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Fair Value Measurements (Details 2) - Level 3
$ in Millions
 12 Months Ended
Dec. 31, 2019
USD ($)
MWh
$ / MWh
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
FTRs      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value | $ $ 3 $ 9 $ 3
NUG contracts      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value | $ (16) $ (44) $ (79)
Model | FTRs      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value | $ 3    
Model | NUG contracts      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value | $ $ (16)    
Model | Minimum | FTRs      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
RTO auction clearing prices (in dollars per unit) 0.70    
Model | Minimum | NUG contracts      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Power (in dollars per MWH) | MWh 400    
Power, Regional prices (in dollars per unit) 25.30    
Model | Maximum | FTRs      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
RTO auction clearing prices (in dollars per unit) 3.40    
Model | Maximum | NUG contracts      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Power (in dollars per MWH) | MWh 330,000    
Power, Regional prices (in dollars per unit) 35.20    
Model | Weighted Average | FTRs      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
RTO auction clearing prices (in dollars per unit) 1.30    
Model | Weighted Average | NUG contracts      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Power (in dollars per MWH) | MWh 115,000    
Power, Regional prices (in dollars per unit) 26.30