PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSIONS (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, PBOP and SERP	$ 896,844,000	$ 2,130,497,000
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Amortization of actuarial gain or loss	10,500,000	8,900,000	5,700,000
OCI Amortization of Prior Service Cost	200,000	200,000	300,000
Defined Benefit Plan, Estimated Future Benefit Payments [Abstract]
Description of allocation of assets to subsidiaries	Effective January 1, 2013, the NSTAR Pension Plan assets were transferred into the NUSCO Pension Plan master trust. The, NUSCO Pension Plan is entitled to approximately 66 percent of each asset category in the master trust, the NSTAR Pension Plan is entitled to approximately 30 percent of each asset category in the master trust and the 401(h) plans are entitled to approximately four percent of each asset category in the master trust. CL&P, PSNH and WMECO participate in the NUSCO Pension and PBOP Plans. Each company participating in the plans is allocated a portion of the total plan assets. As of December 31, 2013 and 2012, the NUSCO Pension Plan had total assets of $2,750.4 million and $2,342.6 million, respectively. CL&P’s, PSNH’s and WMECO’s portion of these total Pension Plan assets was 37 percent, 19 percent and 9 percent, respectively, as of December 31, 2013, and 40 percent, 17 percent and 9 percent, respectively, as of December 31, 2012. The NUSCO PBOP Plans had total assets of $391 million and $334.9 million as of December 31, 2013 and 2012, respectively. CL&P’s, PSNH’s and WMECO’s portion of these total PBOP Plan assets was 39 percent, 21 percent and 9 percent, respectively, as of December 31, 2013 and 2012.
Pension Plans Defined Benefit Member
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
us-gaap_DefinedBenefitPlanBusinessCombinationsAndAcquisitionsBenefitObligation	0	(1,409,700,000)
Benefit Obligation Beginning Balance	(5,022,800,000)	(3,098,900,000)
Defined Benefit Plan Service Cost	(102,300,000)	(84,300,000)	(55,400,000)
Defined Benefit Plan Interest Cost	(206,700,000)	(198,300,000)	(153,300,000)
Defined Benefit Plan, Actuarial Net (Gains) Losses	433,600,000	(429,700,000)
Defined Benefit Plan, Benefits Paid	216,600,000	187,700,000
Defined Benefit Plan, Payments SERP	5,100,000	4,200,000
Benefit Obligation Ending Balance	(4,676,500,000)	(5,022,800,000)	(3,098,900,000)
DefinedBenefitPlanSettlementsBenefitObligation	0	6,200,000
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	102,300,000	84,300,000	55,400,000
Defined Benefit Plan Interest Cost	206,700,000	198,300,000	153,300,000
Defined Benefit Plan, Expected Return on Plan Assets	(278,100,000)	(220,900,000)	(170,800,000)
Defined Benefit Plan, Amortization of Gains (Losses)	210,500,000	172,400,000	84,200,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	4,000,000	7,900,000	9,700,000
Defined Benefit Plan Net Periodic Benefit Cost	245,400,000	242,000,000	131,800,000
Amount Capitalized Defined Benefit Expense	73,200,000	70,600,000	29,700,000
us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlementsAndCurtailments	0	2,200,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Defined Benefit Plan, Business Combinations and Acquisitions, Plan Assets		984,700,000
Fair Value of Plan Assets Beginning Balance	3,411,300,000	2,005,900,000
Defined Benefit Plan, Actual Return on Plan Assets	506,500,000	386,000,000
Defined Benefit Plan, Contributions by Employer	284,700,000	222,400,000
Defined Benefit Plan, Benefits Paid	(216,600,000)	(187,700,000)
Fair Value of Plan Assets Ending Balance	3,985,900,000	3,411,300,000	2,005,900,000
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	(690,600,000)	(1,611,500,000)
Defined Benefit Plan, Accumulated Benefit Obligation	4,538,800,000	4,622,100,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	3,411,300,000	2,005,900,000
Fair Value of Plan Assets Ending Balance	3,985,900,000	3,411,300,000	2,005,900,000
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, before Tax	(28,900,000)	19,100,000
Amortization of actuarial gain or loss	(9,400,000)	(7,800,000)
OCI Amortization of Prior Service Cost	(200,000)	(200,000)
RegulatoryAssetDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodBeforeTax	(635,200,000)	245,700,000
RegulatoryAssetReclassificationOfDefinedBenefitPlansNetGainLossRecognizedInNetPeriodicBenefitCostBeforeTax	(201,200,000)	(164,600,000)
RegulatoryAssetAmortizationOfDefinedBenefitPlanNetPriorServiceCostCreditRecognizedInNetPeriodicBenefitCostBeforeTax	(3,800,000)	(7,700,000)
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax [Abstract]
ension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax	43,200,000	81,500,000
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Prior Service Cost (Credit), before Tax	1,000,000	1,200,000
DefinedBenefitPlanRegulatoryAssetIncomeNetGainsLossesBeforeTax	1,137,400,000	1,973,800,000
DefinedBenefitPlanRegulatoryAssetNetPriorServiceCostCreditBeforeTax	17,400,000	21,200,000
Defined Benefit Amounts to be Reclassified from AOCI in next fiscal year
Defined Benefit Plan Future Amortization Of Gain Loss.	5,600,000
Defined Benefit Plan Future Amortization of Prior Service cost	200,000
RegulatoryAssetDefinedBenefitPlanAmortizationOfNetGainsLosses	126,200,000
RegulatoryAssetDefinedBenefitPlanAmortizationOfNetPriorServiceCostCredit	4,200,000
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	8.25%
Defined Benefit Plan, Estimated Future Benefit Payments [Abstract]
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months	263,300,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Two	273,300,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Three	282,900,000
Defined Benefit Plan, Expected Future Benefit Payments, Year Four	287,400,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Five	299,400,000
Defined Benefit Plan, Expected Future Benefit Payments in Five Fiscal Years Thereafter	1,617,000,000
DescriptionOfDefinedContributionPensionAndOtherPostretirementPlans	NU’s policy is to annually fund the NUSCO and NSTAR Pension Plans in an amount at least equal to an amount that will satisfy federal requirements. NU contributed $202.7 million to the NUSCO Pension Plan in 2013, of which $108.3 million was contributed by PSNH. NSTAR Electric contributed $82 million to the NSTAR Pension Plan in 2013. Based on the current status of the NUSCO Pension Plan, NU expects to make a contribution of $68.6 million in 2014. NSTAR Electric expects to make a contribution of $3 million in 2014 to the NSTAR Pension Plan.
