DERIVATIVE INSTURMENTS (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Derivative Instrument Detail Abstract
Derivative Liabilities, Noncurrent	$ (788,929,000)		$ (788,929,000)		$ (882,654,000)
Derivative Assets Noncurrent	89,309,000		89,309,000		90,612,000
Current Derivative Assets Consolidated	9,100,000		9,100,000		11,400,000
DerivativeCollateralRightToReclaimCash	2,100,000		2,100,000		4,100,000
Consolidated Current Derivative Liabilities	109,000,000		109,000,000		117,200,000
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract
Description of Derivative Activity Volume			As of June 30, 2013 and December 31, 2012, NU had approximately 2 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 Instruments Gain Loss Recognized In Income Net	500,000	(200,000)	800,000	(1,000,000)
Derivative Instruments Gain Loss Recognized In Regulatory Asset Liability	22,200,000	(40,800,000)	50,100,000	(33,500,000)
Derivative Credit Risk Related Contingent Features Abstract
Derivative Net Liability Position Aggregate Fair Value	(9,700,000)		(9,700,000)		(15,300,000)
Collateral Already Posted Aggregate Fair Value					0
Additional Collateral Aggregate Fair Value	11,400,000		11,400,000		17,400,000
MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	1.4		1.4		1.4
FairValueInputsEnergyPrices	44		44		43
FairValueInputsRenewableEnergyCreditPrices	25		25		25
FairValueInputsForwardReserve	3		3		0.35
FairValueInputsExitPricePremium			0.11
MaximumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	10.53		10.53		10.53
FairValueInputsEnergyPrices	93		93		90
FairValueInputsRenewableEnergyCreditPrices	85		85		85
FairValueInputsForwardReserve	3		3		0.9
FairValueInputsExitPricePremium			0.32
The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(95,183,000)		(95,183,000)		(96,931,000)
Derivative Liabilities, Noncurrent	(777,144,000)		(777,144,000)		(865,571,000)
Derivative Assets Noncurrent	89,309,000		89,309,000		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		1.4
FairValueInputsEnergyPrices	53		53		51
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 0.3 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 0.3 million MWh of energy through November 2013.
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	2,500,000		2,500,000
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.
Western Massachusetts Electric Company [Member] | MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	1.4		1.4		1.4
FairValueInputsEnergyPrices	44		44		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	85		85		85
Fair Value Inputs Level 2 [Member] | Public Service Company Of New Hampshire [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	400,000		400,000		200,000
Derivative Liabilities - Current	(10,300,000)		(10,300,000)		(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	(95,200,000)		(95,200,000)		(96,900,000)
Derivative Liabilities, Noncurrent	(777,100,000)		(777,100,000)		(865,600,000)
Derivative Assets Noncurrent	89,300,000		89,300,000		90,600,000
Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(1,600,000)		(1,600,000)		(1,000,000)
Derivative Liabilities, Noncurrent	(11,500,000)		(11,500,000)		(13,900,000)
Fair Value Inputs Level 3 [Member] | Western Massachusetts Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(400,000)		(400,000)
Derivative Liabilities, Noncurrent	(300,000)		(300,000)		(3,000,000)
Fair Value Inputs Level 3 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	1,500,000		1,500,000		5,500,000
Netting And Collateral [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	(10,200,000)		(10,200,000)		(12,000,000)
Derivative Liabilities - Current	0		0		600,000
Derivative Assets Noncurrent	(53,900,000)		(53,900,000)		(69,100,000)
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	(10,200,000)		(10,200,000)		(12,000,000)
Derivative Assets Noncurrent	(53,900,000)		(53,900,000)		(69,100,000)
Commodity [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	19,300,000		19,300,000		23,400,000
Derivative Liabilities - Current	(109,000,000)		(109,000,000)		(117,800,000)
Derivative Liabilities, Noncurrent	(788,900,000)		(788,900,000)		(882,700,000)
Derivative Assets Noncurrent	143,200,000		143,200,000		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	(1,500,000)		(1,500,000)
Commodity [Member] | Fair Value Inputs Level 2 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	400,000		400,000		200,000
Derivative Liabilities - Current	(10,300,000)		(10,300,000)		(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	17,400,000		17,400,000		17,700,000
Derivative Liabilities - Current	(95,200,000)		(95,200,000)		(96,900,000)
Derivative Liabilities, Noncurrent	(777,100,000)		(777,100,000)		(865,600,000)
Derivative Assets Noncurrent	143,200,000		143,200,000		159,700,000
Commodity [Member] | Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(1,600,000)		(1,600,000)		(1,000,000)
Derivative Liabilities, Noncurrent	(11,500,000)		(11,500,000)		(13,900,000)
Commodity [Member] | Fair Value Inputs Level 3 [Member] | Western Massachusetts Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(400,000)		(400,000)
Derivative Liabilities, Noncurrent	(300,000)		(300,000)		(3,000,000)
Commodity [Member] | Fair Value Inputs Level 3 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	$ 1,500,000		$ 1,500,000		$ 5,500,000