DERIVATIVE INSTURMENTS (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Derivative Instrument Detail Abstract
Derivative Liabilities, Noncurrent	$ (624,050,000)	$ (882,654,000)
Derivative Assets Noncurrent	74,155,000	90,612,000
Current Derivative Assets Consolidated	10,200,000	11,400,000
Derivative Assets Subject to Credit Risk with investment grade counterparty	80,000,000
DerivativeCollateralRightToReclaimCash		4,100,000
Consolidated Current Derivative Liabilities	93,700,000	117,200,000
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract
Description of Derivative Activity Volume	As of December 31, 2013 and 2012, NU had NYMEX future contracts in order to reduce variability associated with the purchase price of approximately 9.1 million and 11.5 million MMBtu of natural gas, respectively. As of December 31, 2012, NU had approximately 24 thousand MWh of supply volumes remaining in its unregulated wholesale portfolio when expected sales were compared with supply contracts. These contracts expired on December 31, 2013.
Derivative Credit Risk Related Contingent Features Abstract
Derivative Net Liability Position Aggregate Fair Value	0	(15,300,000)
Collateral Already Posted Aggregate Fair Value		0
Additional Collateral Aggregate Fair Value	0	17,400,000
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsExitPricePremium	Exit price premiums of 10 percent through 32 percent are also applied on these contracts and reflect the most recent market activity available for similar type contracts.
The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(92,233,000)	(96,931,000)
Derivative Liabilities, Noncurrent	(617,072,000)	(865,571,000)
Derivative Assets Noncurrent	71,384,000	90,612,000
Derivative Assets Subject to Credit Risk with investment grade counterparty	79,000,000
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 and a capacity related contract to purchase up to 35 MW per year through 2019.
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 has not yet achieved commercial operation.
Fair Value Inputs Level 2 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	1,600,000	200,000
Derivative Liabilities - Current		(19,300,000)
Derivative Liabilities, Noncurrent		(200,000)
Derivative Assets Noncurrent	200,000
Fair Value Inputs Level 3 [Member] | MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	5.07	1.40
FairValueInputsEnergyPrices	49	43
FairValueInputsRenewableEnergyCreditPrices	36	25
FairValueInputsForwardReserve	3.30	0.35
Fair Value Inputs Level 3 [Member] | MaximumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	11.82	10.53
FairValueInputsEnergyPrices	77	90
FairValueInputsRenewableEnergyCreditPrices	87	85
FairValueInputsForwardReserve	3.30	0.90
Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	7,300,000	5,700,000
Derivative Liabilities - Current	(92,200,000)	(96,900,000)
Derivative Liabilities, Noncurrent	(617,100,000)	(865,600,000)
Derivative Assets Noncurrent	71,400,000	90,600,000
Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member] | MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	5.07	1.40
FairValueInputsEnergyPrices	56	50
FairValueInputsForwardReserve	3.30	0.35
Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member] | MaximumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	10.42	9.83
FairValueInputsEnergyPrices	58	55
FairValueInputsForwardReserve	3.30	0.90
Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	1,200,000
Derivative Liabilities - Current	(1,500,000)	(1,000,000)
Derivative Liabilities, Noncurrent	(7,000,000)	(13,900,000)
Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member] | MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	5.07	1.40
FairValueInputsRenewableEnergyCreditPrices	36	25
Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member] | MaximumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	7.38	3.39
FairValueInputsRenewableEnergyCreditPrices	70	71
Fair Value Inputs Level 3 [Member] | Western Massachusetts Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	100,000
Derivative Liabilities, Noncurrent		(3,000,000)
Derivative Assets Noncurrent	2,600,000
Fair Value Inputs Level 3 [Member] | Western Massachusetts Electric Company [Member] | MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	5.07	1.40
FairValueInputsEnergyPrices	49	43
FairValueInputsRenewableEnergyCreditPrices	36	25
Fair Value Inputs Level 3 [Member] | Western Massachusetts Electric Company [Member] | MaximumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	11.82	10.53
FairValueInputsEnergyPrices	77	90
FairValueInputsRenewableEnergyCreditPrices	87	85
Fair Value Inputs Level 3 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current		5,500,000
Netting And Collateral [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	(10,100,000)	(12,000,000)
Derivative Liabilities - Current	0	600,000
Derivative Liabilities, Noncurrent	0	0
Derivative Assets Noncurrent	(42,200,000)	(69,100,000)
Netting And Collateral [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current		600,000
Netting And Collateral [Member] | Fair Value Inputs Level 2 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	(300,000)
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)
Derivative Liabilities, Noncurrent	0
Derivative Assets Noncurrent	(42,200,000)	(69,100,000)
Commodity [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	20,300,000	23,400,000
Derivative Liabilities - Current	(93,700,000)	(117,800,000)
Derivative Liabilities, Noncurrent	(624,100,000)	(882,700,000)
Derivative Assets Noncurrent	116,400,000	159,700,000
Derivative Instruments Not Designated As Hedging Instruments Gain Loss Net Abstract
Derivative Gain Loss On Derivative Net	1,000,000	(700,000)	500,000
Derivative Instruments Gain Loss Recognized In Regulatory Asset Liability	160,600,000	(29,000,000)	(162,000,000)
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 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.
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)
Derivative Assets 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	17,100,000	17,700,000
Derivative Liabilities - Current	(92,200,000)	(96,900,000)
Derivative Liabilities, Noncurrent	(617,100,000)	(865,600,000)
Derivative Assets Noncurrent	113,600,000	159,700,000
Commodity [Member] | Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	1,200,000
Derivative Liabilities - Current	(1,500,000)	(1,000,000)
Derivative Liabilities, Noncurrent	(7,000,000)	(13,900,000)
Commodity [Member] | Fair Value Inputs Level 3 [Member] | Western Massachusetts Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	100,000
Derivative Liabilities, Noncurrent		(3,000,000)
Derivative Assets Noncurrent	2,600,000
Commodity [Member] | Fair Value Inputs Level 3 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current		$ 5,500,000