DERIVATIVE INSTURMENTS (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016 USD ($) $ / KWmo	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) $ / KWmo	Jun. 30, 2015 USD ($)	Dec. 31, 2015 USD ($) $ / KWmo $ / REC
Derivative Instrument Detail Abstract
Derivative Liabilities, Noncurrent	$ (381,572)		$ (381,572)		$ (337,102)
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 June 30, 2016 and December 31, 2015, Eversource had NYMEX financial contracts for natural gas futures in order to reduce variability associated with the purchase price of approximately 7.1 million and 9.1 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	(42,900)	$ (36,600)	$ (74,400)	$ (50,100)
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsExitPricePremium			Exit price premiums of 4 percent through 21 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	(91,603)		$ (91,603)		(91,820)
Derivative Liabilities, Noncurrent	(380,643)		(380,643)		(336,189)
Fair Value Inputs Level 2 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	6,700		6,700
Fair Value Inputs Level 2 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	3,500		3,500
Derivative Liabilities - Current	0		0		(5,800)
Derivative Assets Noncurrent					100
Fair Value Inputs Level 3 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current					5,800
Derivative Liabilities - Current	(93,000)		(93,000)		(92,300)
Derivative Liabilities, Noncurrent	(381,600)		(381,600)		(337,100)
Derivative Assets Noncurrent	$ 55,300		$ 55,300		$ 42,700
Fair Value Inputs Level 3 [Member] | Minimum [Member]
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices | $ / KWmo	5.50		5.50		10.81
FairValueInputsRenewableEnergyCreditPrices | $ / KWmo	38		38		45
FairValueInputsForwardReserve | $ / KWmo	2.00		2.00		2.00
Fair Value Inputs Level 3 [Member] | Maximum [Member]
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices | $ / KWmo	13.23		13.23		15.82
FairValueInputsRenewableEnergyCreditPrices	42		42		51
FairValueInputsForwardReserve | $ / KWmo
Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	$ 5,800		$ 5,800		$ 5,800
Derivative Liabilities - Current	(91,600)		(91,600)		(91,800)
Derivative Liabilities, Noncurrent	(380,600)		(380,600)		(336,200)
Derivative Assets Noncurrent	$ 55,100		$ 55,100		$ 41,400
Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member] | Minimum [Member]
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices | $ / KWmo	5.50		5.50		10.81
FairValueInputsForwardReserve | $ / KWmo	2.00		2.00		2.00
Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member] | Maximum [Member]
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices | $ / KWmo	13.23		13.23		12.60
FairValueInputsForwardReserve | $ / KWmo
Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	$ 900		$ 900
Derivative Liabilities - Current	(1,400)		(1,400)		$ (500)
Derivative Liabilities, Noncurrent	(1,000)		(1,000)		(900)
Derivative Assets Noncurrent	$ 200		$ 200		$ 1,300
Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member] | Minimum [Member]
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices | $ / KWmo	8.50		8.50		10.81
FairValueInputsRenewableEnergyCreditPrices	38		38		45
Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member] | Maximum [Member]
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices | $ / KWmo	12.11		12.11		15.82
FairValueInputsRenewableEnergyCreditPrices	42		42		51
Amount Offset Against Derivatives [Member] | Fair Value Inputs Level 2 [Member] | Subsidiaries [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)		$ (10,900)
Derivative Liabilities - Current	0		0
Derivative Assets Noncurrent	(14,800)		(14,800)		(19,300)
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)		(10,900)
Derivative Assets Noncurrent	(14,800)		(14,800)		(19,300)
Commodity [Member] | Fair Value Inputs Level 2 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	3,500		3,500
Derivative Liabilities - Current	0		0		(5,800)
Derivative Assets Noncurrent					100
Commodity [Member] | Fair Value Inputs Level 3 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	17,500		17,500		16,700
Derivative Liabilities - Current	(93,000)		(93,000)		(92,300)
Derivative Liabilities, Noncurrent	(381,600)		(381,600)		(337,100)
Derivative Assets Noncurrent	70,100		70,100		62,000
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		16,700
Derivative Liabilities - Current	(91,600)		(91,600)		(91,800)
Derivative Liabilities, Noncurrent	(380,600)		(380,600)		(336,200)
Derivative Assets Noncurrent	69,900		69,900		60,700
Commodity [Member] | Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	900		900
Derivative Liabilities - Current	(1,400)		(1,400)		(500)
Derivative Liabilities, Noncurrent	(1,000)		(1,000)		(900)
Derivative Assets Noncurrent	$ 200		$ 200		$ 1,300