SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	$ 532,400	$ 457,800	$ 486,300	$ 417,400
Uncollectible Expense	16,800	10,700	40,945	40,753
Uncollectible Costs Deferred	47,900	25,000	127,500	74,700
Write-Offs	(57,900)	(39,700)	(123,800)	(89,800)
Recoveries Collected	3,500	3,800	11,800	14,500
Ending Balance	542,700	457,600	542,700	457,600
CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	262,800	192,700	225,300	181,300
Uncollectible Expense	2,400	4,100	7,106	10,816
Uncollectible Costs Deferred	32,500	13,300	89,600	24,300
Write-Offs	(22,500)	(12,700)	(50,300)	(23,800)
Recoveries Collected	1,100	1,200	4,600	6,000
Ending Balance	276,300	198,600	276,300	198,600
NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	97,800	100,500	95,000	97,000
Uncollectible Expense	5,200	3,500	12,988	12,159
Uncollectible Costs Deferred	7,900	6,800	19,600	16,700
Write-Offs	(11,400)	(12,600)	(30,800)	(31,300)
Recoveries Collected	1,300	1,300	4,000	4,900
Ending Balance	100,800	99,500	100,800	99,500
PSNH
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	12,700	27,300	29,200	24,300
Uncollectible Expense	2,000	3,500	1,534	8,060
Uncollectible Costs Deferred	1,100	100	(11,100)	1,200
Write-Offs	(3,600)	(2,300)	(7,800)	(5,400)
Recoveries Collected	200	200	600	600
Ending Balance	12,400	28,800	12,400	28,800
Hardship Accounts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	338,200	242,700	284,400	226,100
Uncollectible Expense	0	0	0	0
Uncollectible Costs Deferred	33,900	15,600	104,700	38,000
Write-Offs	(17,500)	(6,500)	(35,300)	(13,500)
Recoveries Collected	200	300	1,000	1,500
Ending Balance	354,800	252,100	354,800	252,100
Hardship Accounts | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	226,500	152,100	188,900	144,600
Uncollectible Expense	0	0	0	0
Uncollectible Costs Deferred	29,400	11,300	80,300	22,300
Write-Offs	(14,900)	(5,000)	(29,000)	(9,400)
Recoveries Collected	200	200	1,000	1,100
Ending Balance	241,200	158,600	241,200	158,600
Hardship Accounts | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	45,600	45,000	43,700	43,300
Uncollectible Expense	0	0	0	0
Uncollectible Costs Deferred	3,400	2,800	5,800	4,900
Write-Offs	100	(300)	(400)	(700)
Recoveries Collected	0	0	0	0
Ending Balance	49,100	47,500	49,100	47,500
Retail (Non-Hardship), Wholesale, and Other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	194,200	215,100	201,900	191,300
Uncollectible Expense	16,800	10,700	40,900	40,800
Uncollectible Costs Deferred	14,000	9,400	22,800	36,700
Write-Offs	(40,400)	(33,200)	(88,500)	(76,300)
Recoveries Collected	3,300	3,500	10,800	13,000
Ending Balance	187,900	205,500	187,900	205,500
Retail (Non-Hardship), Wholesale, and Other | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	36,300	40,600	36,400	36,700
Uncollectible Expense	2,400	4,100	7,100	10,800
Uncollectible Costs Deferred	3,100	2,000	9,300	2,000
Write-Offs	(7,600)	(7,700)	(21,300)	(14,400)
Recoveries Collected	900	1,000	3,600	4,900
Ending Balance	35,100	40,000	35,100	40,000
Retail (Non-Hardship), Wholesale, and Other | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	52,200	55,500	51,300	53,700
Uncollectible Expense	5,200	3,500	13,000	12,200
Uncollectible Costs Deferred	4,500	4,000	13,800	11,800
Write-Offs	(11,500)	(12,300)	(30,400)	(30,600)
Recoveries Collected	1,300	1,300	4,000	4,900
Ending Balance	$ 51,700	$ 52,000	$ 51,700	$ 52,000