SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	$ 432,200	$ 391,700	$ 417,400	$ 358,900
Uncollectible Expense	12,900	11,400	30,032	27,683
Uncollectible Costs Deferred	34,000	41,100	49,700	73,600
Write-Offs	(25,700)	(21,500)	(50,000)	(41,500)
Recoveries Collected	4,400	3,100	10,700	7,100
Ending Balance	457,800	425,800	457,800	425,800
CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	180,500	175,100	181,300	157,400
Uncollectible Expense	2,900	2,800	6,701	6,614
Uncollectible Costs Deferred	17,200	17,600	11,000	36,900
Write-Offs	(9,500)	(7,700)	(11,100)	(14,500)
Recoveries Collected	1,600	1,000	4,800	2,400
Ending Balance	192,700	188,800	192,700	188,800
NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	94,800	89,300	97,000	91,600
Uncollectible Expense	4,000	3,500	8,688	7,394
Uncollectible Costs Deferred	7,800	11,800	9,900	11,700
Write-Offs	(7,800)	(7,800)	(18,700)	(15,500)
Recoveries Collected	1,700	1,200	3,600	2,800
Ending Balance	100,500	98,000	100,500	98,000
PSNH
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	26,200	17,300	24,300	17,200
Uncollectible Expense	2,000	2,000	4,544	3,226
Uncollectible Costs Deferred	100	(300)	1,100	800
Write-Offs	(1,300)	(2,000)	(3,100)	(4,500)
Recoveries Collected	300	200	500	500
Ending Balance	27,300	17,200	27,300	17,200
Hardship Accounts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	225,500	197,300	226,100	194,800
Uncollectible Expense	0	0	0	0
Uncollectible Costs Deferred	21,400	16,600	22,400	22,000
Write-Offs	(4,600)	(3,400)	(6,900)	(6,700)
Recoveries Collected	400	200	1,100	600
Ending Balance	242,700	210,700	242,700	210,700
Hardship Accounts | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	140,100	138,400	144,600	129,100
Uncollectible Expense	0	0	0	0
Uncollectible Costs Deferred	15,000	9,300	11,000	21,200
Write-Offs	(3,300)	(2,300)	(4,400)	(5,200)
Recoveries Collected	300	200	900	500
Ending Balance	152,100	145,600	152,100	145,600
Hardship Accounts | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	39,700	31,100	43,300	39,700
Uncollectible Expense	0	0	0	0
Uncollectible Costs Deferred	5,400	4,900	2,100	(3,500)
Write-Offs	(100)	(100)	(400)	(300)
Recoveries Collected	0	0	0	0
Ending Balance	45,000	35,900	45,000	35,900
Retail (Non-Hardship), Wholesale, and Other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	206,700	194,400	191,300	164,100
Uncollectible Expense	12,900	11,400	30,000	27,700
Uncollectible Costs Deferred	12,600	24,500	27,300	51,600
Write-Offs	(21,100)	(18,100)	(43,100)	(34,800)
Recoveries Collected	4,000	2,900	9,600	6,500
Ending Balance	215,100	215,100	215,100	215,100
Retail (Non-Hardship), Wholesale, and Other | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	40,400	36,700	36,700	28,300
Uncollectible Expense	2,900	2,800	6,700	6,600
Uncollectible Costs Deferred	2,200	8,300	0	15,700
Write-Offs	(6,200)	(5,400)	(6,700)	(9,300)
Recoveries Collected	1,300	800	3,900	1,900
Ending Balance	40,600	43,200	40,600	43,200
Retail (Non-Hardship), Wholesale, and Other | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	55,100	58,200	53,700	51,900
Uncollectible Expense	4,000	3,500	8,700	7,400
Uncollectible Costs Deferred	2,400	6,900	7,800	15,200
Write-Offs	(7,700)	(7,700)	(18,300)	(15,200)
Recoveries Collected	1,700	1,200	3,600	2,800
Ending Balance	$ 55,500	$ 62,100	$ 55,500	$ 62,100