PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSIONS (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, SERP and PBOP	$ 1,407,288	$ 1,638,558
Defined Benefit Plan, Estimated Future Benefit Payments [Abstract]
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year, Description	Eversource currently expects to make contributions of approximately $146 million in 2016, of which $21 million will be contributed by NSTAR Electric and $17 million by PSNH. The remaining $108 million is expected to be contributed by other Eversource subsidiaries, primarily Eversource Service. Eversource contributed $7.9 million to the PBOP Plan in 2015, of which $4.9 million was contributed by NSTAR Electric. Eversource expects to make approximately $9.5 million in contributions in 2016.
Amount That Represents Deferred Regulatory Assets	$ 3,200
United States Equity Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	8.50%	9.00%
Target Allocation	22.00%	24.00%
International Equity Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	8.50%	9.00%
Target Allocation	13.00%	10.00%
Emergining Markets Equity Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	10.00%	10.00%
Target Allocation	5.00%	6.00%
Private Equity Funds [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	12.00%	13.00%
Target Allocation	12.00%	10.00%
Fixed Income Debt Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	4.50%	5.00%
Target Allocation	12.00%	15.00%
High Yield Fixed Income Debt Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	8.50%	7.50%
Target Allocation	13.00%	9.00%
Emerging Markets Debt Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	7.50%	7.50%
Target Allocation	5.00%	6.00%
Hedge Funds [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	7.00%	7.00%
Target Allocation	8.00%	11.00%
Real Estate [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	7.50%	7.50%
Target Allocation	10.00%	9.00%
Pension Plans Defined Benefit Member
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation Beginning Balance	$ (5,486,200)	$ (4,676,500)
Defined Benefit Plan Service Cost	(91,400)	(79,900)	$ (102,300)
Defined Benefit Plan Interest Cost	(227,000)	(225,700)	(206,700)
Defined Benefit Plan, Actuarial Net (Gains) Losses	331,500	(739,600)
Defined Benefit Plan, Benefits Paid	238,500	230,300
Defined Benefit Plan, Payments SERP	5,000	5,200
Defined Benefit Plan Benefits Paid Lump Sum	149,500
Benefit Obligation Ending Balance	(5,080,100)	(5,486,200)	(4,676,500)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	91,400	79,900	102,300
Defined Benefit Plan Interest Cost	227,000	225,700	206,700
Defined Benefit Plan, Expected Return on Plan Assets	(335,900)	(310,800)	(278,100)
Defined Benefit Plan, Amortization of Gains (Losses)	148,500	128,400	210,500
Defined Benefit Plan Amortization Of Prior Service Cost Credit	3,700	4,400	4,000
Defined Benefit Plan Net Periodic Benefit Cost	134,700	127,600	245,400
Amount Capitalized Defined Benefit Expense	41,000	35,200	73,200
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	4,126,500	3,985,900
Defined Benefit Plan, Actual Return on Plan Assets	12,300	199,300
Defined Benefit Plan, Contributions by Employer	154,600	171,600
Defined Benefit Plan, Benefits Paid	(238,500)	(230,300)
Defined Benefit Plan Benefits Paid Lump Sum	(149,500)
Fair Value of Plan Assets Ending Balance	3,905,400	4,126,500	$ 3,985,900
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	(1,174,700)	(1,359,700)
Defined Benefit Plan, Accumulated Benefit Obligation	$ 4,733,200	$ 5,000,100
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate		4.20%
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	4.20%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase	3.50%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	8.25%	8.25%	8.25%
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	$ 4,126,500	$ 3,985,900
Fair Value of Plan Assets Ending Balance	3,905,400	4,126,500	$ 3,985,900
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	(6,200)	55,900
OCI Reclassification Adjustment for Net Actuarial Gain (Loss), before Tax	(6,200)	(5,600)
OCI Reclassification Adjustment for Amortization of Prior Service Cost, before tax	(200)	(200)
Gain Loss Arising During Year Regulatory	(2,000)	797,300
Losses Reclassifed From RegAsset PreTax	(142,300)	(122,800)
AmortizationPriorServiceCostfromRegAsset	(3,500)	(4,200)
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax [Abstract]
Gains Losses included in AOCI, Pre Tax	81,100	93,500
Prior Service Cost balance in AOCI	600	800
RegulatoryAsset Gains Losses Before Tax	1,667,600	1,811,900
Regulatory Asset Prior Service Cost	9,700	$ 13,200
Defined Benefit Amounts to be Reclassified from AOCI in next fiscal year
Future Amortization Of Gain Loss.	5,400
Future Amortization of Prior Service cost	200
Regulatory Asset Amortization Of Net Gains Losses	120,600
Regulatory Asset Defined Benefit Plan Amortization Of Net Prior ServiceCost	$ 3,400
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	$ 253,500
Defined Benefit Plan, Expected Future Benefit Payments in Year Two	272,900
Defined Benefit Plan, Expected Future Benefit Payments in Year Three	273,900
Defined Benefit Plan, Expected Future Benefit Payments, Year Four	283,700
Defined Benefit Plan, Expected Future Benefit Payments in Year Five	292,700
Defined Benefit Plan, Expected Future Benefit Payments in Five Fiscal Years Thereafter	$ 1,604,300
Impact of change in mortality assumption	During 2014, the Society of Actuaries released a series of updated mortality tables resulting from studies that measured mortality rates for various groups of individuals. The updated mortality tables released in 2014 increased the life expectancy of plan participants by three to five years and had the effect of increasing the estimated benefits to be provided to plan participants. The impact of adopting the updated mortality tables on Eversource's liability as of December 31, 2014 was an increase of approximately $340 million. In 2015, a revised scale for the mortality table was released having the effect of decreasing the estimate of benefits to be provided to plan participants. The impact of the adoption of the new mortality scale resulted in a decrease of $48 million on Eversource's liability as of December 31, 2015.
