Other Postretirement Benefits - Additional Information (Detail) (Other Postretirement Benefits [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in postretirement benefit obligations		$ 102
Reduction to postretirement health benefit obligation recorded as actuarial gain		306
Increase in liability for other postretirement benefits	45
Increase to regulatory assets	14
Charge to OCI	3
Net losses unrecognized to be amortized	33
Prior service cost unrecognized to be amortized	(20)
Health care cost trend rate for net periodic benefit cost, current	5.50%
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	5.25%
Health care cost trend rate for benefit obligations	4.50%
Year for final trend rate for benefit obligations	2018
Expected contributions	6
Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in postretirement benefit obligations	10
CECONY [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in postretirement benefit obligations		25
Increase in liability for other postretirement benefits	32
Increase to regulatory assets	6
Charge to OCI	2
Net losses unrecognized to be amortized	29
Prior service cost unrecognized to be amortized	(14)
Expected contributions	$ 6