Environmental Matters - Environmental Remediation Costs (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Environmental Exit Cost [Line Items]
Remediation costs incurred	$ 8	$ 5	$ 15	$ 14
Insurance recoveries received	0	0	0	5
CECONY
Environmental Exit Cost [Line Items]
Remediation costs incurred	7	2	12	10
Insurance recoveries received	$ 0	$ 0	$ 0	$ 5