SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	$ 457,800	$ 425,800	$ 417,400	$ 358,900
Uncollectible Expense	10,700	12,000	40,753	39,690
Uncollectible Costs Deferred	25,000	24,500	74,700	98,100
Write-Offs	(39,700)	(24,400)	(89,800)	(65,800)
Recoveries Collected	3,800	3,800	14,500	10,800
Ending Balance	457,600	441,700	457,600	441,700
CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	192,700	188,800	181,300	157,400
Uncollectible Expense	4,100	3,600	10,816	10,183
Uncollectible Costs Deferred	13,300	10,200	24,300	47,100
Write-Offs	(12,700)	(8,900)	(23,800)	(23,500)
Recoveries Collected	1,200	1,800	6,000	4,300
Ending Balance	198,600	195,500	198,600	195,500
NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	100,500	98,000	97,000	91,600
Uncollectible Expense	3,500	5,100	12,159	12,477
Uncollectible Costs Deferred	6,800	8,300	16,700	20,100
Write-Offs	(12,600)	(7,500)	(31,300)	(23,000)
Recoveries Collected	1,300	1,100	4,900	3,800
Ending Balance	99,500	105,000	99,500	105,000
PSNH
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	27,300	17,200	24,300	17,200
Uncollectible Expense	3,500	1,200	8,060	4,381
Uncollectible Costs Deferred	100	1,200	1,200	2,000
Write-Offs	(2,300)	(2,800)	(5,400)	(7,300)
Recoveries Collected	200	200	600	700
Ending Balance	28,800	17,000	28,800	17,000
Hardship Accounts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	242,700	210,700	226,100	194,800
Uncollectible Expense	0	0	0	0
Uncollectible Costs Deferred	15,600	22,800	38,000	44,800
Write-Offs	(6,500)	(3,900)	(13,500)	(10,600)
Recoveries Collected	300	300	1,500	900
Ending Balance	252,100	229,900	252,100	229,900
Hardship Accounts | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	152,100	145,600	144,600	129,100
Uncollectible Expense	0	0	0	0
Uncollectible Costs Deferred	11,300	4,500	22,300	25,700
Write-Offs	(5,000)	(3,500)	(9,400)	(8,700)
Recoveries Collected	200	200	1,100	700
Ending Balance	158,600	146,800	158,600	146,800
Hardship Accounts | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	45,000	35,900	43,300	39,700
Uncollectible Expense	0	0	0	0
Uncollectible Costs Deferred	2,800	11,800	4,900	8,300
Write-Offs	(300)	(100)	(700)	(400)
Recoveries Collected	0	0	0	0
Ending Balance	47,500	47,600	47,500	47,600
Retail (Non-Hardship), Wholesale, and Other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	215,100	215,100	191,300	164,100
Uncollectible Expense	10,700	12,000	40,800	39,700
Uncollectible Costs Deferred	9,400	1,700	36,700	53,300
Write-Offs	(33,200)	(20,500)	(76,300)	(55,200)
Recoveries Collected	3,500	3,500	13,000	9,900
Ending Balance	205,500	211,800	205,500	211,800
Retail (Non-Hardship), Wholesale, and Other | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	40,600	43,200	36,700	28,300
Uncollectible Expense	4,100	3,600	10,800	10,200
Uncollectible Costs Deferred	2,000	5,700	2,000	21,400
Write-Offs	(7,700)	(5,400)	(14,400)	(14,800)
Recoveries Collected	1,000	1,600	4,900	3,600
Ending Balance	40,000	48,700	40,000	48,700
Retail (Non-Hardship), Wholesale, and Other | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	55,500	62,100	53,700	51,900
Uncollectible Expense	3,500	5,100	12,200	12,500
Uncollectible Costs Deferred	4,000	(3,500)	11,800	11,800
Write-Offs	(12,300)	(7,400)	(30,600)	(22,600)
Recoveries Collected	1,300	1,100	4,900	3,800
Ending Balance	$ 52,000	$ 57,400	$ 52,000	$ 57,400