ACQUISITION OF AQUARION AND GOODWILL - Narrative (Details) - USD ($)				3 Months Ended								12 Months Ended
	Dec. 04, 2017	Oct. 01, 2017	Jun. 02, 2017	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Payments to acquire businesses												$ 877,652,000	$ 0	$ 0
Operating Revenues				$ 1,895,600,000	$ 1,988,500,000	$ 1,762,800,000	$ 2,105,100,000	$ 1,776,600,000	$ 2,039,700,000	$ 1,767,200,000	$ 2,055,600,000	7,751,952,000	7,639,129,000	$ 7,954,827,000
Goodwill				4,427,266,000				$ 3,519,401,000				4,427,266,000	$ 3,519,401,000
Impairment loss		$ 0
Aquarion Water Company
Business Acquisition [Line Items]
Consideration transferred			$ 1,675,000,000
Payments to acquire businesses	$ 880,000,000		880,000,000
Consideration transferred, liabilities incurred			$ 795,000,000
Equity interest	745,000,000
Aquarion shareholder loan paid at closing	135,000,000
Operating Revenues												15,900,000
Income before tax												1,100,000
Goodwill	$ 900,000,000			$ 907,900,000								$ 907,900,000