PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSIONS (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, PBOP and SERP	$ 1,638,558,000	$ 896,844,000
Defined Benefit Plan, Estimated Future Benefit Payments [Abstract]
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year, Description	NU expects to make contributions of approximately $155 million in 2015, of which $5 million will be contributed by NSTAR Electric and $1 million will be contributed by PSNH. The remaining $149 million is expected to be contributed by other NU subsidiaries, primarily NUSCO. NU contributed $40 million to the PBOP Plans in 2014 and expects to make approximately $27 million in contributions in 2015. This amount will be funded into the 401(h) account and VEBAs up to the maximum tax-deductible level permitted
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. For the years ended December 31, 2014 and 2013 the NUSCO Pension Plan was entitled to $2,803.6 million and $2,750.4 million respectively and the NSTAR Pension Plan was entitled to $1,322.9 million and $1,235.3 million, respectively. Also effective January 1, 2013, the NSTAR PBOP Plan assets were transferred into a master trust with the NUSCO PBOP Plan assets and assets were allocated to each plan. For the years ended December 31, 2014 and 2013, the NUSCO PBOP Plan was entitled to $399 million and $391 million, respectively, and the NSTAR PBOP Plan was entitled to $463.6 million and $435.5 million, respectively. CL&P, PSNH and WMECO are allocated a portion of the NUSCO Pension and PBOP Plan assets. NSTAR Electric is entitled to a portion of the NSTAR Pension and PBOP Plan assets.
Pension Plans Defined Benefit Member
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation Beginning Balance	(4,676,500,000)	(5,022,800,000)
Defined Benefit Plan Service Cost	(79,900,000)	(102,300,000)	(84,300,000)
Defined Benefit Plan Interest Cost	(225,700,000)	(206,700,000)	(198,300,000)
Defined Benefit Plan, Actuarial Net (Gains) Losses	(739,600,000)	433,600,000
Defined Benefit Plan, Benefits Paid	230,300,000	216,600,000
Defined Benefit Plan, Payments SERP	5,200,000	5,100,000
Benefit Obligation Ending Balance	(5,486,200,000)	(4,676,500,000)	(5,022,800,000)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	79,900,000	102,300,000	84,300,000
Defined Benefit Plan Interest Cost	225,700,000	206,700,000	198,300,000
Defined Benefit Plan, Expected Return on Plan Assets	(310,800,000)	(278,100,000)	(220,900,000)
Defined Benefit Plan, Amortization of Gains (Losses)	128,400,000	210,500,000	172,400,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	4,400,000	4,000,000	7,900,000
Defined Benefit Plan Net Periodic Benefit Cost	127,600,000	245,400,000	242,000,000
Amount Capitalized Defined Benefit Expense	35,200,000	73,200,000	70,600,000
us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlementsAndCurtailments			2,200,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	3,985,900,000	3,411,300,000
Defined Benefit Plan, Actual Return on Plan Assets	199,300,000	506,500,000
Defined Benefit Plan, Contributions by Employer	171,600,000	284,700,000
Defined Benefit Plan, Benefits Paid	(230,300,000)	(216,600,000)
Fair Value of Plan Assets Ending Balance	4,126,500,000	3,985,900,000	3,411,300,000
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	(1,359,700,000)	(690,600,000)
Defined Benefit Plan, Accumulated Benefit Obligation	5,000,100,000	4,538,800,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	3,985,900,000	3,411,300,000
Fair Value of Plan Assets Ending Balance	4,126,500,000	3,985,900,000	3,411,300,000
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	55,900,000	(28,900,000)
OCI Reclassification Adjustment for Net Actuarial Gain (Loss), before Tax	(5,600,000)	(9,400,000)
OCI Reclassification Adjustment for Amortization of Prior Service Cost, before tax	(200,000)	(200,000)
Gain Loss Arising During Year Regulatory	797,300,000	(635,200,000)
Losses Reclassifed From RegAsset PreTax	(122,800,000)	(201,200,000)
AmortizationPriorServiceCostfromRegAsset	(4,200,000)	(3,800,000)
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax [Abstract]
Gains Losses included in AOCI, Pre Tax	93,500,000	43,200,000
Prior Service Cost balance in AOCI	800,000	1,000,000
RegulatoryAsset Gains Losses Before Tax	1,811,900,000	1,137,400,000
Regulatory Asset Prior Service Cost	13,200,000	17,400,000
Defined Benefit Amounts to be Reclassified from AOCI in next fiscal year
Future Amortization Of Gain Loss.	6,400,000
Future Amortization of Prior Service cost	200,000
Regulatory Asset Amortization Of Net Gains Losses	149,100,000
Regulatory Asset Defined Benefit Plan Amortization Of Net Prior ServiceCost	3,500,000
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	244,500,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Two	253,600,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Three	268,900,000
Defined Benefit Plan, Expected Future Benefit Payments, Year Four	273,400,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Five	285,400,000
Defined Benefit Plan, Expected Future Benefit Payments in Five Fiscal Years Thereafter	1,591,100,000
Impact of Decrease in Discount Rate	In addition, the decreases in the discount rates resulted in an increase on NU's funded status liability of approximately $530 million. Partially offsetting these increases are the impact of other actuarial assumptions
