SCHEDULE II - VALUATION AND QUALIFYING ACCOUNTS AND RESERVES (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Increase due to CMA asset acquisition	$ 24,200
Allowance for uncollectible accounts for late fees and other receivable amounts	$ 358,851	$ 224,821
Restatement Adjustment
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Allowance for uncollectible accounts for late fees and other receivable amounts		23,800
The Connecticut Light and Power Company
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Period of accounts receivable recoverable under financial or medical duress	180 days
Increase due to CMA asset acquisition	$ 0
Allowance for uncollectible accounts for late fees and other receivable amounts	157,447	97,348
The Connecticut Light and Power Company | Restatement Adjustment
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Allowance for uncollectible accounts for late fees and other receivable amounts		22,200
NSTAR Electric Company
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Increase due to CMA asset acquisition	0
Allowance for uncollectible accounts for late fees and other receivable amounts	91,583	75,406
NSTAR Electric Company | Restatement Adjustment
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Allowance for uncollectible accounts for late fees and other receivable amounts		(1,300)
Public Service Company of New Hampshire
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Increase due to CMA asset acquisition	0
Allowance for uncollectible accounts for late fees and other receivable amounts	$ 17,157	10,497
Public Service Company of New Hampshire | Restatement Adjustment
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Allowance for uncollectible accounts for late fees and other receivable amounts		300
Yankee Gas Services Company
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Period of accounts receivable recoverable under financial or medical duress	90 days
Allowance for Doubtful Accounts
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of period	$ 224,821	212,723	$ 195,708
Additions, Charged to Costs and Expense	53,461	63,446	61,337
Additions, Charged to Other Accounts	145,005	57,223	48,671
Deductions	64,436	108,571	92,993
Balance at end of period	358,851	224,821	212,723
Allowance for Doubtful Accounts | The Connecticut Light and Power Company
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of period	97,348	88,034	78,872
Additions, Charged to Costs and Expense	12,882	15,947	15,831
Additions, Charged to Other Accounts	71,223	38,935	29,524
Deductions	24,006	45,568	36,193
Balance at end of period	157,447	97,348	88,034
Allowance for Doubtful Accounts | NSTAR Electric Company
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of period	75,406	74,516	69,666
Additions, Charged to Costs and Expense	15,293	25,079	22,279
Additions, Charged to Other Accounts	23,424	12,556	14,971
Deductions	22,540	36,745	32,400
Balance at end of period	91,583	75,406	74,516
Allowance for Doubtful Accounts | Public Service Company of New Hampshire
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of period	10,497	11,065	10,481
Additions, Charged to Costs and Expense	5,164	6,726	6,383
Additions, Charged to Other Accounts	7,692	872	953
Deductions	6,196	8,166	6,752
Balance at end of period	$ 17,157	$ 10,497	$ 11,065