Environmental Matters - Accrued Liability for Asbestos Suits and Workers' Compensation Proceedings (Detail) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Site Contingency [Line Items]
Regulatory assets	$ 7,663	$ 8,228
Asbestos Suits
Site Contingency [Line Items]
Accrued liability	8	8
Regulatory assets	8	8
Workers’ compensation
Site Contingency [Line Items]
Accrued liability	90	86
Regulatory assets	15	11
CECONY
Site Contingency [Line Items]
Regulatory assets	7,097	7,603
CECONY | Asbestos Suits
Site Contingency [Line Items]
Accrued liability	7	7
Regulatory assets	7	7
CECONY | Workers’ compensation
Site Contingency [Line Items]
Accrued liability	85	81
Regulatory assets	$ 15	$ 11