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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, 2011
Dec. 31, 2010
Current Assets, Commodity Contracts:    
Commodity Contract Asset, Current, Electricity $ 2 $ 4
Commodity Contract Asset, Current, Natural Gas 17 9
Current Derivative Assets 19 13
Noncurrent Assets, Commodity Contracts: [Abstract]    
Commodity Contract Asset, Noncurrent, Electricity 0 1
Commodity Contract Asset, Noncurrent, Natural Gas 0 2
Total noncurrent derivative assets 0 3
Total derivative assets not designated as hedging instruments 19 16
Total derivative assets 19 16
Current liabilities:    
Commodity Contract Liability, Current, Electricity 66 77
Commodity Contract Liability, Current, Natural Gas 150 111
Total current derivative liabilities 216 188
Liabilities, Noncurrent [Abstract]    
Commodity Contract Liability, Noncurrent, Electricity 71 42
Commodity Contract Liability, Noncurrent, Natural Gas 101 146
Total noncurrent derivative liabilities 172 188
Total derivative liabilities not designated as hedging instruments 388 376
Total derivative liabilities $ 388 $ 376