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Summary of Significant Accounting Policies (Details 7) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Summary of Significant Accounting Policies    
Emission reduction credits utilized $ 37  
Finite-Lived Intangible Assets    
Cost 498 311
Accumulated amortization (68) (37)
Net 430 274
Estimated amortization expense related to finite-lived intangible assets for the next five years    
2012 50  
2013 47  
2014 44  
2015 41  
2016 36  
Customer contracts and relationships
    
Finite-Lived Intangible Assets    
Average Depreciable Life, Minimum (in years) 1  
Average Depreciable Life, maximum (in years) 20  
Cost 426 221
Accumulated amortization (61) (34)
Net 365 187
Property tax abatement
    
Finite-Lived Intangible Assets    
Average Depreciable Life, Minimum (in years) 7  
Average Depreciable Life, maximum (in years) 13  
Cost 38 23
Accumulated amortization (6) (2)
Net 32 21
Other agreements
    
Finite-Lived Intangible Assets    
Average Depreciable Life, Minimum (in years) 30  
Average Depreciable Life, maximum (in years) 70  
Cost 26 22
Accumulated amortization (1) (1)
Net 25 21
Emission reduction credits
    
Finite-Lived Intangible Assets    
Cost 8 45
Net $ 8 $ 45