ACQUISITION OF ASSETS OF COLUMBIA GAS OF MASSACHUSETTS - Narrative (Details) - USD ($)			3 Months Ended	12 Months Ended
	Oct. 01, 2021	Oct. 09, 2020	Dec. 31, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Oct. 07, 2020
Business Acquisition [Line Items]
Operating revenues					$ 154,800,000
Pre-tax income					13,900,000
Impairment loss	$ 0
Acquisition of Assets of Columbia Gas of Massachusetts, Net of Restricted Cash				$ 0	(1,113,252,000)	$ 0
TSA
Business Acquisition [Line Items]
TSA and pre-TSA costs			$ 21,400,000	$ 15,900,000
Energy Efficiency expense			2,000,000
DPU
Business Acquisition [Line Items]
Approved rate base							$ 995,000,000
CMA
Business Acquisition [Line Items]
Consideration transferred		$ 1,100,000,000
Operating revenues					9,273,000,000	$ 9,103,000,000
Contingent liability for future refund to customers		6,700,000
Energy relief fund liability		$ 56,800,000
Arrearage Forgiveness Fund			$ 15,400,000		15,400,000
Acquisition of Assets of Columbia Gas of Massachusetts, Net of Restricted Cash					$ (1,110,000,000)