SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	$ 535,100	$ 432,200	$ 486,300	$ 417,400
Uncollectible Expense	1,400	12,900	24,171	30,032
Uncollectible Costs Deferred	21,500	34,000	79,500	49,700
Write-Offs	(29,900)	(25,700)	(65,800)	(50,000)
Recoveries Collected	4,300	4,400	8,200	10,700
Ending Balance	532,400	457,800	532,400	457,800
CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	253,400	180,500	225,300	181,300
Uncollectible Expense	900	2,900	4,750	6,701
Uncollectible Costs Deferred	20,000	17,200	57,000	11,000
Write-Offs	(13,500)	(9,500)	(27,800)	(11,100)
Recoveries Collected	2,000	1,600	3,500	4,800
Ending Balance	262,800	192,700	262,800	192,700
NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	94,600	94,800	95,000	97,000
Uncollectible Expense	3,100	4,000	7,759	8,688
Uncollectible Costs Deferred	7,600	7,800	11,700	9,900
Write-Offs	(8,700)	(7,800)	(19,400)	(18,700)
Recoveries Collected	1,200	1,700	2,700	3,600
Ending Balance	97,800	100,500	97,800	100,500
PSNH
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	33,500	26,200	29,200	24,300
Uncollectible Expense	(5,500)	2,000	(451)	4,544
Uncollectible Costs Deferred	(13,500)	100	(12,200)	1,100
Write-Offs	(2,000)	(1,300)	(4,200)	(3,100)
Recoveries Collected	200	300	400	500
Ending Balance	12,700	27,300	12,700	27,300
Hardship Accounts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	318,700	225,500	284,400	226,100
Uncollectible Expense	0	0	0	0
Uncollectible Costs Deferred	27,000	21,400	70,800	22,400
Write-Offs	(8,100)	(4,600)	(17,800)	(6,900)
Recoveries Collected	600	400	800	1,100
Ending Balance	338,200	242,700	338,200	242,700
Hardship Accounts | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	216,200	140,100	188,900	144,600
Uncollectible Expense	0	0	0	0
Uncollectible Costs Deferred	16,600	15,000	50,900	11,000
Write-Offs	(6,800)	(3,300)	(14,100)	(4,400)
Recoveries Collected	500	300	800	900
Ending Balance	226,500	152,100	226,500	152,100
Hardship Accounts | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	42,000	39,700	43,700	43,300
Uncollectible Expense	0	0	0	0
Uncollectible Costs Deferred	3,700	5,400	2,400	2,100
Write-Offs	(100)	(100)	(500)	(400)
Recoveries Collected	0	0	0	0
Ending Balance	45,600	45,000	45,600	45,000
Retail (Non-Hardship), Wholesale, and Other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	216,400	206,700	201,900	191,300
Uncollectible Expense	1,400	12,900	24,200	30,000
Uncollectible Costs Deferred	(5,500)	12,600	8,700	27,300
Write-Offs	(21,800)	(21,100)	(48,000)	(43,100)
Recoveries Collected	3,700	4,000	7,400	9,600
Ending Balance	194,200	215,100	194,200	215,100
Retail (Non-Hardship), Wholesale, and Other | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	37,200	40,400	36,400	36,700
Uncollectible Expense	900	2,900	4,800	6,700
Uncollectible Costs Deferred	3,400	2,200	6,100	0
Write-Offs	(6,700)	(6,200)	(13,700)	(6,700)
Recoveries Collected	1,500	1,300	2,700	3,900
Ending Balance	36,300	40,600	36,300	40,600
Retail (Non-Hardship), Wholesale, and Other | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	52,600	55,100	51,300	53,700
Uncollectible Expense	3,100	4,000	7,800	8,700
Uncollectible Costs Deferred	3,900	2,400	9,300	7,800
Write-Offs	(8,600)	(7,700)	(18,900)	(18,300)
Recoveries Collected	1,200	1,700	2,700	3,600
Ending Balance	$ 52,200	$ 55,500	$ 52,200	$ 55,500