SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Allowance for Uncollectible Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	$ 224,821
Increase due to CMA asset acquisition	24,200
Uncollectible Expense	53,461	$ 63,446	$ 61,337
Uncollectible Costs Deferred	97,000
Write-Offs	(78,000)
Recoveries Collected	13,600
Ending Balance	$ 358,851	224,821
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Restatement Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	$ 23,800
Ending Balance		23,800
Hardship Accounts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	143,300
Increase due to CMA asset acquisition	0
Uncollectible Expense	0
Uncollectible Costs Deferred	43,100
Write-Offs	(14,700)
Recoveries Collected	1,500
Ending Balance	194,800	143,300
Hardship Accounts | Restatement Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	21,600
Ending Balance		21,600
Retail Non Hardship Wholesaleand Other Receivables
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	81,500
Increase due to CMA asset acquisition	24,200
Uncollectible Expense	53,500
Uncollectible Costs Deferred	53,900
Write-Offs	(63,300)
Recoveries Collected	12,100
Ending Balance	164,100	81,500
Retail Non Hardship Wholesaleand Other Receivables | Restatement Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	2,200
Ending Balance		2,200
The Connecticut Light and Power Company
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	97,348
Increase due to CMA asset acquisition	0
Uncollectible Expense	12,882	15,948	15,831
Uncollectible Costs Deferred	49,000
Write-Offs	(29,700)
Recoveries Collected	5,700
Ending Balance	$ 157,447	97,348
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
The Connecticut Light and Power Company | Restatement Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	$ 22,200
Ending Balance		22,200
The Connecticut Light and Power Company | Hardship Accounts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	80,100
Increase due to CMA asset acquisition	0
Uncollectible Expense	0
Uncollectible Costs Deferred	38,200
Write-Offs	(11,900)
Recoveries Collected	1,400
Ending Balance	129,100	80,100
The Connecticut Light and Power Company | Hardship Accounts | Restatement Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	21,300
Ending Balance		21,300
The Connecticut Light and Power Company | Retail Non Hardship Wholesaleand Other Receivables
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	17,200
Increase due to CMA asset acquisition	0
Uncollectible Expense	12,900
Uncollectible Costs Deferred	10,800
Write-Offs	(17,800)
Recoveries Collected	4,300
Ending Balance	28,300	17,200
The Connecticut Light and Power Company | Retail Non Hardship Wholesaleand Other Receivables | Restatement Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	900
Ending Balance		900
NSTAR Electric Company
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	75,406
Increase due to CMA asset acquisition	0
Uncollectible Expense	15,293	25,079	22,279
Uncollectible Costs Deferred	24,700
Write-Offs	(27,200)
Recoveries Collected	4,700
Ending Balance	$ 91,583	75,406
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
NSTAR Electric Company | Restatement Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	$ (1,300)
Ending Balance		(1,300)
NSTAR Electric Company | Hardship Accounts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	43,900
Increase due to CMA asset acquisition	0
Uncollectible Expense	0
Uncollectible Costs Deferred	(1,700)
Write-Offs	(900)
Recoveries Collected	0
Ending Balance	39,700	43,900
NSTAR Electric Company | Hardship Accounts | Restatement Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	(1,600)
Ending Balance		(1,600)
NSTAR Electric Company | Retail Non Hardship Wholesaleand Other Receivables
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	31,500
Increase due to CMA asset acquisition	0
Uncollectible Expense	15,300
Uncollectible Costs Deferred	26,400
Write-Offs	(26,300)
Recoveries Collected	4,700
Ending Balance	51,900	31,500
NSTAR Electric Company | Retail Non Hardship Wholesaleand Other Receivables | Restatement Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	300
Ending Balance		300
Public Service Company of New Hampshire
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	10,497
Increase due to CMA asset acquisition	0
Uncollectible Expense	5,164	6,726	$ 6,383
Uncollectible Costs Deferred	7,400
Write-Offs	(6,900)
Recoveries Collected	700
Ending Balance	$ 17,157	10,497
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Public Service Company of New Hampshire | Restatement Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	$ 300
Ending Balance		$ 300