Pension Plans Defined Benefit Member | MinimumMember
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	4.20%	5.03%
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	4.85%	4.13%	4.52%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase	8.25%	8.25%	7.30%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	3.50%	3.50%	3.50%
Pension Plans Defined Benefit Member | MaximumMember
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	4.90%	5.03%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase	3.50%	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		4.24%	5.03%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase		4.00%	4.00%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets		0.00%	8.25%
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	443,700,000	434,800,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	443,700,000	434,800,000
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	1,748,400,000	1,865,400,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	1,748,400,000	1,865,400,000
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	1,850,700,000	1,560,000,000
Fair Value of Plan Assets Ending Balance	2,122,900,000	1,850,700,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	1,850,700,000	1,560,000,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	76,800,000	137,600,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	20,800,000	31,100,000
Defined Benefit Plan, Purchases, Sales, and Settlements	174,600,000	122,000,000
Fair Value of Plan Assets Ending Balance	2,122,900,000	1,850,700,000
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 Ending Balance	386,700,000	396,700,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	386,700,000	396,700,000
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	18,800,000	96,400,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	18,800,000	96,400,000
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	300,300,000	267,900,000
Fair Value of Plan Assets Ending Balance	367,900,000	300,300,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	300,300,000	267,900,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	14,000,000	15,400,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	13,900,000	13,700,000
Defined Benefit Plan, Purchases, Sales, and Settlements	39,700,000	3,300,000
Fair Value of Plan Assets Ending Balance	367,900,000	300,300,000
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 Ending Balance	1,293,600,000	1,206,200,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	1,293,600,000	1,206,200,000
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	10,200,000	11,600,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	10,200,000	11,600,000
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	561,400,000	605,100,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	561,400,000	605,100,000
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	589,500,000	315,100,000
Fair Value of Plan Assets Ending Balance	722,000,000	589,500,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	589,500,000	315,100,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	45,200,000	55,300,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	(6,200,000)	(1,000,000)
Defined Benefit Plan, Purchases, Sales, and Settlements	93,500,000	187,600,000
Fair Value of Plan Assets Ending Balance	722,000,000	589,500,000
TransferBetweenCategories		32,500,000
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 Ending Balance	495,000,000	416,900,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	495,000,000	416,900,000
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 Ending Balance	20,000,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	20,000,000
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	416,900,000	418,900,000
Fair Value of Plan Assets Ending Balance	475,000,000	416,900,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	416,900,000	418,900,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	23,500,000	33,400,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	(15,200,000)	0
Defined Benefit Plan, Purchases, Sales, and Settlements	49,800,000	(2,900,000)
Fair Value of Plan Assets Ending Balance	475,000,000	416,900,000
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 Ending Balance	397,800,000	376,700,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	397,800,000	376,700,000
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	132,000,000	88,200,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	132,000,000	88,200,000
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	288,500,000	235,400,000
Fair Value of Plan Assets Ending Balance	265,800,000	288,500,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	288,500,000	235,400,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	(3,600,000)	12,900,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	28,300,000	6,200,000
Defined Benefit Plan, Purchases, Sales, and Settlements	(47,400,000)	34,000,000
Fair Value of Plan Assets Ending Balance	265,800,000	288,500,000
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	188,500,000	165,000,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	188,500,000	165,000,000
