Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2013 MWh MMBTU	Dec. 31, 2012 MWh MMBTU
Derivative Instruments [Abstract]
Derivative, Nonmonetary Notional Amount, Energy Measure	(11)	(16)
Nonmonetary Notional Amount of Price Risk Derivatives Natural Gas	163	66
Price Risk Derivatives [Abstract]
Derivative, Notional Amount	$ 1,597	$ 1,602