SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	$ 425,800	$ 270,000	$ 358,900	$ 224,800
Uncollectible Expense	12,000	10,500	39,690	31,101
Uncollectible Costs Deferred	24,500	30,500	98,100	66,400
Write-Offs	(24,400)	(19,100)	(65,800)	(61,600)
Recoveries Collected	3,800	3,100	10,800	10,500
Ending Balance	441,700	295,000	441,700	295,000
Restatement Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				23,800
CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	188,800	136,700	157,400	97,300
Uncollectible Expense	3,600	3,800	10,183	10,013
Uncollectible Costs Deferred	10,200	16,100	47,100	41,000
Write-Offs	(8,900)	(7,200)	(23,500)	(24,000)
Recoveries Collected	1,800	1,400	4,300	4,300
Ending Balance	195,500	150,800	195,500	150,800
CL&P | Restatement Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				22,200
NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	98,000	68,700	91,600	75,400
Uncollectible Expense	5,100	4,500	12,477	11,321
Uncollectible Costs Deferred	8,300	9,100	20,100	8,900
Write-Offs	(7,500)	(6,500)	(23,000)	(21,300)
Recoveries Collected	1,100	1,000	3,800	3,800
Ending Balance	105,000	76,800	105,000	76,800
NSTAR Electric | Restatement Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				(1,300)
PSNH
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	17,200	11,800	17,200	10,500
Uncollectible Expense	1,200	800	4,381	2,058
Uncollectible Costs Deferred	1,200	3,200	2,000	5,600
Write-Offs	(2,800)	(2,000)	(7,300)	(5,000)
Recoveries Collected	200	200	700	500
Ending Balance	17,000	14,000	17,000	14,000
PSNH | Restatement Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				300
Hardship Accounts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	210,700	175,500	194,800	143,300
Uncollectible Expense	0	0	0	0
Uncollectible Costs Deferred	22,800	10,100	44,800	24,900
Write-Offs	(3,900)	(7,300)	(10,600)	(12,300)
Recoveries Collected	300	400	900	1,200
Ending Balance	229,900	178,700	229,900	178,700
Hardship Accounts | Restatement Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				21,600
Hardship Accounts | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	145,600	116,300	129,100	80,100
Uncollectible Expense	0	0	0	0
Uncollectible Costs Deferred	4,500	11,200	25,700	32,700
Write-Offs	(3,500)	(2,600)	(8,700)	(10,000)
Recoveries Collected	200	300	700	1,100
Ending Balance	146,800	125,200	146,800	125,200
Hardship Accounts | CL&P | Restatement Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				21,300
Hardship Accounts | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	35,900	34,600	39,700	43,900
Uncollectible Expense	0	0	0	0
Uncollectible Costs Deferred	11,800	(1,300)	8,300	(8,400)
Write-Offs	(100)	(200)	(400)	(800)
Recoveries Collected	0	0	0	0
Ending Balance	47,600	33,100	47,600	33,100
Hardship Accounts | NSTAR Electric | Restatement Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				(1,600)
Retail (Non-Hardship), Wholesale, and Other Receivables
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	215,100	94,500	164,100	81,500
Uncollectible Expense	12,000	10,500	39,700	31,100
Uncollectible Costs Deferred	1,700	20,400	53,300	41,500
Write-Offs	(20,500)	(11,800)	(55,200)	(49,300)
Recoveries Collected	3,500	2,700	9,900	9,300
Ending Balance	211,800	116,300	211,800	116,300
Retail (Non-Hardship), Wholesale, and Other Receivables | Restatement Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				2,200
Retail (Non-Hardship), Wholesale, and Other Receivables | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	43,200	20,400	28,300	17,200
Uncollectible Expense	3,600	3,800	10,200	10,000
Uncollectible Costs Deferred	5,700	4,900	21,400	8,300
Write-Offs	(5,400)	(4,600)	(14,800)	(14,000)
Recoveries Collected	1,600	1,100	3,600	3,200
Ending Balance	48,700	25,600	48,700	25,600
Retail (Non-Hardship), Wholesale, and Other Receivables | CL&P | Restatement Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				900
Retail (Non-Hardship), Wholesale, and Other Receivables | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	62,100	34,100	51,900	31,500
Uncollectible Expense	5,100	4,500	12,500	11,300
Uncollectible Costs Deferred	(3,500)	10,400	11,800	17,300
Write-Offs	(7,400)	(6,300)	(22,600)	(20,500)
Recoveries Collected	1,100	1,000	3,800	3,800
Ending Balance	$ 57,400	$ 43,700	$ 57,400	43,700
Retail (Non-Hardship), Wholesale, and Other Receivables | NSTAR Electric | Restatement Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				$ 300