Pension Plans Defined Benefit Member | Fair Value Inputs Level 1 Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	434,800,000	460,100,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	434,800,000	460,100,000
Pension Plans Defined Benefit Member | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	1,865,400,000	1,508,800,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	1,865,400,000	1,508,800,000
Pension Plans Defined Benefit Member | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	1,560,000,000	1,102,500,000
Fair Value of Plan Assets Ending Balance	1,850,700,000	1,560,000,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	1,560,000,000	1,102,500,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	137,600,000	108,500,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	31,100,000	23,000,000
Defined Benefit Plan, Purchases, Sales, and Settlements	122,000,000	47,000,000
Fair Value of Plan Assets Ending Balance	1,850,700,000	1,560,000,000
Level3PlanAssetsAssumedinMerger		279,000,000
Pension Plans Defined Benefit Member | United States Equity Securities [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	909,900,000
Fair Value of Plan Assets Ending Balance	1,086,300,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	909,900,000
Fair Value of Plan Assets Ending Balance	1,086,300,000
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	9.00%
Target Allocation	24.00%
Pension Plans Defined Benefit Member | United States Equity Securities [Member] | Fair Value Inputs Level 1 Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	294,600,000	336,500,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	294,600,000	336,500,000
Pension Plans Defined Benefit Member | United States Equity Securities [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	597,700,000	302,800,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	597,700,000	302,800,000
Pension Plans Defined Benefit Member | United States Equity Securities [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	270,600,000	259,400,000
Fair Value of Plan Assets Ending Balance	194,000,000	270,600,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	270,600,000	259,400,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	11,200,000	11,200,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	12,200,000
Defined Benefit Plan, Purchases, Sales, and Settlements	(100,000,000)
Fair Value of Plan Assets Ending Balance	194,000,000	270,600,000
Pension Plans Defined Benefit Member | International Equity Securities [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	456,700,000
Fair Value of Plan Assets Ending Balance	456,300,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	456,700,000
Fair Value of Plan Assets Ending Balance	456,300,000
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	9.00%
Target Allocation	10.00%
Pension Plans Defined Benefit Member | International Equity Securities [Member] | Fair Value Inputs Level 1 Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	32,200,000	42,000,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	32,200,000	42,000,000
Pension Plans Defined Benefit Member | International Equity Securities [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	362,600,000	362,600,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	362,600,000	362,600,000
Pension Plans Defined Benefit Member | International Equity Securities [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	52,100,000
Fair Value of Plan Assets Ending Balance	61,500,000	52,100,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	52,100,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	9,400,000	10,700,000
Fair Value of Plan Assets Ending Balance	61,500,000	52,100,000
Level3PlanAssetsAssumedinMerger		41,400,000
Pension Plans Defined Benefit Member | Emergining Markets Equity Securities [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	135,300,000
Fair Value of Plan Assets Ending Balance	211,800,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	135,300,000
Fair Value of Plan Assets Ending Balance	211,800,000
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	10.00%
Target Allocation	6.00%
Pension Plans Defined Benefit Member | Emergining Markets Equity Securities [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	211,800,000	135,300,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	211,800,000	135,300,000
Pension Plans Defined Benefit Member | Private Equity Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	294,600,000
Fair Value of Plan Assets Ending Balance	396,700,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	294,600,000
Fair Value of Plan Assets Ending Balance	396,700,000
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	13.00%
Target Allocation	10.00%
Pension Plans Defined Benefit Member | Private Equity Funds [Member] | Fair Value Inputs Level 1 Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	96,400,000	26,700,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	96,400,000	26,700,000
Pension Plans Defined Benefit Member | Private Equity Funds [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	267,900,000	255,100,000
Fair Value of Plan Assets Ending Balance	300,300,000	267,900,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	267,900,000	255,100,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	15,400,000	17,000,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	13,700,000	15,000,000
Defined Benefit Plan, Purchases, Sales, and Settlements	3,300,000	(19,200,000)
Fair Value of Plan Assets Ending Balance	300,300,000	267,900,000
Pension Plans Defined Benefit Member | Fixed Income Debt Securities [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	999,200,000
Fair Value of Plan Assets Ending Balance	1,206,200,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	999,200,000
Fair Value of Plan Assets Ending Balance	1,206,200,000
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	5.00%
Target Allocation	15.00%
Pension Plans Defined Benefit Member | Fixed Income Debt Securities [Member] | Fair Value Inputs Level 1 Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	11,600,000	54,900,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	11,600,000	54,900,000
Pension Plans Defined Benefit Member | Fixed Income Debt Securities [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	605,100,000	629,200,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	605,100,000	629,200,000
Pension Plans Defined Benefit Member | Fixed Income Debt Securities [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	315,100,000	276,200,000
Fair Value of Plan Assets Ending Balance	589,500,000	315,100,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	315,100,000	276,200,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	55,300,000	42,100,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	(1,000,000)	700,000
Defined Benefit Plan, Purchases, Sales, and Settlements	187,600,000	(3,900,000)
Fair Value of Plan Assets Ending Balance	589,500,000	315,100,000
TransferBetweenCategories	32,500,000
Pension Plans Defined Benefit Member | High Yield Fixed Income Debt Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	8.00%
Target Allocation	9.00%
Pension Plans Defined Benefit Member | Emerging Markets Debt Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	8.00%
Target Allocation	6.00%
Pension Plans Defined Benefit Member | Hedge Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	418,900,000
Fair Value of Plan Assets Ending Balance	416,900,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	418,900,000
Fair Value of Plan Assets Ending Balance	416,900,000
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	7.00%
Target Allocation	11.00%
Pension Plans Defined Benefit Member | Hedge Funds [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	418,900,000	240,000,000
