Income Tax - Summary of Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of period	$ 42	$ 34	$ 34
Additions based on tax positions related to the current year	1	2	0
Additions based on tax positions of prior years	1	19	1
Reductions for tax positions of prior years	(24)	(13)	0
Reductions from expiration of statute of limitations	(2)	0	(1)
Settlements	(6)	0	0
Balance at end of period	12	42	34
CECONY
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of period	21	2	2
Additions based on tax positions related to the current year	1	2	0
Additions based on tax positions of prior years	1	19	0
Reductions for tax positions of prior years	(18)	(2)	0
Reductions from expiration of statute of limitations	0	0	0
Settlements	0	0	0
Balance at end of period	$ 5	$ 21	$ 2