SCHEDULE II - VALUATION AND QUALIFYING ACCOUNTS AND RESERVES (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation Allowances and Reserves, Beginning Balance	$ 175,317	$ 171,251	$ 165,549
Valuation Allowances and Reserves, Charged to Cost and Expense	51,077	55,657	55,465
Valuation Allowances and Reserves, Charged to Other Accounts	79,622	51,227	37,744
Valuation Allowances and Reserves, Deductions	115,336	102,818	87,507
Valuation Allowances and Reserves, Ending Balance	190,680	175,317	$ 171,251
Uncollectible Hardship Accounts Receivable Reserves
The Connecticut Light And Power Company [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation Allowances and Reserves, Beginning Balance	84,287	81,995	$ 77,571
Valuation Allowances and Reserves, Charged to Cost and Expense	10,105	6,598	3,947
Valuation Allowances and Reserves, Charged to Other Accounts	30,592	39,706	27,258
Valuation Allowances and Reserves, Deductions	45,505	44,012	26,781
Valuation Allowances and Reserves, Ending Balance	79,479	84,287	81,995
NSTAR Electric Company [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation Allowances and Reserves, Beginning Balance	40,670	41,679	44,115
Valuation Allowances and Reserves, Charged to Cost and Expense	14,228	24,740	28,108
Valuation Allowances and Reserves, Charged to Other Accounts	29,559	627	0
Valuation Allowances and Reserves, Deductions	31,829	26,376	30,544
Valuation Allowances and Reserves, Ending Balance	52,628	40,670	41,679
Public Service Company Of New Hampshire [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation Allowances and Reserves, Beginning Balance	7,663	7,364	6,760
Valuation Allowances and Reserves, Charged to Cost and Expense	8,889	6,815	6,608
Valuation Allowances and Reserves, Charged to Other Accounts	841	797	779
Valuation Allowances and Reserves, Deductions	8,660	7,313	6,783
Valuation Allowances and Reserves, Ending Balance	8,733	7,663	7,364
Western Massachusetts Electric Company [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation Allowances and Reserves, Beginning Balance	9,880	9,984	8,501
Valuation Allowances and Reserves, Charged to Cost and Expense	4,940	2,415	2,580
Valuation Allowances and Reserves, Charged to Other Accounts	7,418	3,608	4,299
Valuation Allowances and Reserves, Deductions	8,190	6,127	5,396
Valuation Allowances and Reserves, Ending Balance	$ 14,048	$ 9,880	$ 9,984
Uncollectible Hardship Accounts Receivable Reserves
Yankee Gas Services Company [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Uncollectible Hardship Accounts Receivable Reserves