SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Allowance for Uncollectible Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	$ 486,300	$ 417,400	$ 358,900
Uncollectible Expense	72,468	61,876	60,886
Uncollectible Costs Deferred	158,200	112,500	110,600
Write-Offs	(178,100)	(124,000)	(129,700)
Recoveries Collected	15,600	18,500	16,700
Ending Balance	554,500	486,300	417,400
Hardship Accounts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	284,400	226,100	194,800
Uncollectible Expense	0	0	0
Uncollectible Costs Deferred	137,000	77,800	51,900
Write-Offs	(55,900)	(21,300)	(22,000)
Recoveries Collected	1,300	1,800	1,400
Ending Balance	366,800	284,400	226,100
Retail (Non-Hardship), Wholesale, and Other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	201,900	191,300	164,100
Uncollectible Expense	72,500	61,900	60,900
Uncollectible Costs Deferred	21,200	34,700	58,700
Write-Offs	(122,200)	(102,700)	(107,700)
Recoveries Collected	14,300	16,700	15,300
Ending Balance	187,700	201,900	191,300
CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	225,300	181,300	157,400
Uncollectible Expense	11,675	15,578	13,495
Uncollectible Costs Deferred	126,400	59,500	57,800
Write-Offs	(73,200)	(38,300)	(54,200)
Recoveries Collected	5,800	7,200	6,800
Ending Balance	296,000	225,300	181,300
CL&P | Hardship Accounts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	188,900	144,600	129,100
Uncollectible Expense	0	0	0
Uncollectible Costs Deferred	114,400	58,300	32,300
Write-Offs	(44,700)	(15,300)	(18,000)
Recoveries Collected	1,100	1,300	1,200
Ending Balance	259,700	188,900	144,600
CL&P | Retail (Non-Hardship), Wholesale, and Other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	36,400	36,700	28,300
Uncollectible Expense	11,700	15,600	13,500
Uncollectible Costs Deferred	12,000	1,200	25,500
Write-Offs	(28,500)	(23,000)	(36,200)
Recoveries Collected	4,700	5,900	5,600
Ending Balance	36,300	36,400	36,700
NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	95,000	97,000	91,600
Uncollectible Expense	22,791	21,550	16,649
Uncollectible Costs Deferred	17,500	12,400	20,100
Write-Offs	(43,300)	(42,300)	(37,000)
Recoveries Collected	5,000	6,300	5,700
Ending Balance	97,000	95,000	97,000
NSTAR Electric | Hardship Accounts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	43,700	43,300	39,700
Uncollectible Expense	0	0	0
Uncollectible Costs Deferred	1,500	1,500	4,300
Write-Offs	(1,600)	(1,100)	(700)
Recoveries Collected	0	0	0
Ending Balance	43,600	43,700	43,300
NSTAR Electric | Retail (Non-Hardship), Wholesale, and Other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	51,300	53,700	51,900
Uncollectible Expense	22,800	21,600	16,600
Uncollectible Costs Deferred	16,000	10,900	15,800
Write-Offs	(41,700)	(41,200)	(36,300)
Recoveries Collected	5,000	6,300	5,700
Ending Balance	53,400	51,300	53,700
PSNH
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	29,200	24,300	17,200
Uncollectible Expense	3,989	9,211	13,113
Uncollectible Costs Deferred	(8,700)	2,500	3,100
Write-Offs	(10,900)	(7,700)	(10,000)
Recoveries Collected	700	900	900
Ending Balance	$ 14,300	$ 29,200	$ 24,300