SCHEDULE II - VALUATION AND QUALIFYING ACCOUNTS AND RESERVES (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Doubtful Accounts
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of period	$ 200,630	$ 190,680	$ 175,317
Additions, Charged to Costs and Expense	44,665	69,466	51,077
Additions, Charged to Other Accounts	47,630	45,452	79,622
Deductions	97,217	104,968	115,336
Balance at end of period	195,708	200,630	190,680
Allowance for Doubtful Accounts | The Connecticut Light And Power Company
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of period	86,391	79,479	84,287
Additions, Charged to Costs and Expense	5,312	17,572	10,105
Additions, Charged to Other Accounts	25,533	28,801	30,592
Deductions	38,364	39,461	45,505
Balance at end of period	78,872	86,391	79,479
Allowance for Doubtful Accounts | NSTAR Electric Company
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of period	70,284	66,676	50,550
Additions, Charged to Costs and Expense	21,252	31,728	19,168
Additions, Charged to Other Accounts	14,273	11,253	36,977
Deductions	36,143	39,373	40,019
Balance at end of period	69,666	70,284	66,676
Allowance for Doubtful Accounts | Public Service Company Of New Hampshire
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of period	9,941	8,733	7,663
Additions, Charged to Costs and Expense	6,917	7,288	8,889
Additions, Charged to Other Accounts	464	498	841
Deductions	6,841	6,578	8,660
Balance at end of period	$ 10,481	$ 9,941	$ 8,733
Maximum | CL&P and Yankee Gas
Movement in Valuation Allowances and Reserves [Roll Forward]
Period of accounts receivable recoverable under financial or medical duress	180 days
Minimum | CL&P and Yankee Gas
Movement in Valuation Allowances and Reserves [Roll Forward]
Period of accounts receivable recoverable under financial or medical duress	90 days