Summary of Significant Accounting Policies - Changes in Accumulated Other Comprehensive Income/(Loss) by Component (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE AS OF BEGINNING OF PERIOD	$ 14,306	$ 13,061	$ 12,585
TOTAL OTHER COMPREHENSIVE INCOME, NET OF TAXES	1	7	11
BALANCE AS OF END OF PERIOD	15,425	14,306	13,061
Accumulated OCI
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE AS OF BEGINNING OF PERIOD	(27)	(34)	(45)
TOTAL OTHER COMPREHENSIVE INCOME, NET OF TAXES	1	7	11
BALANCE AS OF END OF PERIOD	(26)	(27)	(34)
Pension Plan Liabilities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
OCI before reclassifications, net of tax	(4)	2	5
Amounts reclassified from accumulated OCI related to pension plan liabilities, net of tax	5	5	6
TOTAL OTHER COMPREHENSIVE INCOME, NET OF TAXES	1	7	11
OCI before reclassifications, tax	3	(1)	(3)
Amounts reclassified from accumulated OCI related to pension plan liabilities, tax	(3)	(3)	(4)
CECONY
AOCI Attributable to Parent, Net of Tax [Roll Forward]
TOTAL OTHER COMPREHENSIVE INCOME, NET OF TAXES	1	2	2
CECONY | Accumulated OCI
AOCI Attributable to Parent, Net of Tax [Roll Forward]
BALANCE AS OF BEGINNING OF PERIOD	(7)	(9)	(11)
TOTAL OTHER COMPREHENSIVE INCOME, NET OF TAXES	1	2	2
BALANCE AS OF END OF PERIOD	(6)	(7)	(9)
CECONY | Pension Plan Liabilities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
OCI before reclassifications, net of tax	0	1	1
Amounts reclassified from accumulated OCI related to pension plan liabilities, net of tax	1	1	1
TOTAL OTHER COMPREHENSIVE INCOME, NET OF TAXES	1	2	2
OCI before reclassifications, tax	1	(1)
Amounts reclassified from accumulated OCI related to pension plan liabilities, tax	$ (1)	$ (1)	$ (1)