Fair Value Measurements (Details 2) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Assets	$ 41	$ 62
Liabilities	90	110
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	(48)	(51)
Total net gains (losses) realized/unrealized:
Included in earnings		2
Included in OCI		(3)
Deferred as a regulatory asset or liability	(10)	7
Settlements	10	(5)
Transfers into Level 3 from Level 2		(2)
Transfers from Level 3 into Level 2	(1)	4
Net derivative balance at end of period	(49)	(48)
Transfers in or out of Level 1 to or from any other hierarchy level	0
Electricity forward contracts
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Assets	40	57
Liabilities	66	82
Electricity forward contracts | Minimum | Discounted cash flows
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Electricity forward price (per MWh)	24.89	23.06
Electricity forward contracts | Maximum | Discounted cash flows
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Electricity forward price (per MWh)	65.04	64.20
Electricity forward contracts | Weighted Average | Discounted cash flows
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Electricity forward price (per MWh)	41.09	43.16
Option Contracts
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Liabilities	19	27
Option Contracts | Minimum | Option model
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Electricity forward price (per MWh)	39.91	36.66
Implied electricity price volatilities (as a percent)	35.00%	15.00%
Natural gas forward price (per MMbtu)	3.57	4.10
Implied natural gas price volatilities (as a percent)	22.00%	17.00%
Option Contracts | Maximum | Option model
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Electricity forward price (per MWh)	85.41	92.19
Implied electricity price volatilities (as a percent)	94.00%	66.00%
Natural gas forward price (per MMbtu)	3.80	4.25
Implied natural gas price volatilities (as a percent)	36.00%	36.00%
Option Contracts | Weighted Average | Option model
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Electricity forward price (per MWh)	58.70	60.97
Implied electricity price volatilities (as a percent)	59.00%	39.00%
Natural gas forward price (per MMbtu)	3.71	4.20
Implied natural gas price volatilities (as a percent)	27.00%	23.00%
Natural gas forward contracts
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Assets	1	5
Liabilities	$ 5	$ 1
Natural gas forward contracts | Minimum | Discounted cash flows
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Natural gas forward price (per MMbtu)	3.47	3.25
Natural gas forward contracts | Maximum | Discounted cash flows
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Natural gas forward price (per MMbtu)	4.31	4.44
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
Natural gas forward price (per MMbtu)	3.87	3.93