ACQUISITION OF AQUARION AND GOODWILL - Schedule of Pro Forma Information (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Operating Revenues	$ 15.9
Aquarion Water Company
Business Acquisition [Line Items]
Operating Revenues	7,947.7	$ 7,849.0
Net Income Attributable to Common Shareholders	$ 1,019.1	$ 969.3
Basic EPS (in dollars per share)	$ 3.21	$ 3.05
Diluted EPS (in dollars per share)	$ 3.20	$ 3.04