ACQUISITION OF AQUARION AND GOODWILL - Schedule of Goodwill by Reportable Segments (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)
Goodwill [Roll Forward]
Beginning balance	$ 3,519,401
Balance as of January 1, 2017	3,500,000
Acquisition of Aquarion	900,000
Ending balance	4,427,266
Electric Distribution
Goodwill [Roll Forward]
Balance as of January 1, 2017	2,500,000
Acquisition of Aquarion	0
Ending balance	2,500,000
Electric Transmission
Goodwill [Roll Forward]
Balance as of January 1, 2017	600,000
Acquisition of Aquarion	0
Ending balance	600,000
Natural Gas Distribution
Goodwill [Roll Forward]
Balance as of January 1, 2017	400,000
Acquisition of Aquarion	0
Ending balance	400,000
Parent and Other
Goodwill [Roll Forward]
Balance as of January 1, 2017	0
Acquisition of Aquarion	900,000
Ending balance	$ 900,000