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Significant Accounting Policies (Details 1) (USD $)
In Millions, unless otherwise specified
Jun. 30, 2013
Dec. 31, 2012
Finite-Lived Intangible Assets [Line Items]    
Emission Allowance $ 4 $ 6
Renewable Energy Credits 47 44
Contract Intangible Assets 141 139
Intangible Assets, Gross 192 189
Less: Accumulated Amortization 40 34
Finite-Lived Intangible Assets, Net 152 155
Intangible Assets, Current 14 20
Less current intangible assets $ 138 $ 135