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Fair Value of Financial Instruments - Derivative Fair Value Measurement (Details) $ in Millions | Dec. 31, 2018 USD ($) $ / MWh | Dec. 31, 2017 USD ($) $ / MWh
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Fair Value Inputs [Abstract] (Deprecated 2018-01-31) |  |
Cash collateral posted in support of energy risk management activities | $ 287 | $ 171
Cash collateral received in support of energy risk management activities | 33 | 37
Commodity contracts |  |
Assets |  |
Derivative assets | 1,042 | 744
Liabilities |  |
Derivative liabilities | 977 | 674
Fair Value Inputs [Abstract] (Deprecated 2018-01-31) |  |
Cash collateral posted in support of energy risk management activities | 114 | 72
Cash collateral received in support of energy risk management activities | 31 | 11
Commodity contracts | Fair Value, Measurements, Recurring |  |
Assets |  |
Derivative assets | 1,042 | 744
Liabilities |  |
Derivative liabilities | 977 | 674
Commodity contracts | Fair Value, Measurements, Recurring | Level 3 |  |
Assets |  |
Derivative assets | 109 | 44
Liabilities |  |
Derivative liabilities | 89 | 59
Commodity contracts | Fair Value, Measurements, Recurring | Power Contracts | Level 3 |  |
Assets |  |
Derivative assets | 89 | 33
Liabilities |  |
Derivative liabilities | $ 75 | $ 47
Commodity contracts | Fair Value, Measurements, Recurring | Power Contracts | Level 3 | Low |  |
Fair Value Inputs [Abstract] (Deprecated 2018-01-31) |  |
Forward Market Price (USD per MWh) | $ / MWh | 1 | 10
Commodity contracts | Fair Value, Measurements, Recurring | Power Contracts | Level 3 | High |  |
Fair Value Inputs [Abstract] (Deprecated 2018-01-31) |  |
Forward Market Price (USD per MWh) | $ / MWh | 214 | 142
Commodity contracts | Fair Value, Measurements, Recurring | Power Contracts | Level 3 | Weighted Average |  |
Fair Value Inputs [Abstract] (Deprecated 2018-01-31) |  |
Forward Market Price (USD per MWh) | $ / MWh | 31 | 24
Commodity contracts | Fair Value, Measurements, Recurring | Financial Transmission Rights | Level 3 |  |
Assets |  |
Derivative assets | $ 20 | $ 11
Liabilities |  |
Derivative liabilities | $ 14 | $ 12
Commodity contracts | Fair Value, Measurements, Recurring | Financial Transmission Rights | Level 3 | Low |  |
Fair Value Inputs [Abstract] (Deprecated 2018-01-31) |  |
Auction Prices (USD per MWh) | $ / MWh | (90) | (28)
Commodity contracts | Fair Value, Measurements, Recurring | Financial Transmission Rights | Level 3 | High |  |
Fair Value Inputs [Abstract] (Deprecated 2018-01-31) |  |
Auction Prices (USD per MWh) | $ / MWh | 34 | 46
Commodity contracts | Fair Value, Measurements, Recurring | Financial Transmission Rights | Level 3 | Weighted Average |  |
Fair Value Inputs [Abstract] (Deprecated 2018-01-31) |  |
Auction Prices (USD per MWh) | $ / MWh | 0 | 0

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