DERIVATIVE INSTURMENTS (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	$ (105,583,000)		$ (105,583,000)		$ (71,501,000)
Derivative Liabilities, Noncurrent	(884,283,000)		(884,283,000)		(909,668,000)
Derivative Assets Noncurrent	86,730,000		86,730,000		123,242,000
Current Derivative Assets Consolidated	15,000,000		15,000,000		17,300,000
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract
Annual Amount of MegaWatt Hours Puchased Under Contract Required By Regulation	The largest of these generation facilities achieved commercial operation in July 2011. In addition, CL&P has a contract to purchase 0.1 million MWh of energy through 2020.		The largest of these generation facilities achieved commercial operation in July 2011. In addition, CL&P has a contract to purchase 0.1 million MWh of energy through 2020.
Effect Of Fair Value Hedges On Results Of Operations [Abstract]
Notional Amount Of Fair Value Hedge Instruments	263,000,000		263,000,000
Increase Decrease In Fair Value Of Interest Rate Fair Value Hedging Instruments	900,000	3,500,000	1,300,000	7,400,000
Increase Decrease In Fair Value Of Hedged Item In Interest Rate Fair Value Hedge	(900,000)	(3,500,000)	(1,300,000)	(7,400,000)
Interest Realized In Earnings On Fair Value Hedge	2,700,000	2,500,000	5,400,000	5,300,000
Cash Flow Hedges Derivative Instruments At Fair Value Net [Abstract]
Other Comprehensive Income Derivatives Qualifying As Hedges Before Tax Period Increase Decrease	(8,700,000)		(6,800,000)
Cash Flow Hedge Gain Loss Reclassified To Interest Expense Net	(100,000)	(100,000)	(200,000)	(200,000)
Derivative Credit Risk Related Contingent Features Abstract
Derivative Net Liability Position Aggregate Fair Value	(29,700,000)		(29,700,000)		(30,900,000)
Collateral Already Posted Aggregate Fair Value	900,000		900,000		500,000
Standby Letters Of Credit Posted	6,000,000		6,000,000		24,000,000
Additional Collateral Aggregate Fair Value	25,800,000		25,800,000		18,500,000
The Connecticut Light and Power Company [Member] | Netting And Collateral [Member] | Fair Value Inputs Level 3 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	(11,100,000)		(11,100,000)
Derivative Liabilities - Current	0		0		7,700,000
Derivative Liabilities, Noncurrent	0		0
Derivative Assets Noncurrent	(77,700,000)		(77,700,000)		(80,000,000)
Netting And Collateral [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	(11,100,000)		(11,100,000)
Derivative Liabilities - Current	900,000		900,000		8,200,000
Derivative Liabilities, Noncurrent	400,000		400,000		200,000
Derivative Assets Noncurrent	(77,700,000)		(77,700,000)		(80,000,000)
Netting And Collateral [Member] | Fair Value Inputs Level 3 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	0		0
Derivative Liabilities - Current	900,000		900,000		500,000
Derivative Liabilities, Noncurrent	400,000		400,000		200,000
Derivative Assets Noncurrent	0		0
The Connecticut Light and Power Company [Member] | Commodity Contracts Required By Regulation [Member]
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract
Derivative Contract Required By Regulation MegaWatts	CL&P has capacity related contracts with generation facilities. These contracts and similar UI contracts, have an expected capacity of 787 MW. CL&P has a sharing agreement with UI, with 80 percent allocated to CL&P and 20 percent allocated to UI. The capacity contracts have terms up to 15 years and obligate the utilities to make or receive payments on a monthly basis to or from the generation facilities the difference between a set capacity price and the forward capacity market price received in the ISO-NE capacity markets.		CL&P has capacity related contracts with generation facilities. These contracts and similar UI contracts, have an expected capacity of 787 MW. CL&P has a sharing agreement with UI, with 80 percent allocated to CL&P and 20 percent allocated to UI. The capacity contracts have terms up to 15 years and obligate the utilities to make or receive payments on a monthly basis to or from the generation facilities the difference between a set capacity price and the forward capacity market price received in the ISO-NE capacity markets.
Annual Amount of MegaWatt Hours Puchased Under Contract Required By Regulation	The largest of these generation facilities achieved commercial operation in July 2011. In addition, CL&P has a contract to purchase 0.1 million MWh of energy through 2020.		The largest of these generation facilities achieved commercial operation in July 2011. In addition, CL&P has a contract to purchase 0.1 million MWh of energy through 2020.
