DERIVATIVE INSTURMENTS (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Derivative Instrument Detail Abstract
Derivative Liabilities, Noncurrent	$ (396,617,000)		$ (409,632,000)
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract
Description of Derivative Activity Volume	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 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 March 31, 2015 and December 31, 2014, Eversource had NYMEX financial contracts for natural gas futures in order to reduce variability associated with the purchase price of approximately 5.3 million and 8.8 million MMBtu of natural gas, respectively.
Derivative Instruments Not Designated As Hedging Instruments Gain Loss Net Abstract
Derivative Instruments Gain Loss Recognized In Regulatory Asset Liability	(16,600,000)	54,100,000
Derivative Credit Risk Related Contingent Features Abstract
Derivative Net Liability Position Aggregate Fair Value	(3,000,000)		(10,000,000)
Additional Collateral Aggregate Fair Value	3,000,000		10,000,000
The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(88,218,000)		(88,459,000)
Derivative Liabilities, Noncurrent	(395,038,000)		(406,199,000)
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract
Description of Derivative Activity Volume	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 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 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.
Fair Value Inputs Level 2 [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(3,200,000)		(9,800,000)
Derivative Liabilities, Noncurrent	(200,000)		(300,000)
Fair Value Inputs Level 3 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	9,400,000		9,600,000
Derivative Liabilities - Current	(90,300,000)		(90,000,000)
Derivative Liabilities, Noncurrent	(396,400,000)		(409,300,000)
Derivative Assets Noncurrent	70,500,000		74,300,000
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsEnergyPrices	48		52
Fair Value Inputs Level 3 [Member] | MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	8.80		5.30
FairValueInputsRenewableEnergyCreditPrices	45		38
FairValueInputsForwardReserve	5.80		5.80
Fair Value Inputs Level 3 [Member] | MaximumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	12.98		12.98
FairValueInputsRenewableEnergyCreditPrices	50		56
FairValueInputsForwardReserve	9.50		9.50
Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	9,400,000		9,500,000
Derivative Liabilities - Current	(88,200,000)		(88,500,000)
Derivative Liabilities, Noncurrent	(395,000,000)		(406,200,000)
Derivative Assets Noncurrent	70,500,000		74,300,000
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsEnergyPrices	48		52
Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member] | MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	11.13		11.08
FairValueInputsForwardReserve	5.80		5.80
Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member] | MaximumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	12.98		12.98
FairValueInputsForwardReserve	9.50		9.50
Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	0		100,000
Derivative Liabilities - Current	(2,100,000)		(1,500,000)
Derivative Liabilities, Noncurrent	(1,400,000)		(3,100,000)
Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member] | MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	8.80		5.30
FairValueInputsRenewableEnergyCreditPrices	45		38
Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member] | MaximumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	11.13		11.10
FairValueInputsRenewableEnergyCreditPrices	50		56
Amount Offset Against Derivatives [Member] | Fair Value Inputs Level 2 [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	0
Amount Offset Against Derivatives [Member] | Fair Value Inputs Level 3 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	(6,600,000)		(6,600,000)
Derivative Liabilities - Current	0
Derivative Assets Noncurrent	(17,800,000)		(19,200,000)
Amount Offset Against Derivatives [Member] | Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	(6,600,000)		(6,600,000)
Derivative Assets Noncurrent	(17,800,000)		(19,200,000)
Commodity [Member] | Fair Value Inputs Level 2 [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(3,200,000)		(9,800,000)
Derivative Liabilities, Noncurrent	(200,000)		(300,000)
Commodity [Member] | Fair Value Inputs Level 3 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	16,000,000		16,200,000
Derivative Liabilities - Current	(90,300,000)		(90,000,000)
Derivative Liabilities, Noncurrent	(396,400,000)		(409,300,000)
Derivative Assets Noncurrent	88,300,000		93,500,000
Commodity [Member] | Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	16,000,000		16,100,000
Derivative Liabilities - Current	(88,200,000)		(88,500,000)
Derivative Liabilities, Noncurrent	(395,000,000)		(406,200,000)
Derivative Assets Noncurrent	88,300,000		93,500,000
Commodity [Member] | Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	0		100,000
Derivative Liabilities - Current	(2,100,000)		(1,500,000)
Derivative Liabilities, Noncurrent	$ (1,400,000)		$ (3,100,000)