ACQUISITION OF AQUARION AND GOODWILL - Schedule of Pro Forma Information (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares
Business Acquisition [Line Items]
Operating Revenues	$ 15.9
Aquarion Water Company
Business Acquisition [Line Items]
Operating Revenues	7,947.7
Net Income Attributable to Common Shareholders	$ 1,019.1
Basic EPS (in dollars per share) | $ / shares	$ 3.21
Diluted EPS (in dollars per share) | $ / shares	$ 3.20