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Fair Value Measurements (Quantitative Information About Level 3 Fair Value Measurements) (Details) - Level 3 $ in Millions | 12 Months Ended
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Dec. 31, 2015 USD ($) $ / MWh MW | Dec. 31, 2014 USD ($) $ / MWh MW
Congestion revenue rights | Market simulation model and auction prices |  |  |
Quantitative Information About Level 3 Measurements [Line Items] |  |  |
Fair Value, Assets | $ |  | $ 152 | $ 317
Fair Value, Liabilities | $ |  | $ 0 | $ 0
Congestion revenue rights | Market simulation model and auction prices | Load forecast | Minimum |  |  |
Quantitative Information About Level 3 Measurements [Line Items] |  |  |
Fair value inputs, power units (in megawatts) | MW |  | 6,289 | 7,630
Congestion revenue rights | Market simulation model and auction prices | Load forecast | Maximum |  |  |
Quantitative Information About Level 3 Measurements [Line Items] |  |  |
Fair value inputs, power units (in megawatts) | MW |  | 24,349 | 25,431
Congestion revenue rights | Market simulation model and auction prices | Gas Prices | Minimum |  |  |
Quantitative Information About Level 3 Measurements [Line Items] |  |  |
Fair Value Inputs, Price Level ($ per MWh) | [1] | 1.98 | 3.65
Congestion revenue rights | Market simulation model and auction prices | Gas Prices | Maximum |  |  |
Quantitative Information About Level 3 Measurements [Line Items] |  |  |
Fair Value Inputs, Price Level ($ per MWh) | [1] | 5.72 | 6.53
Congestion revenue rights | Market simulation model and auction prices | Power Prices | Minimum |  |  |
Quantitative Information About Level 3 Measurements [Line Items] |  |  |
Fair Value Inputs, Price Level ($ per MWh) | [2] | 0.00 | 1.65
Congestion revenue rights | Market simulation model and auction prices | Power Prices | Maximum |  |  |
Quantitative Information About Level 3 Measurements [Line Items] |  |  |
Fair Value Inputs, Price Level ($ per MWh) | [2] | 110.44 | 109.95
Tolling | Option model |  |  |
Quantitative Information About Level 3 Measurements [Line Items] |  |  |
Fair Value, Assets | $ |  | $ 10 | $ 4
Fair Value, Liabilities | $ |  | $ 1,297 | $ 1,207
Tolling | Option model | Gas Prices | Minimum |  |  |
Quantitative Information About Level 3 Measurements [Line Items] |  |  |
Expected volatility rate (as a percent) |  | 15.00% | 13.00%
Tolling | Option model | Gas Prices | Maximum |  |  |
Quantitative Information About Level 3 Measurements [Line Items] |  |  |
Expected volatility rate (as a percent) |  | 58.00% | 53.00%
Tolling | Option model | Gas Prices | Weighted Average |  |  |
Quantitative Information About Level 3 Measurements [Line Items] |  |  |
Expected volatility rate (as a percent) |  | 20.00% | 20.00%
Tolling | Option model | Power Prices | Minimum |  |  |
Quantitative Information About Level 3 Measurements [Line Items] |  |  |
Fair Value Inputs, Price Level ($ per MWh) |  | 24.15 | 30.60
Expected volatility rate (as a percent) |  | 26.00% | 25.00%
Tolling | Option model | Power Prices | Maximum |  |  |
Quantitative Information About Level 3 Measurements [Line Items] |  |  |
Fair Value Inputs, Price Level ($ per MWh) |  | 46.93 | 61.40
Expected volatility rate (as a percent) |  | 38.00% | 42.00%
Tolling | Option model | Power Prices | Weighted Average |  |  |
Quantitative Information About Level 3 Measurements [Line Items] |  |  |
Fair Value Inputs, Price Level ($ per MWh) |  | 34.80 | 44.60
Expected volatility rate (as a percent) |  | 30.00% | 30.00%
[1] | Prices are in dollars per million British thermal units.
[2] | Prices are in dollars per megawatt-hour.

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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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- Definition Measure of dispersion, in percentage terms (for instance, the standard deviation or variance), for a given stock price. + 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 (e) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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