Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2014 t MWh MMBTU	Dec. 31, 2013 t MWh MMBTU
Derivative Instruments [Abstract]
Derivative, Nonmonetary Notional Amount, Energy Measure	(29)	(29)
Nonmonetary Notional Amount of Price Risk Derivatives Natural Gas	541	448
Derivative, Nonmonetary Notional Amount, Mass	2	0
Price Risk Derivatives [Abstract]
Derivative, Notional Amount	$ 1,517	$ 1,527