PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSIONS (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, SERP and PBOP	$ 1,294,548		$ 1,294,548		$ 1,407,288
Defined Benefit Plan, Estimated Future Benefit Payments [Abstract]
Amount That Represents Deferred Regulatory Assets		$ 800		$ 1,600
Pension Plans Defined Benefit Member
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	18,700	22,800	37,900	45,900
Defined Benefit Plan Interest Cost	46,400	56,900	92,800	113,300
Defined Benefit Plan, Expected Return on Plan Assets	(79,500)	(83,900)	(159,100)	(168,200)
Defined Benefit Plan, Amortization of Gains (Losses)	31,300	36,500	63,000	75,500
Defined Benefit Plan Amortization Of Prior Service Cost Credit	900	900	1,800	1,800
Defined Benefit Plan Net Periodic Benefit Cost	17,800	33,200	36,400	68,300
Amount Capitalized Defined Benefit Expense	5,400	9,800	$ 11,400	21,100
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Service Cost Discount Rate Under Spot Rate			4.89%
Interest Cost Discount Rate under spot rate methodology			3.80%
Defined Benefit Plan, Estimated Future Benefit Payments [Abstract]
Impact of Change in Accounting Estimate Spot Rate Discount Rate	11,500		$ 23,000
PBOP Plan
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	3,100	3,900	6,300	8,100
Defined Benefit Plan Interest Cost	9,400	11,700	19,000	23,600
Defined Benefit Plan, Expected Return on Plan Assets	(15,700)	(16,900)	(31,500)	(33,700)
Defined Benefit Plan, Amortization of Gains (Losses)	900	1,700	2,100	3,400
Defined Benefit Plan Amortization Of Prior Service Cost Credit	0	(200)	(100)	(200)
Defined Benefit Plan Net Periodic Benefit Cost	(2,300)	200	(4,200)	1,200
Amount Capitalized Defined Benefit Expense	(1,000)	(100)	$ (1,900)	0
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Service Cost Discount Rate Under Spot Rate			5.14%
Interest Cost Discount Rate under spot rate methodology			3.72%
Defined Benefit Plan, Estimated Future Benefit Payments [Abstract]
Impact of Change in Accounting Estimate Spot Rate Discount Rate	2,500		$ 5,000
DefinedBenefitPlanDescriptionOfPlanAmendment			On August 5, 2016, the Company amended its PBOP plan. The amendment standardized separate benefit structures that existed within the plan and made other benefit changes. Overall, the plan amendment is expected to reduce the PBOP plan's accumulated projected benefit obligation by approximately $220 million. The PBOP liability will be remeasured as of August 5, 2016, and the PBOP expense for the remainder of 2016 will be recorded using updated assumptions and actuarial calculations.
DefinedBenefitPlanEffectOfPlanAmendmentOnAccumulatedBenefitObligation	220,000		$ 220,000
The Connecticut Light And Power Company [Member]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, SERP and PBOP	247,419		247,419		271,056
The Connecticut Light And Power Company [Member] | Pension Plans Defined Benefit Member
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	4,600	6,200	9,600	12,200
Defined Benefit Plan Interest Cost	10,200	12,900	21,000	25,400
Defined Benefit Plan, Expected Return on Plan Assets	(18,000)	(19,800)	(36,200)	(39,400)
Defined Benefit Plan, Amortization of Gains (Losses)	6,300	8,000	13,000	16,200
Defined Benefit Plan Amortization Of Prior Service Cost Credit	400	400	800	800
Defined Benefit Plan Net Periodic Benefit Cost	3,500	7,700	8,200	15,200
Related Intercompany Allocationsof Defined Benefit Expense	3,500	5,800	6,800	12,200
Amount Capitalized Defined Benefit Expense	2,200	4,700	4,900	9,400
The Connecticut Light And Power Company [Member] | PBOP Plan
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	400	500	900	1,100
Defined Benefit Plan Interest Cost	1,300	1,800	2,700	3,500
Defined Benefit Plan, Expected Return on Plan Assets	(2,500)	(2,800)	(5,100)	(5,500)
Defined Benefit Plan, Amortization of Gains (Losses)	300	200	500	300
Defined Benefit Plan Amortization Of Prior Service Cost Credit			0	0
Defined Benefit Plan Net Periodic Benefit Cost	(500)	(300)	(1,000)	(600)
Related Intercompany Allocationsof Defined Benefit Expense	200	400	300	900
Amount Capitalized Defined Benefit Expense	(200)	(100)	(500)	(100)