Pension Plans Defined Benefit Member | Equity Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	1,741,900,000	1,754,400,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	1,741,900,000	1,754,400,000
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 Ending Balance	414,700,000	326,800,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	414,700,000	326,800,000
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 Ending Balance	1,035,000,000	1,172,100,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	1,035,000,000	1,172,100,000
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	255,500,000	322,700,000
Fair Value of Plan Assets Ending Balance	292,200,000	255,500,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	255,500,000	322,700,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	(2,300,000)	20,600,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	0	12,200,000
Defined Benefit Plan, Purchases, Sales, and Settlements	39,000,000	(100,000,000)
Fair Value of Plan Assets Ending Balance	292,200,000	255,500,000
PBOP Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation Beginning Balance	(1,038,000,000)	(1,233,300,000)
Defined Benefit Plan Service Cost	(12,500,000)	(16,900,000)	(15,700,000)
Defined Benefit Plan Interest Cost	(49,500,000)	(47,200,000)	(49,000,000)
Defined Benefit Plan, Actuarial Net (Gains) Losses	(95,500,000)	200,900,000
Defined Benefit Plan, Benefits Paid	47,600,000	58,500,000
Benefit Obligation Ending Balance	(1,147,900,000)	(1,038,000,000)	(1,233,300,000)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	12,500,000	16,900,000	15,700,000
Defined Benefit Plan Interest Cost	49,500,000	47,200,000	49,000,000
Defined Benefit Plan, Expected Return on Plan Assets	(63,300,000)	(55,400,000)	(39,200,000)
Defined Benefit Plan, Amortization of Gains (Losses)	12,200,000	26,000,000	36,000,000
Defined Benefit Plan Amortization Of Transition Obligations Assets			12,200,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	(2,800,000)	(2,100,000)	(1,400,000)
Defined Benefit Plan Net Periodic Benefit Cost	8,100,000	32,600,000	72,300,000
Amount Capitalized Defined Benefit Expense	1,400,000	8,800,000	26,600,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	826,500,000	709,100,000
Defined Benefit Plan, Actual Return on Plan Assets	43,700,000	118,300,000
Defined Benefit Plan, Contributions by Employer	40,000,000	57,600,000
Defined Benefit Plan, Benefits Paid	(47,600,000)	(58,500,000)
Fair Value of Plan Assets Ending Balance	862,600,000	826,500,000	709,100,000
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	(285,300,000)	(211,500,000)
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	As of December 31, 2014 and 2013, the health care cost trend rate assumption used to determine the PBOP Plans’ funded status was 6.5 percent and 7 percent, respectively, subsequently decreasing to an ultimate rate of 4.5 percent in 2023.
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	111,200,000
Defined Benefit Plan, Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation	(88,400,000)
Defined Benefit Plan, Effect of One Percentage Point Increase on Service and Interest Cost Components	5,300,000
Defined Benefit Plan, Effect of One Percentage Point Decrease on Service and Interest Cost Components	(4,400,000)
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	826,500,000	709,100,000
Fair Value of Plan Assets Ending Balance	862,600,000	826,500,000	709,100,000
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	400,000	(1,900,000)
OCI Reclassification Adjustment for Net Actuarial Gain (Loss), before Tax	(600,000)	(1,100,000)
Gain Loss Arising During Year Regulatory	115,100,000	(262,000,000)
Losses Reclassifed From RegAsset PreTax	(11,600,000)	(24,900,000)
AmortizationPriorServiceCostfromRegAsset	2,800,000	2,100,000
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax [Abstract]
Gains Losses included in AOCI, Pre Tax	6,000,000	6,200,000
RegulatoryAsset Gains Losses Before Tax	192,700,000	89,200,000
Regulatory Asset Prior Service Cost	(1,800,000)	(4,600,000)
Defined Benefit Amounts to be Reclassified from AOCI in next fiscal year
Future Amortization Of Gain Loss.	300,000
Regulatory Asset Amortization Of Net Gains Losses	6,900,000
Regulatory Asset Defined Benefit Plan Amortization Of Net Prior ServiceCost	(500,000)
Defined Benefit Plan, Estimated Future Benefit Payments [Abstract]
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months	58,700,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Two	59,700,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Three	60,600,000
Defined Benefit Plan, Expected Future Benefit Payments, Year Four	61,300,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Five	62,000,000
Defined Benefit Plan, Expected Future Benefit Payments in Five Fiscal Years Thereafter	318,800,000
Impact of change in mortality assumption	During 2014, the Society of Actuaries released a series of updated mortality tables resulting from recent studies that measured mortality rates for various groups of individuals. The updated mortality tables released in 2014 increased life expectancy of plan participants by 3 to 5 years and have the effect of increasing the estimate of benefits to be provided to plan participants. The impact of this adoption on NU's funded status liability for the year ended December 31, 2014 was an increase of approximately $82 million.
