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Fair Value Measurements - Level 3 Rollforward (Details) - Level 3 - USD ($)
$ in Millions
 9 Months Ended 12 Months Ended
Sep. 30, 2020
Dec. 31, 2019
NUG contracts    
Reconciliation of changes in the fair value of FTRs contracts    
Beginning Balance, Derivative Assets $ 0 $ 0
Beginning Balance, Derivative Liabilities (16) (44)
Beginning Balance, Net (16) (44)
Unrealized gain (loss), Derivative Assets 0 0
Unrealized loss, Derivative Liabilities (3) (11)
Unrealized loss, Net (3) (11)
Purchases, Derivative Assets 0 0
Purchases, Derivative Liabilities 0 0
Purchases, Net 0 0
Settlements, Derivative Assets 0 0
Settlements, Derivative Liabilities 19 39
Settlements, Net 19 39
Ending Balance, Derivative Assets 0 0
Ending Balance, Derivative Liabilities 0 (16)
Ending Balance, Net 0 (16)
FTRs    
Reconciliation of changes in the fair value of FTRs contracts    
Beginning Balance, Derivative Assets 4 10
Beginning Balance, Derivative Liabilities (1) (1)
Beginning Balance, Net 3 9
Unrealized gain (loss), Derivative Assets 0 (1)
Unrealized loss, Derivative Liabilities 0 0
Unrealized loss, Net 0 (1)
Purchases, Derivative Assets 7 6
Purchases, Derivative Liabilities (2) (4)
Purchases, Net 5 2
Settlements, Derivative Assets (4) (11)
Settlements, Derivative Liabilities 3 4
Settlements, Net (1) (7)
Ending Balance, Derivative Assets 7 4
Ending Balance, Derivative Liabilities 0 (1)
Ending Balance, Net $ 7 $ 3