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Fair Value Measurements - Level 3 Quantitative Information (Details) - Level 3 - FTRs $ in Millions | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2021 USD ($) $ / MWh | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($)
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value | $ | $ (1) | $ 3 | $ 3
Model |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value | $ | $ (1) |  |
Model | Minimum |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) | 0.20 |  |
Model | Maximum |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) | 1.90 |  |
Model | Weighted Average |  |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |  |
Fair Value Inputs, RTO Auction Clearing Prices (in $/MWH) | 0.60 |  |

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