DERIVATIVE INSTURMENTS (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Derivative Instrument Detail Abstract
Derivative Liabilities, Noncurrent	$ (420,931,000)		$ (420,931,000)		$ (624,050,000)
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, along with UI, has capacity-related contracts with generation facilities. 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 combined capacity of these contracts is 787 MW. 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. 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. As of September 30, 2014 and December 31, 2013, NU had NYMEX future contracts in order to reduce variability associated with the purchase price of approximately 9.7 million and 9.1 million MMBtu of natural gas, respectively.
Derivative Instruments Not Designated As Hedging Instruments Gain Loss Net Abstract
Derivative Gain Loss On Derivative Net		200,000		900,000
Derivative Instruments Gain Loss Recognized In Regulatory Asset Liability	(15,700,000)	300,000	149,900,000	48,800,000
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsExitPricePremium			Exit price premiums of 8 percent through 25 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	(86,759,000)		(86,759,000)		(92,233,000)
Derivative Liabilities, Noncurrent	(418,137,000)		(418,137,000)		(617,072,000)
Fair Value Inputs Level 2 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	400,000		400,000		1,600,000
Derivative Liabilities - Current	(2,000,000)		(2,000,000)
Derivative Assets Noncurrent					200,000
Fair Value Inputs Level 3 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	10,600,000		10,600,000		8,600,000
Derivative Liabilities - Current	(88,300,000)		(88,300,000)		(93,700,000)
Derivative Liabilities, Noncurrent	(420,900,000)		(420,900,000)		(624,100,000)
Derivative Assets Noncurrent	75,100,000		75,100,000		74,000,000
Fair Value Inputs Level 3 [Member] | MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	7.03		7.03		5.07
FairValueInputsEnergyPrices	66		66		49
FairValueInputsRenewableEnergyCreditPrices	41		41		36
FairValueInputsForwardReserve	5.80		5.80		3.30
Fair Value Inputs Level 3 [Member] | MaximumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	12.98		12.98		11.82
FairValueInputsEnergyPrices	68		68		77
FairValueInputsRenewableEnergyCreditPrices	70		70		87
FairValueInputsForwardReserve	9.50		9.50		3.30
Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	9,600,000		9,600,000		7,300,000
Derivative Liabilities - Current	(86,800,000)		(86,800,000)		(92,200,000)
Derivative Liabilities, Noncurrent	(418,100,000)		(418,100,000)		(617,100,000)
Derivative Assets Noncurrent	74,200,000		74,200,000		71,400,000
Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member] | MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	7.03		7.03		5.07
FairValueInputsEnergyPrices	66		66		56
FairValueInputsForwardReserve	5.80		5.80		3.30
Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member] | MaximumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	12.98		12.98		10.42
FairValueInputsEnergyPrices	68		68		58
FairValueInputsForwardReserve	9.50		9.50		3.30
Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	1,000,000		1,000,000		1,200,000
Derivative Liabilities - Current	(1,500,000)		(1,500,000)		(1,500,000)
Derivative Liabilities, Noncurrent	(2,800,000)		(2,800,000)		(7,000,000)
Derivative Assets Noncurrent	900,000		900,000
Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member] | MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	11.00		11.00		5.07
FairValueInputsRenewableEnergyCreditPrices	41		41		36
Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member] | MaximumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	11.10		11.10		7.38
FairValueInputsRenewableEnergyCreditPrices	70		70		70
Netting And Collateral [Member] | Fair Value Inputs Level 2 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	0		0		(300,000)
Derivative Liabilities - Current	400,000		400,000
Netting And Collateral [Member] | Fair Value Inputs Level 3 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	(6,800,000)		(6,800,000)		(9,800,000)
Derivative Assets Noncurrent	(20,900,000)		(20,900,000)		(42,200,000)
Netting And Collateral [Member] | Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	(6,800,000)		(6,800,000)		(9,800,000)
Derivative Assets Noncurrent	(20,900,000)		(20,900,000)		(42,200,000)
Commodity [Member] | Fair Value Inputs Level 2 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	400,000		400,000		1,900,000
Derivative Liabilities - Current	(2,400,000)		(2,400,000)
Derivative Assets Noncurrent					200,000
Commodity [Member] | Fair Value Inputs Level 3 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	17,400,000		17,400,000		18,400,000
Derivative Liabilities - Current	(88,300,000)		(88,300,000)		(93,700,000)
Derivative Liabilities, Noncurrent	(420,900,000)		(420,900,000)		(624,100,000)
Derivative Assets Noncurrent	96,000,000		96,000,000		116,200,000
Commodity [Member] | Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	16,400,000		16,400,000		17,100,000
Derivative Liabilities - Current	(86,800,000)		(86,800,000)		(92,200,000)
Derivative Liabilities, Noncurrent	(418,100,000)		(418,100,000)		(617,100,000)
Derivative Assets Noncurrent	95,100,000		95,100,000		113,600,000
Commodity [Member] | Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	1,000,000		1,000,000		1,200,000
Derivative Liabilities - Current	(1,500,000)		(1,500,000)		(1,500,000)
Derivative Liabilities, Noncurrent	(2,800,000)		(2,800,000)		(7,000,000)
Derivative Assets Noncurrent	$ 900,000		$ 900,000