COMPREHENSIVE INCOME (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI Qualified Cash Flow Hedging Instruments	$ (18,400,000)	$ (4,200,000)
AOCI Unrealized Gains/Losses on Other Securities	1,100,000	600,000
AOCI Pension, SERP and PBOP Benefits	(53,400,000)	(39,800,000)
Accumulated Other Comprehensive Loss	(70,686,000)	(43,370,000)
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	700,000
Cash Flow Transactions Entered into for the Period	(14,900,000)
Qualified Cash Flow Hedging Instruments	(14,177,000)	200,000	200,000
Ending Balance - Accumulated Other Comprehensive Income Loss Cumulative Changes In Net Gain Loss From Cash Flow Hedges Effect Net Of Tax	(18,400,000)	(4,200,000)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent [Abstract]
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Tax Effect	9,500,000	(200,000)	(200,000)
Other Comprehensive Income (Loss), Available-for-sale Securities, Tax	(400,000)	(200,000)	700,000
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Tax	7,900,000	0	2,900,000
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	17,000,000	(400,000)	3,400,000
Cash Flow Hedge Gain Loss To Be Reclassified Within Twelve Months	(2,200,000)
Pension Serp And Pbop Gain Loss To Be Reclassified Within Twelve Months	(8,700,000)
The Connecticut Light And Power Company [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI Qualified Cash Flow Hedging Instruments	(2,300,000)	(2,700,000)
AOCI Unrealized Gains/Losses on Other Securities	0
Accumulated Other Comprehensive Loss	(2,251,000)	(2,713,000)
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	(2,700,000)
Qualified Cash Flow Hedging Instruments	445,000	444,000	445,000
Ending Balance - Accumulated Other Comprehensive Income Loss Cumulative Changes In Net Gain Loss From Cash Flow Hedges Effect Net Of Tax	(2,300,000)	(2,700,000)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent [Abstract]
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Tax Effect	(300,000)	(300,000)	(300,000)
Cash Flow Hedge Gain Loss To Be Reclassified Within Twelve Months	(400,000)
Public Service Company Of New Hampshire [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI Qualified Cash Flow Hedging Instruments	(10,900,000)	(600,000)
AOCI Unrealized Gains/Losses on Other Securities	100,000
Accumulated Other Comprehensive Loss	(10,832,000)	(601,000)
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	500,000
Cash Flow Transactions Entered into for the Period	(10,800,000)
Qualified Cash Flow Hedging Instruments	(10,260,000)	87,000	87,000
Ending Balance - Accumulated Other Comprehensive Income Loss Cumulative Changes In Net Gain Loss From Cash Flow Hedges Effect Net Of Tax	(10,900,000)	(600,000)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent [Abstract]
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Tax Effect	7,000,000	(100,000)
Cash Flow Hedge Gain Loss To Be Reclassified Within Twelve Months	(1,200,000)
Western Massachusetts Electric Company [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI Qualified Cash Flow Hedging Instruments	(4,200,000)	(100,000)
AOCI Unrealized Gains/Losses on Other Securities	0
Accumulated Other Comprehensive Loss	(4,186,000)	(83,000)
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	100,000
Cash Flow Transactions Entered into for the Period	(4,200,000)
Qualified Cash Flow Hedging Instruments	(4,108,000)	(79,000)	(79,000)
Ending Balance - Accumulated Other Comprehensive Income Loss Cumulative Changes In Net Gain Loss From Cash Flow Hedges Effect Net Of Tax	(4,200,000)	(100,000)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent [Abstract]
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Tax Effect	2,700,000		100,000
Cash Flow Hedge Gain Loss To Be Reclassified Within Twelve Months	$ (300,000)