Other Postretirement Benefits - Schedule of Change of Assumed Health Care Cost Trend Rate (Detail) (Scenario, Forecast [Member], Other Postretirement Benefits [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Effect on accumulated other postretirement benefit obligation, Increase	$ (21)
Effect on service cost and interest cost components for 2014, Increase	(2)
Effect on accumulated other postretirement benefit obligation, Decrease	40
Effect on service cost and interest cost components for 2014, Decrease	1
CECONY [Member]
Defined Benefit Plan Disclosure [Line Items]
Effect on accumulated other postretirement benefit obligation, Increase	(43)
Effect on service cost and interest cost components for 2014, Increase	(4)
Effect on accumulated other postretirement benefit obligation, Decrease	57
Effect on service cost and interest cost components for 2014, Decrease	$ 3