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Fair values of price risk management assets and liabilities (Details) - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Current Assets, Commodity Contracts: |  |
Electricity | $ 11 | $ 3
Natural Gas | 7 | 3
Total current derivative assets | 18 | 6
Noncurrent Assets, Commodity Contracts: [Abstract] |  |
Commodity Contract Asset, Noncurrent, Electricity | 1 | 0
Commodity Contract Asset, Noncurrent, Natural Gas | 1 | 0
Derivative Asset, Noncurrent | 2 | 0
Total derivative assets not designated as hedging instruments | 20 | 6
Total derivative assets | 20 | 6
Current Liabilities, Commodity Contracts: [Abstract] |  |
Electricity | 16 | 13
Natural Gas | 35 | 46
Total current derivative liabilities | 51 | 59
Noncurrent Liabilities, Commodity Contracts: [Abstract] |  |
Electricity | 78 | 122
Natural Gas | 23 | 29
Total noncurrent derivative liabilities | 101 | 151
Total derivative liabilities not designated as hedging instruments | 152 | 210
Total derivative liabilities | $ 152 | $ 210

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