SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	$ 579,600	$ 532,400	$ 554,500	$ 486,300
Uncollectible Expense	29,200	16,800	55,438	40,945
Uncollectible Costs Deferred	21,400	47,900	96,400	127,500
Write-Offs	(58,100)	(57,900)	(141,600)	(123,800)
Recoveries Collected	2,600	3,500	10,000	11,800
Ending Balance	574,700	542,700	574,700	542,700
CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	308,600	262,800	296,000	225,300
Uncollectible Expense	5,100	2,400	11,432	7,106
Uncollectible Costs Deferred	(5,400)	32,500	38,000	89,600
Write-Offs	(19,600)	(22,500)	(59,400)	(50,300)
Recoveries Collected	1,100	1,100	3,800	4,600
Ending Balance	289,800	276,300	289,800	276,300
NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	106,200	97,800	97,000	95,000
Uncollectible Expense	15,700	5,200	27,746	12,988
Uncollectible Costs Deferred	18,300	7,900	30,900	19,600
Write-Offs	(22,200)	(11,400)	(39,700)	(30,800)
Recoveries Collected	300	1,300	2,400	4,000
Ending Balance	118,300	100,800	118,300	100,800
PSNH
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	14,100	12,700	14,300	29,200
Uncollectible Expense	1,200	2,000	3,420	1,534
Uncollectible Costs Deferred	1,400	1,100	3,800	(11,100)
Write-Offs	(3,200)	(3,600)	(8,400)	(7,800)
Recoveries Collected	200	200	600	600
Ending Balance	13,700	12,400	13,700	12,400
Hardship Accounts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	389,100	338,200	366,800	284,400
Uncollectible Expense	0	0	0	0
Uncollectible Costs Deferred	5,400	33,900	58,900	104,700
Write-Offs	(16,800)	(17,500)	(48,500)	(35,300)
Recoveries Collected	100	200	600	1,000
Ending Balance	377,800	354,800	377,800	354,800
Hardship Accounts | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	274,400	226,500	259,700	188,900
Uncollectible Expense	0	0	0	0
Uncollectible Costs Deferred	(8,800)	29,400	29,300	80,300
Write-Offs	(12,200)	(14,900)	(36,000)	(29,000)
Recoveries Collected	100	200	500	1,000
Ending Balance	253,500	241,200	253,500	241,200
Hardship Accounts | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	45,900	45,600	43,600	43,700
Uncollectible Expense	0	0	0	0
Uncollectible Costs Deferred	10,100	3,400	13,600	5,800
Write-Offs	(1,900)	100	(3,100)	(400)
Recoveries Collected	0	0	0	0
Ending Balance	54,100	49,100	54,100	49,100
Retail (Non-Hardship), Wholesale, and Other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	190,500	194,200	187,700	201,900
Uncollectible Expense	29,200	16,800	55,400	40,900
Uncollectible Costs Deferred	16,000	14,000	37,500	22,800
Write-Offs	(41,300)	(40,400)	(93,100)	(88,500)
Recoveries Collected	2,500	3,300	9,400	10,800
Ending Balance	196,900	187,900	196,900	187,900
Retail (Non-Hardship), Wholesale, and Other | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	34,200	36,300	36,300	36,400
Uncollectible Expense	5,100	2,400	11,400	7,100
Uncollectible Costs Deferred	3,400	3,100	8,700	9,300
Write-Offs	(7,400)	(7,600)	(23,400)	(21,300)
Recoveries Collected	1,000	900	3,300	3,600
Ending Balance	36,300	35,100	36,300	35,100
Retail (Non-Hardship), Wholesale, and Other | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	60,300	52,200	53,400	51,300
Uncollectible Expense	15,700	5,200	27,700	13,000
Uncollectible Costs Deferred	8,200	4,500	17,300	13,800
Write-Offs	(20,300)	(11,500)	(36,600)	(30,400)
Recoveries Collected	300	1,300	2,400	4,000
Ending Balance	$ 64,200	$ 51,700	$ 64,200	$ 51,700