Impact of Decrease in Discount Rate	In addition, the decreases in the discount rates resulted in an increase on NU's funded status liability of approximately $110 million. Partially offsetting these increases are the impact of other actuarial assumptions.
PBOP Plan | MinimumMember
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	4.22%	4.78%
Health Care Trend Assumption Used In Funded Status	6.50%	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	4.78%	4.04%	4.58%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	8.25%	8.25%	7.30%
PBOP Plan | MaximumMember
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate		5.10%
Health Care Trend Assumption Used In Funded Status		0.00%
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	5.10%	4.35%	4.84%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	8.25%	8.25%	8.25%
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	120,200,000	121,400,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	120,200,000	121,400,000
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	302,200,000	310,700,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	302,200,000	310,700,000
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	229,400,000	146,000,000
Fair Value of Plan Assets Ending Balance	251,700,000	229,400,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	229,400,000	146,000,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	7,800,000	35,700,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	(2,100,000)	100,000
Defined Benefit Plan, Purchases, Sales, and Settlements	16,600,000	47,600,000
Fair Value of Plan Assets Ending Balance	251,700,000	229,400,000
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	24,900,000	17,900,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	24,900,000	17,900,000
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	17,900,000	11,300,000
Fair Value of Plan Assets Ending Balance	24,900,000	17,900,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	17,900,000	11,300,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	1,300,000	1,500,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	100,000	200,000
Defined Benefit Plan, Purchases, Sales, and Settlements	5,600,000	4,900,000
Fair Value of Plan Assets Ending Balance	24,900,000	17,900,000
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	204,400,000	171,200,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	204,400,000	171,200,000
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 Ending Balance	16,100,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	16,100,000
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	110,000,000	119,700,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	110,000,000	119,700,000
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	51,500,000	32,100,000
Fair Value of Plan Assets Ending Balance	78,300,000	51,500,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	51,500,000	32,100,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	1,900,000	4,100,000
Defined Benefit Plan, Purchases, Sales, and Settlements	24,900,000	15,300,000
Fair Value of Plan Assets Ending Balance	78,300,000	51,500,000
PBOP Plan | Hedge Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	58,400,000	57,000,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	58,400,000	57,000,000
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	57,000,000	39,600,000
Fair Value of Plan Assets Ending Balance	58,400,000	57,000,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	57,000,000	39,600,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	1,400,000	5,400,000
Defined Benefit Plan, Purchases, Sales, and Settlements	0	12,000,000
Fair Value of Plan Assets Ending Balance	58,400,000	57,000,000
PBOP Plan | Real Estate [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	34,400,000	48,100,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	34,400,000	48,100,000
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	19,400,000	14,200,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	19,400,000	14,200,000
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	33,900,000	26,700,000
Fair Value of Plan Assets Ending Balance	15,000,000	33,900,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	33,900,000	26,700,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	(2,800,000)	3,900,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	(2,200,000)	(100,000)
Defined Benefit Plan, Purchases, Sales, and Settlements	(13,900,000)	3,400,000
