Summary of Significant Accounting Policies (Inventory) (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Significant Accounting Policies [Line Items]
Document Period End Date	Jun. 30, 2013
Inventory
Coal, fuel oil and other raw materials	$ 62	$ 123
Spare parts, materials and supplies	59	52
Total inventory	121	175
Midwest Generation LLC
Significant Accounting Policies [Line Items]
Document Period End Date	Jun. 30, 2013
Inventory
Coal, fuel oil and other raw materials	59	119
Spare parts, materials and supplies	45	46
Total inventory	$ 104	$ 165