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Price Risk Management Fair values of price risk management assets and liabilities (Details) - USD ($) $ in Millions | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Current Assets, Commodity Contracts: |  |
Electricity | $ 2 | $ 3
Natural gas | 2 | 3
Total current derivative assets | 4 | 6
Noncurrent Assets, Commodity Contracts: [Abstract] |  |
Total derivative assets not designated as hedging instruments | 4 | 6
Total derivative assets | 4 | 6
Current Liabilities, Commodity Contracts: [Abstract] |  |
Electricity | 20 | 13
Natural gas | 47 | 46
Total current derivative liabilities | 67 | 59
Noncurrent Liabilities, Commodity Contracts: [Abstract] |  |
Electricity | 114 | 122
Natural gas | 30 | 29
Total noncurrent derivative liabilities | 144 | 151
Total derivative liabilities not designated as hedging instruments | 211 | 210
Total derivative liabilities | $ 211 | $ 210

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