DERIVATIVE INSTURMENTS (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	$ (113,907,000)		$ (113,907,000)		$ (107,558,000)
Derivative Liabilities, Noncurrent	(912,352,000)		(912,352,000)		(959,876,000)
Derivative Assets Noncurrent	93,256,000		93,256,000		98,357,000
Current Derivative Assets Consolidated	13,300,000		13,300,000		11,000,000
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract
Description of Derivative Activity Volume			NU also has NYMEX future contracts in order to reduce variability associated with the purchase price of approximately 9.1 million MMBtu of natural gas.
Effect Of Fair Value Hedges On Results Of Operations [Abstract]
Interest Realized In Earnings On Fair Value Hedge		2,500,000	2,500,000	7,900,000
Cash Flow Hedges Derivative Instruments At Fair Value Net [Abstract]
Cash Flow Hedge Gain Loss Reclassified To Interest Expense Net	900,000	400,000	2,400,000	600,000
Description Of Cash Flow Hedge Activity				In 2011, PSNH and WMECO settled interest rate swaps associated with $280 million and $50 million, respectively, of long-term debt issuances and as a result PSNH and WMECO recorded pre-tax reductions of $18.2 million and $6.9 million, respectively, to AOCI that are being amortized over the remaining lives of the associated debt.
Derivative Credit Risk Related Contingent Features Abstract
Derivative Net Liability Position Aggregate Fair Value	(15,800,000)		(15,800,000)		(23,500,000)
Collateral Already Posted Aggregate Fair Value	1,300,000		1,300,000		4,100,000
Additional Collateral Aggregate Fair Value	15,700,000		15,700,000		19,900,000
Minimum [Member]
Fairvalueinputsquantitativeinformationabstract [Abstract]
FairValueInputsDiscountForLackOfMarketability			10.00%
Fair Value inputs, Capacity Prices	1.4		1.4
Fair Value Inpts, Energy Prices	41		41
Fair Value Inputs, Renewable Energy Credit Prices	25		25
Fair Value Inputs, Forward Reserve	0.35		0.35
Maximum [Member]
Fairvalueinputsquantitativeinformationabstract [Abstract]
FairValueInputsDiscountForLackOfMarketability			32.00%
Fair Value inputs, Capacity Prices	10.53		10.53
Fair Value Inpts, Energy Prices	90		90
Fair Value Inputs, Renewable Energy Credit Prices	85		85
Fair Value Inputs, Forward Reserve	0.9		0.9
The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(97,536,000)		(97,536,000)		(95,881,000)
Derivative Liabilities, Noncurrent	(891,160,000)		(891,160,000)		(935,849,000)
Derivative Assets Noncurrent	91,989,000		91,989,000		93,755,000
The Connecticut Light And Power Company [Member] | Minimum [Member]
Fairvalueinputsquantitativeinformationabstract [Abstract]
Fair Value inputs, Capacity Prices	1.4		1.4
Fair Value Inpts, Energy Prices	47		47
Fair Value Inputs, Forward Reserve	0.35		0.35
The Connecticut Light And Power Company [Member] | Maximum [Member]
Fairvalueinputsquantitativeinformationabstract [Abstract]
Fair Value inputs, Capacity Prices	9.51		9.51
Fair Value Inpts, Energy Prices	55		55
Fair Value Inputs, Forward Reserve	0.9		0.9
Western Massachusetts Electric Company [Member] | Minimum [Member]
Fairvalueinputsquantitativeinformationabstract [Abstract]
Fair Value inputs, Capacity Prices	1.4		1.4
Fair Value Inpts, Energy Prices	41		41
Fair Value Inputs, Renewable Energy Credit Prices	25		25
Western Massachusetts Electric Company [Member] | Maximum [Member]
Fairvalueinputsquantitativeinformationabstract [Abstract]
Fair Value inputs, Capacity Prices	10.53		10.53
Fair Value Inpts, Energy Prices	90		90
Fair Value Inputs, Renewable Energy Credit Prices	85		85
NSTAR Electric Company [Member] | Minimum [Member]
Fairvalueinputsquantitativeinformationabstract [Abstract]
Fair Value inputs, Capacity Prices	1.4		1.4
Fair Value Inputs, Renewable Energy Credit Prices	25		25
NSTAR Electric Company [Member] | Maximum [Member]
Fairvalueinputsquantitativeinformationabstract [Abstract]
Fair Value inputs, Capacity Prices	10.18		10.18
Fair Value Inputs, Renewable Energy Credit Prices	63		63
Fair Value Inputs Level 2 [Member] | Public Service Company Of New Hampshire [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	600,000		600,000
Fair Value Inputs Level 2 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	2,100,000		2,100,000
Derivative Liabilities - Current	(15,800,000)		(15,800,000)
Derivative Liabilities, Noncurrent	(4,300,000)		(4,300,000)
Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	5,800,000		5,800,000		6,300,000
Derivative Liabilities - Current	(97,500,000)		(97,500,000)		(95,900,000)
Derivative Liabilities, Noncurrent	(891,200,000)		(891,200,000)		(935,800,000)
Derivative Assets Noncurrent	92,000,000		92,000,000		93,800,000
