Fair Value Measurements - Significant Unobservable Inputs Used to Value Level 3 Instruments (Details 2) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2015 USD ($) $ / MMBTU $ / MWh	Dec. 31, 2014 USD ($) $ / MMBTU $ / MWh
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Assets	$ 30	$ 33
Liabilities	73	74
Electricity forward contracts
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Assets	28	30
Liabilities	$ 57	$ 56
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) | $ / MWh	21.07	19.51
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) | $ / MWh	67.74	56.72
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) | $ / MWh	31.46	35.27
Option Contracts
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Assets	$ 0	$ 0
Liabilities	$ 12	$ 15
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) | $ / MWh	32.85	32.14
Natural gas forward price (per MMbtu) | $ / MMBTU		3.18
Electricity price volatilities (as a percent)	26.00%	23.00%
Natural gas price volatilities (as a percent)	27.00%	23.00%
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) | $ / MWh	67.74	66.09
Natural gas forward price (per MMbtu) | $ / MMBTU		3.29
Electricity price volatilities (as a percent)	115.00%	63.00%
Natural gas price volatilities (as a percent)	42.00%	41.00%
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) | $ / MWh	46.13	45.83
Natural gas forward price (per MMbtu) | $ / MMBTU		3.25
Electricity price volatilities (as a percent)	68.00%	41.00%
Natural gas price volatilities (as a percent)	31.00%	31.00%
Natural gas forward contracts
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Assets	$ 2	$ 3
Liabilities	$ 4	$ 3
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) | $ / MMBTU	2.69	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) | $ / MMBTU	3.61	4.13
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) | $ / MMBTU	3.23	3.45