Fair Value of Plan Assets Ending Balance	416,900,000	418,900,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	418,900,000	240,000,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	33,400,000	21,800,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	0	(300,000)
Defined Benefit Plan, Purchases, Sales, and Settlements	(2,900,000)	30,800,000
Fair Value of Plan Assets Ending Balance	416,900,000	418,900,000
Level3PlanAssetsAssumedinMerger		126,600,000
TransferBetweenCategories	(32,500,000)
Pension Plans Defined Benefit Member | Real Estate [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	314,300,000
Fair Value of Plan Assets Ending Balance	376,700,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	314,300,000
Fair Value of Plan Assets Ending Balance	376,700,000
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	7.50%
Target Allocation	9.00%
Pension Plans Defined Benefit Member | Real Estate [Member] | Fair Value Inputs Level 1 Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	0	0
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	0	0
Pension Plans Defined Benefit Member | Real Estate [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	88,200,000	78,900,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	88,200,000	78,900,000
Pension Plans Defined Benefit Member | Real Estate [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	235,400,000	71,800,000
Fair Value of Plan Assets Ending Balance	288,500,000	235,400,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	235,400,000	71,800,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	12,900,000	5,700,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	6,200,000	7,600,000
Defined Benefit Plan, Purchases, Sales, and Settlements	34,000,000	39,300,000
Fair Value of Plan Assets Ending Balance	288,500,000	235,400,000
Level3PlanAssetsAssumedinMerger		111,000,000
Pension Plans Defined Benefit Member | Assetsrelatedto401h [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	165,000,000	117,600,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	165,000,000	117,600,000
PBOP Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
us-gaap_DefinedBenefitPlanBusinessCombinationsAndAcquisitionsBenefitObligation		(770,600,000)
Benefit Obligation Beginning Balance	(1,233,300,000)	(520,900,000)
Defined Benefit Plan Service Cost	(16,900,000)	(15,700,000)	(9,200,000)
Defined Benefit Plan Interest Cost	(47,200,000)	(49,000,000)	(25,700,000)
Defined Benefit Plan, Actuarial Net (Gains) Losses	200,900,000	70,900,000
Defined Benefit Plan, Benefits Paid	58,500,000	58,200,000
Defined Benefit Plan, Gross Prescription Drug Subsidy Receipts Received	0	(6,200,000)
Benefit Obligation Ending Balance	(1,038,000,000)	(1,233,300,000)	(520,900,000)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	16,900,000	15,700,000	9,200,000
Defined Benefit Plan Interest Cost	47,200,000	49,000,000	25,700,000
Defined Benefit Plan, Expected Return on Plan Assets	(55,400,000)	(39,200,000)	(21,600,000)
Defined Benefit Plan, Amortization of Gains (Losses)	26,000,000	36,000,000	19,000,000
Defined Benefit Plan Amortization Of Transition Obligations Assets	0	12,200,000	11,600,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	(2,100,000)	(1,400,000)	(300,000)
Defined Benefit Plan Net Periodic Benefit Cost	32,600,000	72,300,000	43,600,000
Amount Capitalized Defined Benefit Expense	8,800,000	26,600,000	12,700,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Defined Benefit Plan, Business Combinations and Acquisitions, Plan Assets		330,400,000
Fair Value of Plan Assets Beginning Balance	709,100,000	285,400,000
Defined Benefit Plan, Actual Return on Plan Assets	118,300,000	78,800,000
Defined Benefit Plan, Contributions by Employer	57,600,000	72,700,000
Defined Benefit Plan, Benefits Paid	(58,500,000)	(58,200,000)
Fair Value of Plan Assets Ending Balance	826,500,000	709,100,000	285,400,000
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	(211,500,000)	(524,200,000)
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Defined Benefit Plan, Description of Direction and Pattern of Change for Assumed Health Care Cost Trend Rate	The health care cost trend rate assumption used to calculate the 2013 PBOP expense amounts was 7 percent for the NUSCO PBOP Plan, subsequently decreasing by 50 basis points per year to an ultimate rate of 5 percent in 2017, and 7.10 percent for the NSTAR PBOP Plan, subsequently decreasing to an ultimate rate of 4.5 percent in 2024. As of December 31, 2013, the health care cost trend rate assumption used to determine the NUSCO and NSTAR PBOP Plans’ year end funded status is 7 percent, subsequently decreasing to an ultimate rate of 4.5 percent in 2024.
Defined Benefit Plan, Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates [Abstract]
Defined Benefit Plan, Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation	85,800,000
Defined Benefit Plan, Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation	(70,400,000)
Defined Benefit Plan, Effect of One Percentage Point Increase on Service and Interest Cost Components	7,100,000
Defined Benefit Plan, Effect of One Percentage Point Decrease on Service and Interest Cost Components	(5,500,000)
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	709,100,000	285,400,000
Fair Value of Plan Assets Ending Balance	826,500,000	709,100,000	285,400,000
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, before Tax	(1,900,000)	(1,800,000)
Amortization of actuarial gain or loss	(1,100,000)	(1,100,000)
Other Comprehensive Income (Loss), Reclassification, Pension and Other Postretirement Benefit Plans, Net Transition Asset (Obligation), Recognized in Net Periodic Benefit Cost, before Tax	0	(200,000)
RegulatoryAssetDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodBeforeTax	(262,000,000)	(108,600,000)
RegulatoryAssetReclassificationOfDefinedBenefitPlansNetGainLossRecognizedInNetPeriodicBenefitCostBeforeTax	(24,900,000)	(34,900,000)
RegulatoryAssetAmortizationOfDefinedBenefitPlanNetPriorServiceCostCreditRecognizedInNetPeriodicBenefitCostBeforeTax	2,100,000	1,400,000
RegulatoryAssetReclassificationOfDefinedBenefitPlanNetTransitionAssetObligationRecognizedInNetPeriodicBenefitCostBeforeTax	0	(11,900,000)
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax [Abstract]
ension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax	6,200,000	9,200,000
DefinedBenefitPlanRegulatoryAssetIncomeNetGainsLossesBeforeTax	89,200,000	376,100,000
DefinedBenefitPlanRegulatoryAssetNetPriorServiceCostCreditBeforeTax	(4,600,000)	(6,700,000)
Defined Benefit Amounts to be Reclassified from AOCI in next fiscal year
Defined Benefit Plan Future Amortization Of Gain Loss.	700,000
RegulatoryAssetDefinedBenefitPlanAmortizationOfNetGainsLosses	11,400,000
RegulatoryAssetDefinedBenefitPlanAmortizationOfNetPriorServiceCostCredit	(2,800,000)
Defined Benefit Plan, Estimated Future Benefit Payments [Abstract]
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months	61,600,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Two	63,300,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Three	64,500,000
Defined Benefit Plan, Expected Future Benefit Payments, Year Four	65,600,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Five	66,600,000
Defined Benefit Plan, Expected Future Benefit Payments in Five Fiscal Years Thereafter	344,400,000
DescriptionOfDefinedContributionPensionAndOtherPostretirementPlans	For the PBOP Plans, it is NU’s policy to annually fund the NUSCO PBOP Plans in an amount equal to the PBOP Plans' postretirement benefit cost, excluding curtailment and termination benefits, and the NSTAR PBOP Plan in an amount that approximates annual benefit payments. NU contributed $57.6 million to the PBOP Plans in 2013 and expects to make $39.7 million in contributions in 2014.
