SCHEDULE II - VALUATION AND QUALIFYING ACCOUNTS AND RESERVES (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation Allowances and Reserves, Beginning Balance	$ 115,689,000	$ 119,190,000	$ 127,517,000
Valuation Allowances and Reserves, Charged to Cost and Expense	36,275,000	16,420,000	31,352,000
Valuation Allowances and Reserves, Charged to Other Accounts	34,761,000	40,663,000	45,953,000
Valuation Allowances and Reserves, Deductions	80,462,000	60,584,000	85,632,000
Valuation Allowances and Reserves, Ending Balance	165,549,000	115,689,000	119,190,000
Uncollectible Hardship Accounts Receivable Reserves		68,600,000	65,000,000
Impact Related To Nstar Merger	59,286,000
The Connecticut Light And Power Company [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation Allowances and Reserves, Beginning Balance	83,475,000	82,173,000	80,586,000
Valuation Allowances and Reserves, Charged to Cost and Expense	2,080,000	3,215,000	7,484,000
Valuation Allowances and Reserves, Charged to Other Accounts	27,084,000	33,911,000	41,639,000
Valuation Allowances and Reserves, Deductions	35,068,000	35,824,000	47,536,000
Valuation Allowances and Reserves, Ending Balance	77,571,000	83,475,000	82,173,000
Public Service Company Of New Hampshire [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation Allowances and Reserves, Beginning Balance	7,190,000	6,824,000	5,086,000
Valuation Allowances and Reserves, Charged to Cost and Expense	6,457,000	7,035,000	8,858,000
Valuation Allowances and Reserves, Charged to Other Accounts	2,481,000	1,334,000	1,017,000
Valuation Allowances and Reserves, Deductions	9,368,000	8,003,000	8,137,000
Valuation Allowances and Reserves, Ending Balance	6,760,000	7,190,000	6,824,000
Western Massachusetts Electric Company [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation Allowances and Reserves, Beginning Balance	10,018,000	12,891,000	16,304,000
Valuation Allowances and Reserves, Charged to Cost and Expense	2,294,000	3,133,000	9,747,000
Valuation Allowances and Reserves, Charged to Other Accounts	2,428,000	1,141,000	243,000
Valuation Allowances and Reserves, Deductions	6,239,000	7,147,000	13,403,000
Valuation Allowances and Reserves, Ending Balance	8,501,000	10,018,000	12,891,000
Uncollectible Hardship Accounts Receivable Reserves		5,400,000	6,900,000
Yankee Gas Services Company [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Uncollectible Hardship Accounts Receivable Reserves		6,800,000	7,500,000
NSTAR Electric Company [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation Allowances and Reserves, Beginning Balance	27,118,000	29,033,000	26,379,000
Valuation Allowances and Reserves, Charged to Cost and Expense	40,301,000	22,582,000	29,417,000
Valuation Allowances and Reserves, Charged to Other Accounts	0	0	0
Valuation Allowances and Reserves, Deductions	23,304,000	24,497,000	26,763,000
Valuation Allowances and Reserves, Ending Balance	$ 44,115,000	$ 27,118,000	$ 29,033,000