SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 580,500	$ 556,200
Uncollectible Expense	30,795	22,863
Uncollectible Costs Deferred	46,600	33,200
Write-Offs	(50,700)	(50,400)
Recoveries Collected	4,500	3,700
Ending balance	611,700	565,600
CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	258,500	279,100
Uncollectible Expense	5,491	4,671
Uncollectible Costs Deferred	24,900	10,700
Write-Offs	(16,400)	(19,400)
Recoveries Collected	1,500	1,100
Ending balance	274,000	276,200
NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	132,600	114,900
Uncollectible Expense	8,299	7,485
Uncollectible Costs Deferred	3,900	9,000
Write-Offs	(19,200)	(16,800)
Recoveries Collected	1,500	1,400
Ending balance	127,100	116,000
PSNH
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	23,500	14,100
Uncollectible Expense	1,442	1,315
Uncollectible Costs Deferred	2,200	1,400
Write-Offs	(3,100)	(2,700)
Recoveries Collected	300	200
Ending balance	24,300	14,300
Hardship Accounts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	367,200	364,600
Uncollectible Expense	0	0
Uncollectible Costs Deferred	27,800	19,400
Write-Offs	(12,200)	(12,600)
Recoveries Collected	400	100
Ending balance	383,200	371,500
Hardship Accounts | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	225,700	240,700
Uncollectible Expense	0	0
Uncollectible Costs Deferred	22,100	8,800
Write-Offs	(9,500)	(9,300)
Recoveries Collected	300	100
Ending balance	238,600	240,300
Hardship Accounts | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	60,700	55,200
Uncollectible Expense	0	0
Uncollectible Costs Deferred	(3,500)	3,500
Write-Offs	(600)	(900)
Recoveries Collected	0	0
Ending balance	56,600	57,800
Retail (Non-Hardship), Wholesale, and Other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	213,300	191,600
Uncollectible Expense	30,800	22,900
Uncollectible Costs Deferred	18,800	13,800
Write-Offs	(38,500)	(37,800)
Recoveries Collected	4,100	3,600
Ending balance	228,500	194,100
Retail (Non-Hardship), Wholesale, and Other | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	32,800	38,400
Uncollectible Expense	5,500	4,700
Uncollectible Costs Deferred	2,800	1,900
Write-Offs	(6,900)	(10,100)
Recoveries Collected	1,200	1,000
Ending balance	35,400	35,900
Retail (Non-Hardship), Wholesale, and Other | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	71,900	59,700
Uncollectible Expense	8,300	7,500
Uncollectible Costs Deferred	7,400	5,500
Write-Offs	(18,600)	(15,900)
Recoveries Collected	1,500	1,400
Ending balance	$ 70,500	$ 58,200