PBOP Plan | Fair Value Inputs Level 1 Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	121,400,000	201,700,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	121,400,000	201,700,000
PBOP Plan | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	310,700,000	243,800,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	310,700,000	243,800,000
PBOP Plan | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	146,000,000	60,400,000
Fair Value of Plan Assets Ending Balance	229,400,000	146,000,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	146,000,000	60,400,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	35,700,000	16,600,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	100,000
Defined Benefit Plan, Purchases, Sales, and Settlements	47,600,000	9,500,000
Fair Value of Plan Assets Ending Balance	229,400,000	146,000,000
Level3PlanAssetsAssumedinMerger		59,500,000
PBOP Plan | United States Equity Securities [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	256,700,000	210,300,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	256,700,000	210,300,000
PBOP Plan | United States Equity Securities [Member] | Fair Value Inputs Level 1 Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	67,000,000	116,300,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	67,000,000	116,300,000
PBOP Plan | United States Equity Securities [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	120,600,000	57,700,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	120,600,000	57,700,000
PBOP Plan | United States Equity Securities [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	36,300,000	10,700,000
Fair Value of Plan Assets Ending Balance	69,100,000	36,300,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	36,300,000	10,700,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	20,800,000	5,900,000
Defined Benefit Plan, Purchases, Sales, and Settlements	12,000,000	0
Fair Value of Plan Assets Ending Balance	69,100,000	36,300,000
Level3PlanAssetsAssumedinMerger		19,700,000
PBOP Plan | International Equity Securities [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	70,900,000	97,700,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	70,900,000	97,700,000
PBOP Plan | International Equity Securities [Member] | Fair Value Inputs Level 1 Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	28,100,000	68,000,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	28,100,000	68,000,000
PBOP Plan | International Equity Securities [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	42,800,000	29,700,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	42,800,000	29,700,000
PBOP Plan | Emergining Markets Equity Securities [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	28,600,000	21,700,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	28,600,000	21,700,000
PBOP Plan | Emergining Markets Equity Securities [Member] | Fair Value Inputs Level 1 Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	15,200,000	7,700,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	15,200,000	7,700,000
PBOP Plan | Emergining Markets Equity Securities [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	13,400,000	14,000,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	13,400,000	14,000,000
PBOP Plan | Private Equity Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	17,900,000	11,300,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	17,900,000	11,300,000
PBOP Plan | Private Equity Funds [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	11,300,000	5,100,000
Fair Value of Plan Assets Ending Balance	17,900,000	11,300,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	11,300,000	5,100,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	1,500,000	1,600,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	200,000
Defined Benefit Plan, Purchases, Sales, and Settlements	4,900,000	4,600,000
Fair Value of Plan Assets Ending Balance	17,900,000	11,300,000
PBOP Plan | Fixed Income Debt Securities [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	171,200,000	169,800,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	171,200,000	169,800,000
PBOP Plan | Fixed Income Debt Securities [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	119,700,000	137,700,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	119,700,000	137,700,000
PBOP Plan | Fixed Income Debt Securities [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	32,100,000	26,000,000
Fair Value of Plan Assets Ending Balance	51,500,000	32,100,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	32,100,000	26,000,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	4,100,000	4,000,000
Defined Benefit Plan, Purchases, Sales, and Settlements	15,300,000	2,100,000
Fair Value of Plan Assets Ending Balance	51,500,000	32,100,000
PBOP Plan | Hedge Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	57,000,000	39,600,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	57,000,000	39,600,000
PBOP Plan | Hedge Funds [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	39,600,000	16,100,000
Fair Value of Plan Assets Ending Balance	57,000,000	39,600,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	39,600,000	16,100,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	5,400,000	2,100,000
Defined Benefit Plan, Purchases, Sales, and Settlements	12,000,000
Fair Value of Plan Assets Ending Balance	57,000,000	39,600,000
Level3PlanAssetsAssumedinMerger		21,400,000
PBOP Plan | Real Estate [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	48,100,000	31,400,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	48,100,000	31,400,000
PBOP Plan | Real Estate [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	14,200,000	4,700,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	14,200,000	4,700,000
PBOP Plan | Real Estate [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	26,700,000	2,500,000
Fair Value of Plan Assets Ending Balance	33,900,000	26,700,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	26,700,000	2,500,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	3,900,000	3,000,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	(100,000)
Defined Benefit Plan, Purchases, Sales, and Settlements	3,400,000	2,800,000
Fair Value of Plan Assets Ending Balance	33,900,000	26,700,000
Level3PlanAssetsAssumedinMerger		18,400,000
PBOP Plan | Cash And Cash Equivalents Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	11,100,000	9,700,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	11,100,000	9,700,000
PBOP Plan | Cash And Cash Equivalents Member | Fair Value Inputs Level 1 Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	11,100,000	9,700,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	11,100,000	9,700,000
PBOP Plan | Assetsrelatedto401h [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	165,000,000	117,600,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	165,000,000	117,600,000
NUSCO Pension Plan [Member]
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	5.03%	4.24%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase	3.50%	3.50%
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate	4.24%	5.03%	5.57%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase	3.50%	3.50%	3.50%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	8.25%	8.25%	8.25%
Nstar pbop Plans [Member]
