DERIVATIVE INSTURMENTS (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) $ / KWmo $ / REC	Dec. 31, 2014 USD ($) $ / KWmo $ / REC $ / MWh	Dec. 31, 2013 USD ($)
Derivative Instrument Detail Abstract
Derivative Liabilities, Noncurrent	$ (337,102)	$ (409,632)
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 the costs or benefits of each contract borne by or allocated to CL&P and 20 percent borne by or 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 December 31, 2015 and 2014, Eversource had NYMEX financial contracts for natural gas futures in order to reduce variability associated with the purchase price of approximately 9.1 million and 8.8 million MMBtu of natural gas, respectively.
Derivative Credit Risk Related Contingent Features Abstract
Derivative Net Liability Position Aggregate Fair Value	$ (5,800)	(10,000)
Additional Collateral Aggregate Fair Value	$ 5,800	10,000
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsExitPricePremium	Exit price premiums of 5 percent to 22 percent are also applied on these contracts and reflect the uncertainty and illiquidity premiums that would be required based on the most recent market activity available for similar type contracts
The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	$ (91,820)	(88,459)
Derivative Liabilities, Noncurrent	(336,189)	(406,199)
Derivative Assets Subject to Credit Risk with investment grade counterparty	$ 47,000
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 the costs or benefits of each contract borne by or allocated to CL&P and 20 percent borne by or 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.
Fair Value Inputs Level 2 [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	$ (5,800)	(9,800)
Derivative Liabilities, Noncurrent	0	(300)
Derivative Assets Noncurrent	100	0
Fair Value Inputs Level 3 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	5,800	9,600
Derivative Liabilities - Current		(90,000)
Derivative Liabilities, Noncurrent	(337,100)	(409,300)
Derivative Assets Noncurrent	42,700	$ 74,300
Consolidated Current Derivative Liabilities	$ (92,300)
Fair Value Inputs Level 3 [Member] | Minimum [Member]
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices | $ / KWmo	10.81	5.30
FairValueInputsRenewableEnergyCreditPrices | $ / REC	45	38
FairValueInputsForwardReserve | $ / KWmo		5.80
Fair Value Inputs Level 3 [Member] | Maximum [Member]
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	15.82	12.98
FairValueInputsRenewableEnergyCreditPrices | $ / REC	51	56
FairValueInputsForwardReserve | $ / KWmo		9.50
Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	$ 5,800	$ 9,500
Derivative Liabilities - Current	(91,800)	(88,500)
Derivative Liabilities, Noncurrent	(336,200)	(406,200)
Derivative Assets Noncurrent	$ 41,400	$ 74,300
Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member] | Minimum [Member]
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	10.81	11.08
FairValueInputsForwardReserve | $ / KWmo		5.80
Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member] | Maximum [Member]
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	12.60	12.98
FairValueInputsForwardReserve | $ / KWmo		9.50
Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	$ 0	$ 100
Derivative Liabilities - Current	(500)	(1,500)
Derivative Liabilities, Noncurrent	(900)	$ (3,100)
Derivative Assets Noncurrent	$ 1,300
Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member] | Minimum [Member]
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	10.81	5.30
Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member] | Maximum [Member]
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	15.82	11.10
Amount Offset Against Derivatives [Member] | 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.
Amount Offset Against Derivatives [Member] | Fair Value Inputs Level 3 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	$ (10,900)	$ (6,600)
Derivative Assets Noncurrent	(19,300)	(19,200)
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	(10,900)	(6,600)
Derivative Liabilities, Noncurrent	0
Derivative Assets Noncurrent	(19,300)	(19,200)
Commodity [Member]
Derivative Instruments Not Designated As Hedging Instruments Gain Loss Net Abstract
Derivative Gain Loss On Derivative Net	0	0
Derivative Instruments Gain Loss Recognized In Regulatory Asset Liability	60,200	134,400	$ 160,600
Commodity [Member] | Fair Value Inputs Level 2 [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(5,800)	(9,800)
Derivative Liabilities, Noncurrent	0	(300)
Derivative Assets Noncurrent	100	0
Commodity [Member] | Fair Value Inputs Level 3 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	16,700	16,200
Derivative Liabilities - Current	(92,300)	(90,000)
Derivative Liabilities, Noncurrent	(337,100)	(409,300)
Derivative Assets Noncurrent	62,000	93,500
Commodity [Member] | Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	16,700	16,100
Derivative Liabilities - Current	(91,800)	(88,500)
Derivative Liabilities, Noncurrent	(336,200)	(406,200)
Derivative Assets Noncurrent	60,700	93,500
Commodity [Member] | Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	0	100
Derivative Liabilities - Current	(500)	(1,500)
Derivative Liabilities, Noncurrent	(900)	$ (3,100)
Derivative Assets Noncurrent	$ 1,300