SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	$ 570,100	$ 535,100	$ 554,500	$ 486,300
Uncollectible Expense	9,900	1,400	26,279	24,171
Uncollectible Costs Deferred	36,000	21,500	74,900	79,500
Write-Offs	(39,800)	(29,900)	(83,500)	(65,800)
Recoveries Collected	3,400	4,300	7,400	8,200
Ending Balance	579,600	532,400	579,600	532,400
CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	307,800	253,400	296,000	225,300
Uncollectible Expense	2,800	900	6,326	4,750
Uncollectible Costs Deferred	17,900	20,000	43,400	57,000
Write-Offs	(21,200)	(13,500)	(39,800)	(27,800)
Recoveries Collected	1,300	2,000	2,700	3,500
Ending Balance	308,600	262,800	308,600	262,800
NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	97,700	94,600	97,000	95,000
Uncollectible Expense	5,900	3,100	12,007	7,759
Uncollectible Costs Deferred	9,000	7,600	12,600	11,700
Write-Offs	(7,300)	(8,700)	(17,500)	(19,400)
Recoveries Collected	900	1,200	2,100	2,700
Ending Balance	106,200	97,800	106,200	97,800
PSNH
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	13,800	33,500	14,300	29,200
Uncollectible Expense	1,000	(5,500)	2,178	(451)
Uncollectible Costs Deferred	1,100	(13,500)	2,400	(12,200)
Write-Offs	(2,000)	(2,000)	(5,200)	(4,200)
Recoveries Collected	200	200	400	400
Ending Balance	14,100	12,700	14,100	12,700
Hardship Accounts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	379,400	318,700	366,800	284,400
Uncollectible Expense	0	0	0	0
Uncollectible Costs Deferred	26,000	27,000	53,500	70,800
Write-Offs	(16,500)	(8,100)	(31,700)	(17,800)
Recoveries Collected	200	600	500	800
Ending Balance	389,100	338,200	389,100	338,200
Hardship Accounts | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	271,800	216,200	259,700	188,900
Uncollectible Expense	0	0	0	0
Uncollectible Costs Deferred	15,300	16,600	38,100	50,900
Write-Offs	(12,900)	(6,800)	(23,800)	(14,100)
Recoveries Collected	200	500	400	800
Ending Balance	274,400	226,500	274,400	226,500
Hardship Accounts | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	41,800	42,000	43,600	43,700
Uncollectible Expense	0	0	0	0
Uncollectible Costs Deferred	4,300	3,700	3,500	2,400
Write-Offs	(200)	(100)	(1,200)	(500)
Recoveries Collected	0	0	0	0
Ending Balance	45,900	45,600	45,900	45,600
Retail (Non-Hardship), Wholesale, and Other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	190,700	216,400	187,700	201,900
Uncollectible Expense	9,900	1,400	26,300	24,200
Uncollectible Costs Deferred	10,000	(5,500)	21,400	8,700
Write-Offs	(23,300)	(21,800)	(51,800)	(48,000)
Recoveries Collected	3,200	3,700	6,900	7,400
Ending Balance	190,500	194,200	190,500	194,200
Retail (Non-Hardship), Wholesale, and Other | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	36,000	37,200	36,300	36,400
Uncollectible Expense	2,800	900	6,300	4,800
Uncollectible Costs Deferred	2,600	3,400	5,300	6,100
Write-Offs	(8,300)	(6,700)	(16,000)	(13,700)
Recoveries Collected	1,100	1,500	2,300	2,700
Ending Balance	34,200	36,300	34,200	36,300
Retail (Non-Hardship), Wholesale, and Other | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	55,900	52,600	53,400	51,300
Uncollectible Expense	5,900	3,100	12,000	7,800
Uncollectible Costs Deferred	4,700	3,900	9,100	9,300
Write-Offs	(7,100)	(8,600)	(16,300)	(18,900)
Recoveries Collected	900	1,200	2,100	2,700
Ending Balance	$ 60,300	$ 52,200	$ 60,300	$ 52,200