DERIVATIVE INSTURMENTS (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	$ (107,558,000)	$ (71,501,000)
Derivative Liabilities, Noncurrent	(959,876,000)	(909,668,000)
Derivative Assets Noncurrent	98,357,000	123,242,000
Current Derivative Assets Consolidated	13,300,000	17,300,000
Derivative Assets Subject to Credit Risk with investment grade counterparty	102,000,000
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract
Description of Derivative Activity Volume	As of December 31, 2011 and 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 are compared with contracted supply, both of which extend through 2013.
Effect Of Fair Value Hedges On Results Of Operations [Abstract]
Notional Amount Of Fair Value Hedge Instruments	263,000,000
Increase Decrease In Fair Value Of Interest Rate Fair Value Hedging Instruments	1,000,000	9,500,000	1,600,000
Increase Decrease In Fair Value Of Hedged Item In Interest Rate Fair Value Hedge	(1,000,000)	(9,500,000)	(1,600,000)
Interest Realized In Earnings On Fair Value Hedge	10,500,000	10,900,000	9,100,000
Cash Flow Hedges Derivative Instruments At Fair Value Net [Abstract]
Other Comprehensive Income Derivatives Qualifying As Hedges Before Tax Period Increase Decrease	(25,100,000)
Cash Flow Hedge Gain Loss Reclassified To Interest Expense Net	(1,300,000)	(400,000)	(400,000)
Cash Flow Hedge Gain Loss To Be Reclassified Within Twelve Months	(2,200,000)
Derivative Credit Risk Related Contingent Features Abstract
Derivative Net Liability Position Aggregate Fair Value	(23,500,000)	(30,900,000)
Collateral Already Posted Aggregate Fair Value	4,100,000	500,000
Standby Letters Of Credit Posted	0	24,000,000
Additional Collateral Aggregate Fair Value	19,900,000	18,500,000
The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(95,881,000)	(46,781,000)
Derivative Liabilities, Noncurrent	(935,849,000)	(883,091,000)
Derivative Assets Noncurrent	93,755,000	115,870,000
Derivative Assets Subject to Credit Risk with investment grade counterparty	99,700,000
Cash Flow Hedges Derivative Instruments At Fair Value Net [Abstract]
Other Comprehensive Income Derivatives Qualifying As Hedges Before Tax Period Increase Decrease	0
Cash Flow Hedge Gain Loss Reclassified To Interest Expense Net	(700,000)	(700,000)	(700,000)
Cash Flow Hedge Gain Loss To Be Reclassified Within Twelve Months	(400,000)
Public Service Company Of New Hampshire [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	0	(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	(18,200,000)
Cash Flow Hedge Gain Loss Reclassified To Interest Expense Net	(800,000)	(200,000)	(200,000)
Cash Flow Hedge Gain Loss To Be Reclassified Within Twelve Months	(1,200,000)
Description Of Cash Flow Hedge Activity	PSNH entered into three forward starting swaps to fix the U.S. dollar LIBOR swap rate of 3.749 percent on $80 million of a planned $160 million long-term debt issuance, 2.804 percent on the remaining $80 million of the planned $160 million long-term debt issuance and 3.6 percent on $120 million of long-term debt to be issued to refinance outstanding PCRBs. In May 2011, PSNH settled the swap associated with the $120 million refinancing of PCRBs and a $2.9 million pre-tax reduction in AOCI is being amortized over the life of the debt. In September 2011, PSNH settled the two remaining swaps associated with the $160 million long-term debt issuance and a $15.3 million pre-tax reduction in AOCI is being amortized over the life of the debt.
Derivative Credit Risk Related Contingent Features Abstract
Derivative Net Liability Position Aggregate Fair Value		(12,800,000)
Collateral Already Posted Aggregate Fair Value		0
Standby Letters Of Credit Posted		24,000,000
Additional Collateral Aggregate Fair Value		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	(6,900,000)
Cash Flow Hedge Gain Loss Reclassified To Interest Expense Net	(100,000)	100,000	100,000
Cash Flow Hedge Gain Loss To Be Reclassified Within Twelve Months	(300,000)
Description Of Cash Flow Hedge Activity	WMECO entered into a forward starting swap to fix the U.S. dollar LIBOR swap rate of 3.7624 percent associated with $50 million of a planned $100 million long-term debt issuance. In September 2011, WMECO settled the swap and a $6.9 million pre-tax reduction in AOCI is being amortized over the life of the debt.