Fair Value of Plan Assets Ending Balance	15,000,000	33,900,000
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	0	11,100,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	0	11,100,000
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	0	11,100,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	0	11,100,000
PBOP Plan | Assetsrelatedto401h [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	188,500,000	165,000,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	188,500,000	165,000,000
PBOP Plan | Equity Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	352,000,000	356,200,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	352,000,000	356,200,000
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 Ending Balance	104,100,000	110,300,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	104,100,000	110,300,000
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 Ending Balance	172,800,000	176,800,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	172,800,000	176,800,000
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	69,100,000	36,300,000
Fair Value of Plan Assets Ending Balance	75,100,000	69,100,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	69,100,000	36,300,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	6,000,000	20,800,000
Defined Benefit Plan, Purchases, Sales, and Settlements	0	12,000,000
Fair Value of Plan Assets Ending Balance	75,100,000	69,100,000
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%	9.00%
Target Allocation	24.00%	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%	9.00%
Target Allocation	10.00%	10.00%
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%	10.00%
Target Allocation	6.00%	6.00%
NUSCOPensionandPBOP [Member] | Private Equity Funds [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	13.00%	13.00%
Target Allocation	10.00%	10.00%
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%	5.00%
Target Allocation	15.00%	15.00%
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%	7.50%
Target Allocation	9.00%	9.00%
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%	7.50%
Target Allocation	6.00%	6.00%
NUSCOPensionandPBOP [Member] | 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	11.00%	11.00%
NUSCOPensionandPBOP [Member] | 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	9.00%	9.00%
The Connecticut Light And Power Company [Member]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, PBOP and SERP	273,854,000	95,895,000
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	(1,083,400,000)	(1,178,000,000)
ChangeInLiabilityDueToServiceCompanyMerger	26,400,000
Defined Benefit Plan Service Cost	(20,200,000)	(24,900,000)	(21,800,000)
Defined Benefit Plan Interest Cost	(50,500,000)	(48,300,000)	(51,200,000)
Defined Benefit Plan, Actuarial Net (Gains) Losses	(161,000,000)	110,700,000
Defined Benefit Plan, Benefits Paid	58,300,000	56,600,000
Defined Benefit Plan, Payments SERP	300,000	500,000
Benefit Obligation Ending Balance	(1,230,100,000)	(1,083,400,000)	(1,178,000,000)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	20,200,000	24,900,000	21,800,000
Defined Benefit Plan Interest Cost	50,500,000	48,300,000	51,200,000
Defined Benefit Plan, Expected Return on Plan Assets	(75,400,000)	(73,800,000)	(70,600,000)
Defined Benefit Plan, Amortization of Gains (Losses)	33,700,000	55,900,000	49,600,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	1,800,000	1,800,000	3,600,000
Defined Benefit Plan Net Periodic Benefit Cost	30,800,000	57,100,000	55,600,000
Related Intercompany Allocationsof Defined Benefit Expense	26,700,000	44,900,000	42,800,000
Amount Capitalized Defined Benefit Expense	17,600,000	28,000,000	26,800,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	1,016,300,000	937,600,000
Defined Benefit Plan, Actual Return on Plan Assets	49,200,000	135,300,000
Defined Benefit Plan, Contributions by Employer		0
Defined Benefit Plan, Benefits Paid	(58,300,000)	(56,600,000)
Fair Value of Plan Assets Ending Balance	980,800,000	1,016,300,000	937,600,000
ChangeInAssetsDueToServiceCompanyMerger	(26,400,000)
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	(249,300,000)	(67,100,000)
Defined Benefit Plan, Accumulated Benefit Obligation	1,101,400,000	1,058,000,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	1,016,300,000	937,600,000
Fair Value of Plan Assets Ending Balance	980,800,000	1,016,300,000	937,600,000