Impact of Decrease in Discount Rate	The increase in the discount rate used to calculate the funded status resulted in a decrease on Eversource's liability of approximately $267 million as of December 31, 2015. Decreases in the discount rates resulted in an increase on Eversource's liability of approximately $530 million as of December 31, 2014.
Pension Plans Defined Benefit Member | Minimum [Member]
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	4.21%
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		4.85%	4.13%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase		3.50%	3.50%
Pension Plans Defined Benefit Member | Maximum [Member]
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	4.60%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase
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		5.03%	4.24%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase		4.00%	4.00%
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 Beginning Balance	$ 443,700
Fair Value of Plan Assets Ending Balance	404,100	$ 443,700
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	443,700
Fair Value of Plan Assets Ending Balance	404,100	443,700
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 Beginning Balance	1,748,400
Fair Value of Plan Assets Ending Balance	1,584,900	1,748,400
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	1,748,400
Fair Value of Plan Assets Ending Balance	1,584,900	1,748,400
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	2,122,900	1,850,700
Fair Value of Plan Assets Ending Balance	2,105,800	2,122,900	$ 1,850,700
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	2,122,900	1,850,700
Defined Benefit Plan, Actual Return on Plan Assets Still Held	15,900	76,800
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	68,200	20,800
Defined Benefit Plan, Purchases, Sales, and Settlements	(101,200)	174,600
Fair Value of Plan Assets Ending Balance	2,105,800	2,122,900	1,850,700
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	386,700
Fair Value of Plan Assets Ending Balance	472,300	386,700
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	386,700
Fair Value of Plan Assets Ending Balance	472,300	386,700
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 Beginning Balance	18,800
Fair Value of Plan Assets Ending Balance	7,600	18,800
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	18,800
Fair Value of Plan Assets Ending Balance	7,600	18,800
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	367,900	300,300
Fair Value of Plan Assets Ending Balance	464,700	367,900	300,300
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	367,900	300,300
Defined Benefit Plan, Actual Return on Plan Assets Still Held	24,400	14,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	27,300	13,900
Defined Benefit Plan, Purchases, Sales, and Settlements	45,100	39,700
Fair Value of Plan Assets Ending Balance	464,700	367,900	300,300
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	1,293,600
Fair Value of Plan Assets Ending Balance	1,216,800	1,293,600
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	1,293,600
Fair Value of Plan Assets Ending Balance	1,216,800	1,293,600
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 Beginning Balance	10,200
Fair Value of Plan Assets Ending Balance	0	10,200
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	10,200
Fair Value of Plan Assets Ending Balance	0	10,200
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 Beginning Balance	561,400
Fair Value of Plan Assets Ending Balance	432,000	561,400
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	561,400
Fair Value of Plan Assets Ending Balance	432,000	561,400
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	722,000	589,500
Fair Value of Plan Assets Ending Balance	784,800	722,000	589,500
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	722,000	589,500
Defined Benefit Plan, Actual Return on Plan Assets Still Held	(6,700)	45,200
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	17,000	(6,200)
Defined Benefit Plan, Purchases, Sales, and Settlements	52,500	93,500
Fair Value of Plan Assets Ending Balance	784,800	722,000	589,500
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	495,000
Fair Value of Plan Assets Ending Balance	340,500	495,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	495,000
Fair Value of Plan Assets Ending Balance	340,500	495,000
Pension Plans Defined Benefit Member | Hedge Funds [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	20,000
Fair Value of Plan Assets Ending Balance	49,700	20,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	20,000
Fair Value of Plan Assets Ending Balance	49,700	20,000
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	475,000	416,900
Fair Value of Plan Assets Ending Balance	290,800	475,000	416,900
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	475,000	416,900
