Leases - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Operating Leased Assets [Line Items]
Accumulated amortization	$ 3	$ 3
CECONY
Operating Leased Assets [Line Items]
Accumulated amortization	$ 2	$ 1
Increase in annual payments, percentage	2.18%
Decrease in transformer installations, percentage	0.50%