SCHEDULE II - VALUATION AND QUALIFYING ACCOUNTS AND RESERVES (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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
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	$ 195,708	$ 200,630	$ 190,680
Additions, Charged to Costs and Expense	61,337	44,665	69,466
Additions, Charged to Other Accounts	48,671	47,630	45,452
Deductions	92,993	97,217	104,968
Balance at end of period	212,723	195,708	200,630
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	78,872	86,391	79,479
Additions, Charged to Costs and Expense	15,831	5,312	17,572
Additions, Charged to Other Accounts	29,524	25,533	28,801
Deductions	36,193	38,364	39,461
Balance at end of period	88,034	78,872	86,391
Allowance for Doubtful Accounts | NSTAR Electric Company
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of period	69,666	70,284	66,676
Additions, Charged to Costs and Expense	22,279	21,252	31,728
Additions, Charged to Other Accounts	14,971	14,273	11,253
Deductions	32,400	36,143	39,373
Balance at end of period	74,516	69,666	70,284
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	10,481	9,941	8,733
Additions, Charged to Costs and Expense	6,383	6,917	7,288
Additions, Charged to Other Accounts	953	464	498
Deductions	6,752	6,841	6,578
Balance at end of period	$ 11,065	$ 10,481	$ 9,941