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Commitments (Long-Term Purchase Commitments) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Electricity Purchase Contracts      
Long-Term Purchase Commitment [Line Items]      
Purchases Under Long Term Contracts $ 2,592 $ 3,104 $ 2,665
Future Commitments 2016 2,623    
Future Commitments 2017 2,444    
Future Commitments 2018 2,634    
Future Commitments 2019 2,799    
Future Commitments 2020 2,918    
Future Commitments Thereafter 24,176    
Future Commitments Total 37,594    
Fuel Purchase Contracts      
Long-Term Purchase Commitment [Line Items]      
Purchases Under Long Term Contracts 1,262 1,521 1,590
Future Commitments 2016 1,120    
Future Commitments 2017 835    
Future Commitments 2018 532    
Future Commitments 2019 314    
Future Commitments 2020 311    
Future Commitments Thereafter 2,141    
Future Commitments Total 5,253    
Other Purchase Contracts      
Long-Term Purchase Commitment [Line Items]      
Purchases Under Long Term Contracts 2,121 $ 1,386 $ 1,743
Future Commitments 2016 1,332    
Future Commitments 2017 1,047    
Future Commitments 2018 1,081    
Future Commitments 2019 873    
Future Commitments 2020 655    
Future Commitments Thereafter 4,395    
Future Commitments Total $ 9,383