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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

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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 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 + Details
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