DERIVATIVE INSTURMENTS (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	$ (117,194,000)	$ (107,558,000)
Derivative Liabilities, Noncurrent	(882,654,000)	(959,876,000)
Derivative Assets Noncurrent	90,612,000	98,357,000
Current Derivative Assets Consolidated	11,400,000	11,000,000
Derivative Assets Subject to Credit Risk with investment grade counterparty	96,500,000
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract
Description of Derivative Activity Volume	As of December 31, 2012 and 2011, NU had approximately 24 thousand MWh and 123 thousand MWh, respectively, of supply volumes remaining in its unregulated wholesale portfolio when expected sales are compared with supply contracts.
Effect Of Fair Value Hedges On Results Of Operations [Abstract]
Notional Amount Of Fair Value Hedge Instruments		263,000,000
Interest Realized In Earnings On Fair Value Hedge	2,500,000	10,500,000	10,900,000
Cash Flow Hedges Derivative Instruments At Fair Value Net [Abstract]
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, before Tax	(25,100,000)
Cash Flow Hedge Gain Loss Reclassified To Interest Expense Net	(3,300,000)	(1,300,000)	(400,000)
Cash Flow Hedge Loss To Be Reclassified Within Twelve Months	(2,000,000)
Description Of Cash Flow Hedge Activity		In 2011, PSNH and WMECO settled interest rate swaps associated with $280 million and $50 million, respectively, of long-term debt issuances and as a result PSNH and WMECO recorded pre-tax reductions of $18.2 million and $6.9 million, respectively, to AOCI that are being amortized over the remaining lives of the associated debt. In addition, NU, CL&P, PSNH and WMECO continue to amortize interest rate swaps settled in prior years from AOCI into Interest Expense over the remaining life of the associated long-term debt.
Derivative Credit Risk Related Contingent Features Abstract
Derivative Net Liability Position Aggregate Fair Value	(15,300,000)	(23,500,000)
Collateral Already Posted Aggregate Fair Value		4,100,000
Additional Collateral Aggregate Fair Value	17,400,000	19,900,000
MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	1.4
FairValueInputsEnergyPrices	43
FairValueInputsRenewableEnergyCreditPrices	25
FairValueInputsForwardReserve	0.35
FairValueInputsExitPricePremium	0.1
MaximumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	10.53
FairValueInputsEnergyPrices	90
FairValueInputsRenewableEnergyCreditPrices	85
FairValueInputsForwardReserve	0.9
FairValueInputsExitPricePremium	0.32
The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(96,931,000)	(95,881,000)
Derivative Liabilities, Noncurrent	(865,571,000)	(935,849,000)
Derivative Assets Noncurrent	90,612,000	93,755,000
Derivative Assets Subject to Credit Risk with investment grade counterparty	96,300,000
Cash Flow Hedges Derivative Instruments At Fair Value Net [Abstract]
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, before Tax	0
Cash Flow Hedge Gain Loss Reclassified To Interest Expense Net	(700,000)	(700,000)	(700,000)
Cash Flow Hedge Loss To Be Reclassified Within Twelve Months	(400,000)
The Connecticut Light And Power Company [Member] | MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	1.4
FairValueInputsEnergyPrices	50
FairValueInputsForwardReserve	0.35
The Connecticut Light And Power Company [Member] | MaximumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	9.83
FairValueInputsEnergyPrices	55
FairValueInputsForwardReserve	0.9
Public Service Company Of New Hampshire [Member]
Cash Flow Hedges Derivative Instruments At Fair Value Net [Abstract]
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, before Tax	(18,200,000)
Cash Flow Hedge Gain Loss Reclassified To Interest Expense Net	(2,000,000)	(800,000)	(200,000)
Cash Flow Hedge Loss To Be Reclassified Within Twelve Months	(1,200,000)
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 2028 with a facility that is expected to achieve commercial operation by November 2013.
