Summary of Significant Accounting Policies - Changes in Accumulated Other Comprehensive Income by Component (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning, accumulated OCI, net of taxes	$ (45)	$ (25)
OCI before reclassifications, net of tax of $18 and $4 in 2014 for Con Edison and CECONY and $3 in 2015 for Con Edison	5	(26)
Amounts reclassified from accumulated OCI related to pension plan liabilities, net of tax of $(4) and $(1) in 2014 for Con Edison and CECONY and $4 and $1 in 2015 for Con Edison and CECONY	6	6
Total OCI, net of taxes	11	(20)	$ 28
Ending, accumulated OCI, net of taxes	(34)	(45)	(25)
OCI before reclassifications, tax	(3)	18
Amounts reclassified from accumulated OCI related to pension plan liabilities, tax	(4)	(4)
CECONY
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning, accumulated OCI, net of taxes	(11)	(6)
OCI before reclassifications, net of tax of $18 and $4 in 2014 for Con Edison and CECONY and $3 in 2015 for Con Edison	1	(6)
Amounts reclassified from accumulated OCI related to pension plan liabilities, net of tax of $(4) and $(1) in 2014 for Con Edison and CECONY and $4 and $1 in 2015 for Con Edison and CECONY	1	1
Total OCI, net of taxes	2	(5)	3
Ending, accumulated OCI, net of taxes	(9)	(11)	$ (6)
OCI before reclassifications, tax		4
Amounts reclassified from accumulated OCI related to pension plan liabilities, tax	$ (1)	$ (1)