Summary of Significant Accounting Policies - Utility Plant (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Accumulated amortization	$ 19	$ 15
Software Licenses
Finite-Lived Intangible Assets [Line Items]
Gross asset	$ 94
Amortization period	15 years
Estimated aggregate annual amortization expense	$ 6
Accumulated amortization	0
Software Licenses | CECONY
Finite-Lived Intangible Assets [Line Items]
Gross asset	89
Estimated aggregate annual amortization expense	$ 6