Fair Value Measurements - Significant Unobservable Inputs Used to Value Level 3 Instruments (Details 2) $ in Millions	9 Months Ended	12 Months Ended
Sep. 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	$ 29	$ 33
Liabilities	70	74
Electricity forward contracts
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments
Assets	27	30
Liabilities	$ 56	$ 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	17.37	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	40.62	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	28.25	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	$ 7	$ 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	28.36	32.14
Electricity price volatilities (as a percent)	48.00%	23.00%
Natural gas price volatilities (as a percent)	30.00%	23.00%
Natural gas forward price (per MMbtu) \| $ / MMBTU		3.18
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	46.50	66.09
Electricity price volatilities (as a percent)	61.00%	63.00%
Natural gas price volatilities (as a percent)	47.00%	41.00%
Natural gas forward price (per MMbtu) \| $ / MMBTU		3.29
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	35.55	45.83
Electricity price volatilities (as a percent)	55.00%	41.00%
Natural gas price volatilities (as a percent)	34.00%	31.00%
Natural gas forward price (per MMbtu) \| $ / MMBTU		3.25
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	$ 7	$ 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	2.32	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	3.17	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	2.73	3.45

Represents the electricity forward price per Megawatt Hours, used as an input to measure fair value.

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- Definition

Represents the gas price per Metric Million British Thermal Units, used as an input to measure fair value.

+ References

No definition available.

+ Details

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- Definition

Represents the percentage of implied electricity volatilities.

+ References

No definition available.

+ Details

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- Definition

Represents the percentage of implied electricity natural gas price volatilities.

+ References

No definition available.

+ Details

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- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Fair value of financial and nonfinancial obligations.

+ References

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