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Environmental Matters - Additional Information (Detail) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Site Contingency [Line Items]          
Insurance recoveries $ 1,000,000 $ 0 $ 1,000,000 $ 0  
Manufactured gas plant sites | Maximum          
Site Contingency [Line Items]          
Estimated aggregate undiscounted potential liability related environmental contaminants (up to)         $ 2,800,000,000
CECONY          
Site Contingency [Line Items]          
Insurance recoveries $ 1,000,000 $ 0 $ 1,000,000 $ 0  
CECONY | Manufactured gas plant sites | Maximum          
Site Contingency [Line Items]          
Estimated aggregate undiscounted potential liability related environmental contaminants (up to)         $ 2,700,000,000
Asbestos Related          
Site Contingency [Line Items]          
Estimated undiscounted asbestos liability (years)     15 years    
Asbestos Related | CECONY          
Site Contingency [Line Items]          
Estimated undiscounted asbestos liability (years)     15 years