Fair Value Measurements (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended					9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended			9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended			9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013 Electricity forward contracts	Dec. 31, 2012 Electricity forward contracts	Sep. 30, 2013 Electricity forward contracts Minimum Discounted cash flows	Dec. 31, 2012 Electricity forward contracts Minimum Discounted cash flows	Sep. 30, 2013 Electricity forward contracts Maximum Discounted cash flows	Dec. 31, 2012 Electricity forward contracts Maximum Discounted cash flows	Sep. 30, 2013 Electricity forward contracts Weighted Average Discounted cash flows	Dec. 31, 2012 Electricity forward contracts Weighted Average Discounted cash flows	Sep. 30, 2013 Option Contracts	Dec. 31, 2012 Option Contracts	Sep. 30, 2013 Option Contracts Minimum Option model	Dec. 31, 2012 Option Contracts Minimum Option model	Sep. 30, 2013 Option Contracts Maximum Option model	Dec. 31, 2012 Option Contracts Maximum Option model	Sep. 30, 2013 Option Contracts Weighted Average Option model	Dec. 31, 2012 Option Contracts Weighted Average Option model	Sep. 30, 2013 Natural gas forward contracts	Dec. 31, 2012 Natural gas forward contracts	Sep. 30, 2013 Natural gas forward contracts Minimum Discounted cash flows	Dec. 31, 2012 Natural gas forward contracts Minimum Discounted cash flows	Sep. 30, 2013 Natural gas forward contracts Maximum Discounted cash flows	Dec. 31, 2012 Natural gas forward contracts Maximum Discounted cash flows	Sep. 30, 2013 Natural gas forward contracts Weighted Average Discounted cash flows	Dec. 31, 2012 Natural gas forward contracts Weighted Average Discounted cash flows
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Assets	$ 48		$ 48		$ 62	$ 47	$ 57															$ 1	$ 5
Liabilities	92		92		110	74	82							16	27							2	1
Electricity forward price (per MWh)								24.55	23.06	60.97	64.20	40.23	43.16			38.56	36.66	82.07	92.19	55.74	60.97
Electricity price volatilities (as a percent)																51.00%	15.00%	106.00%	66.00%	74.00%	39.00%
Natural gas forward price (per mmbtu)																3.82	4.10	3.96	4.25	3.91	4.20			3.31	3.25	4.25	4.44	3.97	3.93
Natural gas price volatilities (as a percent)																24.00%	17.00%	51.00%	36.00%	31.00%	23.00%
Changes in fair value for our risk management activities assets and liabilities that are measured at fair value on a recurring basis using Level 3 inputs
Net derivative balance at beginning of period	(53)	(45)	(48)	(51)
Total net gains (losses) realized/unrealized:
Included in earnings				2
Included in OCI				(2)
Deferred as a regulatory asset or liability	4	(3)	(2)	4
Settlements	6	(1)	8	(1)
Transfers into Level 3 from Level 2	(1)	(4)	(1)	(2)
Transfers from Level 3 into Level 2		3	(1)
Net derivative balance at end of period	(44)	(50)	(44)	(50)
Transfers in or out of Level 1 to or from any other hierarchy level			$ 0