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Fair Value Measurements - Level 3 Rollforward (Details) - Level 3 - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2021 | Dec. 31, 2020
NUG contracts |  |
Reconciliation of changes in the fair value of FTRs contracts |  |
Beginning Balance, Derivative Liabilities | $ 0 | $ (16)
Unrealized loss, Derivative Liabilities | 0 | (3)
Purchases, Derivative Liabilities |  | 0
Settlements, Derivative Liabilities | 0 | 19
Ending Balance, Derivative Liabilities | 0 | 0
FTRs |  |
Reconciliation of changes in the fair value of FTRs contracts |  |
Beginning Balance, Derivative Assets | 3 | 4
Beginning Balance, Derivative Liabilities | 0 | (1)
Beginning Balance, Net | 3 | 3
Unrealized loss, Derivative Assets | 0 | (3)
Unrealized loss, Derivative Liabilities | (1) | 0
Unrealized loss, Net | (1) | (3)
Purchases, Derivative Assets |  | 7
Purchases, Derivative Liabilities |  | (2)
Purchases, Net |  | 5
Settlements, Derivative Assets | (3) | (5)
Settlements, Derivative Liabilities | 0 | 3
Settlements, Net | (3) | (2)
Ending Balance, Derivative Assets | 0 | 3
Ending Balance, Derivative Liabilities | (1) | 0
Ending Balance, Net | $ (1) | $ 3

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