DERIVATIVE INSTURMENTS (Details) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Derivative Instrument Detail Abstract
Derivative Liabilities, Noncurrent	$ (546,387,000)		$ (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 has capacity-related contracts with generation facilities. These contracts and similar UI contracts have an expected capacity of 787 MW. 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 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 forward 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, 2014 and December 31, 2013, NU had NYMEX future contracts in order to reduce variability associated with the purchase price of approximately 7.4 million and 9.1 million MMBtu of natural gas, respectively.
Derivative Instruments Not Designated As Hedging Instruments Gain Loss Net Abstract
Derivative Gain Loss On Derivative Net		300,000
Derivative Instruments Gain Loss Recognized In Regulatory Asset Liability	54,100,000	28,000,000
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsExitPricePremium	Exit price premiums of 9 percent through 26 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	(92,040,000)		(92,233,000)
Derivative Liabilities, Noncurrent	(539,444,000)		(617,072,000)
Fair Value Inputs Level 2 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	1,100,000		1,600,000
Derivative Liabilities, Noncurrent	(200,000)
Derivative Assets Noncurrent			200,000
Fair Value Inputs Level 3 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	8,100,000		8,600,000
Derivative Liabilities - Current	(93,300,000)		(93,700,000)
Derivative Liabilities, Noncurrent	(546,200,000)		(624,100,000)
Derivative Assets Noncurrent	67,100,000		74,000,000
Fair Value Inputs Level 3 [Member] | MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	1.70		5.07
FairValueInputsEnergyPrices	57		49
FairValueInputsRenewableEnergyCreditPrices	38		36
FairValueInputsForwardReserve	3.30		3.30
Fair Value Inputs Level 3 [Member] | MaximumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	10.42		11.82
FairValueInputsEnergyPrices	60		77
FairValueInputsRenewableEnergyCreditPrices	70		87
FairValueInputsForwardReserve	3.30		3.30
Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	7,300,000		7,300,000
Derivative Liabilities - Current	(92,000,000)		(92,200,000)
Derivative Liabilities, Noncurrent	(539,400,000)		(617,100,000)
Derivative Assets Noncurrent	67,100,000		71,400,000
Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member] | MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	5.23		5.07
FairValueInputsEnergyPrices	57		56
FairValueInputsForwardReserve	3.30		3.30
Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member] | MaximumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	10.42		10.42
FairValueInputsEnergyPrices	60		58
FairValueInputsForwardReserve	3.30		3.30
Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	800,000		1,200,000
Derivative Liabilities - Current	(1,300,000)		(1,500,000)
Derivative Liabilities, Noncurrent	(6,800,000)		(7,000,000)
Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member] | MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	1.70		5.07
FairValueInputsRenewableEnergyCreditPrices	38		36
Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member] | MaximumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	7.38		7.38
FairValueInputsRenewableEnergyCreditPrices	70		70
Netting And Collateral [Member] | Fair Value Inputs Level 2 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	(100,000)		(300,000)
Netting And Collateral [Member] | Fair Value Inputs Level 3 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	(9,700,000)		(9,800,000)
Derivative Assets Noncurrent	(31,700,000)		(42,200,000)
Netting And Collateral [Member] | Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	(9,700,000)		(9,800,000)
Derivative Assets Noncurrent	(31,700,000)		(42,200,000)
Commodity [Member] | Fair Value Inputs Level 2 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	1,200,000		1,900,000
Derivative Liabilities, Noncurrent	(200,000)
Derivative Assets Noncurrent			200,000
Commodity [Member] | Fair Value Inputs Level 3 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	17,800,000		18,400,000
Derivative Liabilities - Current	(93,300,000)		(93,700,000)
Derivative Liabilities, Noncurrent	(546,200,000)		(624,100,000)
Derivative Assets Noncurrent	98,800,000		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	17,000,000		17,100,000
Derivative Liabilities - Current	(92,000,000)		(92,200,000)
Derivative Liabilities, Noncurrent	(539,400,000)		(617,100,000)
Derivative Assets Noncurrent	98,800,000		113,600,000
Commodity [Member] | Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	800,000		1,200,000
Derivative Liabilities - Current	(1,300,000)		(1,500,000)
Derivative Liabilities, Noncurrent	$ (6,800,000)		$ (7,000,000)