Other Material Contingencies - Con Edison Transmission (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2014
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	$ 1,991	$ 2,073
Payment Guarantee by CET Electric of Contributions to New York Transco LLC
Guarantor Obligations [Line Items]
Ownership interest, percentage			45.70%
Estimated project cost percentage	175.00%
Financial Guarantee in Behalf of CET Gas for Proposed Gas Transmission Project
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	$ 125