GOODWILL - Summary of Goodwill by Reportable Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Goodwill [Roll Forward]
Balance	$ 4,445,988	$ 4,427,000
Goodwill acquired	21,000	42,000
Sale of Hingham water system		(23,000)
Goodwill measurement period adjustments	10,000
Balance	4,477,269	4,445,988
Electric Distribution
Goodwill [Roll Forward]
Balance	2,544,000	2,544,000
Goodwill acquired	0	0
Sale of Hingham water system		0
Goodwill measurement period adjustments	0
Balance	2,544,000	2,544,000
Electric Transmission
Goodwill [Roll Forward]
Balance	577,000	577,000
Goodwill acquired	0	0
Sale of Hingham water system		0
Goodwill measurement period adjustments	0
Balance	577,000	577,000
Water Distribution
Goodwill [Roll Forward]
Balance	884,000	907,000
Goodwill acquired	21,700	0
Sale of Hingham water system		(23,000)
Goodwill measurement period adjustments	0
Balance	905,000	884,000
Natural Gas Distribution
Goodwill [Roll Forward]
Balance	441,000	399,000
Goodwill acquired	0	42,000
Sale of Hingham water system		0
Goodwill measurement period adjustments	10,000
Balance	$ 451,000	$ 441,000