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Acquisitions, Investments and Dispositions - NY Transco (Details) - USD ($)
$ in Millions
1 Months Ended
May 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]      
Equity method investment   $ 467 $ 488
CET Electric      
Schedule of Equity Method Investments [Line Items]      
Ownership interest, percentage   45.70%  
CET Electric | NY Transco      
Schedule of Equity Method Investments [Line Items]      
Ownership interest, percentage 45.70%    
Equity method investment   $ 53 51
CECONY      
Schedule of Equity Method Investments [Line Items]      
Equity method investment   $ 0 $ 0
CECONY | NY Transco | Equity Method Investee | Purchase Price of TOTS Project Transfer      
Schedule of Equity Method Investments [Line Items]      
Amount of transaction $ 122    
CECONY | NY Transco | Equity Method Investee | TOTS Project, Lease Payments      
Schedule of Equity Method Investments [Line Items]      
Payment for easement rights $ 8