Asset Impairment Expense (Details) (USD $) In Millions, unless otherwise specified	0 Months Ended	3 Months Ended		6 Months Ended
	Jan. 09, 2013	Jun. 30, 2013 MW	Jun. 30, 2012	Jun. 30, 2013 MW	Jun. 30, 2012
Asset Impairment Expense [Line Items]
Asset impairment expense		$ 0	$ 18	$ 48	$ 28
Beaver Valley [Member]
Asset Impairment Expense [Line Items]
Asset impairment expense		0	0	46	0
Generation Capacity (MW)		125		125
Lump Sum Payment Received For Termination Of PPA	60
Kelanitissa [ Member]
Asset Impairment Expense [Line Items]
Asset impairment expense		0	7	0	12
St. Patrick [Member]
Asset Impairment Expense [Line Items]
Asset impairment expense		0	11	0	11
Other Impairment [Member]
Asset Impairment Expense [Line Items]
Asset impairment expense		$ 0	$ 0	$ 2	$ 5