Fair Value Inputs Level 3 [Member] | Western Massachusetts Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current					(100,000)
Derivative Liabilities, Noncurrent	(3,700,000)		(3,700,000)		(7,200,000)
Fair Value Inputs Level 3 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	4,800,000		4,800,000		4,700,000
Derivative Liabilities - Current					(11,600,000)
Derivative Liabilities, Noncurrent					(16,900,000)
Derivative Assets Noncurrent	1,300,000		1,300,000		4,600,000
Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(600,000)		(600,000)
Derivative Liabilities, Noncurrent	(13,200,000)		(13,200,000)
Netting And Collateral [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	(12,400,000)		(12,400,000)		(11,600,000)
Derivative Liabilities - Current	1,700,000		1,700,000		4,500,000
Derivative Liabilities, Noncurrent	0		0		400,000
Derivative Assets Noncurrent	(72,000,000)		(72,000,000)		(80,400,000)
Netting And Collateral [Member] | Fair Value Inputs Level 2 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	(300,000)		(300,000)
Derivative Liabilities - Current	1,700,000		1,700,000
Netting And Collateral [Member] | Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	(12,100,000)		(12,100,000)		(11,600,000)
Derivative Assets Noncurrent	(72,000,000)		(72,000,000)		(80,400,000)
Netting And Collateral [Member] | Fair Value Inputs Level 3 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current					4,500,000
Derivative Liabilities, Noncurrent					400,000
Derivative Assets Noncurrent	0		0
Designated As Hedging Instrument [Member] | Fair Value Inputs Level 2 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current					2,300,000
Commodity [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	25,700,000		25,700,000		22,600,000
Derivative Liabilities - Current	(115,600,000)		(115,600,000)		(112,100,000)
Derivative Liabilities, Noncurrent	(912,400,000)		(912,400,000)		(960,300,000)
Derivative Assets Noncurrent	165,300,000		165,300,000		178,800,000
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract
Description of Derivative Activity Volume			As of September 30, 2012 and December 31, 2011, NU had approximately 38 thousand MWh and 123 thousand MWh, respectively, of supply volumes remaining in its unregulated wholesale portfolio when expected sales are compared with supply contracts. 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 have terms up to 15 years and obligate the utilities 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 2017. NSTAR Electric also has a capacity related contract for approximately 25 MW to 35 MW that extends through 2019. WMECO has a renewable energy contract to purchase 0.1 million MWh of energy per year through 2028 with a facility that is expected to achieve commercial operation by November 2013.
Derivative Instruments Not Designated As Hedging Instruments Gain Loss Net Abstract
Derivative Instruments Gain Loss Recognized In Income Net	200,000	300,000	(800,000)	1,000,000
Derivative Instruments Gain Loss Recognized In Regulatory Asset Liability	11,700,000	(47,700,000)	(25,000,000)	(92,800,000)
Commodity [Member] | Fair Value Inputs Level 2 [Member] | Public Service Company Of New Hampshire [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	600,000		600,000
Commodity [Member] | Fair Value Inputs Level 2 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	2,400,000		2,400,000
Derivative Liabilities - Current	(17,500,000)		(17,500,000)
Derivative Liabilities, Noncurrent	(4,300,000)		(4,300,000)
Commodity [Member] | Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	17,900,000		17,900,000		17,900,000
Derivative Liabilities - Current	(97,500,000)		(97,500,000)		(95,900,000)
Derivative Liabilities, Noncurrent	(891,200,000)		(891,200,000)		(935,800,000)
Derivative Assets Noncurrent	164,000,000		164,000,000		174,200,000
Commodity [Member] | Fair Value Inputs Level 3 [Member] | Western Massachusetts Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current					(100,000)
Derivative Liabilities, Noncurrent	(3,700,000)		(3,700,000)		(7,200,000)
Commodity [Member] | Fair Value Inputs Level 3 [Member] | Subsidiaries [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	4,800,000		4,800,000		4,700,000
Derivative Liabilities - Current					(16,100,000)
Derivative Liabilities, Noncurrent					(17,300,000)
Derivative Assets Noncurrent	1,300,000		1,300,000		4,600,000
Commodity [Member] | Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	(600,000)		(600,000)
Derivative Liabilities, Noncurrent	$ (13,200,000)		$ (13,200,000)		$ (3,400,000)