SCHEDULE II - VALUATION AND QUALIFYING ACCOUNTS AND RESERVES (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation Allowances and Reserves, Beginning Balance	$ 39,797,000	$ 55,300,000	$ 43,275,000
Valuation Allowances and Reserves, Charged to Cost and Expense	16,420,000	31,352,000	53,947,000
Valuation Allowances and Reserves, Charged to Other Accounts	8,664,000	10,714,000	24,136,000
Valuation Allowances and Reserves, Deductions	30,020,000	57,569,000	66,058,000
Valuation Allowances and Reserves, Ending Balance	34,861,000	39,797,000	55,300,000
The Connecticut Light And Power Company [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation Allowances and Reserves, Beginning Balance	17,174,000	26,057,000	23,956,000
Valuation Allowances and Reserves, Charged to Cost and Expense	3,215,000	7,484,000	15,276,000
Valuation Allowances and Reserves, Charged to Other Accounts	5,659,000	9,919,000	20,115,000
Valuation Allowances and Reserves, Deductions	11,209,000	26,286,000	33,290,000
Valuation Allowances and Reserves, Ending Balance	14,839,000	17,174,000	26,057,000
Uncollectible Hardship Accounts Receivable Reserves	68,600,000	65,000,000	54,500,000
Public Service Company Of New Hampshire [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation Allowances and Reserves, Beginning Balance	6,824,000	5,086,000	4,165,000
Valuation Allowances and Reserves, Charged to Cost and Expense	7,035,000	8,858,000	10,084,000
Valuation Allowances and Reserves, Charged to Other Accounts	1,334,000	1,017,000	652,000
Valuation Allowances and Reserves, Deductions	8,003,000	8,137,000	9,815,000
Valuation Allowances and Reserves, Ending Balance	7,190,000	6,824,000	5,086,000
Western Massachusetts Electric Company [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation Allowances and Reserves, Beginning Balance	5,975,000	7,217,000	6,571,000
Valuation Allowances and Reserves, Charged to Cost and Expense	3,133,000	9,747,000	7,590,000
Valuation Allowances and Reserves, Charged to Other Accounts	1,038,000	243,000	103,000
Valuation Allowances and Reserves, Deductions	5,555,000	11,232,000	7,047,000
Valuation Allowances and Reserves, Ending Balance	4,591,000	5,975,000	7,217,000
Uncollectible Hardship Accounts Receivable Reserves	5,400,000	6,900,000	9,100,000
Yankee Gas Services Company [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Uncollectible Hardship Accounts Receivable Reserves	$ 6,800,000	$ 7,500,000	$ 8,600,000