Other Postretirement Benefits - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended	3 Months Ended	12 Months Ended		12 Months Ended	3 Months Ended	12 Months Ended
	Dec. 31, 2013	Mar. 31, 2012 Other Postretirement Benefits [Member]	Dec. 31, 2013 Other Postretirement Benefits [Member]	Dec. 31, 2012 Other Postretirement Benefits [Member]	Dec. 31, 2013 CECONY [Member]	Dec. 31, 2012 CECONY [Member] Other Postretirement Benefits [Member]	Dec. 31, 2013 CECONY [Member] Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Reduction in postretirement life and health benefit plans		$ 102				$ 25
Reduction to postretirement health benefit obligation recorded as actuarial gain				306
Decreased liability for other postretirement benefits			(125)				(95)
Decrease to regulatory assets	2,799		(148)				120
Credit to OCI	24		4
Net losses unrecognized to be amortized			59		586		(19)
Prior service cost unrecognized to be amortized			52		2		(15)
Health care cost trend rate for net periodic benefit cost, current			5.75%
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			5.50%
Health care cost trend rate for benefit obligations			4.50%
Year for final trend rate for benefit obligations			2018
Expected contributions			$ 7				$ 7