Cash Flow Hedges Derivative Instruments At Fair Value Net [Abstract]
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, before Tax	(6,900,000)
Cash Flow Hedge Gain Loss Reclassified To Interest Expense Net	(500,000)	(100,000)	100,000
Cash Flow Hedge Loss To Be Reclassified Within Twelve Months	(300,000)
Western Massachusetts Electric Company [Member] | MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	1.4
FairValueInputsEnergyPrices	43
FairValueInputsRenewableEnergyCreditPrices	25
Western Massachusetts Electric Company [Member] | MaximumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	10.53
FairValueInputsEnergyPrices	90
FairValueInputsRenewableEnergyCreditPrices	85
NSTAR Electric Company [Member]
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract
Description of Derivative Activity Volume	NSTAR Electric has a renewable energy contract to purchase 0.1 million MWh of energy per year through 2018. NSTAR Electric also has a capacity related contract for up to 35 MW that extends through 2019.
NSTAR Electric Company [Member] | MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	1.4
FairValueInputsRenewableEnergyCreditPrices	25
NSTAR Electric Company [Member] | MaximumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	3.39
FairValueInputsRenewableEnergyCreditPrices	71
Fair Value Inputs Level 2 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	200,000
Derivative Liabilities - Current	(19,300,000)
Derivative Liabilities, Noncurrent	(200,000)
Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	5,700,000	6,300,000
Derivative Liabilities - Current	(96,900,000)	(95,900,000)
Derivative Liabilities, Noncurrent	(865,600,000)	(935,800,000)
Derivative Assets Noncurrent	90,600,000	93,800,000
Fair Value Inputs Level 3 [Member] | Western Massachusetts Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current		(100,000)
Derivative Liabilities, Noncurrent	(3,000,000)	(7,200,000)
Fair Value Inputs Level 3 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	5,500,000	4,700,000
Derivative Liabilities - Current		(11,600,000)
Derivative Liabilities, Noncurrent		(16,900,000)
Derivative Assets Noncurrent		4,600,000
Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(1,000,000)
Derivative Liabilities, Noncurrent	(13,900,000)
Netting And Collateral [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	(12,000,000)	(11,600,000)
Derivative Liabilities - Current	600,000	4,500,000
Derivative Liabilities, Noncurrent	0	400,000
Derivative Assets Noncurrent	(69,100,000)	(80,400,000)
Netting And Collateral [Member] | Fair Value Inputs Level 2 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	600,000
Netting And Collateral [Member] | Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	(12,000,000)	(11,600,000)
Derivative Liabilities, Noncurrent	0
Derivative Assets Noncurrent	(69,100,000)	(80,400,000)
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
Derivative Liabilities, Noncurrent		400,000
Designated As Hedging Instrument [Member] | Fair Value Inputs Level 2 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current		2,300,000
Commodity [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	23,400,000	22,600,000
Derivative Liabilities - Current	(117,800,000)	(112,100,000)
Derivative Liabilities, Noncurrent	(882,700,000)	(960,300,000)
Derivative Assets Noncurrent	159,700,000	178,800,000
Derivative Instruments Not Designated As Hedging Instruments Gain Loss Net Abstract
Derivative Instruments Gain Loss Recognized In Income Net	(700,000)	500,000	2,700,000
Derivative Instruments Gain Loss Recognized In Regulatory Asset Liability	(29,000,000)	(162,000,000)	(95,700,000)
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	Commodity Supply and Price Risk Management: As required by regulation, 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 extend through 2026 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. In addition, CL&P has a contract to purchase 0.1 million MWh of energy per year through 2020.
Commodity [Member] | Fair Value Inputs Level 2 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	200,000
Derivative Liabilities - Current	(19,900,000)
Derivative Liabilities, Noncurrent	(200,000)
Commodity [Member] | Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	17,700,000	17,900,000
Derivative Liabilities - Current	(96,900,000)	(95,900,000)
Derivative Liabilities, Noncurrent	(865,600,000)	(935,800,000)
Derivative Assets Noncurrent	159,700,000	174,200,000
Commodity [Member] | Fair Value Inputs Level 3 [Member] | Western Massachusetts Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current		(100,000)
Derivative Liabilities, Noncurrent	(3,000,000)	(7,200,000)
Commodity [Member] | Fair Value Inputs Level 3 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	5,500,000	4,700,000
Derivative Liabilities - Current		(16,100,000)
Derivative Liabilities, Noncurrent		(17,300,000)
Derivative Assets Noncurrent		4,600,000
Commodity [Member] | Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(1,000,000)
Derivative Liabilities, Noncurrent	$ (13,900,000)	$ (3,400,000)