NSTAR Electric Company [Member]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, SERP and PBOP	194,734		194,734		209,153
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	3,300	3,700	6,700	7,500
Defined Benefit Plan Interest Cost	8,500	10,000	16,800	20,100
Defined Benefit Plan, Expected Return on Plan Assets	(16,900)	(17,500)	(33,800)	(35,100)
Defined Benefit Plan, Amortization of Gains (Losses)	8,700	8,700	17,000	18,400
Defined Benefit Plan Amortization Of Prior Service Cost Credit	0	0	0
Defined Benefit Plan Net Periodic Benefit Cost	3,600	4,900	6,700	10,900
Related Intercompany Allocationsof Defined Benefit Expense	2,200	3,400	4,500	6,900
Amount Capitalized Defined Benefit Expense	2,000	2,700	3,700	5,900
NSTAR Electric Company [Member] | PBOP Plan
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	1,000	1,300	1,800	2,700
Defined Benefit Plan Interest Cost	3,800	4,700	7,800	9,500
Defined Benefit Plan, Expected Return on Plan Assets	(6,400)	(6,900)	(12,700)	(13,700)
Defined Benefit Plan, Amortization of Gains (Losses)	300	400	500	1,200
Defined Benefit Plan Amortization Of Prior Service Cost Credit		(100)	0	(100)
Defined Benefit Plan Net Periodic Benefit Cost	(1,300)	(600)	(2,600)	(400)
Related Intercompany Allocationsof Defined Benefit Expense	100	200	100	400
Amount Capitalized Defined Benefit Expense	(600)	(200)	(1,100)	(100)
Public Service Company Of New Hampshire [Member]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, SERP and PBOP	75,721		75,721		89,579
Public Service Company Of New Hampshire [Member] | Pension Plans Defined Benefit Member
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	2,500	3,100	5,000	6,000
Defined Benefit Plan Interest Cost	5,100	6,100	10,300	12,100
Defined Benefit Plan, Expected Return on Plan Assets	(9,600)	(10,100)	(19,300)	(20,200)
Defined Benefit Plan, Amortization of Gains (Losses)	2,600	2,900	4,900	5,800
Defined Benefit Plan Amortization Of Prior Service Cost Credit	100	100	200	200
Defined Benefit Plan Net Periodic Benefit Cost	700	2,100	1,100	3,900
Related Intercompany Allocationsof Defined Benefit Expense	1,000	1,600	2,000	3,400
Amount Capitalized Defined Benefit Expense	400	900	700	1,700
Public Service Company Of New Hampshire [Member] | PBOP Plan
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	300	300	600	700
Defined Benefit Plan Interest Cost	700	1,000	1,500	1,900
Defined Benefit Plan, Expected Return on Plan Assets	(1,400)	(1,500)	(2,800)	(3,000)
Defined Benefit Plan, Amortization of Gains (Losses)	100	100	200	300
Defined Benefit Plan Amortization Of Prior Service Cost Credit			0	0
Defined Benefit Plan Net Periodic Benefit Cost	(300)	(100)	(500)	(100)
Related Intercompany Allocationsof Defined Benefit Expense		100		200
Amount Capitalized Defined Benefit Expense	(100)	0	(100)	100
Western Massachusetts Electric Company [Member]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, SERP and PBOP	12,135		12,135		$ 19,515
Western Massachusetts Electric Company [Member] | Pension Plans Defined Benefit Member
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	800	1,000	1,600	2,200
Defined Benefit Plan Interest Cost	2,100	2,600	4,200	5,200
Defined Benefit Plan, Expected Return on Plan Assets	(4,400)	(4,700)	(8,800)	(9,400)
Defined Benefit Plan, Amortization of Gains (Losses)	1,300	1,600	2,800	3,200
Defined Benefit Plan Amortization Of Prior Service Cost Credit	100	100	200	100
Defined Benefit Plan Net Periodic Benefit Cost	(100)	600	0	1,300
Related Intercompany Allocationsof Defined Benefit Expense	600	1,100	1,200	2,300
Amount Capitalized Defined Benefit Expense	100	500	200	1,000
Western Massachusetts Electric Company [Member] | PBOP Plan
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	100	100	200	200
Defined Benefit Plan Interest Cost	300	300	500	800
Defined Benefit Plan, Expected Return on Plan Assets	(600)	(600)	(1,100)	(1,300)
Defined Benefit Plan, Amortization of Gains (Losses)	0	0	0
Defined Benefit Plan Amortization Of Prior Service Cost Credit			0	0
Defined Benefit Plan Net Periodic Benefit Cost	(200)	(200)	(400)	(300)
Related Intercompany Allocationsof Defined Benefit Expense	0	100	100	200
Amount Capitalized Defined Benefit Expense	$ (100)	$ 0	$ (200)	$ (100)