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Price Risk Management Fair values of price risk management assets and liabilities (Details) - USD ($) $ in Millions | Sep. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Current Assets, Commodity Contracts: |  |
Electricity | $ 3 | $ 6
Natural gas | 1 | 12
Total current derivative assets | 4 | 18
Noncurrent Assets, Commodity Contracts: [Abstract] |  |
Commodity Contract Asset, Noncurrent, Electricity | 0 | 1
Commodity Contract Asset, Noncurrent, Natural Gas | 0 | 4
Total noncurrent derivative assets | 0 | 5
Total derivative assets not designated as hedging instruments | 4 | 23
Total derivative assets | 4 | 23
Current Liabilities, Commodity Contracts: [Abstract] |  |
Electricity | 11 | 12
Natural gas | 32 | 32
Total current derivative liabilities | 43 | 44
Noncurrent Liabilities, Commodity Contracts: [Abstract] |  |
Electricity | 132 | 106
Natural gas | 18 | 19
Total noncurrent derivative liabilities | 150 | 125
Total derivative liabilities not designated as hedging instruments | 193 | 169
Total derivative liabilities | $ 193 | $ 169

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