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, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, accumulated OCI, net of taxes	$ (39)	$ (20)	$ (45)	$ (25)
OCI before reclassifications, net of tax of $(2) and $(1) for Con Edison in 2015 and 2014, respectively			3	2
Amounts reclassified from accumulated OCI related to pension plan liabilities, net of tax	1	1	4	4
Current period OCI, net of taxes	1	1	7	6
Ending balance, accumulated OCI, net of taxes	(38)	(19)	(38)	(19)
OCI before reclassifications, tax			(2)	(1)
Amounts reclassified from accumulated OCI related to pension plan liabilities, tax	(1)	(1)	(3)	(3)
CECONY
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, accumulated OCI, net of taxes	(10)	(5)	(11)	(6)
OCI before reclassifications, net of tax of $(2) and $(1) for Con Edison in 2015 and 2014, respectively			0	0
Amounts reclassified from accumulated OCI related to pension plan liabilities, net of tax	1	0	2	1
Current period OCI, net of taxes	1	0	2	1
Ending balance, accumulated OCI, net of taxes	$ (9)	$ (5)	$ (9)	$ (5)