Defined Benefit Plan, Actual Return on Plan Assets Still Held	0	23,500
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	(900)	(15,200)
Defined Benefit Plan, Purchases, Sales, and Settlements	(106,800)	49,800
Fair Value of Plan Assets Ending Balance	290,800	475,000	416,900
TransferBetweenCategories	(76,500)
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	397,800
Fair Value of Plan Assets Ending Balance	377,800	397,800
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	397,800
Fair Value of Plan Assets Ending Balance	377,800	397,800
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 Beginning Balance	0
Fair Value of Plan Assets Ending Balance	0	0
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	0
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 Beginning Balance	132,000
Fair Value of Plan Assets Ending Balance	117,500	132,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	132,000
Fair Value of Plan Assets Ending Balance	117,500	132,000
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	265,800	288,500
Fair Value of Plan Assets Ending Balance	260,300	265,800	288,500
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	265,800	288,500
Defined Benefit Plan, Actual Return on Plan Assets Still Held	(7,100)	(3,600)
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	24,800	28,300
Defined Benefit Plan, Purchases, Sales, and Settlements	(23,200)	(47,400)
Fair Value of Plan Assets Ending Balance	260,300	265,800	288,500
Pension Plans Defined Benefit Member | Assetsrelatedto401h [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	188,500
Fair Value of Plan Assets Ending Balance	189,400	188,500
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	188,500
Fair Value of Plan Assets Ending Balance	189,400	188,500
Pension Plans Defined Benefit Member | Equity Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	1,741,900
Fair Value of Plan Assets Ending Balance	1,687,400	1,741,900
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	1,741,900
Fair Value of Plan Assets Ending Balance	1,687,400	1,741,900
Pension Plans Defined Benefit Member | Equity Member | Fair Value Inputs Level 1 Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	414,700
Fair Value of Plan Assets Ending Balance	396,500	414,700
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	414,700
Fair Value of Plan Assets Ending Balance	396,500	414,700
Pension Plans Defined Benefit Member | Equity Member | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	1,035,000
Fair Value of Plan Assets Ending Balance	985,700	1,035,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	1,035,000
Fair Value of Plan Assets Ending Balance	985,700	1,035,000
Pension Plans Defined Benefit Member | Equity 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	292,200	255,500
Fair Value of Plan Assets Ending Balance	305,200	292,200	255,500
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	292,200	255,500
Defined Benefit Plan, Actual Return on Plan Assets Still Held	5,300	(2,300)
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	0	0
Defined Benefit Plan, Purchases, Sales, and Settlements	(68,800)	39,000
Fair Value of Plan Assets Ending Balance	305,200	292,200	255,500
TransferBetweenCategories	76,500
PBOP Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation Beginning Balance	(1,147,900)	(1,038,000)
Defined Benefit Plan Service Cost	(16,300)	(12,500)	(16,900)
Defined Benefit Plan Interest Cost	(47,200)	(49,500)	(47,200)
Defined Benefit Plan, Actuarial Net (Gains) Losses	106,000	(95,500)
Defined Benefit Plan, Benefits Paid	54,000	47,600
Benefit Obligation Ending Balance	(1,051,400)	(1,147,900)	(1,038,000)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	16,300	12,500	16,900
Defined Benefit Plan Interest Cost	47,200	49,500	47,200
Defined Benefit Plan, Expected Return on Plan Assets	(67,400)	(63,300)	(55,400)
Defined Benefit Plan, Amortization of Gains (Losses)	6,800	12,200	26,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	(500)	(2,800)	(2,100)
Defined Benefit Plan Net Periodic Benefit Cost	2,400	8,100	32,600
Amount Capitalized Defined Benefit Expense	100	1,400	8,800
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	862,600	826,500
Defined Benefit Plan, Actual Return on Plan Assets	(4,300)	43,700
Defined Benefit Plan, Contributions by Employer	7,900	40,000
Defined Benefit Plan, Benefits Paid	(54,000)	(47,600)
Fair Value of Plan Assets Ending Balance	812,200	862,600	$ 826,500
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	$ (239,200)	$ (285,300)
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	As of December 31, 2015 and 2014, the health care cost trend rate assumptions used to determine the PBOP Plan's funded status was 6.25 percent and 6.5 percent, respectively, subsequently decreasing to an ultimate rate of 4.5 percent in 2023.
