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Price Risk Management Fair values of price risk management assets and liabilities (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2012
Dec. 31, 2011
Current Assets, Commodity Contracts:    
Commodity Contract Asset, Current, Electricity $ 1 $ 2
Commodity Contract Asset, Current, Natural Gas 3 17
Current Derivative Assets 4 19
Noncurrent Assets, Commodity Contracts: [Abstract]    
Commodity Contract Asset, Noncurrent, Natural Gas 2 0
Total derivative assets not designated as hedging instruments 6 19
Total derivative assets 6 19
Current liabilities:    
Commodity Contract Liability, Current, Electricity 44 66
Commodity Contract Liability, Current, Natural Gas 83 150
Total current derivative liabilities 127 216
Liabilities, Noncurrent [Abstract]    
Commodity Contract Liability, Noncurrent, Electricity 38 71
Commodity Contract Liability, Noncurrent, Natural Gas 35 101
Total noncurrent derivative liabilities 73 172
Total derivative liabilities not designated as hedging instruments 200 388
Total derivative liabilities $ 200 $ 388