ACQUISITION OF ASSETS OF COLUMBIA GAS OF MASSACHUSETTS - Schedule of Pro Forma Information (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2017
Business Acquisition [Line Items]
Operating Revenues			$ 154.8
CMA
Business Acquisition [Line Items]
Operating Revenues	$ 9,273.0	$ 9,103.0
Net Income Attributable to Common Shareholders	$ 1,265.0	$ 909.0
Basic EPS (in dollars per share)	$ 3.73	$ 2.83
Diluted EPS (in dollars per share)	$ 3.72	$ 2.82