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	4.22%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	8.25%	8.25%	8.25%
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	$ 115,300
Defined Benefit Plan, Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation	(90,800)
Defined Benefit Plan, Effect of One Percentage Point Increase on Service and Interest Cost Components	8,500
Defined Benefit Plan, Effect of One Percentage Point Decrease on Service and Interest Cost Components	(6,300)
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	862,600	$ 826,500
Fair Value of Plan Assets Ending Balance	812,200	862,600	$ 826,500
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	700	400
OCI Reclassification Adjustment for Net Actuarial Gain (Loss), before Tax	(400)	(600)
Gain Loss Arising During Year Regulatory	(34,100)	115,100
Losses Reclassifed From RegAsset PreTax	(6,400)	(11,600)
AmortizationPriorServiceCostfromRegAsset	500	2,800
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax [Abstract]
Gains Losses included in AOCI, Pre Tax	6,300	6,000
RegulatoryAsset Gains Losses Before Tax	152,200	192,700
Regulatory Asset Prior Service Cost	(1,300)	$ (1,800)
Defined Benefit Amounts to be Reclassified from AOCI in next fiscal year
Future Amortization Of Gain Loss.	400
Regulatory Asset Amortization Of Net Gains Losses	4,000
Regulatory Asset Defined Benefit Plan Amortization Of Net Prior ServiceCost	(200)
Defined Benefit Plan, Estimated Future Benefit Payments [Abstract]
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months	60,800
Defined Benefit Plan, Expected Future Benefit Payments in Year Two	61,200
Defined Benefit Plan, Expected Future Benefit Payments in Year Three	61,400
Defined Benefit Plan, Expected Future Benefit Payments, Year Four	61,800
Defined Benefit Plan, Expected Future Benefit Payments in Year Five	62,400
Defined Benefit Plan, Expected Future Benefit Payments in Five Fiscal Years Thereafter	$ 315,400
Impact of change in mortality assumption	During 2014, the Society of Actuaries released a series of updated mortality tables resulting from studies that measured mortality rates for various groups of individuals. The updated mortality tables released in 2014 increased the life expectancy of plan participants by three to five years and had the effect of increasing the estimated benefits to be provided to plan participants. The impact of adopting the updated mortality tables on Eversource's liability as of December 31, 2014 was an increase of approximately $82 million. In 2015, a revised scale for the mortality table was released having the effect of decreasing the estimate of benefits to be provided to plan participants. The impact of the adoption of the new mortality scale resulted in a decrease of $23 million on Eversource's liability as of December 31, 2015.
Impact of Decrease in Discount Rate	The increase in the discount rate used to calculate the funded status resulted in a decrease on Eversource's liability of approximately $60 million as of December 31, 2015. Decreases in the discount rates resulted in an increase on Eversource's liability of approximately $110 million as of December 31, 2014.
PBOP Plan | Minimum [Member]
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	4.62%	4.22%
Health Care Trend Assumption Used In Funded Status	6.25%	6.50%
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		4.78%	4.04%
PBOP Plan | Maximum [Member]
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		5.10%	4.35%
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 Beginning Balance	$ 120,200
Fair Value of Plan Assets Ending Balance	119,400	$ 120,200
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	120,200
Fair Value of Plan Assets Ending Balance	119,400	120,200
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 Beginning Balance	302,200
Fair Value of Plan Assets Ending Balance	238,500	302,200
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	302,200
Fair Value of Plan Assets Ending Balance	238,500	302,200
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	251,700	229,400
Fair Value of Plan Assets Ending Balance	264,900	251,700	$ 229,400
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	251,700	229,400
Defined Benefit Plan, Actual Return on Plan Assets Still Held	1,500	7,800
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	(300)	(2,100)
Defined Benefit Plan, Purchases, Sales, and Settlements	12,000	16,600
Fair Value of Plan Assets Ending Balance	264,900	251,700	229,400
PBOP Plan | Private Equity Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	24,900
Fair Value of Plan Assets Ending Balance	32,900	24,900
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	24,900
Fair Value of Plan Assets Ending Balance	32,900	24,900
