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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