Fair Value Inputs Level 2 [Member] | Public Service Company Of New Hampshire [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current		(12,800,000)
Fair Value Inputs Level 2 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	2,300,000	7,700,000
Derivative Assets Noncurrent		4,100,000
Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	6,300,000	7,900,000
Derivative Liabilities - Current	(95,900,000)	(46,800,000)
Derivative Liabilities, Noncurrent	(935,800,000)	(883,100,000)
Derivative Assets Noncurrent	93,800,000	115,900,000
Fair Value Inputs Level 3 [Member] | Western Massachusetts Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(100,000)
Derivative Liabilities, Noncurrent	(7,200,000)
Fair Value Inputs Level 3 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	4,700,000	1,700,000
Derivative Liabilities - Current	(11,600,000)	(11,900,000)
Derivative Liabilities, Noncurrent	(16,900,000)	(26,600,000)
Derivative Assets Noncurrent	4,600,000	3,200,000
Netting And Collateral [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	(11,600,000)
Derivative Liabilities - Current	4,500,000	8,200,000
Derivative Liabilities, Noncurrent	400,000	200,000
Derivative Assets Noncurrent	(80,400,000)	(80,000,000)
Netting And Collateral [Member] | Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	(11,600,000)
Derivative Liabilities - Current	0	7,700,000
Derivative Liabilities, Noncurrent	0
Derivative Assets Noncurrent	(80,400,000)	(80,000,000)
Netting And Collateral [Member] | Fair Value Inputs Level 3 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	0
Derivative Liabilities - Current	4,500,000	500,000
Derivative Liabilities, Noncurrent	400,000	200,000
Derivative Assets Noncurrent	0
Commodity Contracts Required By Regulation [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	17,500,000	5,800,000
Derivative Liabilities - Current	(96,000,000)	(54,300,000)
Derivative Liabilities, Noncurrent	(943,000,000)	(883,100,000)
Derivative Assets Noncurrent	174,200,000	195,900,000
Derivative Instruments Not Designated As Hedging Instruments Gain Loss Net Abstract
Derivative Instruments Gain Loss Recognized In Regulatory Asset Liability	(158,100,000)	(74,000,000)	(99,900,000)
Commodity Contracts Required By Regulation [Member] | The Connecticut Light And Power Company [Member]
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract
Description of Derivative Activity Volume	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 of each contract 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 based on the difference between a set capacity price and the forward capacity market price received in the ISO-NE capacity markets. 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 per year through 2020.
Derivative Instruments Not Designated As Hedging Instruments Gain Loss Net Abstract
Derivative Instruments Gain Loss Recognized In Regulatory Asset Liability	(150,800,000)	(74,000,000)	(99,900,000)
Commodity Contracts Required By Regulation [Member] | Western Massachusetts Electric Company [Member]
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract
Description of Derivative Activity Volume	WMECO has a renewable energy contract to purchase 0.1 million MWh of energy per year through 2027 with a facility that is expected to achieve commercial operation by December 2012.
Derivative Instruments Not Designated As Hedging Instruments Gain Loss Net Abstract
Derivative Instruments Gain Loss Recognized In Regulatory Asset Liability	(7,300,000)
Commodity Contracts Required By Regulation [Member] | Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	17,500,000	5,800,000
Derivative Liabilities - Current	(95,900,000)	(54,300,000)
Derivative Liabilities, Noncurrent	(935,800,000)	(883,100,000)
Derivative Assets Noncurrent	174,200,000	195,900,000
Commodity Contracts Required By Regulation [Member] | Fair Value Inputs Level 3 [Member] | Western Massachusetts Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(100,000)
Derivative Liabilities, Noncurrent	(7,200,000)
Designated As Hedging Instrument [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	2,300,000	7,700,000
Derivative Liabilities - Current	0
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	2,300,000	7,700,000
Derivative Assets Noncurrent		4,100,000
Commodity [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	5,100,000	3,800,000
Derivative Liabilities - Current	(16,100,000)	(25,400,000)
Derivative Liabilities, Noncurrent	(17,300,000)	(26,800,000)
Derivative Assets Noncurrent	4,600,000	3,200,000
Derivative Instruments Not Designated As Hedging Instruments Gain Loss Net Abstract
Derivative Instruments Gain Loss Recognized In Income Net	500,000	2,700,000	6,200,000
Derivative Instruments Gain Loss Recognized In Regulatory Asset Liability	(3,900,000)	(21,700,000)	(73,200,000)
Commodity [Member] | The Connecticut Light And Power Company [Member]
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract
Description of Derivative Activity Volume	As of December 31, 2011 and 2010, CL&P had 0.6 million and 1.8 million MWh, respectively, remaining under FTRs that extend through December 2012 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	(2,800,000)	(6,200,000)	(7,800,000)
Commodity [Member] | Public Service Company Of New Hampshire [Member]
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract
Description of Derivative Activity Volume	PSNH has 0.3 million MWh remaining under FTRs as of December 31, 2011 and 2010 that extend through December 2012 and require monthly payments or receipts. PSNH had electricity procurement contracts with delivery dates through 2011 to purchase an aggregate amount of 0.4 million MWh of power as of December 31, 2010.
Derivative Instruments Not Designated As Hedging Instruments Gain Loss Net Abstract
Derivative Instruments Gain Loss Recognized In Regulatory Asset Liability	(1,000,000)	(15,000,000)	(62,600,000)
Commodity [Member] | Fair Value Inputs Level 2 [Member] | Public Service Company Of New Hampshire [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current		(12,800,000)
Commodity [Member] | Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	400,000	2,100,000
Derivative Liabilities - Current	0	(200,000)
Derivative Assets Noncurrent	0
Commodity [Member] | Fair Value Inputs Level 3 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	4,700,000	1,700,000
Derivative Liabilities - Current	(16,100,000)	(12,400,000)
Derivative Liabilities, Noncurrent	(17,300,000)	(26,800,000)
Derivative Assets Noncurrent	$ 4,600,000	$ 3,200,000