Fair Value Measurements (Details 3) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2012 item
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Liabilities	114
Electricity forward contracts
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Liabilities	88
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)	20.67
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)	67.18
Option Contracts | Option Model
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Liabilities	25
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)	29.13
Natural gas forward price (per mmbtu)	2.69
Implied electricity price volatilities (as a percent)	15.00%
Implied natural gas price volatilities (as a percent)	18.00%
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)	99.27
Natural gas forward price (per mmbtu)	4.11
Implied electricity price volatilities (as a percent)	151.00%
Implied natural gas price volatilities (as a percent)	56.00%
Natural gas forward contracts
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Liabilities	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)	2.60
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)	4.18