Fair Value Measurements - Significant Unobservable Inputs Used to Value Level 3 Instruments (Details 2) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Assets	$ 30	$ 41
Liabilities	68	90
Electricity forward contracts
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Assets	27	40
Liabilities	50	66
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	24.89
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)	63.85	65.04
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)	40.42	41.09
Option Contracts
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Assets	0	0
Liabilities	15	19
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)	38.96	39.91
Natural gas forward price (per MMbtu)	3.76	3.57
Electricity price volatilities (as a percent)	29.00%	35.00%
Natural gas price volatilities (as a percent)	21.00%	22.00%
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)	78.85	85.41
Natural gas forward price (per MMbtu)	3.86	3.80
Electricity price volatilities (as a percent)	64.00%	94.00%
Natural gas price volatilities (as a percent)	67.00%	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)	53.76	58.70
Natural gas forward price (per MMbtu)	3.82	3.71
Electricity price volatilities (as a percent)	46.00%	59.00%
Natural gas price volatilities (as a percent)	28.00%	27.00%
Natural gas forward contracts
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Assets	3	1
Liabilities	$ 3	$ 5
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)	3.77	3.47
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)	4.34	4.31
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)	3.99	3.87