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Fair Value of Financial Instruments - Derivative Fair Value Measurement (Details) $ in Millions | Dec. 31, 2019 USD ($) $ / MWh | Dec. 31, 2018 USD ($) $ / MWh
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Fair Value Measurement Inputs and Valuation Techniques [Abstract |  |
Cash collateral posted in support of energy risk management activities | $ 190 | $ 287
Cash collateral received in support of energy risk management activities | 32 | 33
Commodity contracts |  |
Assets |  |
Derivative assets | 1,170 | 1,042
Liabilities |  |
Commodity contracts | 1,103 | 977
Fair Value Measurement Inputs and Valuation Techniques [Abstract |  |
Cash collateral posted in support of energy risk management activities | 73 | 114
Cash collateral received in support of energy risk management activities | 7 | 31
Commodity contracts | Level 3 |  |
Assets |  |
Derivative assets | 193 | 109
Liabilities |  |
Commodity contracts | 155 | 89
Commodity contracts | Fair Value, Measurements, Recurring | Level 3 |  |
Assets |  |
Derivative assets | 193 | 109
Liabilities |  |
Commodity contracts | 155 | 89
Commodity contracts | Fair Value, Measurements, Recurring | Power Contracts | Level 3 |  |
Assets |  |
Derivative assets | 151 | 89
Liabilities |  |
Commodity contracts | $ 139 | $ 75
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 | 8 | 1
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 | 218 | 214
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 | 24 | 31
Commodity contracts | Fair Value, Measurements, Recurring | Financial Transmission Rights | Level 3 |  |
Assets |  |
Derivative assets | $ 42 | $ 20
Liabilities |  |
Commodity contracts | $ 16 | $ 14
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 | (105) | (90)
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 | 213 | 34
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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