SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	$ 391,700	$ 262,100	$ 358,900	$ 224,800
Uncollectible Expense	11,400	9,200	27,683	20,579
Uncollectible Costs Deferred	41,100	15,600	73,600	35,900
Write-Offs	(21,500)	(21,000)	(41,500)	(42,500)
Recoveries Collected	3,100	4,100	7,100	7,400
Ending Balance	425,800	270,000	425,800	270,000
CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	175,100	130,900	157,400	97,300
Uncollectible Expense	2,800	3,000	6,614	6,187
Uncollectible Costs Deferred	17,600	10,200	36,900	24,900
Write-Offs	(7,700)	(9,600)	(14,500)	(16,900)
Recoveries Collected	1,000	2,200	2,400	3,000
Ending Balance	188,800	136,700	188,800	136,700
NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	89,300	71,100	91,600	75,400
Uncollectible Expense	3,500	3,100	7,394	6,826
Uncollectible Costs Deferred	11,800	(200)	11,700	(200)
Write-Offs	(7,800)	(6,400)	(15,500)	(14,800)
Recoveries Collected	1,200	1,100	2,800	2,800
Ending Balance	98,000	68,700	98,000	68,700
PSNH
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	17,300	11,200	17,200	10,500
Uncollectible Expense	2,000	600	3,226	1,255
Uncollectible Costs Deferred	(300)	1,100	800	2,400
Write-Offs	(2,000)	(1,300)	(4,500)	(3,000)
Recoveries Collected	200	200	500	300
Ending Balance	17,200	11,800	17,200	11,800
Hardship Accounts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	197,300	172,200	194,800	143,300
Uncollectible Expense	0	0	0	0
Uncollectible Costs Deferred	16,600	7,100	22,000	18,900
Write-Offs	(3,400)	(4,400)	(6,700)	(9,100)
Recoveries Collected	200	600	600	800
Ending Balance	210,700	175,500	210,700	175,500
Hardship Accounts | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	138,400	110,700	129,100	80,100
Uncollectible Expense	0	0	0	0
Uncollectible Costs Deferred	9,300	8,600	21,200	21,500
Write-Offs	(2,300)	(3,600)	(5,200)	(7,400)
Recoveries Collected	200	600	500	800
Ending Balance	145,600	116,300	145,600	116,300
Hardship Accounts | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	31,100	38,500	39,700	43,900
Uncollectible Expense	0	0	0	0
Uncollectible Costs Deferred	4,900	(3,700)	(3,500)	(7,100)
Write-Offs	(100)	(200)	(300)	(600)
Recoveries Collected	0	0	0	0
Ending Balance	35,900	34,600	35,900	34,600
Retail (Non-Hardship), Wholesale, and Other Receivables
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	194,400	89,900	164,100	81,500
Uncollectible Expense	11,400	9,200	27,700	20,600
Uncollectible Costs Deferred	24,500	8,500	51,600	17,000
Write-Offs	(18,100)	(16,600)	(34,800)	(33,400)
Recoveries Collected	2,900	3,500	6,500	6,600
Ending Balance	215,100	94,500	215,100	94,500
Retail (Non-Hardship), Wholesale, and Other Receivables | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	36,700	20,200	28,300	17,200
Uncollectible Expense	2,800	3,000	6,600	6,200
Uncollectible Costs Deferred	8,300	1,600	15,700	3,400
Write-Offs	(5,400)	(6,000)	(9,300)	(9,500)
Recoveries Collected	800	1,600	1,900	2,200
Ending Balance	43,200	20,400	43,200	20,400
Retail (Non-Hardship), Wholesale, and Other Receivables | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	58,200	32,600	51,900	31,500
Uncollectible Expense	3,500	3,100	7,400	6,800
Uncollectible Costs Deferred	6,900	3,500	15,200	6,900
Write-Offs	(7,700)	(6,200)	(15,200)	(14,200)
Recoveries Collected	1,200	1,100	2,800	2,800
Ending Balance	$ 62,100	$ 34,100	$ 62,100	34,100
Restatement Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				23,800
Restatement Adjustment | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				22,200
Restatement Adjustment | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				(1,300)
Restatement Adjustment | PSNH
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				300
Restatement Adjustment | Hardship Accounts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				21,600
Restatement Adjustment | Hardship Accounts | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				21,300
Restatement Adjustment | Hardship Accounts | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				(1,600)
Restatement Adjustment | Retail (Non-Hardship), Wholesale, and Other Receivables
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				2,200
Restatement Adjustment | Retail (Non-Hardship), Wholesale, and Other Receivables | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				900
Restatement Adjustment | Retail (Non-Hardship), Wholesale, and Other Receivables | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance				$ 300