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GOODWILL - Summary of Goodwill by Reportable Segment (Details) - USD ($) $ in Thousands		12 Months Ended
	Oct. 03, 2022	Dec. 31, 2023	Dec. 31, 2022
Goodwill [Roll Forward]
Beginning Balance		$ 4,522,632	$ 4,477,300
Ending Balance		4,532,100	4,522,632
New England Service Company (NESC)
Goodwill [Roll Forward]
Measurement Period Adjustments			500
The Torrington Water Company (TWC)
Goodwill [Roll Forward]
Goodwill acquired			44,800
Water Acquisitions
Goodwill [Roll Forward]
Goodwill acquired		9,500
Electric Distribution
Goodwill [Roll Forward]
Beginning Balance		2,543,600	2,543,600
Ending Balance		2,543,600	2,543,600
Electric Distribution \| New England Service Company (NESC)
Goodwill [Roll Forward]
Measurement Period Adjustments			0
Electric Distribution \| The Torrington Water Company (TWC)
Goodwill [Roll Forward]
Goodwill acquired			0
Electric Distribution \| Water Acquisitions
Goodwill [Roll Forward]
Goodwill acquired		0
Electric Transmission
Goodwill [Roll Forward]
Beginning Balance		576,800	576,800
Ending Balance		576,800	576,800
Electric Transmission \| New England Service Company (NESC)
Goodwill [Roll Forward]
Measurement Period Adjustments			0
Electric Transmission \| The Torrington Water Company (TWC)
Goodwill [Roll Forward]
Goodwill acquired			0
Electric Transmission \| Water Acquisitions
Goodwill [Roll Forward]
Goodwill acquired		0
Natural Gas Distribution
Goodwill [Roll Forward]
Beginning Balance		451,000	451,000
Ending Balance		451,000	451,000
Natural Gas Distribution \| New England Service Company (NESC)
Goodwill [Roll Forward]
Measurement Period Adjustments			0
Natural Gas Distribution \| The Torrington Water Company (TWC)
Goodwill [Roll Forward]
Goodwill acquired			0
Natural Gas Distribution \| Water Acquisitions
Goodwill [Roll Forward]
Goodwill acquired		0
Water Distribution
Goodwill [Roll Forward]
Beginning Balance		951,200	905,900
Ending Balance		960,700	951,200
Water Distribution \| New England Service Company (NESC)
Goodwill [Roll Forward]
Measurement Period Adjustments		500	500
Goodwill acquired		22,200
Water Distribution \| The Torrington Water Company (TWC)
Goodwill [Roll Forward]
Goodwill acquired	$ 44,800		$ 44,800
Water Distribution \| Water Acquisitions
Goodwill [Roll Forward]
Goodwill acquired		$ 9,500