SCHEDULE II - VALUATION AND QUALIFYING ACCOUNTS AND RESERVES (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Allowance for Doubtful Accounts [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of period	$ 190,680	$ 175,317	$ 171,251
Additions, Charged to Costs and Expense	69,466	51,077	55,657
Additions, Charged to Other Accounts	45,452	79,622	51,227
Deductions	104,968	115,336	102,818
Balance at end of period	200,630	190,680	175,317
Allowance for Doubtful Accounts [Member] | The Connecticut Light And Power Company [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of period	79,479	84,287	81,995
Additions, Charged to Costs and Expense	17,572	10,105	6,598
Additions, Charged to Other Accounts	28,801	30,592	39,706
Deductions	39,461	45,505	44,012
Balance at end of period	86,391	79,479	84,287
Allowance for Doubtful Accounts [Member] | NSTAR Electric Company [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of period	52,628	40,670	41,679
Additions, Charged to Costs and Expense	27,978	14,228	24,740
Additions, Charged to Other Accounts	4,050	29,559	627
Deductions	29,884	31,829	26,376
Balance at end of period	54,772	52,628	40,670
Allowance for Doubtful Accounts [Member] | Public Service Company Of New Hampshire [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of period	8,733	7,663	7,364
Additions, Charged to Costs and Expense	7,288	8,889	6,815
Additions, Charged to Other Accounts	498	841	797
Deductions	6,578	8,660	7,313
Balance at end of period	9,941	8,733	7,663
Allowance for Doubtful Accounts [Member] | Western Massachusetts Electric Company [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of period	14,048	9,880	9,984
Additions, Charged to Costs and Expense	3,750	4,940	2,415
Additions, Charged to Other Accounts	7,203	7,418	3,608
Deductions	9,489	8,190	6,127
Balance at end of period	$ 15,512	$ 14,048	$ 9,880
Maximum [Member] | CL&P and Yankee Gas [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Period of uncollectible hardship accounts receivable Reserves	180 days
Minimum [Member] | CL&P and Yankee Gas [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Period of uncollectible hardship accounts receivable Reserves	90 days