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Fair Value of Financial Instruments - Derivative Fair Value Measurement (Details) $ in Millions | Dec. 31, 2020 USD ($) $ / MWh | Dec. 31, 2019 USD ($) $ / MWh
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Fair Value Measurement Inputs and Valuation Techniques [Abstract |  |
Cash collateral posted in support of energy risk management activities | $ 50 | $ 190
Cash collateral received in support of energy risk management activities | 19 | 32
Commodity contracts |  |
Assets |  |
Derivative assets | 821 | 1,170
Liabilities |  |
Commodity contracts | 884 | 1,103
Fair Value Measurement Inputs and Valuation Techniques [Abstract |  |
Cash collateral posted in support of energy risk management activities | 0 | 73
Cash collateral received in support of energy risk management activities | 5 | 7
Commodity contracts | Level 3 |  |
Assets |  |
Derivative assets | 139 | 193
Liabilities |  |
Commodity contracts | 155 | 155
Commodity contracts | Fair Value, Measurements, Recurring | Level 3 |  |
Assets |  |
Derivative assets | 139 | 193
Liabilities |  |
Commodity contracts | 155 | 155
Commodity contracts | Fair Value, Measurements, Recurring | Power Contracts | Level 3 |  |
Assets |  |
Derivative assets | 111 | 151
Liabilities |  |
Commodity contracts | $ 143 | $ 139
Commodity contracts | Fair Value, Measurements, Recurring | Power Contracts | Level 3 | Low |  |
Fair Value Measurement Inputs and Valuation Techniques [Abstract |  |
Forward Market Price (USD per MWh) | $ / MWh | 10 | 8
Commodity contracts | Fair Value, Measurements, Recurring | Power Contracts | Level 3 | High |  |
Fair Value Measurement Inputs and Valuation Techniques [Abstract |  |
Forward Market Price (USD per MWh) | $ / MWh | 105 | 218
Commodity contracts | Fair Value, Measurements, Recurring | Power Contracts | Level 3 | Weighted Average |  |
Fair Value Measurement Inputs and Valuation Techniques [Abstract |  |
Forward Market Price (USD per MWh) | $ / MWh | 21 | 24
Commodity contracts | Fair Value, Measurements, Recurring | Financial Transmission Rights | Level 3 |  |
Assets |  |
Derivative assets | $ 28 | $ 42
Liabilities |  |
Commodity contracts | $ 12 | $ 16
Commodity contracts | Fair Value, Measurements, Recurring | Financial Transmission Rights | Level 3 | Low |  |
Fair Value Measurement Inputs and Valuation Techniques [Abstract |  |
Auction Prices (USD per MWh) | $ / MWh | (28) | (105)
Commodity contracts | Fair Value, Measurements, Recurring | Financial Transmission Rights | Level 3 | High |  |
Fair Value Measurement Inputs and Valuation Techniques [Abstract |  |
Auction Prices (USD per MWh) | $ / MWh | 43 | 213
Commodity contracts | Fair Value, Measurements, Recurring | Financial Transmission Rights | Level 3 | Weighted Average |  |
Fair Value Measurement Inputs and Valuation Techniques [Abstract |  |
Auction Prices (USD per MWh) | $ / MWh | 0 | 0

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