SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	$ 270,000		$ 224,800
Uncollectible Expense	10,500	$ 14,900	31,101	$ 46,417
Uncollectible Costs Deferred	30,500		66,400
Write-Offs	(19,100)		(61,600)
Recoveries Collected	3,100		10,500
Ending Balance	295,000		295,000
CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	136,700		97,300
Uncollectible Expense	3,800	4,500	10,013	12,084
Uncollectible Costs Deferred	16,100		41,000
Write-Offs	(7,200)		(24,000)
Recoveries Collected	1,400		4,300
Ending Balance	150,800		150,800
NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	68,700		75,400
Uncollectible Expense	4,500	7,000	11,321	18,598
Uncollectible Costs Deferred	9,100		8,900
Write-Offs	(6,500)		(21,300)
Recoveries Collected	1,000		3,800
Ending Balance	76,800		76,800
PSNH
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	11,800		10,500
Uncollectible Expense	800	$ 1,800	2,058	$ 4,977
Uncollectible Costs Deferred	3,200		5,600
Write-Offs	(2,000)		(5,000)
Recoveries Collected	200		500
Ending Balance	14,000		14,000
Hardship Accounts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	175,500		143,300
Uncollectible Expense	0		0
Uncollectible Costs Deferred	10,100		24,900
Write-Offs	(7,300)		(12,300)
Recoveries Collected	400		1,200
Ending Balance	178,700		178,700
Hardship Accounts | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	116,300		80,100
Uncollectible Expense	0		0
Uncollectible Costs Deferred	11,200		32,700
Write-Offs	(2,600)		(10,000)
Recoveries Collected	300		1,100
Ending Balance	125,200		125,200
Hardship Accounts | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	34,600		43,900
Uncollectible Expense	0		0
Uncollectible Costs Deferred	(1,300)		(8,400)
Write-Offs	(200)		(800)
Recoveries Collected	0		0
Ending Balance	33,100		33,100
Retail (Non-Hardship), Wholesale, and Other Receivables
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	94,500		81,500
Uncollectible Expense	10,500		31,100
Uncollectible Costs Deferred	20,400		41,500
Write-Offs	(11,800)		(49,300)
Recoveries Collected	2,700		9,300
Ending Balance	116,300		116,300
Retail (Non-Hardship), Wholesale, and Other Receivables | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	20,400		17,200
Uncollectible Expense	3,800		10,000
Uncollectible Costs Deferred	4,900		8,300
Write-Offs	(4,600)		(14,000)
Recoveries Collected	1,100		3,200
Ending Balance	25,600		25,600
Retail (Non-Hardship), Wholesale, and Other Receivables | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	34,100		31,500
Uncollectible Expense	4,500		11,300
Uncollectible Costs Deferred	10,400		17,300
Write-Offs	(6,300)		(20,500)
Recoveries Collected	1,000		3,800
Ending Balance	$ 43,700		43,700
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