Acquisitions, Investments and Dispositions - Coram Wind (Details) - Coram Wind $ in Millions	1 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) MW
Business Acquisition [Line Items]
Generating capacity (MW AC) | MW		102
Net assets	$ 87	$ 96
Con Edison Development
Business Acquisition [Line Items]
Percentage of voting interest acquired	100.00%
Purchase price		97
Non-utility property		191
Intangible assets		78
Restricted cash, current		8
Long-term debt		$ 180
Increase in intangible asset	$ 22
Decrease in non-utility property	$ 22