SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Allowance for Uncollectible Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	$ 556,200	$ 554,500	$ 486,300
Uncollectible Expense	101,141	74,069	72,468
Uncollectible Costs Deferred	120,000	119,700	158,200
Write-Offs	(216,100)	(203,800)	(178,100)
Recoveries Collected	17,000	14,000	15,600
Assets Held for Sale	2,300	(2,300)
Ending Balance	580,500	556,200	554,500
Hardship Accounts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	364,600	366,800	284,400
Uncollectible Expense	0	0	0
Uncollectible Costs Deferred	62,400	71,400	137,000
Write-Offs	(61,900)	(74,300)	(55,900)
Recoveries Collected	2,100	700	1,300
Assets Held for Sale	0	0
Ending Balance	367,200	364,600	366,800
Retail (Non-Hardship), Wholesale, and Other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	191,600	187,700	201,900
Uncollectible Expense	101,100	74,100	72,500
Uncollectible Costs Deferred	57,600	48,300	21,200
Write-Offs	(154,200)	(129,500)	(122,200)
Recoveries Collected	14,900	13,300	14,300
Assets Held for Sale	2,300	(2,300)
Ending Balance	213,300	191,600	187,700
CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	279,100	296,000	225,300
Uncollectible Expense	17,949	17,190	11,675
Uncollectible Costs Deferred	43,100	46,800	126,400
Write-Offs	(87,600)	(86,000)	(73,200)
Recoveries Collected	6,000	5,100	5,800
Assets Held for Sale	0	0
Ending Balance	258,500	279,100	296,000
CL&P | Hardship Accounts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	240,700	259,700	188,900
Uncollectible Expense	0	0	0
Uncollectible Costs Deferred	32,000	35,500	114,400
Write-Offs	(48,500)	(55,100)	(44,700)
Recoveries Collected	1,500	600	1,100
Assets Held for Sale	0	0
Ending Balance	225,700	240,700	259,700
CL&P | Retail (Non-Hardship), Wholesale, and Other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	38,400	36,300	36,400
Uncollectible Expense	17,900	17,200	11,700
Uncollectible Costs Deferred	11,100	11,300	12,000
Write-Offs	(39,100)	(30,900)	(28,500)
Recoveries Collected	4,500	4,500	4,700
Assets Held for Sale	0	0
Ending Balance	32,800	38,400	36,300
NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	114,900	97,000	95,000
Uncollectible Expense	41,430	33,607	22,791
Uncollectible Costs Deferred	37,100	37,700	17,500
Write-Offs	(66,600)	(57,000)	(43,300)
Recoveries Collected	5,800	3,600	5,000
Assets Held for Sale	0	0
Ending Balance	132,600	114,900	97,000
NSTAR Electric | Hardship Accounts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	55,200	43,600	43,700
Uncollectible Expense	0	0	0
Uncollectible Costs Deferred	8,300	16,200	1,500
Write-Offs	(2,900)	(4,600)	(1,600)
Recoveries Collected	100	0	0
Assets Held for Sale	0	0
Ending Balance	60,700	55,200	43,600
NSTAR Electric | Retail (Non-Hardship), Wholesale, and Other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	59,700	53,400	51,300
Uncollectible Expense	41,400	33,600	22,800
Uncollectible Costs Deferred	28,800	21,500	16,000
Write-Offs	(63,700)	(52,400)	(41,700)
Recoveries Collected	5,700	3,600	5,000
Assets Held for Sale	0	0
Ending Balance	71,900	59,700	53,400
PSNH
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	14,100	14,300	29,200
Uncollectible Expense	11,973	4,688	3,989
Uncollectible Costs Deferred	7,200	5,100	(8,700)
Write-Offs	(10,500)	(10,900)	(10,900)
Recoveries Collected	700	900	700
Assets Held for Sale	0	0
Ending Balance	$ 23,500	$ 14,100	$ 14,300