Derivative Instruments Not Designated As Hedging Instruments Gain Loss Net Abstract
Derivative Instruments Gain Loss Recognized In Regulatory Asset Liability	(13,000,000)	(23,100,000)	(43,200,000)	(91,800,000)
The Connecticut Light and Power Company [Member] | Commodity Contracts Required By Regulation [Member] | Fair Value Inputs Level 3 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	15,200,000		15,200,000		5,800,000
Derivative Liabilities - Current	(83,300,000)		(83,300,000)		(54,300,000)
Derivative Liabilities, Noncurrent	(863,300,000)		(863,300,000)		(883,100,000)
Derivative Assets Noncurrent	160,600,000		160,600,000		195,900,000
Commodity Contracts Required By Regulation [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	15,200,000		15,200,000		5,800,000
Derivative Liabilities - Current	(83,300,000)		(83,300,000)		(54,300,000)
Derivative Liabilities, Noncurrent	(863,300,000)		(863,300,000)		(883,100,000)
Derivative Assets Noncurrent	160,600,000		160,600,000		195,900,000
Derivative Instruments Not Designated As Hedging Instruments Gain Loss Net Abstract
Derivative Instruments Gain Loss Recognized In Regulatory Asset Liability	(13,000,000)	(23,100,000)	(43,200,000)	(91,800,000)
Public Service Company of New Hampshire [Member] | Designated As Hedging Instrument [Member] | Fair Value Inputs Level 2 [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(2,400,000)		(2,400,000)
Western Massachusetts Electric Company [Member] | Designated As Hedging Instrument [Member] | Fair Value Inputs Level 2 [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(1,500,000)		(1,500,000)
Designated As Hedging Instrument [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	7,700,000		7,700,000		7,700,000
Derivative Liabilities - Current	(3,900,000)		(3,900,000)
Derivative Assets Noncurrent					4,100,000
Designated As Hedging Instrument [Member] | Fair Value Inputs Level 2 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	7,700,000		7,700,000		7,700,000
Derivative Assets Noncurrent					4,100,000
The Connecticut Light and Power Company [Member] | Commodity [Member]
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract
Description Of Volumes Remaining Under Derivative Contracts	As of June 30, 2011 and December 31, 2010, CL&P had 1 million and 1.8 million MWh, respectively, remaining under FTRs that extend through December 2011 and require monthly payments or receipts.		As of June 30, 2011 and December 31, 2010, CL&P had 1 million and 1.8 million MWh, respectively, remaining under FTRs that extend through December 2011 and require monthly payments or receipts.
Derivative Instruments Not Designated As Hedging Instruments Gain Loss Net Abstract
Derivative Instruments Gain Loss Recognized In Regulatory Asset Liability	(900,000)	(600,000)	(1,900,000)	(3,600,000)
The Connecticut Light and Power Company [Member] | Commodity [Member] | Fair Value Inputs Level 3 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	1,000,000		1,000,000		2,100,000
Derivative Liabilities - Current	(100,000)		(100,000)		(200,000)
Derivative Assets Noncurrent	0		0
Public Service Company of New Hampshire [Member] | Commodity [Member]
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract
Description Of Volumes Remaining Under Derivative Contracts	PSNH has electricity procurement contracts with delivery dates through 2011 to purchase an aggregate amount of 0.2 million and 0.4 million MWh of power as of June 30, 2011 and December 31, 2010, respectively. In addition, PSNH has 0.2 million and 0.3 million MWh remaining under FTRs as of June 30, 2011 and December 31, 2010, respectively, that extend through December 2011 and require monthly payments or receipts.		PSNH has electricity procurement contracts with delivery dates through 2011 to purchase an aggregate amount of 0.2 million and 0.4 million MWh of power as of June 30, 2011 and December 31, 2010, respectively. In addition, PSNH has 0.2 million and 0.3 million MWh remaining under FTRs as of June 30, 2011 and December 31, 2010, respectively, that extend through December 2011 and require monthly payments or receipts.
Derivative Instruments Not Designated As Hedging Instruments Gain Loss Net Abstract
Derivative Instruments Gain Loss Recognized In Regulatory Asset Liability	(800,000)	1,900,000	0	(15,700,000)
Public Service Company of New Hampshire [Member] | Commodity [Member] | Fair Value Inputs Level 2 [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(6,700,000)		(6,700,000)		(12,800,000)
Commodity [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	3,200,000		3,200,000		3,800,000
Derivative Liabilities - Current	(19,300,000)		(19,300,000)		(25,400,000)
Derivative Liabilities, Noncurrent	(21,400,000)		(21,400,000)		(26,800,000)
Derivative Assets Noncurrent	3,800,000		3,800,000		3,200,000
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract
Description of Remaining Volumes Under Unregulated Wholesale Contract	As of June 30, 2011 and December 31, 2010, NU had approximately 0.1 million and 0.3 million MWh, respectively, of supply volumes remaining in its unregulated wholesale portfolio when expected sales to an agency that is comprised of municipalities are compared with contracted supply, both of which extend through 2013.		As of June 30, 2011 and December 31, 2010, NU had approximately 0.1 million and 0.3 million MWh, respectively, of supply volumes remaining in its unregulated wholesale portfolio when expected sales to an agency that is comprised of municipalities are compared with contracted supply, both of which extend through 2013.
