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Price Risk Management Fair values of price risk management assets and liabilities (Details) - USD ($)
$ in Millions
Jun. 30, 2015
Dec. 31, 2014
Current Assets, Commodity Contracts:    
Electricity $ 3 $ 4
Natural gas 0 2
Total current derivative assets 3 6
Noncurrent Assets, Commodity Contracts: [Abstract]    
Electricity 1 1
Total noncurrent derivative assets 1 1
Total derivative assets not designated as hedging instruments 4 7
Total derivative assets 4 7
Current Liabilities, Commodity Contracts: [Abstract]    
Electricity 38 54
Natural gas 63 52
Total current derivative liabilities 101 106
Noncurrent Liabilities, Commodity Contracts: [Abstract]    
Electricity 114 58
Natural gas 73 64
Total noncurrent derivative liabilities 187 122
Total derivative liabilities not designated as hedging instruments 288 228
Total derivative liabilities $ 288 $ 228