SCHEDULE II - VALUATION AND QUALIFYING ACCOUNTS AND RESERVES (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
CL&P
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Period of accounts receivable recoverable under financial or medical duress		180 days
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		$ 212,723	$ 195,708	$ 200,630
Additions, Charged to Costs and Expense		63,446	61,337	44,665
Additions, Charged to Other Accounts	[1]	57,223	48,671	47,630
Deductions	[2]	108,571	92,993	97,217
Balance at end of period		224,821	212,723	195,708
Allowance for Doubtful Accounts | CL&P
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of period		88,034	78,872	86,391
Additions, Charged to Costs and Expense		15,947	15,831	5,312
Additions, Charged to Other Accounts	[1]	38,935	29,524	25,533
Deductions	[2]	45,568	36,193	38,364
Balance at end of period		97,348	88,034	78,872
Allowance for Doubtful Accounts | NSTAR Electric
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of period		74,516	69,666	70,284
Additions, Charged to Costs and Expense		25,079	22,279	21,252
Additions, Charged to Other Accounts	[1]	12,556	14,971	14,273
Deductions	[2]	36,745	32,400	36,143
Balance at end of period		75,406	74,516	69,666
Allowance for Doubtful Accounts | PSNH
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of period		11,065	10,481	9,941
Additions, Charged to Costs and Expense		6,726	6,383	6,917
Additions, Charged to Other Accounts	[1]	872	953	464
Deductions	[2]	8,166	6,752	6,841
Balance at end of period		$ 10,497	$ 11,065	$ 10,481

[1] Amounts relate to uncollectible accounts receivables reserved for that are not charged to bad debt expense.  The PURA allows CL&P and Yankee Gas to accelerate the recovery of accounts receivable balances attributable to qualified customers under financial or medical duress (uncollectible hardship accounts receivable) outstanding for greater than 180 days and 90 days, respectively.  The DPU allows NSTAR Electric and NSTAR Gas to recover in rates, amounts associated with certain uncollectible hardship accounts receivable. 

[2] Amounts written off, net of recoveries.