SCHEDULE II - VALUATION AND QUALIFYING ACCOUNTS AND RESERVES (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation Allowances and Reserves, Beginning Balance	$ 171,251	$ 165,549	$ 115,689
Valuation Allowances and Reserves, Charged to Cost and Expense	55,657	55,465	36,275
Valuation Allowances and Reserves, Charged to Other Accounts	51,227	37,744	34,761
Valuation Allowances and Reserves, Deductions	102,818	87,507	80,462
Valuation Allowances and Reserves, Ending Balance	175,317	171,251	165,549
Uncollectible Hardship Accounts Receivable Reserves
Impact Related To Nstar Merger	0	0	59,286
The Connecticut Light And Power Company [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation Allowances and Reserves, Beginning Balance	81,995	77,571	83,475
Valuation Allowances and Reserves, Charged to Cost and Expense	6,598	3,947	2,080
Valuation Allowances and Reserves, Charged to Other Accounts	39,706	27,258	27,084
Valuation Allowances and Reserves, Deductions	44,012	26,781	35,068
Valuation Allowances and Reserves, Ending Balance	84,287	81,995	77,571
NSTAR Electric Company [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation Allowances and Reserves, Beginning Balance	41,679	44,115	27,118
Valuation Allowances and Reserves, Charged to Cost and Expense	24,740	28,108	40,301
Valuation Allowances and Reserves, Charged to Other Accounts	627	0	0
Valuation Allowances and Reserves, Deductions	26,376	30,544	23,304
Valuation Allowances and Reserves, Ending Balance	40,670	41,679	44,115
Public Service Company Of New Hampshire [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation Allowances and Reserves, Beginning Balance	7,364	6,760	7,190
Valuation Allowances and Reserves, Charged to Cost and Expense	6,815	6,608	6,457
Valuation Allowances and Reserves, Charged to Other Accounts	797	779	2,481
Valuation Allowances and Reserves, Deductions	7,313	6,783	9,368
Valuation Allowances and Reserves, Ending Balance	7,663	7,364	6,760
Western Massachusetts Electric Company [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation Allowances and Reserves, Beginning Balance	9,984	8,501	10,018
Valuation Allowances and Reserves, Charged to Cost and Expense	2,415	2,580	2,294
Valuation Allowances and Reserves, Charged to Other Accounts	3,608	4,299	2,428
Valuation Allowances and Reserves, Deductions	6,127	5,396	6,239
Valuation Allowances and Reserves, Ending Balance	9,880	9,984	8,501
Uncollectible Hardship Accounts Receivable Reserves
Yankee Gas Services Company [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Uncollectible Hardship Accounts Receivable Reserves