Derivative Instruments Not Designated As Hedging Instruments Gain Loss Net Abstract
Derivative Instruments Gain Loss Recognized In Income Net	500,000	700,000	800,000	500,000
Derivative Instruments Gain Loss Recognized In Regulatory Asset Liability	(1,700,000)	1,300,000	(2,000,000)	(19,700,000)
Commodity [Member] | Fair Value Inputs Level 3 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	2,200,000		2,200,000		1,700,000
Derivative Liabilities - Current	(12,500,000)		(12,500,000)		(12,400,000)
Derivative Liabilities, Noncurrent	(21,400,000)		(21,400,000)		(26,800,000)
Derivative Assets Noncurrent	3,800,000		3,800,000		3,200,000
Public Service Company of New Hampshire [Member] | Fair Value Inputs Level 2 [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(9,100,000)		(9,100,000)		(12,800,000)
Western Massachusetts Electric Company [Member] | Fair Value Inputs Level 2 [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(1,500,000)		(1,500,000)
Fair Value Inputs Level 2 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	7,700,000		7,700,000		7,700,000
Derivative Assets Noncurrent					4,100,000
The Connecticut Light and Power Company [Member] | Fair Value Inputs Level 3 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	5,100,000		5,100,000		7,900,000
Derivative Liabilities - Current	(83,400,000)		(83,400,000)		(46,800,000)
Derivative Liabilities, Noncurrent	(863,300,000)		(863,300,000)		(883,100,000)
Derivative Assets Noncurrent	82,900,000		82,900,000		115,900,000
Fair Value Inputs Level 3 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	2,200,000		2,200,000		1,700,000
Derivative Liabilities - Current	(11,600,000)		(11,600,000)		(11,900,000)
Derivative Liabilities, Noncurrent	(21,000,000)		(21,000,000)
Derivative Assets Noncurrent	3,800,000		3,800,000		3,200,000
The Connecticut Light and Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(83,442,000)		(83,442,000)		(46,781,000)
Derivative Liabilities, Noncurrent	(863,292,000)		(863,292,000)		(883,091,000)
Derivative Assets Noncurrent	82,902,000		82,902,000		115,870,000
Cash Flow Hedges Derivative Instruments At Fair Value Net [Abstract]
Other Comprehensive Income Derivatives Qualifying As Hedges Before Tax Period Increase Decrease	0		0
Cash Flow Hedge Gain Loss Reclassified To Interest Expense Net	(200,000)	(200,000)	(400,000)	(400,000)
Public Service Company of New Hampshire [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(9,097,000)		(9,097,000)		(12,834,000)
Cash Flow Hedges Derivative Instruments At Fair Value Net [Abstract]
Other Comprehensive Income Derivatives Qualifying As Hedges Before Tax Period Increase Decrease	(6,800,000)		(5,300,000)
Cash Flow Hedge Gain Loss Reclassified To Interest Expense Net	0	0	(100,000)	(100,000)
Description Of Cash Flow Hedge Activity			PSNH entered into two forward starting swaps to fix the U.S. dollar LIBOR swap rate of 3.73 percent on $80 million of a planned $160 million long-term debt issuance and 3.60 percent on $120 million of planned refinancing of PCRBs. On May 19, 2011, PSNH settled one of the cash flow hedges and the $2.9 million pre-tax reduction in AOCI will be amortized over the life of the debt.
Derivative Credit Risk Related Contingent Features Abstract
Derivative Net Liability Position Aggregate Fair Value	(9,100,000)		(9,100,000)		(12,800,000)
Collateral Already Posted Aggregate Fair Value	0		0		0
Standby Letters Of Credit Posted	6,000,000		6,000,000		24,000,000
Additional Collateral Aggregate Fair Value	6,100,000		6,100,000		0
Western Massachusetts Electric Company [Member]
Cash Flow Hedges Derivative Instruments At Fair Value Net [Abstract]
Other Comprehensive Income Derivatives Qualifying As Hedges Before Tax Period Increase Decrease	(1,900,000)		(1,500,000)
Cash Flow Hedge Gain Loss Reclassified To Interest Expense Net	0	0	100,000	100,000
Description Of Cash Flow Hedge Activity			WMECO entered into a forward starting swap to fix the U.S. dollar LIBOR swap rate of 3.75 percent associated with $50 million of a planned $100 million long-term debt issuance.
Derivative Credit Risk Related Contingent Features Abstract
Derivative Net Liability Position Aggregate Fair Value	(1,500,000)		(1,500,000)
Additional Collateral Aggregate Fair Value	1,500,000		1,500,000
Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities, Noncurrent					$ (26,600,000)