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	24,900	17,900
Fair Value of Plan Assets Ending Balance	32,900	24,900	17,900
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	24,900	17,900
Defined Benefit Plan, Actual Return on Plan Assets Still Held	2,600	1,300
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	0	100
Defined Benefit Plan, Purchases, Sales, and Settlements	5,400	5,600
Fair Value of Plan Assets Ending Balance	32,900	24,900	17,900
PBOP Plan | Fixed Income Debt Securities [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	204,400
Fair Value of Plan Assets Ending Balance	191,200	204,400
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	204,400
Fair Value of Plan Assets Ending Balance	191,200	204,400
PBOP Plan | 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 Beginning Balance	16,100
Fair Value of Plan Assets Ending Balance	9,700	16,100
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	16,100
Fair Value of Plan Assets Ending Balance	9,700	16,100
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 Beginning Balance	110,000
Fair Value of Plan Assets Ending Balance	99,900	110,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	110,000
Fair Value of Plan Assets Ending Balance	99,900	110,000
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	78,300	51,500
Fair Value of Plan Assets Ending Balance	81,600	78,300	51,500
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	78,300	51,500
Defined Benefit Plan, Actual Return on Plan Assets Still Held	2,100	1,900
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	(300)
Defined Benefit Plan, Purchases, Sales, and Settlements	1,500	24,900
Fair Value of Plan Assets Ending Balance	81,600	78,300	51,500
PBOP Plan | Hedge Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	58,400
Fair Value of Plan Assets Ending Balance	52,200	58,400
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	58,400
Fair Value of Plan Assets Ending Balance	52,200	58,400
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	58,400	57,000
Fair Value of Plan Assets Ending Balance	52,200	58,400	57,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	58,400	57,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	(1,500)	1,400
Defined Benefit Plan, Purchases, Sales, and Settlements	(4,700)	0
Fair Value of Plan Assets Ending Balance	52,200	58,400	57,000
PBOP Plan | Real Estate [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	34,400
Fair Value of Plan Assets Ending Balance	37,400	34,400
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	34,400
Fair Value of Plan Assets Ending Balance	37,400	34,400
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 Beginning Balance	19,400
Fair Value of Plan Assets Ending Balance	17,000	19,400
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	19,400
Fair Value of Plan Assets Ending Balance	17,000	19,400
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	15,000	33,900
Fair Value of Plan Assets Ending Balance	20,400	15,000	33,900
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	15,000	33,900
Defined Benefit Plan, Actual Return on Plan Assets Still Held	300	(2,800)
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	0	(2,200)
Defined Benefit Plan, Purchases, Sales, and Settlements	5,100	(13,900)
Fair Value of Plan Assets Ending Balance	20,400	15,000	33,900
PBOP Plan | Assetsrelatedto401h [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	188,500
Fair Value of Plan Assets Ending Balance	189,400	188,500
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	188,500
Fair Value of Plan Assets Ending Balance	189,400	188,500
PBOP Plan | Equity Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	352,000
Fair Value of Plan Assets Ending Balance	309,100	352,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	352,000
Fair Value of Plan Assets Ending Balance	309,100	352,000
PBOP Plan | Equity Member | Fair Value Inputs Level 1 Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	104,100
Fair Value of Plan Assets Ending Balance	109,700	104,100
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	104,100
Fair Value of Plan Assets Ending Balance	109,700	104,100
PBOP Plan | Equity Member | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	172,800
Fair Value of Plan Assets Ending Balance	121,600	172,800
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	172,800
Fair Value of Plan Assets Ending Balance	121,600	172,800
PBOP Plan | Equity 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	75,100	69,100
Fair Value of Plan Assets Ending Balance	77,800	75,100	69,100
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	75,100	69,100
Defined Benefit Plan, Actual Return on Plan Assets Still Held	(2,000)	6,000
Defined Benefit Plan, Purchases, Sales, and Settlements	4,700	0
Fair Value of Plan Assets Ending Balance	77,800	75,100	69,100
