Environmental Matters - Accrued Liability for Asbestos Suits and Workers' Compensation Proceedings (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Site Contingency [Line Items]
Regulatory assets	$ 8,512	$ 9,304
Asbestos Suits
Site Contingency [Line Items]
Accrued liability	8	8
Regulatory assets	8	8
Workers’ compensation
Site Contingency [Line Items]
Accrued liability	86	83
Regulatory assets	11	8
CECONY
Site Contingency [Line Items]
Regulatory assets	7,873	8,613
CECONY | Asbestos Suits
Site Contingency [Line Items]
Accrued liability	7	7
Regulatory assets	7	7
CECONY | Workers’ compensation
Site Contingency [Line Items]
Accrued liability	81	78
Regulatory assets	$ 11	$ 8