SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		$ 224,800
Uncollectible Expense	$ 13,000	20,579	$ 31,546
Uncollectible Costs Deferred		35,900
Write-Offs		(42,500)
Recoveries Collected		7,400
Ending Balance		270,000
CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		97,300
Uncollectible Expense	3,500	6,187	7,627
Uncollectible Costs Deferred		24,900
Write-Offs		(16,900)
Recoveries Collected		3,000
Ending Balance		136,700
NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		75,400
Uncollectible Expense	5,700	6,826	11,605
Uncollectible Costs Deferred		(200)
Write-Offs		(14,800)
Recoveries Collected		2,800
Ending Balance		68,700
PSNH
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		10,500
Uncollectible Expense	$ 1,400	1,255	$ 3,143
Uncollectible Costs Deferred		2,400
Write-Offs		(3,000)
Recoveries Collected		300
Ending Balance		11,800
Hardship Accounts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		143,300
Uncollectible Expense		0
Uncollectible Costs Deferred		18,900
Write-Offs		(9,100)
Recoveries Collected		800
Ending Balance		175,500
Hardship Accounts | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		80,100
Uncollectible Expense		0
Uncollectible Costs Deferred		21,500
Write-Offs		(7,400)
Recoveries Collected		800
Ending Balance		116,300
Hardship Accounts | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		43,900
Uncollectible Expense		0
Uncollectible Costs Deferred		(7,100)
Write-Offs		(600)
Recoveries Collected		0
Ending Balance		34,600
Retail (Non-Hardship), Wholesale, and Other Receivables
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		81,500
Uncollectible Expense		20,600
Uncollectible Costs Deferred		17,000
Write-Offs		(33,400)
Recoveries Collected		6,600
Ending Balance		94,500
Retail (Non-Hardship), Wholesale, and Other Receivables | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		17,200
Uncollectible Expense		6,200
Uncollectible Costs Deferred		3,400
Write-Offs		(9,500)
Recoveries Collected		2,200
Ending Balance		20,400
Retail (Non-Hardship), Wholesale, and Other Receivables | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance		31,500
Uncollectible Expense		6,800
Uncollectible Costs Deferred		6,900
Write-Offs		(14,200)
Recoveries Collected		2,800
Ending Balance		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