The Connecticut Light And Power Company [Member]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, SERP and PBOP	271,056	273,854
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	(1,230,100)	(1,083,400)
ChangeInLiabilityDueToTransferOfEmployees	(4,600)	26,400
Defined Benefit Plan Service Cost	(24,700)	(20,200)	(24,900)
Defined Benefit Plan Interest Cost	(51,100)	(50,500)	(48,300)
Defined Benefit Plan, Actuarial Net (Gains) Losses	77,800	(161,000)
Defined Benefit Plan, Benefits Paid	60,200	58,300
Defined Benefit Plan, Payments SERP	400	300
Defined Benefit Plan Benefits Paid Lump Sum	14,500
Benefit Obligation Ending Balance	(1,157,600)	(1,230,100)	(1,083,400)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	24,700	20,200	24,900
Defined Benefit Plan Interest Cost	51,100	50,500	48,300
Defined Benefit Plan, Expected Return on Plan Assets	(78,900)	(75,400)	(73,800)
Defined Benefit Plan, Amortization of Gains (Losses)	32,200	33,700	55,900
Defined Benefit Plan Amortization Of Prior Service Cost Credit	1,500	1,800	1,800
Defined Benefit Plan Net Periodic Benefit Cost	30,600	30,800	57,100
Related Intercompany Allocationsof Defined Benefit Expense	22,500	26,700	44,900
Amount Capitalized Defined Benefit Expense	18,800	17,600	28,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	980,800	1,016,300
Defined Benefit Plan, Actual Return on Plan Assets	2,800	49,200
Defined Benefit Plan, Contributions by Employer		0
Defined Benefit Plan, Benefits Paid	(60,200)	(58,300)
Defined Benefit Plan Benefits Paid Lump Sum	(14,500)
Fair Value of Plan Assets Ending Balance	913,500	980,800	1,016,300
ChangeInAssetsDueToTransferOfEmployees	4,600	(26,400)
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	(244,100)	(249,300)
Defined Benefit Plan, Accumulated Benefit Obligation	1,062,700	1,101,400
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	980,800	1,016,300
Fair Value of Plan Assets Ending Balance	913,500	980,800	1,016,300
The Connecticut Light And Power Company [Member] | PBOP Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation Beginning Balance	(173,900)	(180,400)
ChangeInLiabilityDueToTransferOfEmployees	100	3,700
Defined Benefit Plan Service Cost	(2,100)	(2,200)	(3,400)
Defined Benefit Plan Interest Cost	(7,200)	(8,100)	(7,900)
Defined Benefit Plan, Actuarial Net (Gains) Losses	7,200	3,500
Defined Benefit Plan, Benefits Paid	11,900	9,600
Benefit Obligation Ending Balance	(164,000)	(173,900)	(180,400)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	2,100	2,200	3,400
Defined Benefit Plan Interest Cost	7,200	8,100	7,900
Defined Benefit Plan, Expected Return on Plan Assets	(11,100)	(10,500)	(10,100)
Defined Benefit Plan, Amortization of Gains (Losses)	700	4,200	7,400
Defined Benefit Plan Amortization Of Prior Service Cost Credit		0	0
Defined Benefit Plan Net Periodic Benefit Cost	(1,100)	4,000	8,600
Related Intercompany Allocationsof Defined Benefit Expense	1,900	3,800	7,100
Amount Capitalized Defined Benefit Expense	(200)	1,800	3,900
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	149,000	151,300
Defined Benefit Plan, Actual Return on Plan Assets	(400)	6,300
Defined Benefit Plan, Contributions by Employer	0	4,200
Defined Benefit Plan, Benefits Paid	(11,900)	(9,600)
Fair Value of Plan Assets Ending Balance	136,700	149,000	151,300
ChangeInAssetsDueToTransferOfEmployees	0	(3,200)
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	(27,300)	(24,900)
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	149,000	151,300
Fair Value of Plan Assets Ending Balance	136,700	149,000	151,300
NSTAR Electric Company [Member]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, SERP and PBOP	209,153	235,529
Defined Benefit Plan, Estimated Future Benefit Payments [Abstract]
Pension Contribution	9,886	120,306	82,000
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation Beginning Balance	(982,600)	(1,353,300)
ChangeInLiabilityDueToTransferOfEmployees	6,200	479,900
Defined Benefit Plan Service Cost	(14,900)	(13,600)	(33,100)
Defined Benefit Plan Interest Cost	(40,200)	(41,300)	(58,000)
Defined Benefit Plan, Actuarial Net (Gains) Losses	34,100	(107,000)
Defined Benefit Plan, Benefits Paid	47,600	52,400
Defined Benefit Plan, Payments SERP	100
Benefit Obligation Ending Balance	(949,700)	(982,600)	(1,353,300)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	14,900	13,600	33,100
Defined Benefit Plan Interest Cost	40,200	41,300	58,000
Defined Benefit Plan, Expected Return on Plan Assets	(70,000)	(63,000)	(84,400)
Defined Benefit Plan, Amortization of Gains (Losses)	35,800	23,500	58,100
Defined Benefit Plan Amortization Of Prior Service Cost Credit	(100)	0	(300)
Defined Benefit Plan Net Periodic Benefit Cost	20,800	15,400	64,500
Related Intercompany Allocationsof Defined Benefit Expense	13,600	10,400	(8,400)
