SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	$ 486,300	$ 417,400
Uncollectible Expense	22,821	17,135
Uncollectible Costs Deferred	58,000	15,700
Write-Offs	(36,000)	(24,300)
Recoveries Collected	4,000	6,300
Ending Balance	535,100	432,200
CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	225,300	181,300
Uncollectible Expense	3,876	3,776
Uncollectible Costs Deferred	37,000	(6,100)
Write-Offs	(14,300)	(1,600)
Recoveries Collected	1,500	3,100
Ending Balance	253,400	180,500
NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	95,000	97,000
Uncollectible Expense	4,697	4,662
Uncollectible Costs Deferred	4,100	2,100
Write-Offs	(10,700)	(10,900)
Recoveries Collected	1,500	1,900
Ending Balance	94,600	94,800
PSNH
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	29,200	24,300
Uncollectible Expense	5,060	2,496
Uncollectible Costs Deferred	1,200	1,000
Write-Offs	(2,200)	(1,800)
Recoveries Collected	200	200
Ending Balance	33,500	26,200
Hardship Accounts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	284,400	226,100
Uncollectible Expense	0	0
Uncollectible Costs Deferred	43,700	900
Write-Offs	(9,700)	(2,300)
Recoveries Collected	300	800
Ending Balance	318,700	225,500
Hardship Accounts | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	188,900	144,600
Uncollectible Expense	0	0
Uncollectible Costs Deferred	34,300	(4,000)
Write-Offs	(7,300)	(1,100)
Recoveries Collected	300	600
Ending Balance	216,200	140,100
Hardship Accounts | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	43,700	43,300
Uncollectible Expense	0	0
Uncollectible Costs Deferred	(1,300)	(3,300)
Write-Offs	(400)	(300)
Recoveries Collected	0	0
Ending Balance	42,000	39,700
Retail (Non-Hardship), Wholesale, and Other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	201,900	191,300
Uncollectible Expense	22,800	17,100
Uncollectible Costs Deferred	14,300	14,800
Write-Offs	(26,300)	(22,000)
Recoveries Collected	3,700	5,500
Ending Balance	216,400	206,700
Retail (Non-Hardship), Wholesale, and Other | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	36,400	36,700
Uncollectible Expense	3,900	3,800
Uncollectible Costs Deferred	2,700	(2,100)
Write-Offs	(7,000)	(500)
Recoveries Collected	1,200	2,500
Ending Balance	37,200	40,400
Retail (Non-Hardship), Wholesale, and Other | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	51,300	53,700
Uncollectible Expense	4,700	4,700
Uncollectible Costs Deferred	5,400	5,400
Write-Offs	(10,300)	(10,600)
Recoveries Collected	1,500	1,900
Ending Balance	$ 52,600	$ 55,100