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Price Risk Management Fair values of price risk management assets and liabilities (Details) (USD $)
In Millions, unless otherwise specified
Sep. 30, 2014
Dec. 31, 2013
Current Assets, Commodity Contracts:    
Electricity $ 2 $ 9
Natural gas 2 4
Total current derivative assets 4 13
Noncurrent Assets, Commodity Contracts: [Abstract]    
Electricity 0 1
Natural gas 2 0
Total noncurrent derivative assets 2 1
Total derivative assets not designated as hedging instruments 6 14
Total derivative assets 6 14
Current Liabilities, Commodity Contracts: [Abstract]    
Electricity 29 20
Natural gas 15 29
Total current derivative liabilities 44 49
Noncurrent Liabilities, Commodity Contracts: [Abstract]    
Electricity 55 107
Natural gas 23 34
Total noncurrent derivative liabilities 78 141
Total derivative liabilities not designated as hedging instruments 122 190
Total derivative liabilities $ 122 $ 190