Regulatory Matters - Cecony - Steam - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2011
Schedule Of Regulatory Assets And Liabilities [Line Items]
Regulatory liability	$ 183,000,000				$ 118,000,000
Deferred net regulatory liabilities	2,061,000,000				1,971,000,000
CECONY - Steam [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Steam rate increase	43,700,000
Return on common equity	9.30%
Difference between actual costs and costs reflected in rates	90.00%
Decreased expenses		14,900,000	14,400,000
Increased expenses				3,100,000
Non-cash increase in steam revenues	20,300,000
Deferred net regulatory liabilities	200,000				300,000
Potential refund from customers	6,000,000
Potential operational penalties	1,000,000
Return on common equity	9.30%
CECONY - Steam [Member] | September Two Thousand And Nine [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Regulatory liability	0
CECONY - Steam [Member] | September 2010 [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Regulatory liability	4,000,000
CECONY - Steam [Member] | October 2010 [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Increase rate	49,500,000
CECONY - Steam [Member] | October 2011 [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Increase rate	49,500,000
CECONY - Steam [Member] | October 2012 [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Increase rate	17,800,000
CECONY - Steam [Member] | September 2013 [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Surcharge collected from customers	31.7
Reduction of cost	1,500,000
Average base rate	1,613,000,000
CECONY - Steam [Member] | September 2011 [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Reduction of cost	4,500,000
Average base rate	1,589,000,000
CECONY - Steam [Member] | September 2012 [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Reduction of cost	3,000,000
Average base rate	1,603,000,000
CECONY - Steam [Member] | January 2014 [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Return on common equity	10.35%
Gas base rate increase	5,000,000
Common equity ratio	50.00%
Gas base rate increase	5,000,000
Return on common equity	10.35%
CECONY - Steam [Member] | January 2015 [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Return on common equity	10.85%
Gas base rate increase	22,000,000
Gas base rate increase	22,000,000
Return on common equity	10.85%
CECONY - Steam [Member] | January 2016 [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Gas base rate increase	18,000,000
Common equity ratio	50.00%
Gas base rate increase	18,000,000
CECONY - Steam [Member] | Maximum [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Return on common equity	10.10%
Average common equity ratio	50.00%
Potential negative earnings adjustments	1,000,000
Return on common equity	10.10%
CECONY - Steam [Member] | Minimum [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Potential negative earnings adjustments	$ 950,000