midwest Generation LLC
Sale Leaseback Transaction [Line Items]
Remaining lessor debt held by pass-through trustees	$ 345
Fixed interest rate	8.56%