Summary of Significant Accounting Policies - Changes in Accumulated Other Comprehensive Income/(Loss) by Component (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 13,958	$ 12,797	$ 13,061	$ 12,585
TOTAL OTHER COMPREHENSIVE INCOME, NET OF TAXES	1	1	2	7
Ending Balance	14,275	13,049	14,275	13,049
OCI before reclassifications, tax			1	(2)
Amounts reclassified from accumulated OCI related to pension plan liabilities, tax	(1)	(1)	(2)	(3)
CECONY
AOCI Attributable to Parent, Net of Tax [Roll Forward]
TOTAL OTHER COMPREHENSIVE INCOME, NET OF TAXES	0	1	1	2
Accumulated Other Comprehensive Income/(Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(33)	(39)	(34)	(45)
OCI before reclassifications, net of tax			(1)	3
Amounts reclassified from accumulated OCI related to pension plan liabilities, net of tax	1	1	3	4
TOTAL OTHER COMPREHENSIVE INCOME, NET OF TAXES	1	1	2	7
Ending Balance	(32)	(38)	(32)	(38)
Accumulated Other Comprehensive Income/(Loss) | CECONY
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(8)	(10)	(9)	(11)
OCI before reclassifications, net of tax			0	0
Amounts reclassified from accumulated OCI related to pension plan liabilities, net of tax	0	1	1	2
TOTAL OTHER COMPREHENSIVE INCOME, NET OF TAXES	0	1	1	2
Ending Balance	$ (8)	$ (9)	$ (8)	$ (9)