DERIVATIVE INSTURMENTS (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($) $ / KWmo	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) $ / KWmo	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($) $ / KWmo $ / REC
Derivative Instrument Detail Abstract
Derivative Liabilities, Noncurrent	$ (365,692)		$ (365,692)		$ (409,632)
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 September 30, 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 9.9 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	(8,800)	$ (15,700)	$ (58,900)	$ 149,900
Derivative Credit Risk Related Contingent Features Abstract
Derivative Net Liability Position Aggregate Fair Value	(4,600)		(4,600)		(10,000)
Additional Collateral Aggregate Fair Value	4,700		$ 4,700		10,000
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsExitPricePremium			Exit price premiums of 6 percent through 23 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	(91,372)		$ (91,372)		(88,459)
Derivative Liabilities, Noncurrent	$ (364,691)		$ (364,691)		(406,199)
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	$ (4,600)		$ (4,600)		(9,800)
Derivative Liabilities, Noncurrent	0		0		(300)
Fair Value Inputs Level 3 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	5,800		5,800		9,600
Derivative Liabilities - Current	(92,600)		(92,600)		(90,000)
Derivative Liabilities, Noncurrent	(365,700)		(365,700)		(409,300)
Derivative Assets Noncurrent	$ 45,500		$ 45,500		$ 74,300
Fair Value Inputs Level 3 [Member] | Minimum [Member]
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices | $ / KWmo	10.30		10.30		5.30
FairValueInputsRenewableEnergyCreditPrices | $ / KWmo	45		45		38
FairValueInputsForwardReserve | $ / KWmo	2.00		2.00		5.80
Fair Value Inputs Level 3 [Member] | Maximum [Member]
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices | $ / KWmo	13.13		13.13		12.98
FairValueInputsRenewableEnergyCreditPrices	50		50		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		$ 5,800		$ 9,500
Derivative Liabilities - Current	(91,400)		(91,400)		(88,500)
Derivative Liabilities, Noncurrent	(364,700)		(364,700)		(406,200)
Derivative Assets Noncurrent	$ 43,400		$ 43,400		$ 74,300
Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member] | Minimum [Member]
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices | $ / KWmo	10.81		10.81		11.08
FairValueInputsForwardReserve | $ / KWmo	2.00		2.00		5.80
Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member] | Maximum [Member]
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices | $ / KWmo	12.60		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					$ 100
Derivative Liabilities - Current	$ (1,200)		$ (1,200)		(1,500)
Derivative Liabilities, Noncurrent	(1,000)		(1,000)		$ (3,100)
Derivative Assets Noncurrent	$ 2,100		$ 2,100
Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member] | Minimum [Member]
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices | $ / KWmo	10.30		10.30		5.30
FairValueInputsRenewableEnergyCreditPrices	45		45		38
Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member] | Maximum [Member]
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices | $ / KWmo	13.13		13.13		11.10
FairValueInputsRenewableEnergyCreditPrices	50		50		56
Amount Offset Against Derivatives [Member] | Fair Value Inputs Level 2 [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	$ 0		$ 0
Amount Offset Against Derivatives [Member] | Fair Value Inputs Level 3 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	(10,800)		(10,800)		$ (6,600)
Derivative Liabilities - Current	0		0
Derivative Assets Noncurrent	(22,200)		(22,200)		(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	(10,800)		(10,800)		(6,600)
Derivative Assets Noncurrent	(22,200)		(22,200)		(19,200)
Commodity [Member] | Fair Value Inputs Level 2 [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(4,600)		(4,600)		(9,800)
Derivative Liabilities, Noncurrent	0		0		(300)
Commodity [Member] | Fair Value Inputs Level 3 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	16,600		16,600		16,200
Derivative Liabilities - Current	(92,600)		(92,600)		(90,000)
Derivative Liabilities, Noncurrent	(365,700)		(365,700)		(409,300)
Derivative Assets Noncurrent	67,700		67,700		93,500
Commodity [Member] | Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	16,600		16,600		16,100
Derivative Liabilities - Current	(91,400)		(91,400)		(88,500)
Derivative Liabilities, Noncurrent	(364,700)		(364,700)		(406,200)
Derivative Assets Noncurrent	65,600		65,600		93,500
Commodity [Member] | Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current					100
Derivative Liabilities - Current	(1,200)		(1,200)		(1,500)
Derivative Liabilities, Noncurrent	(1,000)		(1,000)		$ (3,100)
Derivative Assets Noncurrent	$ 2,100		$ 2,100