SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	$ 554,500	$ 486,300
Uncollectible Expense	16,387	22,821
Uncollectible Costs Deferred	38,900	58,000
Write-Offs	(43,600)	(36,000)
Recoveries Collected	3,900	4,000
Ending Balance	570,100	535,100
CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	296,000	225,300
Uncollectible Expense	3,507	3,876
Uncollectible Costs Deferred	25,400	37,000
Write-Offs	(18,500)	(14,300)
Recoveries Collected	1,400	1,500
Ending Balance	307,800	253,400
NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	97,000	95,000
Uncollectible Expense	6,109	4,697
Uncollectible Costs Deferred	3,600	4,100
Write-Offs	(10,200)	(10,700)
Recoveries Collected	1,200	1,500
Ending Balance	97,700	94,600
PSNH
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	14,300	29,200
Uncollectible Expense	1,163	5,060
Uncollectible Costs Deferred	1,300	1,200
Write-Offs	(3,200)	(2,200)
Recoveries Collected	200	200
Ending Balance	13,800	33,500
Hardship Accounts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	366,800	284,400
Uncollectible Expense	0	0
Uncollectible Costs Deferred	27,500	43,700
Write-Offs	(15,100)	(9,700)
Recoveries Collected	200	300
Ending Balance	379,400	318,700
Hardship Accounts | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	259,700	188,900
Uncollectible Expense	0	0
Uncollectible Costs Deferred	22,700	34,300
Write-Offs	(10,800)	(7,300)
Recoveries Collected	200	300
Ending Balance	271,800	216,200
Hardship Accounts | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	43,600	43,700
Uncollectible Expense	0	0
Uncollectible Costs Deferred	(800)	(1,300)
Write-Offs	(1,000)	(400)
Recoveries Collected	0	0
Ending Balance	41,800	42,000
Retail (Non-Hardship), Wholesale, and Other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	187,700	201,900
Uncollectible Expense	16,400	22,800
Uncollectible Costs Deferred	11,400	14,300
Write-Offs	(28,500)	(26,300)
Recoveries Collected	3,700	3,700
Ending Balance	190,700	216,400
Retail (Non-Hardship), Wholesale, and Other | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	36,300	36,400
Uncollectible Expense	3,500	3,900
Uncollectible Costs Deferred	2,700	2,700
Write-Offs	(7,700)	(7,000)
Recoveries Collected	1,200	1,200
Ending Balance	36,000	37,200
Retail (Non-Hardship), Wholesale, and Other | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	53,400	51,300
Uncollectible Expense	6,100	4,700
Uncollectible Costs Deferred	4,400	5,400
Write-Offs	(9,200)	(10,300)
Recoveries Collected	1,200	1,500
Ending Balance	$ 55,900	$ 52,600