Amount Capitalized Defined Benefit Expense	11,400	7,900	28,900
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	879,000	1,235,300
Defined Benefit Plan, Actual Return on Plan Assets	2,700	36,500
Defined Benefit Plan, Contributions by Employer	5,000	101,000
Defined Benefit Plan, Benefits Paid	(47,600)	(52,400)
Fair Value of Plan Assets Ending Balance	832,900	879,000	1,235,300
ChangeInAssetsDueToTransferOfEmployees	(6,200)	(441,400)
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	(116,800)	(103,600)
Defined Benefit Plan, Accumulated Benefit Obligation	888,800	910,400
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	879,000	1,235,300
Fair Value of Plan Assets Ending Balance	832,900	879,000	1,235,300
NSTAR Electric Company [Member] | PBOP Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation Beginning Balance	(468,700)	0
ChangeInLiabilityDueToTransferOfEmployees	2,300	(395,500)
Defined Benefit Plan Service Cost	(5,400)	(3,100)
Defined Benefit Plan Interest Cost	(19,000)	(19,400)
Defined Benefit Plan, Actuarial Net (Gains) Losses	59,100	(68,600)
Defined Benefit Plan, Benefits Paid	18,900	17,900
Benefit Obligation Ending Balance	(412,800)	(468,700)	0
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	5,400	3,100
Defined Benefit Plan Interest Cost	19,000	19,400
Defined Benefit Plan, Expected Return on Plan Assets	(27,300)	(25,900)
Defined Benefit Plan, Amortization of Gains (Losses)	2,300	(500)
Defined Benefit Plan Amortization Of Prior Service Cost Credit	(200)	(1,900)
Defined Benefit Plan Net Periodic Benefit Cost	(800)	(5,800)	4,600
Related Intercompany Allocationsof Defined Benefit Expense	800	800
Amount Capitalized Defined Benefit Expense	(200)	(2,300)
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	336,500	0
Defined Benefit Plan, Actual Return on Plan Assets	(2,800)	18,400
Defined Benefit Plan, Contributions by Employer	4,900	19,300
Defined Benefit Plan, Benefits Paid	(18,900)	(17,900)
Fair Value of Plan Assets Ending Balance	320,300	336,500	0
ChangeInAssetsDueToTransferOfEmployees	600	316,700
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	(92,500)	(132,200)
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	336,500	0
Fair Value of Plan Assets Ending Balance	320,300	336,500	$ 0
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
Public Service Company Of New Hampshire [Member]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, SERP and PBOP	89,579	93,243
Defined Benefit Plan, Estimated Future Benefit Payments [Abstract]
Amount That Represents Deferred Regulatory Assets	3,200
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	(580,700)	(529,000)
ChangeInLiabilityDueToTransferOfEmployees	(1,900)	32,200
Defined Benefit Plan Service Cost	(12,100)	(9,700)	$ (13,100)
Defined Benefit Plan Interest Cost	(24,300)	(23,800)	(23,600)
Defined Benefit Plan, Actuarial Net (Gains) Losses	38,900	(73,300)
Defined Benefit Plan, Benefits Paid	23,200	22,800
Defined Benefit Plan, Payments SERP	200	100
Defined Benefit Plan Benefits Paid Lump Sum	9,100
Benefit Obligation Ending Balance	(547,600)	(580,700)	(529,000)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	12,100	9,700	13,100
Defined Benefit Plan Interest Cost	24,300	23,800	23,600
Defined Benefit Plan, Expected Return on Plan Assets	(40,400)	(38,100)	(35,400)
Defined Benefit Plan, Amortization of Gains (Losses)	11,600	11,600	21,600
Defined Benefit Plan Amortization Of Prior Service Cost Credit	500	700	700
Defined Benefit Plan Net Periodic Benefit Cost	8,100	7,700	23,600
Related Intercompany Allocationsof Defined Benefit Expense	6,700	7,600	10,500
Amount Capitalized Defined Benefit Expense	3,500	3,000	7,300
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	498,400	528,600
Defined Benefit Plan, Actual Return on Plan Assets	1,500	24,800
Defined Benefit Plan, Contributions by Employer	1,000	0
Defined Benefit Plan, Benefits Paid	(23,200)	(22,800)
Defined Benefit Plan Benefits Paid Lump Sum	(9,100)
Fair Value of Plan Assets Ending Balance	470,500	498,400	528,600
ChangeInAssetsDueToTransferOfEmployees	1,900	(32,200)
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	(77,100)	(82,300)
Defined Benefit Plan, Accumulated Benefit Obligation	506,400	524,500
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	498,400	528,600
Fair Value of Plan Assets Ending Balance	470,500	498,400	528,600
Public Service Company Of New Hampshire [Member] | PBOP Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation Beginning Balance	(91,800)	(93,500)
ChangeInLiabilityDueToTransferOfEmployees	(300)	4,300
Defined Benefit Plan Service Cost	(1,400)	(1,300)	(2,300)
Defined Benefit Plan Interest Cost	(3,900)	(4,300)	(4,000)
Defined Benefit Plan, Actuarial Net (Gains) Losses	3,600	(1,100)
Defined Benefit Plan, Benefits Paid	5,300	4,100
