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Regulatory Matters - Cecony - Electric - Additional Information (Detail) (USD $)
 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2012
CECONY - Electric [Member]
Dec. 31, 2011
CECONY - Electric [Member]
Dec. 31, 2010
CECONY - Electric [Member]
Dec. 31, 2012
CECONY - Electric [Member]
T&D [Member]
Dec. 31, 2012
CECONY - Electric [Member]
Other [Member]
Dec. 31, 2012
CECONY - Electric [Member]
Effective April 2009 [Member]
Dec. 31, 2011
CECONY - Electric [Member]
Effective April 2009 [Member]
Dec. 31, 2009
CECONY - Electric [Member]
Effective April 2009 [Member]
Dec. 31, 2010
CECONY - Electric [Member]
Effective April 2009 [Member]
Dec. 31, 2012
CECONY - Electric [Member]
Effective April 2009 [Member]
Revenue decoupling mechanism [Member]
Dec. 31, 2012
CECONY - Electric [Member]
November 2009 Joint Proposal [Member]
Dec. 31, 2012
CECONY - Electric [Member]
Sale of transmission rights [Member]
Dec. 31, 2011
CECONY - Electric [Member]
Sale of transmission rights [Member]
Dec. 31, 2010
CECONY - Electric [Member]
Sale of transmission rights [Member]
Mar. 31, 2012
CECONY - Electric [Member]
March 2013 [Member]
Dec. 31, 2012
CECONY - Electric [Member]
March 2013 [Member]
75% of earnings [Member]
Dec. 31, 2012
CECONY - Electric [Member]
March 2013 [Member]
90% of earnings [Member]
Dec. 31, 2012
CECONY - Electric [Member]
March 2013 [Member]
60% of earnings [Member]
Dec. 31, 2012
CECONY - Electric [Member]
March 2013 [Member]
T&D [Member]
Dec. 31, 2012
CECONY - Electric [Member]
March 2013 [Member]
Other [Member]
Dec. 31, 2012
CECONY - Electric [Member]
March 2013 [Member]
ERP [Member]
Dec. 31, 2012
CECONY - Electric [Member]
March 2013 [Member]
November 2009 Joint Proposal [Member]
Dec. 31, 2012
CECONY - Electric [Member]
March 2010 [Member]
Effective April 2009 [Member]
Dec. 31, 2012
CECONY - Electric [Member]
March 2011 [Member]
Dec. 31, 2012
CECONY - Electric [Member]
March 2011 [Member]
50% of earnings [Member]
Dec. 31, 2012
CECONY - Electric [Member]
March 2011 [Member]
75% of earnings [Member]
Dec. 31, 2012
CECONY - Electric [Member]
March 2011 [Member]
90% of earnings [Member]
Dec. 31, 2012
CECONY - Electric [Member]
March 2011 [Member]
T&D [Member]
Dec. 31, 2012
CECONY - Electric [Member]
March 2011 [Member]
Other [Member]
Mar. 31, 2011
CECONY - Electric [Member]
March 2011 [Member]
ERP [Member]
Dec. 31, 2012
CECONY - Electric [Member]
March 2011 [Member]
Effective April 2009 [Member]
Dec. 31, 2012
CECONY - Electric [Member]
March 2011 [Member]
November 2009 Joint Proposal [Member]
Dec. 31, 2012
CECONY - Electric [Member]
March 2008 rate order [Member]
Effective April 2009 [Member]
Dec. 31, 2012
CECONY - Electric [Member]
March 2009 [Member]
Effective April 2009 [Member]
Dec. 31, 2012
CECONY - Electric [Member]
April 2010 [Member]
November 2009 Joint Proposal [Member]
Dec. 31, 2012
CECONY - Electric [Member]
April 2011 [Member]
November 2009 Joint Proposal [Member]
Dec. 31, 2012
CECONY - Electric [Member]
April 2012 [Member]
November 2009 Joint Proposal [Member]
Dec. 31, 2012
CECONY - Electric [Member]
March 2012 [Member]
Dec. 31, 2012
CECONY - Electric [Member]
March 2012 [Member]
75% of earnings [Member]
Dec. 31, 2012
CECONY - Electric [Member]
March 2012 [Member]
90% of earnings [Member]
Dec. 31, 2012
CECONY - Electric [Member]
March 2012 [Member]
60% of earnings [Member]
Dec. 31, 2012
CECONY - Electric [Member]
March 2012 [Member]
T&D [Member]
Dec. 31, 2012
CECONY - Electric [Member]
March 2012 [Member]
Other [Member]
Dec. 31, 2012
CECONY - Electric [Member]
March 2012 [Member]
ERP [Member]
Dec. 31, 2012
CECONY - Electric [Member]
March 2012 [Member]
November 2009 Joint Proposal [Member]
Dec. 31, 2012
CECONY - Electric [Member]
February 2010 [Member]
Dec. 31, 2012
CECONY - Electric [Member]
January 2014 [Member]
Dec. 31, 2012
CECONY - Electric [Member]
January 2015 [Member]
Dec. 31, 2012
CECONY - Electric [Member]
January 2016 [Member]
Dec. 31, 2012
CECONY - Electric [Member]
Maximum [Member]
Dec. 31, 2012
CECONY - Electric [Member]
Maximum [Member]
March 2013 [Member]
75% of earnings [Member]
Dec. 31, 2012
CECONY - Electric [Member]
Maximum [Member]
March 2013 [Member]
60% of earnings [Member]
Dec. 31, 2012
CECONY - Electric [Member]
Maximum [Member]
March 2011 [Member]
50% of earnings [Member]
Dec. 31, 2012
CECONY - Electric [Member]
Maximum [Member]
March 2011 [Member]
75% of earnings [Member]
Dec. 31, 2012
CECONY - Electric [Member]
Maximum [Member]
March 2012 [Member]
75% of earnings [Member]
Dec. 31, 2012
CECONY - Electric [Member]
Maximum [Member]
March 2012 [Member]
60% of earnings [Member]
Dec. 31, 2012
CECONY - Electric [Member]
Minimum [Member]
Dec. 31, 2012
CECONY - Electric [Member]
Minimum [Member]
March 2013 [Member]
75% of earnings [Member]
Dec. 31, 2012
CECONY - Electric [Member]
Minimum [Member]
March 2013 [Member]
90% of earnings [Member]
Dec. 31, 2012
CECONY - Electric [Member]
Minimum [Member]
March 2013 [Member]
60% of earnings [Member]
Dec. 31, 2012
CECONY - Electric [Member]
Minimum [Member]
March 2011 [Member]
50% of earnings [Member]
Dec. 31, 2012
CECONY - Electric [Member]
Minimum [Member]
March 2011 [Member]
75% of earnings [Member]
Dec. 31, 2012
CECONY - Electric [Member]
Minimum [Member]
March 2011 [Member]
90% of earnings [Member]
Dec. 31, 2012
CECONY - Electric [Member]
Minimum [Member]
March 2012 [Member]
75% of earnings [Member]
Dec. 31, 2012
CECONY - Electric [Member]
Minimum [Member]
March 2012 [Member]
90% of earnings [Member]
Dec. 31, 2012
CECONY - Electric [Member]
Minimum [Member]
March 2012 [Member]
60% of earnings [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]                                                                                                                                        
Revenue from electric $ 249,000,000                                               $ 254,000,000               $ 249,000,000                                                                      
Regulatory liability 14,000,000                 24,000,000                                                                                                                    
