PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSIONS (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, SERP and PBOP	$ 1,636,769		$ 1,636,769		$ 1,638,558
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	22,800	$ 19,100	45,900	$ 41,500
Defined Benefit Plan Interest Cost	56,900	56,300	113,300	113,000
Defined Benefit Plan, Expected Return on Plan Assets	(83,900)	(77,700)	(168,200)	(155,400)
Defined Benefit Plan, Amortization of Gains (Losses)	36,500	31,700	75,500	64,700
Defined Benefit Plan Amortization Of Prior Service Cost Credit	900	1,100	1,800	2,100
Defined Benefit Plan Net Periodic Benefit Cost	33,200	30,500	68,300	65,900
Amount Capitalized Defined Benefit Expense	9,800	8,700	21,100	18,400
PBOP Plan
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	3,900	3,200	8,100	6,300
Defined Benefit Plan Interest Cost	11,700	12,100	23,600	24,700
Defined Benefit Plan, Expected Return on Plan Assets	(16,900)	(15,800)	(33,700)	(31,600)
Defined Benefit Plan, Amortization of Gains (Losses)	1,700	3,000	3,400	6,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	(200)	(700)	(200)	(1,400)
Defined Benefit Plan Net Periodic Benefit Cost	200	1,800	1,200	4,000
Amount Capitalized Defined Benefit Expense	(100)	400	0	800
The Connecticut Light And Power Company [Member]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, SERP and PBOP	286,131		286,131		273,854
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	6,200	5,000	12,200	10,200
Defined Benefit Plan Interest Cost	12,900	12,400	25,400	25,700
Defined Benefit Plan, Expected Return on Plan Assets	(19,800)	(18,700)	(39,400)	(38,000)
Defined Benefit Plan, Amortization of Gains (Losses)	8,000	8,200	16,200	17,300
Defined Benefit Plan Amortization Of Prior Service Cost Credit	400	500	800	900
Defined Benefit Plan Net Periodic Benefit Cost	7,700	7,400	15,200	16,100
Related Intercompany Allocationsof Defined Benefit Expense	5,800	7,500	12,200	14,300
Amount Capitalized Defined Benefit Expense	4,700	4,400	9,400	9,300
The Connecticut Light And Power Company [Member] | PBOP Plan
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	500	500	1,100	1,100
Defined Benefit Plan Interest Cost	1,800	1,900	3,500	4,000
Defined Benefit Plan, Expected Return on Plan Assets	(2,800)	(2,500)	(5,500)	(5,200)
Defined Benefit Plan, Amortization of Gains (Losses)	200	1,000	300	2,100
Defined Benefit Plan Amortization Of Prior Service Cost Credit			0	0
Defined Benefit Plan Net Periodic Benefit Cost	(300)	900	(600)	2,000
Related Intercompany Allocationsof Defined Benefit Expense	400	1,100	900	2,200
Amount Capitalized Defined Benefit Expense	(100)	500	(100)	1,000
NSTAR Electric Company [Member]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, SERP and PBOP	220,000		220,000		235,529
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	3,700	3,000	7,500	7,600
Defined Benefit Plan Interest Cost	10,000	10,300	20,100	20,600
Defined Benefit Plan, Expected Return on Plan Assets	(17,500)	(15,700)	(35,100)	(31,500)
Defined Benefit Plan, Amortization of Gains (Losses)	8,700	5,900	18,400	11,700
Defined Benefit Plan Amortization Of Prior Service Cost Credit	0	0	0	0
Defined Benefit Plan Net Periodic Benefit Cost	4,900	3,500	10,900	8,400
Related Intercompany Allocationsof Defined Benefit Expense	3,400	1,400	6,900	3,800
Amount Capitalized Defined Benefit Expense	2,700	1,000	5,900	2,900
NSTAR Electric Company [Member] | PBOP Plan
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	1,300	800	2,700	1,600
Defined Benefit Plan Interest Cost	4,700	4,800	9,500	9,700
Defined Benefit Plan, Expected Return on Plan Assets	(6,900)	(6,500)	(13,700)	(13,000)
Defined Benefit Plan, Amortization of Gains (Losses)	400	(200)	1,200	(300)
Defined Benefit Plan Amortization Of Prior Service Cost Credit	(100)	(500)	(100)	(900)
Defined Benefit Plan Net Periodic Benefit Cost	(600)	(1,600)	(400)	(2,900)
Related Intercompany Allocationsof Defined Benefit Expense	200	0	400	100
Amount Capitalized Defined Benefit Expense	(200)	(500)	(100)	(1,000)
Public Service Company Of New Hampshire [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Amortization Of Prior Service Cost Credit		0
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, SERP and PBOP	99,318		99,318		93,243
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	3,100	2,300	6,000	5,100
Defined Benefit Plan Interest Cost	6,100	5,800	12,100	12,300
Defined Benefit Plan, Expected Return on Plan Assets	(10,100)	(9,300)	(20,200)	(19,500)
Defined Benefit Plan, Amortization of Gains (Losses)	2,900	2,800	5,800	6,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	100	100	200	300
Defined Benefit Plan Net Periodic Benefit Cost	2,100	1,700	3,900	4,200
Related Intercompany Allocationsof Defined Benefit Expense	1,600	2,100	3,400	4,200
Amount Capitalized Defined Benefit Expense	900	800	1,700	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	700	700
Defined Benefit Plan Interest Cost	1,000	1,000	1,900	2,100
Defined Benefit Plan, Expected Return on Plan Assets	(1,500)	(1,300)	(3,000)	(2,700)
Defined Benefit Plan, Amortization of Gains (Losses)	100	600	300	1,100
Defined Benefit Plan Amortization Of Prior Service Cost Credit			0	0
Defined Benefit Plan Net Periodic Benefit Cost	(100)	600	(100)	1,200
Related Intercompany Allocationsof Defined Benefit Expense	100	300	200	600
Amount Capitalized Defined Benefit Expense	0	200	100	400
Western Massachusetts Electric Company [Member]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, SERP and PBOP	19,530		19,530		$ 17,705
Western Massachusetts Electric Company [Member] | Pension Plans Defined Benefit Member
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	1,000	800	2,200	1,900
Defined Benefit Plan Interest Cost	2,600	2,500	5,200	5,200
Defined Benefit Plan, Expected Return on Plan Assets	(4,700)	(4,400)	(9,400)	(9,000)
Defined Benefit Plan, Amortization of Gains (Losses)	1,600	1,700	3,200	3,500
Defined Benefit Plan Amortization Of Prior Service Cost Credit	100	100	100	200
Defined Benefit Plan Net Periodic Benefit Cost	600	700	1,300	1,800
Related Intercompany Allocationsof Defined Benefit Expense	1,100	1,400	2,300	2,700
Amount Capitalized Defined Benefit Expense	500	600	1,000	1,400
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	400	800	800
Defined Benefit Plan, Expected Return on Plan Assets	(600)	(600)	(1,300)	(1,100)
Defined Benefit Plan, Amortization of Gains (Losses)	0	100	0	200
Defined Benefit Plan Amortization Of Prior Service Cost Credit			0	0
Defined Benefit Plan Net Periodic Benefit Cost	(200)	0	(300)
Related Intercompany Allocationsof Defined Benefit Expense	100	200	200	400
Amount Capitalized Defined Benefit Expense	$ 0	$ 0	$ (100)	$ 100