The Connecticut Light And Power Company [Member] | PBOP Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation Beginning Balance	(180,400,000)	(196,800,000)
ChangeInLiabilityDueToServiceCompanyMerger	3,700,000
Defined Benefit Plan Service Cost	(2,200,000)	(3,400,000)	(3,000,000)
Defined Benefit Plan Interest Cost	(8,100,000)	(7,900,000)	(9,200,000)
Defined Benefit Plan, Actuarial Net (Gains) Losses	3,500,000	13,300,000
Defined Benefit Plan, Benefits Paid	9,600,000	14,400,000
Benefit Obligation Ending Balance	(173,900,000)	(180,400,000)	(196,800,000)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	2,200,000	3,400,000	3,000,000
Defined Benefit Plan Interest Cost	8,100,000	7,900,000	9,200,000
Defined Benefit Plan, Expected Return on Plan Assets	(10,500,000)	(10,100,000)	(9,100,000)
Defined Benefit Plan, Amortization of Gains (Losses)	4,200,000	7,400,000	7,500,000
Defined Benefit Plan Amortization Of Transition Obligations Assets			6,100,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit		0	0
Defined Benefit Plan Net Periodic Benefit Cost	4,000,000	8,600,000	16,700,000
Related Intercompany Allocationsof Defined Benefit Expense	3,800,000	7,100,000	7,900,000
Amount Capitalized Defined Benefit Expense	1,800,000	3,900,000	8,200,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	151,300,000	132,200,000
Defined Benefit Plan, Actual Return on Plan Assets	6,300,000	24,800,000
Defined Benefit Plan, Contributions by Employer	4,200,000	8,700,000
Defined Benefit Plan, Benefits Paid	(9,600,000)	(14,400,000)
Fair Value of Plan Assets Ending Balance	149,000,000	151,300,000	132,200,000
ChangeInAssetsDueToServiceCompanyMerger	(3,200,000)
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	(24,900,000)	(29,100,000)
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	151,300,000	132,200,000
Fair Value of Plan Assets Ending Balance	149,000,000	151,300,000	132,200,000
NSTAR Electric Company [Member]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, PBOP and SERP	235,529,000	118,010,000
Defined Benefit Plan, Estimated Future Benefit Payments [Abstract]
Pension Contribution	120,306,000	82,000,000	25,000,000
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation Beginning Balance	(1,353,300,000)	(1,430,000,000)
ChangeInLiabilityDueToServiceCompanyMerger	479,900,000
Defined Benefit Plan Service Cost	(13,600,000)	(33,100,000)	(30,300,000)
Defined Benefit Plan Interest Cost	(41,300,000)	(58,000,000)	(58,900,000)
Defined Benefit Plan, Actuarial Net (Gains) Losses	(107,000,000)	96,600,000
Defined Benefit Plan, Benefits Paid	52,400,000	71,200,000
Defined Benefit Plan, Payments SERP	300,000
Benefit Obligation Ending Balance	(982,600,000)	(1,353,300,000)	(1,430,000,000)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	13,600,000	33,100,000	30,300,000
Defined Benefit Plan Interest Cost	41,300,000	58,000,000	58,900,000
Defined Benefit Plan, Expected Return on Plan Assets	(63,000,000)	(84,400,000)	(65,600,000)
Defined Benefit Plan, Amortization of Gains (Losses)	23,500,000	58,100,000	63,100,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	0	(300,000)	(600,000)
Defined Benefit Plan Net Periodic Benefit Cost	15,400,000	64,500,000	86,100,000
Related Intercompany Allocationsof Defined Benefit Expense	10,400,000	(8,400,000)	(12,300,000)
Amount Capitalized Defined Benefit Expense	7,900,000	28,900,000	30,700,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	1,235,300,000	1,069,100,000
Defined Benefit Plan, Actual Return on Plan Assets	36,500,000	155,400,000
Defined Benefit Plan, Contributions by Employer	101,000,000	82,000,000
Defined Benefit Plan, Benefits Paid	(52,400,000)	(71,200,000)
Fair Value of Plan Assets Ending Balance	879,000,000	1,235,300,000	1,069,100,000
ChangeInAssetsDueToServiceCompanyMerger	(441,400,000)
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	(103,600,000)	(118,000,000)
Defined Benefit Plan, Accumulated Benefit Obligation	910,400,000	1,280,600,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	1,235,300,000	1,069,100,000
Fair Value of Plan Assets Ending Balance	879,000,000	1,235,300,000	1,069,100,000
NSTAR Electric Company [Member] | PBOP Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
ChangeInLiabilityDueToServiceCompanyMerger	(395,500,000)
Defined Benefit Plan Service Cost	(3,100,000)
Defined Benefit Plan Interest Cost	(19,400,000)
Defined Benefit Plan, Actuarial Net (Gains) Losses	(68,600,000)
Defined Benefit Plan, Benefits Paid	17,900,000