Regulatory liability recognition after tax charge                   14,000,000                                                                                                                    
Credit to customer               36,000,000                                                                                                                        
Electric base rate increase               523,000,000                                                         420,000,000 420,000,000 287,000,000                   375,000,000 195,000,000 270,000,000                                  
Return of common equity     10.85%         10.00%         10.15%                                                                       10.35%                                      
Common equity ratio     50.00%         48.00%         48.00%                                                                       50.00%     50.00%                                
Revenue requirements               60,000,000                                 60,000,000                                                                                      
Increased revenues                       122,000,000                                                                                                                
Decrease in annual revenue               150,000,000                                                     120,000,000                                                                  
Revenues difference between actual and rate year, recoverable from customers               120,000,000                                                                                                                        
Accrued revenues               7,000,000           45,000,000 26,000,000 9,000,000                                                                                                        
Deferred expenses               36,400,000                                                                                                                        
Surcharge collected from customers               198,000,000                 134,000,000             134,000,000                                                                                        
Potential operational penalties               152,000,000                                                                                                                        
Potential refund to customers                                                 254,000,000                     237,000,000                                                                
Austerity plan costs                                                 47,000,000                                                                                      
Weighted average cost of capital                         7.76%                                                                                                              
Operations and maintenance expenses                                               27,000,000                   13,000,000                         20,000,000                                          
Long term debt cost                         5.65%                                                                                                              
Average base rate                                               16,826,000,000                   14,887,000,000                         15,987,000,000                                          
Deferred regulatory liabilities                                         15,414 1,650 115     8.0       13,818 1,487                 0.1       14,742 1,565 25                                            
Capital expenditures           2,300,000,000 402,000,000                                             1,200,000,000 220,000,000 125,000,000                                                                        
Percentage of revenue allocated to customers                                   75.00% 90.00% 60.00%             50.00% 75.00% 90.00%                       75.00% 90.00% 60.00%                                                  
Percentage of revenue requirement earnings                                                                                                         13.149% 12.149% 12.149% 13.149% 13.149% 12.149%   12.15% 13.15% 10.65% 11.15% 12.15% 13.15% 12.15% 13.15% 10.65%
Percentage of share to reduce regulatory assets     50.00%                                                                                                                                  
Maximum impact on return on common equity and deferral of facility relocation expenses     Not more than a 10 basis point                                                                                                                                  
Deferred net regulatory asset 126,000,000 173,000,000     264,000,000                                                                                                                              
Deferred net regulatory liability 2,061,000,000 1,971,000,000   39,000,000                                                                                                                                
Difference in property taxes                                                                                                       80.00%             20.00%                  
Deferred revenues               59,000,000 90,000,000   124,000,000                                                                                                                  
Potential operations penalties               152,000,000                                                                                                                        
System reliability penalty                 5,000,000                                                                                                                      
Potential refund from customers                                                                                               $ 249,000,000