DERIVATIVE INSTURMENTS (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Derivative Instrument Detail Abstract
Derivative Liabilities, Noncurrent	$ (766,804,000)		$ (766,804,000)		$ (882,654,000)
Derivative Assets Noncurrent	88,887,000		88,887,000		90,612,000
Current Derivative Assets Consolidated	8,900,000		8,900,000		11,400,000
DerivativeCollateralRightToReclaimCash	1,000,000		1,000,000		4,100,000
Consolidated Current Derivative Liabilities	103,500,000		103,500,000		117,200,000
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract
Description of Derivative Activity Volume			As of September 30, 2013 and December 31, 2012, NU had approximately 5 thousand MWh and 24 thousand MWh, respectively, of supply volumes remaining in its unregulated wholesale portfolio when expected sales are compared with supply contracts.
Derivative Instruments Not Designated As Hedging Instruments Gain Loss Net Abstract
Derivative Gain Loss On Derivative Net	200,000	200,000	900,000	(800,000)
Derivative Instruments Gain Loss Recognized In Regulatory Asset Liability	300,000	11,700,000	48,800,000	(25,000,000)
Derivative Credit Risk Related Contingent Features Abstract
Derivative Net Liability Position Aggregate Fair Value	(6,700,000)		(6,700,000)		(15,300,000)
Additional Collateral Aggregate Fair Value	13,400,000		13,400,000		17,400,000
MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	1.4		1.4		1.4
FairValueInputsEnergyPrices	45		45		43
FairValueInputsRenewableEnergyCreditPrices	25		25		25
FairValueInputsForwardReserve	3		3		0.35
FairValueInputsExitPricePremium			0.1
MaximumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	10.53		10.53		10.53
FairValueInputsEnergyPrices	93		93		90
FairValueInputsRenewableEnergyCreditPrices	87		87		85
FairValueInputsForwardReserve	3		3		0.9
FairValueInputsExitPricePremium			0.32
The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(94,123,000)		(94,123,000)		(96,931,000)
Derivative Liabilities, Noncurrent	(756,437,000)		(756,437,000)		(865,571,000)
Derivative Assets Noncurrent	88,018,000		88,018,000		90,612,000
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 both CL&P and UI 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.
The Connecticut Light And Power Company [Member] | MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	1.4		1.4		1.4
FairValueInputsEnergyPrices	52		52		50
FairValueInputsForwardReserve	3		3		0.35
The Connecticut Light And Power Company [Member] | MaximumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	9.51		9.51		9.83
FairValueInputsEnergyPrices	56		56		55
FairValueInputsForwardReserve	3		3		0.9
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 per year that extends through 2019. PSNH has electricity procurement contracts to purchase 0.2 million MWh of energy through November 2013.
NSTAR Electric Company [Member] | MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	1.4		1.4		1.4
FairValueInputsRenewableEnergyCreditPrices	25		25		25
NSTAR Electric Company [Member] | MaximumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	3.39		3.39		3.39
FairValueInputsRenewableEnergyCreditPrices	71		71		71
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 electricity procurement contracts to purchase 0.2 million MWh of energy through November 2013.
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 2029 with a facility that is expected to achieve commercial operation by June 2014.
Western Massachusetts Electric Company [Member] | MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	1.4		1.4		1.4
FairValueInputsEnergyPrices	45		45		43
FairValueInputsRenewableEnergyCreditPrices	25		25		25
Western Massachusetts Electric Company [Member] | MaximumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	10.53		10.53		10.53
FairValueInputsEnergyPrices	93		93		90
FairValueInputsRenewableEnergyCreditPrices	87		87		85
Fair Value Inputs Level 2 [Member] | Public Service Company Of New Hampshire [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(300,000)		(300,000)
Fair Value Inputs Level 2 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current					200,000
Derivative Liabilities - Current	(7,400,000)		(7,400,000)		(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	7,200,000		7,200,000		5,700,000
Derivative Liabilities - Current	(94,100,000)		(94,100,000)		(96,900,000)
Derivative Liabilities, Noncurrent	(756,400,000)		(756,400,000)		(865,600,000)
Derivative Assets Noncurrent	88,000,000		88,000,000		90,600,000
Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	400,000		400,000
Derivative Liabilities - Current	(1,600,000)		(1,600,000)		(1,000,000)
Derivative Liabilities, Noncurrent	(10,400,000)		(10,400,000)		(13,900,000)
Fair Value Inputs Level 3 [Member] | Western Massachusetts Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(100,000)		(100,000)
Derivative Liabilities, Noncurrent					(3,000,000)
Fair Value Inputs Level 3 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	1,300,000		1,300,000		5,500,000
Netting And Collateral [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	(10,100,000)		(10,100,000)		(12,000,000)
Derivative Liabilities - Current	200,000		200,000		600,000
Derivative Assets Noncurrent	(51,500,000)		(51,500,000)		(69,100,000)
Netting And Collateral [Member] | Fair Value Inputs Level 2 [Member] | Public Service Company Of New Hampshire [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	200,000		200,000
Netting And Collateral [Member] | Fair Value Inputs Level 2 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	0		0		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	(10,100,000)		(10,100,000)		(12,000,000)
Derivative Assets Noncurrent	(51,500,000)		(51,500,000)		(69,100,000)
Commodity [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	19,000,000		19,000,000		23,400,000
Derivative Liabilities - Current	(103,700,000)		(103,700,000)		(117,800,000)
Derivative Liabilities, Noncurrent	(766,800,000)		(766,800,000)		(882,700,000)
Derivative Assets Noncurrent	140,400,000		140,400,000		159,700,000
Commodity [Member] | Fair Value Inputs Level 2 [Member] | Public Service Company Of New Hampshire [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(500,000)		(500,000)
Commodity [Member] | Fair Value Inputs Level 2 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current					200,000
Derivative Liabilities - Current	(7,400,000)		(7,400,000)		(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	17,300,000		17,300,000		17,700,000
Derivative Liabilities - Current	(94,100,000)		(94,100,000)		(96,900,000)
Derivative Liabilities, Noncurrent	(756,400,000)		(756,400,000)		(865,600,000)
Derivative Assets Noncurrent	139,500,000		139,500,000		159,700,000
Commodity [Member] | Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	400,000		400,000
Derivative Liabilities - Current	(1,600,000)		(1,600,000)		(1,000,000)
Derivative Liabilities, Noncurrent	(10,400,000)		(10,400,000)		(13,900,000)
Commodity [Member] | Fair Value Inputs Level 3 [Member] | Western Massachusetts Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(100,000)		(100,000)
Derivative Liabilities, Noncurrent					(3,000,000)
Commodity [Member] | Fair Value Inputs Level 3 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	$ 1,300,000		$ 1,300,000		$ 5,500,000