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Significant Accounting Policies (Details 1) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Finite-Lived Intangible Assets [Line Items]    
Emission Allowance $ 5 $ 6
Renewable Energy Credits 46 44
Contract Intangible Assets 140 139
Intangible Assets, Gross 191 189
Less: Accumulated Amortization 37 34
Finite-Lived Intangible Assets, Net 154 155
Intangible Assets, Current 19 20
Intangible Assets, Net $ 135 $ 135