SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	$ 417,400	$ 358,900
Uncollectible Expense	17,135	16,295
Uncollectible Costs Deferred	15,700	32,500
Write-Offs	(24,300)	(20,000)
Recoveries Collected	6,300	4,000
Ending Balance	432,200	391,700
CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	181,300	157,400
Uncollectible Expense	3,776	3,797
Uncollectible Costs Deferred	(6,100)	19,400
Write-Offs	(1,600)	(6,900)
Recoveries Collected	3,100	1,400
Ending Balance	180,500	175,100
NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	97,000	91,600
Uncollectible Expense	4,662	3,868
Uncollectible Costs Deferred	2,100	(100)
Write-Offs	(10,900)	(7,600)
Recoveries Collected	1,900	1,500
Ending Balance	94,800	89,300
PSNH
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	24,300	17,200
Uncollectible Expense	2,496	1,221
Uncollectible Costs Deferred	1,000	1,200
Write-Offs	(1,800)	(2,500)
Recoveries Collected	200	200
Ending Balance	26,200	17,300
Hardship Accounts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	226,100	194,800
Uncollectible Expense	0	0
Uncollectible Costs Deferred	900	5,400
Write-Offs	(2,300)	(3,300)
Recoveries Collected	800	400
Ending Balance	225,500	197,300
Hardship Accounts | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	144,600	129,100
Uncollectible Expense	0	0
Uncollectible Costs Deferred	(4,000)	11,900
Write-Offs	(1,100)	(2,900)
Recoveries Collected	600	300
Ending Balance	140,100	138,400
Hardship Accounts | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	43,300	39,700
Uncollectible Expense	0	0
Uncollectible Costs Deferred	(3,300)	(8,500)
Write-Offs	(300)	(100)
Recoveries Collected	0	0
Ending Balance	39,700	31,100
Retail (Non-Hardship), Wholesale, and Other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	191,300	164,100
Uncollectible Expense	17,100	16,300
Uncollectible Costs Deferred	14,800	27,100
Write-Offs	(22,000)	(16,700)
Recoveries Collected	5,500	3,600
Ending Balance	206,700	194,400
Retail (Non-Hardship), Wholesale, and Other | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	36,700	28,300
Uncollectible Expense	3,800	3,800
Uncollectible Costs Deferred	(2,100)	7,500
Write-Offs	(500)	(4,000)
Recoveries Collected	2,500	1,100
Ending Balance	40,400	36,700
Retail (Non-Hardship), Wholesale, and Other | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	53,700	51,900
Uncollectible Expense	4,700	3,900
Uncollectible Costs Deferred	5,400	8,400
Write-Offs	(10,600)	(7,500)
Recoveries Collected	1,900	1,500
Ending Balance	$ 55,100	$ 58,200