Environmental Matters - Environmental Remediation Costs (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Environmental Exit Cost [Line Items]
Remediation costs incurred	$ 6	$ 5	$ 21	$ 19
Insurance recoveries received	0	0	0	5
CECONY
Environmental Exit Cost [Line Items]
Remediation costs incurred	6	2	18	12
Insurance recoveries received	$ 0	$ 0	$ 0	$ 5