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	5.10%	4.35%
Health Care Trend Assumption Used In Funded Status	7.00%	7.10%
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate	4.35%	4.58%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	8.25%	7.30%
Nstar pbop Plans [Member] | United States Equity Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets		8.30%
Target Allocation		25.00%
Nstar pbop Plans [Member] | International Equity Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets		8.60%
Target Allocation		20.00%
Nstar pbop Plans [Member] | Emergining Markets Equity Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets		8.80%
Target Allocation		5.00%
Nstar pbop Plans [Member] | Fixed Income Debt Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets		4.60%
Target Allocation		30.00%
Nstar pbop Plans [Member] | Hedge Funds [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets		8.40%
Target Allocation		10.00%
Nstar pbop Plans [Member] | Real Estate [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets		7.90%
Target Allocation		10.00%
NUSCO PBOP Plan [Member]
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	4.78%	4.04%
Health Care Trend Assumption Used In Funded Status	7.00%	7.00%
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate	4.04%	4.84%	5.28%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	8.25%	8.25%	8.25%
NUSCOPensionandPBOP [Member] | United States Equity Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets		9.00%
Target Allocation		24.00%
NUSCOPensionandPBOP [Member] | International Equity Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets		9.00%
Target Allocation		13.00%
NUSCOPensionandPBOP [Member] | Emergining Markets Equity Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets		10.00%
Target Allocation		3.00%
NUSCOPensionandPBOP [Member] | Private Equity Funds [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets		13.00%
Target Allocation		12.00%
NUSCOPensionandPBOP [Member] | Fixed Income Debt Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets		5.00%
Target Allocation		20.00%
NUSCOPensionandPBOP [Member] | High Yield Fixed Income Debt Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets		7.50%
Target Allocation		3.50%
NUSCOPensionandPBOP [Member] | Emerging Markets Debt Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets		7.50%
Target Allocation		3.50%
NUSCOPensionandPBOP [Member] | Hedge Funds [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets		7.00%
Target Allocation		13.00%
NUSCOPensionandPBOP [Member] | Real Estate [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets		7.50%
Target Allocation		8.00%
The Connecticut Light And Power Company [Member]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, PBOP and SERP	95,895,000	304,696,000
The Connecticut Light And Power Company [Member] | Pension Plans Defined Benefit Member
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation Beginning Balance	(1,178,000,000)	(1,043,800,000)
Defined Benefit Plan Service Cost	(24,900,000)	(21,800,000)	(19,500,000)
Defined Benefit Plan Interest Cost	(48,300,000)	(51,200,000)	(51,900,000)
Defined Benefit Plan, Actuarial Net (Gains) Losses	110,700,000	(117,400,000)
Defined Benefit Plan, Benefits Paid	56,600,000	55,900,000
Defined Benefit Plan, Payments SERP	500,000	300,000
Benefit Obligation Ending Balance	(1,083,400,000)	(1,178,000,000)	(1,043,800,000)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	24,900,000	21,800,000	19,500,000
Defined Benefit Plan Interest Cost	48,300,000	51,200,000	51,900,000
Defined Benefit Plan, Expected Return on Plan Assets	(73,800,000)	(70,600,000)	(76,600,000)
Defined Benefit Plan, Amortization of Gains (Losses)	55,900,000	49,600,000	33,400,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	1,800,000	3,600,000	4,200,000
Defined Benefit Plan Net Periodic Benefit Cost	57,100,000	55,600,000	32,400,000
Related Intercompany Allocationsof Defined Benefit Expense	44,900,000	42,800,000	34,100,000
Amount Capitalized Defined Benefit Expense	28,000,000	26,800,000	16,600,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	937,600,000	869,600,000
Defined Benefit Plan, Actual Return on Plan Assets	135,300,000	123,900,000
Defined Benefit Plan, Contributions by Employer		0
Defined Benefit Plan, Benefits Paid	(56,600,000)	(55,900,000)
Fair Value of Plan Assets Ending Balance	1,016,300,000	937,600,000	869,600,000
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	(67,100,000)	(240,400,000)
Defined Benefit Plan, Accumulated Benefit Obligation	1,058,000,000	1,061,800,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	937,600,000	869,600,000
Fair Value of Plan Assets Ending Balance	1,016,300,000	937,600,000	869,600,000
The Connecticut Light And Power Company [Member] | PBOP Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation Beginning Balance	(196,800,000)	(198,900,000)
Defined Benefit Plan Service Cost	(3,400,000)	(3,000,000)	(2,900,000)
Defined Benefit Plan Interest Cost	(7,900,000)	(9,200,000)	(10,000,000)
Defined Benefit Plan, Actuarial Net (Gains) Losses	13,300,000	1,200,000
Defined Benefit Plan, Benefits Paid	14,400,000	14,800,000
Defined Benefit Plan, Gross Prescription Drug Subsidy Receipts Received	0	(1,700,000)
Benefit Obligation Ending Balance	(180,400,000)	(196,800,000)	(198,900,000)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	3,400,000	3,000,000	2,900,000
Defined Benefit Plan Interest Cost	7,900,000	9,200,000	10,000,000
Defined Benefit Plan, Expected Return on Plan Assets	(10,100,000)	(9,100,000)	(8,700,000)
Defined Benefit Plan, Amortization of Gains (Losses)	7,400,000	7,500,000	7,200,000
Defined Benefit Plan Amortization Of Transition Obligations Assets	0	6,100,000	6,200,000
Defined Benefit Plan Net Periodic Benefit Cost	8,600,000	16,700,000	17,600,000
Related Intercompany Allocationsof Defined Benefit Expense	7,100,000	7,900,000	8,200,000
Amount Capitalized Defined Benefit Expense	3,900,000	8,200,000	8,700,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	132,200,000	112,200,000
Defined Benefit Plan, Actual Return on Plan Assets	24,800,000	15,000,000
Defined Benefit Plan, Contributions by Employer	8,700,000	19,800,000
Defined Benefit Plan, Benefits Paid	(14,400,000)	(14,800,000)
Fair Value of Plan Assets Ending Balance	151,300,000	132,200,000	112,200,000
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	(29,100,000)	(64,600,000)
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	132,200,000	112,200,000
Fair Value of Plan Assets Ending Balance	151,300,000	132,200,000	112,200,000
NSTAR Electric Company [Member]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, PBOP and SERP	118,010,000	360,932,000
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
RegulatoryAssetAmortizationOfDefinedBenefitPlanNetPriorServiceCostCreditRecognizedInNetPeriodicBenefitCostBeforeTax		(600,000)
Defined Benefit Plan, Estimated Future Benefit Payments [Abstract]
Pension Contribution	82,000,000	25,000,000	125,000,000
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation Beginning Balance	(1,430,000,000)	(1,346,200,000)
Defined Benefit Plan Service Cost	(33,100,000)	(30,300,000)	(26,000,000)
Defined Benefit Plan Interest Cost	(58,000,000)	(58,900,000)	(61,000,000)
Defined Benefit Plan, Actuarial Net (Gains) Losses	96,600,000	(63,600,000)
Defined Benefit Plan, Benefits Paid	71,200,000	69,000,000
