Other PostretirementBenefits - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended		12 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 CECONY [Member]	Mar. 31, 2012 Other Postretirement Benefits [Member]	Dec. 31, 2012 Other Postretirement Benefits [Member]	Dec. 31, 2012 Other Postretirement Benefits [Member] CECONY [Member]	Dec. 31, 2012 Other Postretirement Benefits [Member] CECONY [Member]
Schedule Of Other Postretirement Benefits [Line Items]
Reduction in postretirement life and health benefit plans				$ 102		$ 25
Decrease in postretirement health benefit obligation recorded as actuarial gain					306
Pension liability	246	238			402	355	355
Decrease to regulatory assets	233				408		372
Debit to OCI	1				6
Net loss estimated to be amortized			792		64		54
Prior service cost estimated to be amortized			4		(27)		(23)
Health care cost trend rate for net periodic benefit cost, current					6.00%
Health care cost trend rate for net periodic benefit cost					4.50%
Year for final trend rate for net periodic benefit cost					2018
Health care cost trend rate for benefit obligations, current					5.75%
Health care cost trend rate for benefit obligations					4.50%
Year for final trend rate for benefit obligations					2018
Expected contributions					$ 8	$ 8	$ 8