Benefit Obligation Ending Balance	(468,700,000)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	3,100,000
Defined Benefit Plan Interest Cost	19,400,000
Defined Benefit Plan, Expected Return on Plan Assets	(25,900,000)
Defined Benefit Plan, Amortization of Gains (Losses)	(500,000)
Defined Benefit Plan Amortization Of Prior Service Cost Credit	(1,900,000)
Defined Benefit Plan Net Periodic Benefit Cost	(5,800,000)	4,600,000	34,100,000
Related Intercompany Allocationsof Defined Benefit Expense	800,000
Amount Capitalized Defined Benefit Expense	(2,300,000)
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Defined Benefit Plan, Actual Return on Plan Assets	18,400,000
Defined Benefit Plan, Contributions by Employer	19,300,000
Defined Benefit Plan, Benefits Paid	(17,900,000)
Fair Value of Plan Assets Ending Balance	336,500,000
ChangeInAssetsDueToServiceCompanyMerger	316,700,000
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	(132,200,000)
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	336,500,000
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
Public Service Company Of New Hampshire [Member]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, PBOP and SERP	93,243,000	15,272,000
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	(529,000,000)	(576,000,000)
ChangeInLiabilityDueToServiceCompanyMerger	32,200,000
Defined Benefit Plan Service Cost	(9,700,000)	(13,100,000)	(11,800,000)
Defined Benefit Plan Interest Cost	(23,800,000)	(23,600,000)	(24,400,000)
Defined Benefit Plan, Actuarial Net (Gains) Losses	(73,300,000)	62,400,000
Defined Benefit Plan, Benefits Paid	22,800,000	21,100,000
Defined Benefit Plan, Payments SERP	100,000	200,000
Benefit Obligation Ending Balance	(580,700,000)	(529,000,000)	(576,000,000)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	9,700,000	13,100,000	11,800,000
Defined Benefit Plan Interest Cost	23,800,000	23,600,000	24,400,000
Defined Benefit Plan, Expected Return on Plan Assets	(38,100,000)	(35,400,000)	(28,200,000)
Defined Benefit Plan, Amortization of Gains (Losses)	11,600,000	21,600,000	16,200,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	700,000	700,000	1,500,000
Defined Benefit Plan Net Periodic Benefit Cost	7,700,000	23,600,000	25,700,000
Related Intercompany Allocationsof Defined Benefit Expense	7,600,000	10,500,000	10,100,000
Amount Capitalized Defined Benefit Expense	3,000,000	7,300,000	7,900,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	528,600,000	386,600,000
Defined Benefit Plan, Actual Return on Plan Assets	24,800,000	54,800,000
Defined Benefit Plan, Contributions by Employer	0	108,300,000
Defined Benefit Plan, Benefits Paid	(22,800,000)	(21,100,000)
Fair Value of Plan Assets Ending Balance	498,400,000	528,600,000	386,600,000
ChangeInAssetsDueToServiceCompanyMerger	(32,200,000)
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	(82,300,000)	(400,000)
Defined Benefit Plan, Accumulated Benefit Obligation	524,500,000	520,100,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	528,600,000	386,600,000
Fair Value of Plan Assets Ending Balance	498,400,000	528,600,000	386,600,000
Public Service Company Of New Hampshire [Member] | PBOP Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation Beginning Balance	(93,500,000)	(100,200,000)
ChangeInLiabilityDueToServiceCompanyMerger	4,300,000
Defined Benefit Plan Service Cost	(1,300,000)	(2,300,000)	(2,000,000)
Defined Benefit Plan Interest Cost	(4,300,000)	(4,000,000)	(4,600,000)
Defined Benefit Plan, Actuarial Net (Gains) Losses	(1,100,000)	7,200,000
Defined Benefit Plan, Benefits Paid	4,100,000	5,800,000
Benefit Obligation Ending Balance	(91,800,000)	(93,500,000)	(100,200,000)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	1,300,000	2,300,000	2,000,000
Defined Benefit Plan Interest Cost	4,300,000	4,000,000	4,600,000
Defined Benefit Plan, Expected Return on Plan Assets	(5,400,000)	(5,200,000)	(4,600,000)
Defined Benefit Plan, Amortization of Gains (Losses)	2,200,000	3,600,000	3,600,000
Defined Benefit Plan Amortization Of Transition Obligations Assets			2,500,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit		0	0
Defined Benefit Plan Net Periodic Benefit Cost	2,400,000	4,700,000	8,100,000
Related Intercompany Allocationsof Defined Benefit Expense	1,000,000	1,600,000	2,000,000
Amount Capitalized Defined Benefit Expense	800,000	1,300,000	2,300,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	81,800,000	69,500,000
