DERIVATIVE INSTURMENTS (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	$ (108,964,000)		$ (117,194,000)
Derivative Liabilities, Noncurrent	(840,043,000)		(882,654,000)
Derivative Assets Noncurrent	94,970,000		90,612,000
Current Derivative Assets Consolidated	10,500,000		11,400,000
DerivativeCollateralRightToReclaimCash	3,500,000		4,100,000
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract
Description of Derivative Activity Volume	As of March 31, 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.
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
Derivative Credit Risk Related Contingent Features Abstract
Derivative Net Liability Position Aggregate Fair Value	(8,400,000)		(15,300,000)
Collateral Already Posted Aggregate Fair Value			0
Additional Collateral Aggregate Fair Value	9,400,000		17,400,000
MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	1.4		1.4
FairValueInputsEnergyPrices	44		43
FairValueInputsRenewableEnergyCreditPrices	25		25
FairValueInputsForwardReserve	3		0.35
FairValueInputsExitPricePremium	0.11
MaximumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	10.53		10.53
FairValueInputsEnergyPrices	90		90
FairValueInputsRenewableEnergyCreditPrices	85		85
FairValueInputsForwardReserve	3		0.9
FairValueInputsExitPricePremium	0.32
The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(95,573,000)		(96,931,000)
Derivative Liabilities, Noncurrent	(826,023,000)		(865,571,000)
Derivative Assets Noncurrent	94,970,000		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 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.
The Connecticut Light And Power Company [Member] | MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	1.4		1.4
FairValueInputsEnergyPrices	50		50
FairValueInputsForwardReserve	3		0.35
The Connecticut Light And Power Company [Member] | MaximumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	9.83		9.83
FairValueInputsEnergyPrices	54		55
FairValueInputsForwardReserve	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.
NSTAR Electric Company [Member] | MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	1.4		1.4
FairValueInputsRenewableEnergyCreditPrices	25		25
NSTAR Electric Company [Member] | MaximumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	3.39		3.39
FairValueInputsRenewableEnergyCreditPrices	71		71
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.
Western Massachusetts Electric Company [Member] | MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	1.4		1.4
FairValueInputsEnergyPrices	44		43
FairValueInputsRenewableEnergyCreditPrices	25		25
Western Massachusetts Electric Company [Member] | MaximumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	10.53		10.53
FairValueInputsEnergyPrices	90		90
FairValueInputsRenewableEnergyCreditPrices	85		85
Fair Value Inputs Level 2 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	1,600,000		200,000
Derivative Liabilities - Current	(12,000,000)		(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	7,000,000		5,700,000
Derivative Liabilities - Current	(95,600,000)		(96,900,000)
Derivative Liabilities, Noncurrent	(826,000,000)		(865,600,000)
Derivative Assets Noncurrent	95,000,000		90,600,000
Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(1,300,000)		(1,000,000)
Derivative Liabilities, Noncurrent	(12,300,000)		(13,900,000)
Fair Value Inputs Level 3 [Member] | Western Massachusetts Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(100,000)
Derivative Liabilities, Noncurrent	(1,700,000)		(3,000,000)
Fair Value Inputs Level 3 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	1,900,000		5,500,000
Netting And Collateral [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	(10,500,000)		(12,000,000)
Derivative Liabilities - Current			600,000
Derivative Assets Noncurrent	(57,300,000)		(69,100,000)
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	(10,500,000)		(12,000,000)
Derivative Assets Noncurrent	(57,300,000)		(69,100,000)
Commodity [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	21,000,000		23,400,000
Derivative Liabilities - Current	(109,000,000)		(117,800,000)
Derivative Liabilities, Noncurrent	(840,000,000)		(882,700,000)
Derivative Assets Noncurrent	152,300,000		159,700,000
Derivative Instruments Not Designated As Hedging Instruments Gain Loss Net Abstract
Derivative Instruments Gain Loss Recognized In Income Net	300,000	(800,000)
Derivative Instruments Gain Loss Recognized In Regulatory Asset Liability	28,000,000	6,000,000
Commodity [Member] | Fair Value Inputs Level 2 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	1,600,000		200,000
Derivative Liabilities - Current	(12,000,000)		(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	17,500,000		17,700,000
Derivative Liabilities - Current	(95,600,000)		(96,900,000)
Derivative Liabilities, Noncurrent	(826,000,000)		(865,600,000)
Derivative Assets Noncurrent	152,300,000		159,700,000
Commodity [Member] | Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(1,300,000)		(1,000,000)
Derivative Liabilities, Noncurrent	(12,300,000)		(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)
Derivative Liabilities, Noncurrent	(1,700,000)		(3,000,000)
Commodity [Member] | Fair Value Inputs Level 3 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	$ 1,900,000		$ 5,500,000