Fair Value Measurements (Details 3) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012 item
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Liabilities	110
Electricity forward contracts
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Liabilities	81
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)	22.00
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)	63.71
Option Contracts
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Liabilities	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)	30.50
Natural gas forward price (per mmbtu)	2.98
Implied electricity price volatilities (as a percent)	16.00%
Implied natural gas price volatilities (as a percent)	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)	92.37
Natural gas forward price (per mmbtu)	4.38
Implied electricity price volatilities (as a percent)	57.00%
Implied natural gas price volatilities (as a percent)	42.00%
Natural gas forward contracts
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Liabilities	2
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)	2.98
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)	4.61