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Fair Value Measurements - Level 3 Quantitative Information (Details) - Level 3
$ in Millions
 3 Months Ended
Mar. 31, 2020
USD ($)
MWh
$ / MWh
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
FTRs      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value | $ $ (1) $ 3 $ 9
NUG contracts      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value | $ (6) $ (16) $ (44)
Model | FTRs      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value | $ (1)    
Model | NUG contracts      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value | $ $ (6)    
Model | Minimum | FTRs      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) (0.10)    
Model | Minimum | NUG contracts      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value Inputs, Power (in MWH) | MWh 400    
Fair Value Inputs, Power, Regional Prices (in $/MWH) 20.50    
Model | Maximum | FTRs      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) 2.10    
Model | Maximum | NUG contracts      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value Inputs, Power (in MWH) | MWh 109,000    
Fair Value Inputs, Power, Regional Prices (in $/MWH) 34.70    
Model | Weighted Average | FTRs      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) 0.50    
Model | Weighted Average | NUG contracts      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value Inputs, Power (in MWH) | MWh 27,000    
Fair Value Inputs, Power, Regional Prices (in $/MWH) 21.60