Fair Value Measurements (Details 2) (USD $) In Millions, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Assets	$ 53	$ 62
Liabilities	106	110
Electricity forward contracts
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Assets	53	57
Liabilities	76	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)	24.77	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)	70.12	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)	42.10	43.16
Option Contracts
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Liabilities	27	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)	39.64	36.66
Natural gas forward price (per mmbtu)		4.10
Implied electricity price volatilities (as a percent)	17.00%	15.00%
Implied natural gas price volatilities (as a percent)	16.00%	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)	84.73	92.19
Natural gas forward price (per mmbtu)		4.25
Implied electricity price volatilities (as a percent)	98.00%	66.00%
Implied natural gas price volatilities (as a percent)	32.00%	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)	58.26	60.97
Natural gas forward price (per mmbtu)		4.20
Implied electricity price volatilities (as a percent)	46.00%	39.00%
Implied natural gas price volatilities (as a percent)	21.00%	23.00%
Natural gas forward contracts
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Assets		5
Liabilities	$ 3	$ 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)	3.36	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)	3.36	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)	3.36	3.93