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Price Risk Management Fair values of price risk management assets and liabilities (Details) - USD ($) $ in Millions | Mar. 31, 2016 | Dec. 31, 2015
--- | --- | ---
Current Assets, Commodity Contracts: |  |
Electricity | $ 11 | $ 7
Natural gas | 7 | 3
Total current derivative assets | 18 | 10
Noncurrent Assets, Commodity Contracts: [Abstract] |  |
Commodity Contract Asset, Noncurrent, Natural Gas | 2 | 0
Total noncurrent derivative assets | 2 | 0
Total derivative assets not designated as hedging instruments | 20 | 10
Total derivative assets | 20 | 10
Current Liabilities, Commodity Contracts: [Abstract] |  |
Electricity | 37 | 36
Natural gas | 105 | 94
Total current derivative liabilities | 142 | 130
Noncurrent Liabilities, Commodity Contracts: [Abstract] |  |
Electricity | 109 | 97
Natural gas | 52 | 64
Total noncurrent derivative liabilities | 161 | 161
Total derivative liabilities not designated as hedging instruments | 303 | 291
Total derivative liabilities | $ 303 | $ 291

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