SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	$ 556,200	$ 554,500
Uncollectible Expense	22,863	16,387
Uncollectible Costs Deferred	33,200	38,900
Write-Offs	(50,400)	(43,600)
Recoveries Collected	3,700	3,900
Ending Balance	565,600	570,100
CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	279,100	296,000
Uncollectible Expense	4,671	3,507
Uncollectible Costs Deferred	10,700	25,400
Write-Offs	(19,400)	(18,500)
Recoveries Collected	1,100	1,400
Ending Balance	276,200	307,800
NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	114,900	97,000
Uncollectible Expense	7,485	6,109
Uncollectible Costs Deferred	9,000	3,600
Write-Offs	(16,800)	(10,200)
Recoveries Collected	1,400	1,200
Ending Balance	116,000	97,700
PSNH
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	14,100	14,300
Uncollectible Expense	1,315	1,163
Uncollectible Costs Deferred	1,400	1,300
Write-Offs	(2,700)	(3,200)
Recoveries Collected	200	200
Ending Balance	14,300	13,800
Hardship Accounts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	364,600	366,800
Uncollectible Expense	0	0
Uncollectible Costs Deferred	19,400	27,500
Write-Offs	(12,600)	(15,100)
Recoveries Collected	100	200
Ending Balance	371,500	379,400
Hardship Accounts | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	240,700	259,700
Uncollectible Expense	0	0
Uncollectible Costs Deferred	8,800	22,700
Write-Offs	(9,300)	(10,800)
Recoveries Collected	100	200
Ending Balance	240,300	271,800
Hardship Accounts | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	55,200	43,600
Uncollectible Expense	0	0
Uncollectible Costs Deferred	3,500	(800)
Write-Offs	(900)	(1,000)
Recoveries Collected	0	0
Ending Balance	57,800	41,800
Retail (Non-Hardship), Wholesale, and Other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	191,600	187,700
Uncollectible Expense	22,900	16,400
Uncollectible Costs Deferred	13,800	11,400
Write-Offs	(37,800)	(28,500)
Recoveries Collected	3,600	3,700
Ending Balance	194,100	190,700
Retail (Non-Hardship), Wholesale, and Other | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	38,400	36,300
Uncollectible Expense	4,700	3,500
Uncollectible Costs Deferred	1,900	2,700
Write-Offs	(10,100)	(7,700)
Recoveries Collected	1,000	1,200
Ending Balance	35,900	36,000
Retail (Non-Hardship), Wholesale, and Other | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	59,700	53,400
Uncollectible Expense	7,500	6,100
Uncollectible Costs Deferred	5,500	4,400
Write-Offs	(15,900)	(9,200)
Recoveries Collected	1,400	1,200
Ending Balance	$ 58,200	$ 55,900