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Fair Value of Financial Instruments - Derivative Fair Value Measurements (Details) $ in Millions | Mar. 31, 2021 USD ($) $ / MWh | Dec. 31, 2020 USD ($) $ / MWh
--- | --- | ---
Liabilities |  |
Commodity contracts | $ 2,440 | $ 884
Level 3 |  |
Liabilities |  |
Commodity contracts | 105 | 155
Commodity contracts |  |
Assets |  |
Derivative assets | 2,824 | 821
Liabilities |  |
Commodity contracts | 2,438 | 884
Commodity contracts | Level 3 |  |
Assets |  |
Derivative assets | 264 | 139
Liabilities |  |
Commodity contracts | 105 | 155
Commodity contracts | Level 3 | Natural Gas Contracts |  |
Assets |  |
Derivative assets | 3 |
Liabilities |  |
Commodity contracts | $ 0 |
Commodity contracts | Level 3 | Natural Gas Contracts | Low |  |
Fair Value Inputs / Range |  |
Forward Market Price (per MWh) | $ / MWh | 1 |
Commodity contracts | Level 3 | Natural Gas Contracts | High |  |
Fair Value Inputs / Range |  |
Forward Market Price (per MWh) | $ / MWh | 16 |
Commodity contracts | Level 3 | Natural Gas Contracts | Weighted Average |  |
Fair Value Inputs / Range |  |
Forward Market Price (per MWh) | $ / MWh | 14 |
Commodity contracts | Level 3 | Power Contracts |  |
Assets |  |
Derivative assets | $ 234 | 111
Liabilities |  |
Commodity contracts | $ 91 | $ 143
Commodity contracts | Level 3 | Power Contracts | Low |  |
Fair Value Inputs / Range |  |
Forward Market Price (per MWh) | $ / MWh | 1 | 10
Commodity contracts | Level 3 | Power Contracts | High |  |
Fair Value Inputs / Range |  |
Forward Market Price (per MWh) | $ / MWh | 237 | 105
Commodity contracts | Level 3 | Power Contracts | Weighted Average |  |
Fair Value Inputs / Range |  |
Forward Market Price (per MWh) | $ / MWh | 29 | 21
Commodity contracts | Level 3 | FTRs |  |
Assets |  |
Derivative assets | $ 27 | $ 28
Liabilities |  |
Commodity contracts | $ 14 | $ 12
Commodity contracts | Level 3 | FTRs | Low |  |
Fair Value Inputs / Range |  |
Auction Prices (per MWh) | $ / MWh | (33) | (28)
Commodity contracts | Level 3 | FTRs | High |  |
Fair Value Inputs / Range |  |
Auction Prices (per MWh) | $ / MWh | 320 | 43
Commodity contracts | Level 3 | FTRs | Weighted Average |  |
Fair Value Inputs / Range |  |
Auction Prices (per MWh) | $ / MWh | 0 | 0

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