Benefit Obligation Ending Balance	(88,500)	(91,800)	(93,500)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	1,400	1,300	2,300
Defined Benefit Plan Interest Cost	3,900	4,300	4,000
Defined Benefit Plan, Expected Return on Plan Assets	(6,000)	(5,400)	(5,200)
Defined Benefit Plan, Amortization of Gains (Losses)	500	2,200	3,600
Defined Benefit Plan Amortization Of Prior Service Cost Credit		0	0
Defined Benefit Plan Net Periodic Benefit Cost	(200)	2,400	4,700
Related Intercompany Allocationsof Defined Benefit Expense	400	1,000	1,600
Amount Capitalized Defined Benefit Expense	200	800	1,300
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	80,900	81,800
Defined Benefit Plan, Actual Return on Plan Assets	0	3,800
Defined Benefit Plan, Contributions by Employer	0	2,500
Defined Benefit Plan, Benefits Paid	(5,300)	(4,100)
Fair Value of Plan Assets Ending Balance	75,800	80,900	81,800
ChangeInAssetsDueToTransferOfEmployees	200	(3,100)
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	(12,700)	(10,900)
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	80,900	81,800
Fair Value of Plan Assets Ending Balance	75,800	80,900	81,800
Western Massachusetts Electric Company [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Defined Benefit Plan Benefits Paid Lump Sum	2,500
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Defined Benefit Plan Benefits Paid Lump Sum	(2,500)
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, SERP and PBOP	19,515
Western Massachusetts Electric Company [Member] | Pension Plans Defined Benefit Member
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation Beginning Balance	(249,400)	(223,900)
ChangeInLiabilityDueToTransferOfEmployees	(1,300)	6,200
Defined Benefit Plan Service Cost	(4,300)	(3,500)	(4,700)
Defined Benefit Plan Interest Cost	(10,400)	(10,300)	(10,000)
Defined Benefit Plan, Actuarial Net (Gains) Losses	12,600	(29,800)
Defined Benefit Plan, Benefits Paid	12,700	11,900
Defined Benefit Plan, Payments SERP	0
Benefit Obligation Ending Balance	(237,600)	(249,400)	(223,900)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	4,300	3,500	4,700
Defined Benefit Plan Interest Cost	10,400	10,300	10,000
Defined Benefit Plan, Expected Return on Plan Assets	(18,900)	(17,900)	(17,400)
Defined Benefit Plan, Amortization of Gains (Losses)	6,400	6,900	11,800
Defined Benefit Plan Amortization Of Prior Service Cost Credit	300	400	400
Defined Benefit Plan Net Periodic Benefit Cost	2,500	3,200	9,500
Related Intercompany Allocationsof Defined Benefit Expense	4,400	5,100	8,000
Amount Capitalized Defined Benefit Expense	1,900	2,400	5,200
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	234,000	240,400
Defined Benefit Plan, Actual Return on Plan Assets	700	11,700
Defined Benefit Plan, Contributions by Employer	0
Defined Benefit Plan, Benefits Paid	(12,700)	(11,900)
Fair Value of Plan Assets Ending Balance	220,800	234,000	240,400
ChangeInAssetsDueToTransferOfEmployees	1,300	(6,200)
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	(16,800)	(15,400)
Defined Benefit Plan, Accumulated Benefit Obligation	222,300	226,400
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	234,000	240,400
Fair Value of Plan Assets Ending Balance	220,800	234,000	240,400
Western Massachusetts Electric Company [Member] | PBOP Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation Beginning Balance	(36,600)	(38,700)
ChangeInLiabilityDueToTransferOfEmployees	0	1,000
Defined Benefit Plan Service Cost	(400)	(400)	(700)
Defined Benefit Plan Interest Cost	(1,500)	(1,700)	(1,700)
Defined Benefit Plan, Actuarial Net (Gains) Losses	1,500	1,300
Defined Benefit Plan, Benefits Paid	2,600	1,900
Benefit Obligation Ending Balance	(34,400)	(36,600)	(38,700)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	400	400	700
Defined Benefit Plan Interest Cost	1,500	1,700	1,700
Defined Benefit Plan, Expected Return on Plan Assets	(2,500)	(2,300)	(2,300)
Defined Benefit Plan, Amortization of Gains (Losses)	0	500	1,100
Defined Benefit Plan Amortization Of Prior Service Cost Credit			0
Defined Benefit Plan Net Periodic Benefit Cost	(600)	300	1,200
Related Intercompany Allocationsof Defined Benefit Expense	300	700	1,300
Amount Capitalized Defined Benefit Expense	(200)	200	600
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	34,400	35,300
Defined Benefit Plan, Actual Return on Plan Assets	(100)	1,600
Defined Benefit Plan, Contributions by Employer	0	400
Defined Benefit Plan, Benefits Paid	(2,600)	(1,900)
Fair Value of Plan Assets Ending Balance	31,700	34,400	35,300
ChangeInAssetsDueToTransferOfEmployees	0	(1,000)
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	(2,700)	(2,200)
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	34,400	35,300
Fair Value of Plan Assets Ending Balance	$ 31,700	$ 34,400	$ 35,300