Income Tax - Summary of Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of period	$ 12	$ 42	$ 34
Additions based on tax positions related to the current year	2	1	2
Additions based on tax positions of prior years	1	1	19
Reductions for tax positions of prior years	(2)	(24)	(13)
Reductions from expiration of statute of limitations	(4)	(2)	0
Settlements	(3)	(6)	0
Balance at end of period	6	12	42
CECONY
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of period	5	21	2
Additions based on tax positions related to the current year	2	1	2
Additions based on tax positions of prior years	1	1	19
Reductions for tax positions of prior years	(1)	(18)	(2)
Reductions from expiration of statute of limitations	0	0	0
Settlements	(3)	0	0
Balance at end of period	$ 4	$ 5	$ 21