Defined Benefit Plan, Payments SERP		0
Benefit Obligation Ending Balance	(1,353,300,000)	(1,430,000,000)	(1,346,200,000)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	33,100,000	30,300,000	26,000,000
Defined Benefit Plan Interest Cost	58,000,000	58,900,000	61,000,000
Defined Benefit Plan, Expected Return on Plan Assets	(84,400,000)	(65,600,000)	(71,400,000)
Defined Benefit Plan, Amortization of Gains (Losses)	58,100,000	63,100,000	48,600,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	(300,000)	(600,000)	(700,000)
Defined Benefit Plan Net Periodic Benefit Cost	64,500,000	86,100,000	63,500,000
Related Intercompany Allocationsof Defined Benefit Expense	(8,400,000)	(12,300,000)	(10,200,000)
Amount Capitalized Defined Benefit Expense	28,900,000	30,700,000	19,800,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	1,069,100,000	988,500,000
Defined Benefit Plan, Actual Return on Plan Assets	155,400,000	124,600,000
Defined Benefit Plan, Contributions by Employer	82,000,000	25,000,000
Defined Benefit Plan, Benefits Paid	(71,200,000)	(69,000,000)
Fair Value of Plan Assets Ending Balance	1,235,300,000	1,069,100,000	988,500,000
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	(118,000,000)	(360,900,000)
Defined Benefit Plan, Accumulated Benefit Obligation	1,280,600,000	1,353,100,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	1,069,100,000	988,500,000
Fair Value of Plan Assets Ending Balance	1,235,300,000	1,069,100,000	988,500,000
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
RegulatoryAssetDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodBeforeTax	168,000,000	4,600,000
RegulatoryAssetReclassificationOfDefinedBenefitPlansNetGainLossRecognizedInNetPeriodicBenefitCostBeforeTax	(58,100,000)	(62,800,000)
RegulatoryAssetAmortizationOfDefinedBenefitPlanNetPriorServiceCostCreditRecognizedInNetPeriodicBenefitCostBeforeTax		(300,000)
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax [Abstract]
DefinedBenefitPlanRegulatoryAssetIncomeNetGainsLossesBeforeTax	497,900,000	724,000,000
Defined Benefit Plan, Estimated Future Benefit Payments [Abstract]
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months	88,000,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Two	90,600,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Three	88,400,000
Defined Benefit Plan, Expected Future Benefit Payments, Year Four	88,500,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Five	90,000,000
Defined Benefit Plan, Expected Future Benefit Payments in Five Fiscal Years Thereafter	449,200,000
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member | Fair Value Inputs Level 1 Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	129,400,000	151,600,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	129,400,000	151,600,000
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	555,300,000	693,100,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	555,300,000	693,100,000
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	302,000,000	279,000,000
Fair Value of Plan Assets Ending Balance	550,600,000	302,000,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	302,000,000	279,000,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	40,700,000	26,200,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	9,300,000	(300,000)
Defined Benefit Plan, Purchases, Sales, and Settlements	36,300,000	(2,900,000)
Fair Value of Plan Assets Ending Balance	550,600,000	302,000,000
TransferResultingFromPlanAssetConsolidations	162,300,000
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member | United States Equity Securities [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	343,100,000
Fair Value of Plan Assets Ending Balance	323,400,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	343,100,000
Fair Value of Plan Assets Ending Balance	323,400,000
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	9.00%
Target Allocation	24.00%
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member | United States Equity Securities [Member] | Fair Value Inputs Level 1 Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	87,700,000	96,700,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	87,700,000	96,700,000
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member | United States Equity Securities [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	177,900,000	246,400,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	177,900,000	246,400,000
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member | United States Equity Securities [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	57,800,000
Level 3 Rollforwards
Defined Benefit Plan, Actual Return on Plan Assets Still Held	3,500,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	3,600,000
Defined Benefit Plan, Purchases, Sales, and Settlements	(29,800,000)
Fair Value of Plan Assets Ending Balance	57,800,000
TransferResultingFromPlanAssetConsolidations	80,500,000
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member | International Equity Securities [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	150,400,000
Fair Value of Plan Assets Ending Balance	135,900,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	150,400,000
Fair Value of Plan Assets Ending Balance	135,900,000
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	9.00%
Target Allocation	10.00%
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member | International Equity Securities [Member] | Fair Value Inputs Level 1 Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance		0
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance		0
Level3PlanAssetsAssumedinMerger	9,600,000
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member | International Equity Securities [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	108,000,000	98,300,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	108,000,000	98,300,000
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member | International Equity Securities [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	52,100,000	41,400,000
Fair Value of Plan Assets Ending Balance	18,300,000	52,100,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	52,100,000	41,400,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	2,800,000	10,700,000
Fair Value of Plan Assets Ending Balance	18,300,000	52,100,000
TransferResultingFromPlanAssetConsolidations	(36,600,000)
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member | Emergining Markets Equity Securities [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	55,900,000
Fair Value of Plan Assets Ending Balance	63,100,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	55,900,000
Fair Value of Plan Assets Ending Balance	63,100,000
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	10.00%
Target Allocation	6.00%
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member | Emergining Markets Equity Securities [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	63,100,000	55,900,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	63,100,000	55,900,000
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member | Private Equity Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	118,100,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	118,100,000
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	13.00%
Target Allocation	10.00%
