Regulatory Matters - Cecony - Gas - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Schedule Of Regulatory Assets And Liabilities [Line Items]
Deferred regulatory liabilities	$ 9,644,000,000	$ 8,604,000,000
Deferred net regulatory assets	126,000,000	173,000,000
Deferred net regulatory liabilities	2,061,000,000	1,971,000,000
CECONY - Gas [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Increase in annual funding for new energy efficiency program	14,000,000
Accrued revenues			24,000,000	25,000,000	17,000,000
Return on common equity	9.60%	10.70%
Percentage of share to reduce regulatory assets	50.00%
Common equity basis points	10
Deferred net regulatory assets	46,000,000		67,000,000
Deferred net regulatory liabilities		300,000
Amount of revenues retained	58,000,000
Percentage of revenue reserve	25.00%
Accrual of regulatory asset	33,000,000
Percentage of difference between targeted and least revenue reserve	80.00%
Potential operational penalties	12,600,000
Potential refund from customers	32,000,000
CECONY - Gas [Member] | Gas Distribution Revenues [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Deferred net regulatory assets			14,000,000
Deferred net regulatory liabilities	22,000,000	20,000,000
CECONY - Gas [Member] | Maximum [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Common equity ratio	50.00%
Difference in property taxes	80.00%
CECONY - Gas [Member] | Minimum [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Difference in property taxes	20.00%
CECONY - Gas [Member] | September 2007 [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Amount of revenues retained	57,000,000	70,000,000	40,000,000
CECONY - Gas [Member] | September 2010 [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Weighted average cost of capital, percentage	7.46%
October One Two Thousand Seven [Member] | CECONY - Gas [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Gas base rate increase	84,600,000
Increase in annual gas rate	67,500,000
Gas base rate increase	84,600,000
October One Two Thousand Eight [Member] | CECONY - Gas [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Gas base rate increase	32,700,000
Increase in annual gas rate	67,500,000
Gas base rate increase	32,700,000
October One Two Thousand Nine [Member] | CECONY - Gas [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Gas base rate increase	42,700,000
Increase in annual gas rate	67,500,000
Gas base rate increase	42,700,000
September 2010 [Member] | CECONY - Gas [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Decrease in earnings	6,000,000
September 2009 [Member] | CECONY - Gas [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Decrease in earnings	0
September 2013 [Member] | CECONY - Gas [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Decrease in earnings	9,000,000
Reductions in costs	2,000,000
Average base rate	3,434,000,000
Deferred regulatory liabilities	3,346,000,000
September 2011 [Member] | CECONY - Gas [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Reductions in costs	6,000,000
Average base rate	3,027,000,000
Deferred regulatory liabilities	2,934,000,000
September 2011 [Member] | CECONY - Gas [Member] | 60% of earnings [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Percentage of revenue allocated to customers	60.00%
September 2011 [Member] | CECONY - Gas [Member] | 75% of earnings [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Percentage of revenue allocated to customers	75.00%
September 2011 [Member] | CECONY - Gas [Member] | 90% of earnings [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Percentage of revenue allocated to customers	90.00%
September 2011 [Member] | CECONY - Gas [Member] | Maximum [Member] | 60% of earnings [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Percentage of revenue requirement earnings	11.59%
September 2011 [Member] | CECONY - Gas [Member] | Maximum [Member] | 75% of earnings [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Percentage of revenue requirement earnings	12.59%
September 2011 [Member] | CECONY - Gas [Member] | Maximum [Member] | 90% of earnings [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Percentage of revenue requirement earnings	10.35%
September 2011 [Member] | CECONY - Gas [Member] | Minimum [Member] | 60% of earnings [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Percentage of revenue requirement earnings	10.35%
September 2011 [Member] | CECONY - Gas [Member] | Minimum [Member] | 75% of earnings [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Percentage of revenue requirement earnings	11.60%
September 2011 [Member] | CECONY - Gas [Member] | Minimum [Member] | 90% of earnings [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Percentage of revenue requirement earnings	12.60%
September 2012 [Member] | CECONY - Gas [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Reductions in costs	4,000,000
Average base rate	3,245,000,000
Deferred regulatory liabilities	3,148,000,000
May 2010 [Member] | CECONY - Gas [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Cost of long term debt	5.57%
Common equity ratio	48.00%
September 2012, 2013 [Member] | CECONY - Gas [Member] | 60% of earnings [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Percentage of revenue allocated to customers	60.00%
September 2012, 2013 [Member] | CECONY - Gas [Member] | 75% of earnings [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Percentage of revenue allocated to customers	75.00%
September 2012, 2013 [Member] | CECONY - Gas [Member] | 90% of earnings [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Percentage of revenue allocated to customers	90.00%
September 2012, 2013 [Member] | CECONY - Gas [Member] | Maximum [Member] | 60% of earnings [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Percentage of revenue requirement earnings	11.59%
September 2012, 2013 [Member] | CECONY - Gas [Member] | Maximum [Member] | 75% of earnings [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Percentage of revenue requirement earnings	12.59%
September 2012, 2013 [Member] | CECONY - Gas [Member] | Maximum [Member] | 90% of earnings [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Percentage of revenue requirement earnings	12.60%
September 2012, 2013 [Member] | CECONY - Gas [Member] | Minimum [Member] | 60% of earnings [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Percentage of revenue requirement earnings	10.10%
September 2012, 2013 [Member] | CECONY - Gas [Member] | Minimum [Member] | 75% of earnings [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Percentage of revenue requirement earnings	11.60%
January 2014 [Member] | CECONY - Gas [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Return on common equity	10.35%
Common equity ratio	50.00%
Gas base rate increase	25,000,000
January 2015 [Member] | CECONY - Gas [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Return on common equity	10.85%
Common equity ratio	50.00%
Gas base rate increase	55,000,000
January 2016 [Member] | CECONY - Gas [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Gas base rate increase	63,000,000
October 2010 [Member] | CECONY - Gas [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Gas base rate increase	47,100,000
Gas base rate increase	47,100,000
October 2011 [Member] | CECONY - Gas [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Gas base rate increase	47,900,000
Gas base rate increase	47,900,000
October 2012 [Member] | CECONY - Gas [Member]
Schedule Of Regulatory Assets And Liabilities [Line Items]
Gas base rate increase	46,700,000
Gas base rate increase	$ 46,700,000