SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	$ 358,900	$ 224,800
Uncollectible Expense	16,295	11,353
Uncollectible Costs Deferred	32,500	20,300
Write-Offs	(20,000)	(21,500)
Recoveries Collected	4,000	3,300
Ending Balance	391,700	262,100
CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	157,400	97,300
Uncollectible Expense	3,797	3,230
Uncollectible Costs Deferred	19,400	14,700
Write-Offs	(6,900)	(7,300)
Recoveries Collected	1,400	800
Ending Balance	175,100	130,900
NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	91,600	75,400
Uncollectible Expense	3,868	3,704
Uncollectible Costs Deferred	(100)	0
Write-Offs	(7,600)	(8,400)
Recoveries Collected	1,500	1,700
Ending Balance	89,300	71,100
PSNH
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	17,200	10,500
Uncollectible Expense	1,221	666
Uncollectible Costs Deferred	1,200	1,300
Write-Offs	(2,500)	(1,700)
Recoveries Collected	200	100
Ending Balance	17,300	11,200
Hardship Accounts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	194,800	143,300
Uncollectible Expense	0	0
Uncollectible Costs Deferred	5,400	11,800
Write-Offs	(3,300)	(4,700)
Recoveries Collected	400	200
Ending Balance	197,300	172,200
Hardship Accounts | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	129,100	80,100
Uncollectible Expense	0	0
Uncollectible Costs Deferred	11,900	12,900
Write-Offs	(2,900)	(3,800)
Recoveries Collected	300	200
Ending Balance	138,400	110,700
Hardship Accounts | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	39,700	43,900
Uncollectible Expense	0	0
Uncollectible Costs Deferred	(8,500)	(3,400)
Write-Offs	(100)	(400)
Recoveries Collected	0	0
Ending Balance	31,100	38,500
Retail (Non-Hardship), Wholesale, and Other Receivables
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	164,100	81,500
Uncollectible Expense	16,300	11,400
Uncollectible Costs Deferred	27,100	8,500
Write-Offs	(16,700)	(16,800)
Recoveries Collected	3,600	3,100
Ending Balance	194,400	89,900
Retail (Non-Hardship), Wholesale, and Other Receivables | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	28,300	17,200
Uncollectible Expense	3,800	3,200
Uncollectible Costs Deferred	7,500	1,800
Write-Offs	(4,000)	(3,500)
Recoveries Collected	1,100	600
Ending Balance	36,700	20,200
Retail (Non-Hardship), Wholesale, and Other Receivables | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	51,900	31,500
Uncollectible Expense	3,900	3,700
Uncollectible Costs Deferred	8,400	3,400
Write-Offs	(7,500)	(8,000)
Recoveries Collected	1,500	1,700
Ending Balance	$ 58,200	32,600
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