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member | Private Equity Funds [Member] | Fair Value Inputs Level 1 Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	28,700,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	28,700,000
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member | Private Equity Funds [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	89,400,000
Level 3 Rollforwards
Defined Benefit Plan, Actual Return on Plan Assets Still Held	4,600,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	4,200,000
Defined Benefit Plan, Purchases, Sales, and Settlements	900,000
Fair Value of Plan Assets Ending Balance	89,400,000
TransferResultingFromPlanAssetConsolidations	79,700,000
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member | Fixed Income Debt Securities [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	347,400,000
Fair Value of Plan Assets Ending Balance	358,800,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	347,400,000
Fair Value of Plan Assets Ending Balance	358,800,000
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	5.00%
Target Allocation	15.00%
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member | Fixed Income Debt Securities [Member] | Fair Value Inputs Level 1 Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	3,400,000	54,900,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	3,400,000	54,900,000
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member | Fixed Income Debt Securities [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	180,000,000	292,500,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	180,000,000	292,500,000
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member | Fixed Income Debt Securities [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	175,400,000
Level 3 Rollforwards
Defined Benefit Plan, Actual Return on Plan Assets Still Held	16,400,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	(300,000)
Defined Benefit Plan, Purchases, Sales, and Settlements	55,800,000
Fair Value of Plan Assets Ending Balance	175,400,000
TransferResultingFromPlanAssetConsolidations	93,800,000
TransferBetweenCategories	9,700,000
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member | Hedge Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	122,700,000
Fair Value of Plan Assets Ending Balance	124,100,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	122,700,000
Fair Value of Plan Assets Ending Balance	124,100,000
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	7.00%
Target Allocation	11.00%
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member | Hedge Funds [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	122,700,000	126,600,000
Fair Value of Plan Assets Ending Balance	124,100,000	122,700,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	122,700,000	126,600,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	9,900,000	5,600,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period		(300,000)
Defined Benefit Plan, Purchases, Sales, and Settlements	(800,000)	(9,200,000)
Fair Value of Plan Assets Ending Balance	124,100,000	122,700,000
TransferResultingFromPlanAssetConsolidations	2,000,000
TransferBetweenCategories	(9,700,000)
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member | Real Estate [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	127,200,000
Fair Value of Plan Assets Ending Balance	111,900,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	127,200,000
Fair Value of Plan Assets Ending Balance	111,900,000
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	7.50%
Target Allocation	9.00%
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member | Real Estate [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	26,300,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	26,300,000
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member | Real Estate [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	127,200,000	111,000,000
Fair Value of Plan Assets Ending Balance	85,600,000	127,200,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	127,200,000	111,000,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	3,500,000	9,900,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	1,800,000
Defined Benefit Plan, Purchases, Sales, and Settlements	10,200,000	6,300,000
Fair Value of Plan Assets Ending Balance	85,600,000	127,200,000
TransferResultingFromPlanAssetConsolidations	(57,100,000)
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member | Assetsrelatedto401h [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance		77,600,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance		77,600,000
NSTAR Electric Company [Member] | Supplemental Employee Retirement Plans, Defined Benefit [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Net Periodic Benefit Cost	3,200,000	3,600,000	4,100,000
NSTAR Electric Company [Member] | NSTAR Pension Plan [Member]
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	4.85%	4.13%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase	4.00%	4.00%
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate	4.13%	4.52%	5.30%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase	4.00%	4.00%	4.00%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	8.25%	7.30%	8.00%
NSTAR Electric Company [Member] | NSTAR Pension Plan [Member] | United States Equity Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets		8.30%
Target Allocation		25.00%
NSTAR Electric Company [Member] | NSTAR Pension Plan [Member] | International Equity Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets		8.60%
Target Allocation		13.00%
NSTAR Electric Company [Member] | NSTAR Pension Plan [Member] | Emergining Markets Equity Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets		8.80%
Target Allocation		5.00%
NSTAR Electric Company [Member] | NSTAR Pension Plan [Member] | Fixed Income Debt Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets		4.60%
Target Allocation		21.00%
NSTAR Electric Company [Member] | NSTAR Pension Plan [Member] | High Yield Fixed Income Debt Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets		6.50%
Target Allocation		9.00%
NSTAR Electric Company [Member] | NSTAR Pension Plan [Member] | Emerging Markets Debt Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets		6.40%
Target Allocation		4.00%
NSTAR Electric Company [Member] | NSTAR Pension Plan [Member] | Hedge Funds [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets		8.40%
Target Allocation		13.00%
NSTAR Electric Company [Member] | NSTAR Pension Plan [Member] | Real Estate [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets		7.90%
Target Allocation		10.00%
Public Service Company Of New Hampshire [Member]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, PBOP and SERP	15,272,000	33,981,000
Public Service Company Of New Hampshire [Member] | Pension Plans Defined Benefit Member
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation Beginning Balance	(576,000,000)	(497,900,000)
Defined Benefit Plan Service Cost	(13,100,000)	(11,800,000)	(10,600,000)
Defined Benefit Plan Interest Cost	(23,600,000)	(24,400,000)	(24,400,000)
Defined Benefit Plan, Actuarial Net (Gains) Losses	62,400,000	(61,300,000)
Defined Benefit Plan, Benefits Paid	21,100,000	19,700,000
Defined Benefit Plan, Payments SERP	200,000	0
