ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI Qualified Cash Flow Hedging Instruments	$ (16,400,000)	$ (18,400,000)	$ (4,200,000)
AOCI Unrealized Gains/Losses on Other Securities	1,300,000	1,100,000
AOCI Pension, SERP and PBOP Benefits	(57,800,000)	(53,400,000)
Accumulated Other Comprehensive Loss	(72,854,000)	(70,686,000)
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	(18,400,000)	(4,200,000)
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, Net of Tax		700,000
Cash Flow Transactions Entered into for the Period		(14,900,000)
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	1,971,000	(14,177,000)	200,000
Ending Balance - Accumulated Other Comprehensive Income Loss Cumulative Changes In Net Gain Loss From Cash Flow Hedges Effect Net Of Tax	(16,400,000)	(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	1,300,000	(9,500,000)	200,000
Other Comprehensive Income (Loss), Available-for-sale Securities, Tax	100,000	400,000	200,000
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Tax	(2,700,000)	(7,900,000)	0
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(1,300,000)	(17,000,000)	400,000
Cash Flow Hedge Loss To Be Reclassified Within Twelve Months	(2,000,000)
Pension Serp And Pbop Gain Loss To Be Reclassified Within Twelve Months	(10,500,000)
The Connecticut Light And Power Company [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI Qualified Cash Flow Hedging Instruments	(1,800,000)	(2,300,000)
Accumulated Other Comprehensive Loss	(1,800,000)	(2,251,000)
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	(2,300,000)
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	444,000	445,000	444,000
Ending Balance - Accumulated Other Comprehensive Income Loss Cumulative Changes In Net Gain Loss From Cash Flow Hedges Effect Net Of Tax	(1,800,000)	(2,300,000)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent [Abstract]
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Tax	300,000	300,000	300,000
Cash Flow Hedge 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	(9,700,000)	(10,900,000)	(600,000)
Accumulated Other Comprehensive Loss	(9,655,000)	(10,832,000)
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	(10,900,000)	(600,000)
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, Net of Tax		500,000
Cash Flow Transactions Entered into for the Period		(10,800,000)
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	1,162,000	(10,260,000)	87,000
Ending Balance - Accumulated Other Comprehensive Income Loss Cumulative Changes In Net Gain Loss From Cash Flow Hedges Effect Net Of Tax	(9,700,000)	(10,900,000)	(600,000)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent [Abstract]
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Tax	800,000	(7,000,000)	100,000
Cash Flow Hedge 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	(3,800,000)	(4,200,000)	(100,000)
Accumulated Other Comprehensive Loss	(3,846,000)	(4,186,000)
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)	(100,000)
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, Net of Tax		100,000
Cash Flow Transactions Entered into for the Period		(4,200,000)
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	338,000	(4,108,000)	(79,000)
Ending Balance - Accumulated Other Comprehensive Income Loss Cumulative Changes In Net Gain Loss From Cash Flow Hedges Effect Net Of Tax	(3,800,000)	(4,200,000)	(100,000)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent [Abstract]
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Tax	200,000	(2,700,000)	0
Cash Flow Hedge Loss To Be Reclassified Within Twelve Months	$ (300,000)