DERIVATIVE INSTURMENTS (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Derivative Instrument Detail Abstract
Derivative Liabilities, Noncurrent	$ (409,632,000)	$ (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 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, 2014 and 2013, NU had NYMEX future contracts in order to reduce variability associated with the purchase price of approximately 8.8 million and 9.1 million MMBtu of natural gas, respectively.
Derivative Credit Risk Related Contingent Features Abstract
Derivative Net Liability Position Aggregate Fair Value	(10,000,000)
Additional Collateral Aggregate Fair Value	10,000,000
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsExitPricePremium	Exit price premiums of 7 percent through 24 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	(88,459,000)	(92,233,000)
Derivative Liabilities, Noncurrent	(406,199,000)	(617,072,000)
Derivative Assets Subject to Credit Risk with investment grade counterparty	64,000,000
Fair Value Inputs Level 2 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current		1,600,000
Derivative Liabilities - Current	(9,800,000)
Derivative Liabilities, Noncurrent	(300,000)
Derivative Assets Noncurrent	0	200,000
Fair Value Inputs Level 3 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	9,600,000	8,600,000
Derivative Liabilities - Current		(93,700,000)
Derivative Liabilities, Noncurrent	(409,300,000)	(624,100,000)
Derivative Assets Noncurrent	74,300,000	74,000,000
Consolidated Current Derivative Liabilities	90,000,000
Fair Value Inputs Level 3 [Member] | MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	5.30	5.07
FairValueInputsEnergyPrices		49
FairValueInputsRenewableEnergyCreditPrices	38	36
FairValueInputsForwardReserve	5.80	3.30
Fair Value Inputs Level 3 [Member] | MaximumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	12.98	11.82
FairValueInputsEnergyPrices		77
FairValueInputsRenewableEnergyCreditPrices	56	87
FairValueInputsForwardReserve	9.50	3.30
Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	9,500,000	7,300,000
Derivative Liabilities - Current	(88,500,000)	(92,200,000)
Derivative Liabilities, Noncurrent	(406,200,000)	(617,100,000)
Derivative Assets Noncurrent	74,300,000	71,400,000
Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member] | MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	11.08	5.07
FairValueInputsEnergyPrices		56
FairValueInputsForwardReserve	5.80	3.30
Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member] | MaximumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	12.98	10.42
FairValueInputsEnergyPrices		58
FairValueInputsForwardReserve	9.50	3.30
Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	100,000	1,200,000
Derivative Liabilities - Current	(1,500,000)	(1,500,000)
Derivative Liabilities, Noncurrent	(3,100,000)	(7,000,000)
Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member] | MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	5.30	5.07
FairValueInputsRenewableEnergyCreditPrices	38	36
Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member] | MaximumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	11.10	7.38
FairValueInputsRenewableEnergyCreditPrices	56	70
Amount Offset Against Derivatives [Member] | Fair Value Inputs Level 2 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current		(300,000)
Amount Offset Against Derivatives [Member] | Fair Value Inputs Level 3 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	(6,600,000)	(9,800,000)
Derivative Assets Noncurrent	(19,200,000)	(42,200,000)
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)	(9,800,000)
Derivative Liabilities, Noncurrent	0
Derivative Assets Noncurrent	(19,200,000)	(42,200,000)
Commodity [Member]
Derivative Instruments Not Designated As Hedging Instruments Gain Loss Net Abstract
Derivative Gain Loss On Derivative Net	0	1,000,000	(700,000)
Derivative Instruments Gain Loss Recognized In Regulatory Asset Liability	134,400,000	160,600,000	(29,000,000)
Commodity [Member] | Fair Value Inputs Level 2 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current		1,900,000
Derivative Liabilities - Current	(9,800,000)
Derivative Liabilities, Noncurrent	(300,000)
Derivative Assets Noncurrent	0	200,000
Commodity [Member] | Fair Value Inputs Level 3 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	16,200,000	18,400,000
Derivative Liabilities - Current	(90,000,000)	(93,700,000)
Derivative Liabilities, Noncurrent	(409,300,000)	(624,100,000)
Derivative Assets Noncurrent	93,500,000	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	16,100,000	17,100,000
Derivative Liabilities - Current	(88,500,000)	(92,200,000)
Derivative Liabilities, Noncurrent	(406,200,000)	(617,100,000)
Derivative Assets Noncurrent	93,500,000	113,600,000
Commodity [Member] | Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	100,000	1,200,000
Derivative Liabilities - Current	(1,500,000)	(1,500,000)
Derivative Liabilities, Noncurrent	$ (3,100,000)	$ (7,000,000)