Merger of NU and NSTAR (Details) (USD $) In Millions, except Share data, unless otherwise specified	0 Months Ended	3 Months Ended		9 Months Ended
	Apr. 09, 2012	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Business Acquisition, Cost of Acquired Entity, Purchase Price [Abstract]
Number of NSTAR Shares Outstanding	103,696,000
Exchange Ratio	1.312
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares	136,049,000
Business Acquisition Share Price	$ 36.79
Business Acquisition, Cost of Acquired Entity, Equity Interests Issued and Issuable	$ 5,005.0
Business Acquisition, Cost of Acquired Entity, Other Noncash Consideration	33.0
Business Acquisition, Cost of Acquired Entity, Purchase Price	5,038.0	5,038.0		5,038.0
Business Acquisition, Purchase Price Allocation [Abstract]
Business Acquisition, Purchase Price Allocation, Current Assets		754.0		754.0
Business Acquisition, Purchase Price Allocation, Property, Plant and Equipment		5,155.0		5,155.0
Business Acquisition, Purchase Price Allocation, Goodwill Amount		3,231.0		3,231.0
Business Acquisition, Purchase Price Allocation, Other Noncurrent Assets		2,115.0		2,115.0
Business Acquisition, Purchase Price Allocation, Current Liabilities		(1,332.0)		(1,332.0)
Business Acquisition, Purchase Price Allocation, Other Noncurrent Liabilities		(2,723.0)		(2,723.0)
Business Acquisition, Purchase Price Allocation, Noncurrent Liabilities, Long-term Debt		(2,123.0)		(2,123.0)
Business Aquisition, Purchase Price Allocation, Preferred Stock		(39.0)		(39.0)
Total Purchase Price	5,038.0	5,038.0		5,038.0
Business Acquisition, Pro Forma Information [Abstract]
Business Acquisition, Pro Forma Revenue		1,862.0	1,883.0	5,321.0	5,637.0
Business Acquisition, Pro Forma Net Income (Loss)		208.0	190.0	455.0	518.0
Business Acquisition, Pro Forma Earnings Per Share, Basic		$ 0.66	$ 0.61	$ 1.45	$ 1.65
Business Acquisition, Pro Forma Earnings Per Share, Diluted		$ 0.66	$ 0.60	$ 1.44	$ 1.64
CustomerRateCreditsForBusinessCombination				46.0
StormCostDeferralReduction				40.0
EnergyEfficiencyFundEstablishment				15.0
TotalImpactOfSettlementAgreements				101.0
BusinessCombinationAfterTaxTransactionCosts		1.0	1.0	31.0	17.0
AfterTaxSettlementCostsIncurred		0	0	60.0	0
AfterTaxCostsIncurredinConnectionwithMerger		1.0	1.0	91.0	17.0
Business Combination, Description [Abstract]
Business Acquisition, Percentage of Voting Interests Acquired	100.00%
Common Stock, Shares, Outstanding	314,000,000
Business Acquisition, Number of Shares Awards Vested Immediately Prior to Merger				109,000
Business Aquisition, Description of Settlement Agreements				Regulatory Approvals: On February 15, 2012, NU and NSTAR reached comprehensive settlement agreements with the Massachusetts Attorney General and the DOER related to the merger. On April 4, 2012, the DPU approved the settlement agreements and the merger of NU and NSTAR. On March 13, 2012, NU and NSTAR reached a comprehensive settlement agreement with both the Connecticut Attorney General and the Connecticut Office of Consumer Counsel. On April 2, 2012, the PURA approved the settlement agreement and the merger of NU and NSTAR. The settlement agreements included base distribution rate freezes until December 1, 2014 for CL&P and through 2015 for NSTAR Electric, NSTAR Gas and WMECO.
Business Combination, Goodwill Recognized, Segment Allocation		Goodwill Allocation: NU's reporting units consist of the combined Electric Distribution, Electric Transmission and Natural Gas Distribution. The preliminary estimate of goodwill from the merger with NSTAR of $3.2 billion has been assigned to NU's reporting units based on relative fair values. As of September 30, 2012, the preliminary allocation of goodwill to each reporting unit was $2.3 billion to Electric Distribution, $0.7 billion to Electric Transmission and $0.5 billion to Natural Gas Distribution, including $0.3 billion of pre-existing goodwill applicable to Yankee Gas. NU's reporting units are consistent with its reportable business segments.
Nstar [Member]
Business Combination, Description [Abstract]
RevenuesfromAcquiredCompany		736.2		1,337.6
NetIncomeFromAcquiredCompany		105.0		141.0
The Connecticut Light And Power Company [Member]
Business Acquisition, Pro Forma Information [Abstract]
CustomerRateCreditsForBusinessCombination				25.0
StormCostDeferralReduction				40.0
TotalImpactOfSettlementAgreements				65.0
NSTAR Electric Company [Member]
Business Acquisition, Pro Forma Information [Abstract]
CustomerRateCreditsForBusinessCombination				15.0
TotalImpactOfSettlementAgreements				15.0
Western Massachusetts Electric Company [Member]
Business Acquisition, Pro Forma Information [Abstract]
CustomerRateCreditsForBusinessCombination				3.0
TotalImpactOfSettlementAgreements				$ 3.0