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Commitments and Contingencies - Operating Leases (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Coal, Gas and Transportation Commitments      
Out-of-market contracts, net $ 207 $ 230  
Future commitments under coal, gas and transportation contractual agreements      
Future minimum sublease rentals 49    
Powerton and Joliet      
Future commitments under coal, gas and transportation contractual agreements      
2018 1    
2019 1    
2020 1    
2021 3    
2022 6    
Thereafter 228    
Total 240    
Other Leased Property      
Coal, Gas and Transportation Commitments      
Lease expense 81 $ 96 $ 97
Future commitments under coal, gas and transportation contractual agreements      
2018 78    
2019 80    
2020 75    
2021 65    
2022 64    
Thereafter 479    
Total [1] $ 841    
EME Project Financings      
Coal, Gas and Transportation Commitments      
Leased Interest 100.00%    
Out-of-market contracts, net     $ 159
Lease expense $ 14    
[1] Amounts in the table exclude future sublease income of $49 million associated with long-term leases for office locations.