Summary of Significant Accounting Policies -Changes in Accumulated Other Comprehensive Income/(Loss) by Component (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, accumulated OCI, net of taxes	$ (40)	$ (21)	$ (45)	$ (25)
OCI before reclassifications, net of tax			3	2
Amounts reclassified from accumulated OCI related to pension plan liabilities, net of tax	1	1	3	3
Current period OCI, net of taxes	1	1	6	5
Ending balance, accumulated OCI, net of taxes	(39)	(20)	(39)	(20)
OCI before reclassifications, tax			(2)	(1)
Amounts reclassified from accumulated OCI related to pension plan liabilities, tax	(1)	(1)	(2)	(2)
CECONY
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, accumulated OCI, net of taxes	(11)	(5)	(11)	(6)
OCI before reclassifications, net of tax			0	0
Amounts reclassified from accumulated OCI related to pension plan liabilities, net of tax	1	0	1	1
Current period OCI, net of taxes	1	0	1	1
Ending balance, accumulated OCI, net of taxes	$ (10)	$ (5)	$ (10)	$ (5)