SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Allowance for Uncollectible Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance as of January 1, 2020	$ 358,851	$ 224,800
Increase due to CMA acquisition		24,200
Uncollectible Expense	60,886	53,461	$ 63,446
Uncollectible Costs Deferred	110,600	97,000
Write-Offs	(129,700)	(78,000)
Recoveries Collected	16,700	13,600
Balance as of December 31, 2020	417,406	358,851	224,800
Restatement Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance as of January 1, 2020	23,800
Balance as of December 31, 2020		23,800
Hardship Accounts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance as of January 1, 2020	194,800	143,300
Increase due to CMA acquisition		0
Uncollectible Expense	0	0
Uncollectible Costs Deferred	51,900	43,100
Write-Offs	(22,000)	(14,700)
Recoveries Collected	1,400	1,500
Balance as of December 31, 2020	226,100	194,800	143,300
Hardship Accounts | Restatement Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance as of January 1, 2020		21,600
Balance as of December 31, 2020			21,600
Retail Non Hardship Wholesaleand Other Receivables
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance as of January 1, 2020	164,100	81,500
Increase due to CMA acquisition		24,200
Uncollectible Expense	60,900	53,500
Uncollectible Costs Deferred	58,700	53,900
Write-Offs	(107,700)	(63,300)
Recoveries Collected	15,300	12,100
Balance as of December 31, 2020	191,300	164,100	81,500
Retail Non Hardship Wholesaleand Other Receivables | Restatement Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance as of January 1, 2020		2,200
Balance as of December 31, 2020			2,200
The Connecticut Light and Power Company
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance as of January 1, 2020	157,447	97,300
Increase due to CMA acquisition		0
Uncollectible Expense	13,495	12,882	15,948
Uncollectible Costs Deferred	57,800	49,000
Write-Offs	(54,200)	(29,700)
Recoveries Collected	6,800	5,700
Balance as of December 31, 2020	181,319	157,447	97,300
The Connecticut Light and Power Company | Restatement Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance as of January 1, 2020	22,200
Balance as of December 31, 2020		22,200
The Connecticut Light and Power Company | Hardship Accounts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance as of January 1, 2020	129,100	80,100
Increase due to CMA acquisition		0
Uncollectible Expense	0	0
Uncollectible Costs Deferred	32,300	38,200
Write-Offs	(18,000)	(11,900)
Recoveries Collected	1,200	1,400
Balance as of December 31, 2020	144,600	129,100	80,100
The Connecticut Light and Power Company | Hardship Accounts | Restatement Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance as of January 1, 2020		21,300
Balance as of December 31, 2020			21,300
The Connecticut Light and Power Company | Retail Non Hardship Wholesaleand Other Receivables
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance as of January 1, 2020	28,300	17,200
Increase due to CMA acquisition		0
Uncollectible Expense	13,500	12,900
Uncollectible Costs Deferred	25,500	10,800
Write-Offs	(36,200)	(17,800)
Recoveries Collected	5,600	4,300
Balance as of December 31, 2020	36,700	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]
Balance as of January 1, 2020		900
Balance as of December 31, 2020			900
NSTAR Electric Company
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance as of January 1, 2020	91,583	75,400
Increase due to CMA acquisition		0
Uncollectible Expense	16,649	15,293	25,079
Uncollectible Costs Deferred	20,100	24,700
Write-Offs	(37,000)	(27,200)
Recoveries Collected	5,700	4,700
Balance as of December 31, 2020	97,005	91,583	75,400
NSTAR Electric Company | Restatement Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance as of January 1, 2020	(1,300)
Balance as of December 31, 2020		(1,300)
NSTAR Electric Company | Hardship Accounts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance as of January 1, 2020	39,700	43,900
Increase due to CMA acquisition		0
Uncollectible Expense	0	0
Uncollectible Costs Deferred	4,300	(1,700)
Write-Offs	(700)	(900)
Recoveries Collected	0	0
Balance as of December 31, 2020	43,300	39,700	43,900
NSTAR Electric Company | Hardship Accounts | Restatement Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance as of January 1, 2020		(1,600)
Balance as of December 31, 2020			(1,600)
NSTAR Electric Company | Retail Non Hardship Wholesaleand Other Receivables
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance as of January 1, 2020	51,900	31,500
Increase due to CMA acquisition		0
Uncollectible Expense	16,600	15,300
Uncollectible Costs Deferred	15,800	26,400
Write-Offs	(36,300)	(26,300)
Recoveries Collected	5,700	4,700
Balance as of December 31, 2020	53,700	51,900	31,500
NSTAR Electric Company | Retail Non Hardship Wholesaleand Other Receivables | Restatement Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance as of January 1, 2020		300
Balance as of December 31, 2020			300
Public Service Company of New Hampshire
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance as of January 1, 2020	17,157	10,500
Increase due to CMA acquisition		0
Uncollectible Expense	13,113	5,164	6,726
Uncollectible Costs Deferred	3,100	7,400
Write-Offs	(10,000)	(6,900)
Recoveries Collected	900	700
Balance as of December 31, 2020	24,331	17,157	$ 10,500
Public Service Company of New Hampshire | Restatement Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance as of January 1, 2020	$ 300
Balance as of December 31, 2020		$ 300