Merger of NU and NSTAR (Details) (USD $) In Millions, except Share data, unless otherwise specified	0 Months Ended	3 Months Ended		6 Months Ended
	Apr. 09, 2012	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 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		1,629.0	1,691.0	3,459.0	3,754.0
Business Acquisition, Pro Forma Net Income (Loss)		133.0	142.0	243.0	328.0
Business Acquisition, Pro Forma Earnings Per Share, Basic		$ 0.42	$ 0.45	$ 0.77	$ 1.05
Business Acquisition, Pro Forma Earnings Per Share, Diluted		$ 0.42	$ 0.45	$ 0.77	$ 1.04
CustomerRateCreditsForBusinessCombination		46.0		46.0
StormCostDeferralReduction		40.0		40.0
EnergyEfficiencyFundEstablishment		15.0		15.0
TotalImpactOfSettlementAgreements		101.0		101.0
BusinessCombinationAfterTaxTransactionCosts		29.0	2.0	31.0	16.0
AfterTaxSettlementCostsIncurred		60.0	0	60.0	0
AfterTaxCostsIncurredinConnectionwithMerger		89.0	2.0	91.0	16.0
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.
Nstar [Member]
Business Combination, Description [Abstract]
RevenuesfromAcquiredCompany				601.3
NetIncomeFromAcquiredCompany				35.9
The Connecticut Light And Power Company [Member]
Business Acquisition, Pro Forma Information [Abstract]
CustomerRateCreditsForBusinessCombination		25.0		25.0
StormCostDeferralReduction		40.0		40.0
TotalImpactOfSettlementAgreements		65.0		65.0
NSTAR Electric Company [Member]
Business Acquisition, Pro Forma Information [Abstract]
CustomerRateCreditsForBusinessCombination		15.0		15.0
TotalImpactOfSettlementAgreements		15.0		15.0
Western Massachusetts Electric Company [Member]
Business Acquisition, Pro Forma Information [Abstract]
CustomerRateCreditsForBusinessCombination		3.0		3.0
TotalImpactOfSettlementAgreements		$ 3.0		$ 3.0