Other Postretirement Benefits - Additional Information (Detail) - Other Postretirement Benefits - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Decrease in liability for other postretirement benefits	$ 34
Increase (decrease) to regulatory assets	30
Charge to OCI	(1)
Net losses unrecognized to be amortized	12
Prior service cost estimated to be amortized	$ 20
Health care cost trend rate for net periodic benefit cost, current	5.25%
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	6.00%
Health care cost trend rate for benefit obligations	4.50%
Year for final trend rate for benefit obligations	2024
Expected contributions	$ 5
Maximum
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) in postretirement benefit obligations		$ 10
CECONY
Defined Benefit Plan Disclosure [Line Items]
Decrease in liability for other postretirement benefits	30
Increase (decrease) to regulatory assets	27
Net losses unrecognized to be amortized	10
Prior service cost estimated to be amortized	14
Expected contributions	$ 5