SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	$ 575,600	$ 579,600	$ 556,200	$ 554,500
Uncollectible Expense	18,100	29,200	59,614	55,438
Uncollectible Costs Deferred	13,600	21,400	73,500	96,400
Write-Offs	(60,200)	(58,100)	(149,800)	(141,600)
Recoveries Collected	3,800	2,600	11,400	10,000
Ending Balance	550,900	574,700	550,900	574,700
CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	273,600	308,600	279,100	296,000
Uncollectible Expense	6,000	5,100	16,105	11,432
Uncollectible Costs Deferred	(400)	(5,400)	16,400	38,000
Write-Offs	(22,900)	(19,600)	(57,700)	(59,400)
Recoveries Collected	1,400	1,100	3,800	3,800
Ending Balance	257,700	289,800	257,700	289,800
NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	120,200	106,200	114,900	97,000
Uncollectible Expense	8,200	15,700	22,176	27,746
Uncollectible Costs Deferred	7,200	18,300	25,200	30,900
Write-Offs	(19,100)	(22,200)	(48,700)	(39,700)
Recoveries Collected	1,300	300	4,200	2,400
Ending Balance	117,800	118,300	117,800	118,300
PSNH
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	15,000	14,100	14,100	14,300
Uncollectible Expense	1,500	1,200	3,871	3,420
Uncollectible Costs Deferred	2,000	1,400	4,600	3,800
Write-Offs	(3,200)	(3,200)	(7,700)	(8,400)
Recoveries Collected	200	200	600	600
Ending Balance	15,500	13,700	15,500	13,700
Hardship Accounts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	377,000	389,100	364,600	366,800
Uncollectible Expense	0	0	0	0
Uncollectible Costs Deferred	(100)	5,400	34,200	58,900
Write-Offs	(18,100)	(16,800)	(40,300)	(48,500)
Recoveries Collected	500	100	800	600
Ending Balance	359,300	377,800	359,300	377,800
Hardship Accounts | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	236,300	274,400	240,700	259,700
Uncollectible Expense	0	0	0	0
Uncollectible Costs Deferred	(3,200)	(8,800)	8,800	29,300
Write-Offs	(14,200)	(12,200)	(30,800)	(36,000)
Recoveries Collected	200	100	400	500
Ending Balance	219,100	253,500	219,100	253,500
Hardship Accounts | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	60,300	45,900	55,200	43,600
Uncollectible Expense	0	0	0	0
Uncollectible Costs Deferred	(1,000)	10,100	5,700	13,600
Write-Offs	(500)	(1,900)	(2,100)	(3,100)
Recoveries Collected	0	0	0	0
Ending Balance	58,800	54,100	58,800	54,100
Retail (Non-Hardship), Wholesale, and Other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	198,600	190,500	191,600	187,700
Uncollectible Expense	18,100	29,200	59,600	55,400
Uncollectible Costs Deferred	13,700	16,000	39,300	37,500
Write-Offs	(42,100)	(41,300)	(109,500)	(93,100)
Recoveries Collected	3,300	2,500	10,600	9,400
Ending Balance	191,600	196,900	191,600	196,900
Retail (Non-Hardship), Wholesale, and Other | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	37,300	34,200	38,400	36,300
Uncollectible Expense	6,000	5,100	16,100	11,400
Uncollectible Costs Deferred	2,800	3,400	7,600	8,700
Write-Offs	(8,700)	(7,400)	(26,900)	(23,400)
Recoveries Collected	1,200	1,000	3,400	3,300
Ending Balance	38,600	36,300	38,600	36,300
Retail (Non-Hardship), Wholesale, and Other | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	59,900	60,300	59,700	53,400
Uncollectible Expense	8,200	15,700	22,200	27,700
Uncollectible Costs Deferred	8,200	8,200	19,500	17,300
Write-Offs	(18,600)	(20,300)	(46,600)	(36,600)
Recoveries Collected	1,300	300	4,200	2,400
Ending Balance	$ 59,000	$ 64,200	$ 59,000	$ 64,200