COMPREHENSIVE INCOME (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Net Income	$ 78,719,000	$ 73,348,000	$ 194,303,000	$ 160,948,000
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Qualified Cash Flow Hedging Instruments	(5,100,000)	0	(3,900,000)	100,000
Other Comprehensive Income Available For Sale Securities Adjustment Net Of Tax Period Increase Decrease	100,000	300,000	100,000	600,000
Change in Funded Status of Pension, SERP and Other Postretirement Benefit Plans	400,000	600,000	1,400,000	1,000,000
Other Comprehensive Income/ (Loss)	(4,600,000)	900,000	(2,400,000)	1,700,000
Total Comprehensive Income	74,100,000	74,200,000	191,900,000	162,600,000
Comprehensive Income Attributable to Noncontrolling Interests	(1,400,000)	(1,400,000)	(2,900,000)	(2,800,000)
Comprehensive Income	72,700,000	72,800,000	189,000,000	159,800,000
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			100,000
Change In Fair Value of Interest Rate Swap Agreements			(5,100,000)
Cash Flow Transactions Entered into for the Period			1,100,000
Qualified Cash Flow Hedging Instruments	(5,100,000)	0	(3,900,000)	100,000
Ending Balance - Accumulated Other Comprehensive Income Loss Cumulative Changes In Net Gain Loss From Cash Flow Hedges Effect Net Of Tax	(8,100,000)		(8,100,000)
The Connecticut Light and Power Company [Member]
Net Income	52,635,000	44,085,000	116,972,000	92,498,000
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Qualified Cash Flow Hedging Instruments	100,000	100,000	200,000	200,000
Other Comprehensive Income/ (Loss)	100,000	100,000	200,000	200,000
Comprehensive Income	52,700,000	44,200,000	117,200,000	92,700,000
Other Comprehensive Income Derivatives Qualifying As Hedges Net Of Tax Period Increase Decrease Abstract
Qualified Cash Flow Hedging Instruments	100,000	100,000	200,000	200,000
Public Service Company of New Hampshire [Member]
Net Income	21,665,000	21,616,000	49,129,000	37,426,000
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Qualified Cash Flow Hedging Instruments	(4,000,000)		(3,100,000)	100,000
Other Comprehensive Income/ (Loss)	(4,000,000)		(3,100,000)	100,000
Comprehensive Income	17,700,000	21,600,000	46,000,000	37,500,000
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			0
Change In Fair Value of Interest Rate Swap Agreements			(4,000,000)
Cash Flow Transactions Entered into for the Period			900,000
Qualified Cash Flow Hedging Instruments	(4,000,000)		(3,100,000)	100,000
Ending Balance - Accumulated Other Comprehensive Income Loss Cumulative Changes In Net Gain Loss From Cash Flow Hedges Effect Net Of Tax	(3,700,000)		(3,700,000)
Western Massachusetts Electric Company [Member]
Net Income	8,171,000	5,249,000	18,137,000	10,908,000
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Qualified Cash Flow Hedging Instruments	(1,100,000)		(900,000)	0
Other Comprehensive Income/ (Loss)	(1,100,000)		(900,000)	0
Comprehensive Income	7,100,000	5,200,000	17,200,000	10,900,000
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
Change In Fair Value of Interest Rate Swap Agreements			(1,100,000)
Cash Flow Transactions Entered into for the Period			200,000
Qualified Cash Flow Hedging Instruments	(1,100,000)		(900,000)	0
Ending Balance - Accumulated Other Comprehensive Income Loss Cumulative Changes In Net Gain Loss From Cash Flow Hedges Effect Net Of Tax	$ (1,000,000)		$ (1,000,000)