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Fair Value Measurements - Level 3 Quantitative Information (Details) - Level 3 - FTRs
$ in Millions
 9 Months Ended
Sep. 30, 2020
USD ($)
$ / MWh
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value | $ $ 7 $ 3 $ 9
Model      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value | $ $ 7    
Model | Minimum      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) 0.10    
Model | Maximum      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) 2.40    
Model | Weighted Average      
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]      
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) 1.20