Defined Benefit Plan, Actual Return on Plan Assets	3,800,000	13,400,000
Defined Benefit Plan, Contributions by Employer	2,500,000	4,700,000
Defined Benefit Plan, Benefits Paid	(4,100,000)	(5,800,000)
Fair Value of Plan Assets Ending Balance	80,900,000	81,800,000	69,500,000
ChangeInAssetsDueToServiceCompanyMerger	(3,100,000)
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	(10,900,000)	(11,700,000)
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	81,800,000	69,500,000
Fair Value of Plan Assets Ending Balance	80,900,000	81,800,000	69,500,000
Western Massachusetts Electric Company [Member]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, PBOP and SERP	17,705,000	3,911,000
Western Massachusetts Electric Company [Member] | Pension Plans Defined Benefit Member
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation Beginning Balance	(223,900,000)	(243,100,000)
ChangeInLiabilityDueToServiceCompanyMerger	6,200,000
Defined Benefit Plan Service Cost	(3,500,000)	(4,700,000)	(4,100,000)
Defined Benefit Plan Interest Cost	(10,300,000)	(10,000,000)	(10,500,000)
Defined Benefit Plan, Actuarial Net (Gains) Losses	(29,800,000)	22,400,000
Defined Benefit Plan, Benefits Paid	11,900,000	11,500,000
Defined Benefit Plan, Payments SERP	0
Benefit Obligation Ending Balance	(249,400,000)	(223,900,000)	(243,100,000)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	3,500,000	4,700,000	4,100,000
Defined Benefit Plan Interest Cost	10,300,000	10,000,000	10,500,000
Defined Benefit Plan, Expected Return on Plan Assets	(17,900,000)	(17,400,000)	(16,400,000)
Defined Benefit Plan, Amortization of Gains (Losses)	6,900,000	11,800,000	10,700,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	400,000	400,000	800,000
Defined Benefit Plan Net Periodic Benefit Cost	3,200,000	9,500,000	9,700,000
Related Intercompany Allocationsof Defined Benefit Expense	5,100,000	8,000,000	8,100,000
Amount Capitalized Defined Benefit Expense	2,400,000	5,200,000	5,100,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	240,400,000	218,500,000
Defined Benefit Plan, Actual Return on Plan Assets	11,700,000	33,400,000
Defined Benefit Plan, Benefits Paid	(11,900,000)	(11,500,000)
Fair Value of Plan Assets Ending Balance	234,000,000	240,400,000	218,500,000
ChangeInAssetsDueToServiceCompanyMerger	(6,200,000)
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	(15,400,000)	16,500,000
Defined Benefit Plan, Accumulated Benefit Obligation	226,400,000	220,600,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	240,400,000	218,500,000
Fair Value of Plan Assets Ending Balance	234,000,000	240,400,000	218,500,000
Western Massachusetts Electric Company [Member] | PBOP Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation Beginning Balance	(38,700,000)	(42,500,000)
ChangeInLiabilityDueToServiceCompanyMerger	1,000,000
Defined Benefit Plan Service Cost	(400,000)	(700,000)	(600,000)
Defined Benefit Plan Interest Cost	(1,700,000)	(1,700,000)	(2,000,000)
Defined Benefit Plan, Actuarial Net (Gains) Losses	1,300,000	3,300,000
Defined Benefit Plan, Benefits Paid	1,900,000	2,900,000
Benefit Obligation Ending Balance	(36,600,000)	(38,700,000)	(42,500,000)
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	400,000	700,000	600,000
Defined Benefit Plan Interest Cost	1,700,000	1,700,000	2,000,000
Defined Benefit Plan, Expected Return on Plan Assets	(2,300,000)	(2,300,000)	(2,100,000)
Defined Benefit Plan, Amortization of Gains (Losses)	500,000	1,100,000	1,200,000
Defined Benefit Plan Amortization Of Transition Obligations Assets			1,300,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit		0	0
Defined Benefit Plan Net Periodic Benefit Cost	300,000	1,200,000	3,000,000
Related Intercompany Allocationsof Defined Benefit Expense	700,000	1,300,000	1,500,000
Amount Capitalized Defined Benefit Expense	200,000	600,000	1,600,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	35,300,000	31,000,000
Defined Benefit Plan, Actual Return on Plan Assets	1,600,000	6,000,000
Defined Benefit Plan, Contributions by Employer	400,000	1,200,000
Defined Benefit Plan, Benefits Paid	(1,900,000)	(2,900,000)
Fair Value of Plan Assets Ending Balance	34,400,000	35,300,000	31,000,000
ChangeInAssetsDueToServiceCompanyMerger	(1,000,000)
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	(2,200,000)	(3,400,000)
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	35,300,000	31,000,000
Fair Value of Plan Assets Ending Balance	$ 34,400,000	$ 35,300,000	$ 31,000,000