Summary of Significant Accounting Policies - Utility Plant (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Accumulated amortization	$ 21	$ 15
Software Licenses
Finite-Lived Intangible Assets [Line Items]
Gross asset	$ 102
Amortization period	15 years
Estimated aggregate annual amortization expense	$ 7
Accumulated amortization	2
Software Licenses | CECONY
Finite-Lived Intangible Assets [Line Items]
Gross asset	97
Estimated aggregate annual amortization expense	7
Accumulated amortization	$ 1