GOODWILL - Summary of Goodwill by Reportable Segment (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($)
Goodwill [Roll Forward]
Balance	$ 4,427,266
Acquisition of CMA Assets	42,000
Sale of Hingham water system	(23,000)
Balance	4,445,988
Electric Distribution
Goodwill [Roll Forward]
Balance	2,544,000
Acquisition of CMA Assets	0
Sale of Hingham water system	0
Balance	2,544,000
Electric Transmission
Goodwill [Roll Forward]
Balance	577,000
Acquisition of CMA Assets	0
Sale of Hingham water system	0
Balance	577,000
Water Distribution
Goodwill [Roll Forward]
Balance	907,000
Acquisition of CMA Assets	0
Sale of Hingham water system	(23,000)
Balance	884,000
Natural Gas Distribution
Goodwill [Roll Forward]
Balance	399,000
Acquisition of CMA Assets	42,000
Sale of Hingham water system	0
Balance	$ 441,000