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Fair Value Measurements (Quantitative Information About Level 3 Fair Value Measurements) (Details) - Level 3
$ in Millions
12 Months Ended
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.