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Price Risk Management Fair values of price risk management assets and liabilities (Details) (USD $)
In Millions, unless otherwise specified
Jun. 30, 2014
Dec. 31, 2013
Current Assets, Commodity Contracts:    
Electricity $ 8 $ 9
Natural gas 10 4
Total current derivative assets 18 13
Noncurrent Assets, Commodity Contracts: [Abstract]    
Electricity 0 1
Natural gas 5 0
Total noncurrent derivative assets 5 1
Total derivative assets not designated as hedging instruments 23 14
Total derivative assets 23 14
Current Liabilities, Commodity Contracts: [Abstract]    
Electricity 18 20
Natural gas 14 29
Total current derivative liabilities 32 49
Noncurrent Liabilities, Commodity Contracts: [Abstract]    
Electricity 66 107
Natural gas 17 34
Total noncurrent derivative liabilities 83 141
Total derivative liabilities not designated as hedging instruments 115 190
Total derivative liabilities $ 115 $ 190