Benefit Obligation Ending Balance	(529,000,000)	(576,000,000)	(497,900,000)
DefinedBenefitPlanSettlementsBenefitObligation	0	(300,000)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	13,100,000	11,800,000	10,600,000
Defined Benefit Plan Interest Cost	23,600,000	24,400,000	24,400,000
Defined Benefit Plan, Expected Return on Plan Assets	(35,400,000)	(28,200,000)	(19,800,000)
Defined Benefit Plan, Amortization of Gains (Losses)	21,600,000	16,200,000	10,700,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	700,000	1,500,000	1,800,000
Defined Benefit Plan Net Periodic Benefit Cost	23,600,000	25,700,000	27,700,000
Related Intercompany Allocationsof Defined Benefit Expense	10,500,000	10,100,000	7,600,000
Amount Capitalized Defined Benefit Expense	7,300,000	7,900,000	7,600,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	386,600,000	279,700,000
Defined Benefit Plan, Actual Return on Plan Assets	54,800,000	38,900,000
Defined Benefit Plan, Contributions by Employer	108,300,000	87,700,000
Defined Benefit Plan, Benefits Paid	(21,100,000)	(19,700,000)
Fair Value of Plan Assets Ending Balance	528,600,000	386,600,000	279,700,000
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	(400,000)	(189,400,000)
Defined Benefit Plan, Accumulated Benefit Obligation	520,100,000	515,900,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	386,600,000	279,700,000
Fair Value of Plan Assets Ending Balance	528,600,000	386,600,000	279,700,000
Public Service Company Of New Hampshire [Member] | PBOP Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation Beginning Balance	(100,200,000)	(99,200,000)
Defined Benefit Plan Service Cost	(2,300,000)	(2,000,000)	(1,900,000)
Defined Benefit Plan Interest Cost	(4,000,000)	(4,600,000)	(4,800,000)
Defined Benefit Plan, Actuarial Net (Gains) Losses	7,200,000	300,000
Defined Benefit Plan, Benefits Paid	5,800,000	5,900,000
Defined Benefit Plan, Gross Prescription Drug Subsidy Receipts Received	0	(600,000)
Benefit Obligation Ending Balance	(93,500,000)	(100,200,000)	(99,200,000)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	2,300,000	2,000,000	1,900,000
Defined Benefit Plan Interest Cost	4,000,000	4,600,000	4,800,000
Defined Benefit Plan, Expected Return on Plan Assets	(5,200,000)	(4,600,000)	(4,300,000)
Defined Benefit Plan, Amortization of Gains (Losses)	3,600,000	3,600,000	3,200,000
Defined Benefit Plan Amortization Of Transition Obligations Assets	0	2,500,000	2,500,000
Defined Benefit Plan Net Periodic Benefit Cost	4,700,000	8,100,000	8,100,000
Related Intercompany Allocationsof Defined Benefit Expense	1,600,000	2,000,000	2,000,000
Amount Capitalized Defined Benefit Expense	1,300,000	2,300,000	2,200,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	69,500,000	58,700,000
Defined Benefit Plan, Actual Return on Plan Assets	13,400,000	7,500,000
Defined Benefit Plan, Contributions by Employer	4,700,000	9,200,000
Defined Benefit Plan, Benefits Paid	(5,800,000)	(5,900,000)
Fair Value of Plan Assets Ending Balance	81,800,000	69,500,000	58,700,000
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	(11,700,000)	(30,700,000)
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	69,500,000	58,700,000
Fair Value of Plan Assets Ending Balance	81,800,000	69,500,000	58,700,000
Public Service Company Of New Hampshire [Member] | NUSCO Pension Plan [Member]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, PBOP and SERP	3,000,000
Western Massachusetts Electric Company [Member]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, PBOP and SERP	3,911,000	11,884,000
Western Massachusetts Electric Company [Member] | Pension Plans Defined Benefit Member
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation Beginning Balance	(243,100,000)	(215,800,000)
Defined Benefit Plan Service Cost	(4,700,000)	(4,100,000)	(3,900,000)
Defined Benefit Plan Interest Cost	(10,000,000)	(10,500,000)	(10,700,000)
Defined Benefit Plan, Actuarial Net (Gains) Losses	22,400,000	(24,000,000)
Defined Benefit Plan, Benefits Paid	11,500,000	11,300,000
Defined Benefit Plan, Payments SERP	0
Benefit Obligation Ending Balance	(223,900,000)	(243,100,000)	(215,800,000)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	4,700,000	4,100,000	3,900,000
Defined Benefit Plan Interest Cost	10,000,000	10,500,000	10,700,000
Defined Benefit Plan, Expected Return on Plan Assets	(17,400,000)	(16,400,000)	(17,700,000)
Defined Benefit Plan, Amortization of Gains (Losses)	11,800,000	10,700,000	7,100,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	400,000	800,000	900,000
Defined Benefit Plan Net Periodic Benefit Cost	9,500,000	9,700,000	4,900,000
Related Intercompany Allocationsof Defined Benefit Expense	8,000,000	8,100,000	6,200,000
Amount Capitalized Defined Benefit Expense	5,200,000	5,100,000	2,700,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	218,500,000	202,000,000
Defined Benefit Plan, Actual Return on Plan Assets	33,400,000	27,800,000
Defined Benefit Plan, Benefits Paid	(11,500,000)	(11,300,000)
Fair Value of Plan Assets Ending Balance	240,400,000	218,500,000	202,000,000
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	16,500,000	(24,600,000)
Defined Benefit Plan, Accumulated Benefit Obligation	220,600,000	221,300,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	218,500,000	202,000,000
Fair Value of Plan Assets Ending Balance	240,400,000	218,500,000	202,000,000
Western Massachusetts Electric Company [Member] | PBOP Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation Beginning Balance	(42,500,000)	(42,900,000)
Defined Benefit Plan Service Cost	(700,000)	(600,000)	(600,000)
Defined Benefit Plan Interest Cost	(1,700,000)	(2,000,000)	(2,200,000)
Defined Benefit Plan, Actuarial Net (Gains) Losses	3,300,000	100,000
Defined Benefit Plan, Benefits Paid	2,900,000	3,200,000
Defined Benefit Plan, Gross Prescription Drug Subsidy Receipts Received	0	(300,000)
Benefit Obligation Ending Balance	(38,700,000)	(42,500,000)	(42,900,000)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	700,000	600,000	600,000
Defined Benefit Plan Interest Cost	1,700,000	2,000,000	2,200,000
Defined Benefit Plan, Expected Return on Plan Assets	(2,300,000)	(2,100,000)	(2,000,000)
Defined Benefit Plan, Amortization of Gains (Losses)	1,100,000	1,200,000	1,100,000
Defined Benefit Plan Amortization Of Transition Obligations Assets	0	1,300,000	1,300,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit		0
Defined Benefit Plan Net Periodic Benefit Cost	1,200,000	3,000,000	3,200,000
Related Intercompany Allocationsof Defined Benefit Expense	1,300,000	1,500,000	1,500,000
Amount Capitalized Defined Benefit Expense	600,000	1,600,000	1,500,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	31,000,000	27,100,000
Defined Benefit Plan, Actual Return on Plan Assets	6,000,000	3,500,000
Defined Benefit Plan, Contributions by Employer	1,200,000	3,600,000
Defined Benefit Plan, Benefits Paid	(2,900,000)	(3,200,000)
Fair Value of Plan Assets Ending Balance	35,300,000	31,000,000	27,100,000
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	(3,400,000)	(11,500,000)
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	31,000,000	27,100,000
Fair Value of Plan Assets Ending Balance	35,300,000	31,000,000	27,100,000
Western Massachusetts Electric Company [Member] | NUSCO Pension Plan [Member]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, PBOP and SERP	$ 17,000,000