COMPREHENSIVE INCOME (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Net Income	$ 91,431,000	$ 101,935,000	$ 285,734,000	$ 262,884,000
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Qualified Cash Flow Hedging Instruments	(10,700,000)	100,000	(14,600,000)	100,000
Other Comprehensive Income Available For Sale Securities Adjustment Net Of Tax Period Increase Decrease	300,000	100,000	500,000	800,000
Change in Funded Status of Pension, SERP and Other Postretirement Benefit Plans	1,000,000	600,000	2,300,000	1,600,000
Other Comprehensive Income/ (Loss)	(9,400,000)	800,000	(11,800,000)	2,500,000
Total Comprehensive Income	82,000,000	102,700,000	273,900,000	265,400,000
Comprehensive Income Attributable to Noncontrolling Interests	(1,500,000)	(1,400,000)	(4,300,000)	(4,200,000)
Comprehensive Income	80,500,000	101,300,000	269,600,000	261,200,000
Other Comprehensive Income Derivatives Qualifying As Hedges Net Of Tax Period Increase Decrease Abstract
Beginning Balance - Accumulated Other Comprehensive Income Loss Cumulative Changes In Net Gain Loss From Cash Flow Hedges Effect Net Of Tax			(4,200,000)
Hedged Transactions Recognized into Earnings			300,000
Cash Flow Transactions Entered into for the Period			(14,900,000)
Qualified Cash Flow Hedging Instruments	(10,700,000)	100,000	(14,600,000)	100,000
Ending Balance - Accumulated Other Comprehensive Income Loss Cumulative Changes In Net Gain Loss From Cash Flow Hedges Effect Net Of Tax	(18,800,000)		(18,800,000)
The Connecticut Light and Power Company [Member]
Net Income	66,528,000	69,046,000	183,500,000	161,543,000
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Qualified Cash Flow Hedging Instruments	100,000	100,000	300,000	400,000
Other Comprehensive Income/ (Loss)	100,000	100,000	300,000	400,000
Comprehensive Income	66,600,000	69,100,000	183,800,000	161,900,000
Other Comprehensive Income Derivatives Qualifying As Hedges Net Of Tax Period Increase Decrease Abstract
Qualified Cash Flow Hedging Instruments	100,000	100,000	300,000	400,000
Public Service Company of New Hampshire [Member]
Net Income	25,634,000	28,754,000	74,763,000	66,180,000
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Qualified Cash Flow Hedging Instruments	(7,500,000)		(10,600,000)	100,000
Other Comprehensive Income/ (Loss)	(7,500,000)		(10,600,000)	100,000
Comprehensive Income	18,100,000	28,800,000	64,200,000	66,300,000
Other Comprehensive Income Derivatives Qualifying As Hedges Net Of Tax Period Increase Decrease Abstract
Beginning Balance - Accumulated Other Comprehensive Income Loss Cumulative Changes In Net Gain Loss From Cash Flow Hedges Effect Net Of Tax			(600,000)
Hedged Transactions Recognized into Earnings			200,000
Cash Flow Transactions Entered into for the Period			(10,800,000)
Qualified Cash Flow Hedging Instruments	(7,500,000)		(10,600,000)	100,000
Ending Balance - Accumulated Other Comprehensive Income Loss Cumulative Changes In Net Gain Loss From Cash Flow Hedges Effect Net Of Tax	(11,200,000)		(11,200,000)
Western Massachusetts Electric Company [Member]
Net Income	8,434,000	7,337,000	26,571,000	18,245,000
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Qualified Cash Flow Hedging Instruments	(3,200,000)		(4,200,000)	(100,000)
Other Comprehensive Income/ (Loss)	(3,200,000)		(4,200,000)	(100,000)
Comprehensive Income	5,200,000	7,300,000	22,400,000	18,100,000
Other Comprehensive Income Derivatives Qualifying As Hedges Net Of Tax Period Increase Decrease Abstract
Beginning Balance - Accumulated Other Comprehensive Income Loss Cumulative Changes In Net Gain Loss From Cash Flow Hedges Effect Net Of Tax			(100,000)
Hedged Transactions Recognized into Earnings			0
Cash Flow Transactions Entered into for the Period			(4,200,000)
Qualified Cash Flow Hedging Instruments	(3,200,000)		(4,200,000)	(100,000)
Ending Balance - Accumulated Other Comprehensive Income Loss Cumulative Changes In Net Gain Loss From Cash Flow Hedges Effect Net Of Tax	$ (4,300,000)		$ (4,300,000)