PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSIONS (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, SERP and PBOP	$ 1,498,346		$ 1,498,346		$ 1,638,558
Defined Benefit Plan, Estimated Future Benefit Payments [Abstract]
Amount That Represents Deferred Regulatory Assets	800		2,400
Pension Plans Defined Benefit Member
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	22,700	$ 19,100	68,700	$ 60,700
Defined Benefit Plan Interest Cost	56,900	56,400	170,300	169,500
Defined Benefit Plan, Expected Return on Plan Assets	(83,900)	(77,700)	(252,100)	(233,100)
Defined Benefit Plan, Amortization of Gains (Losses)	36,500	31,700	111,900	96,500
Defined Benefit Plan Amortization Of Prior Service Cost Credit	900	1,100	2,700	3,300
Defined Benefit Plan Net Periodic Benefit Cost	33,100	30,600	101,500	96,900
Amount Capitalized Defined Benefit Expense	9,800	8,300	31,300	26,700
PBOP Plan
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	4,100	3,100	10,300	9,300
Defined Benefit Plan Interest Cost	11,800	12,400	30,400	37,100
Defined Benefit Plan, Expected Return on Plan Assets	(16,900)	(15,800)	(43,200)	(47,400)
Defined Benefit Plan, Amortization of Gains (Losses)	1,700	3,000	4,500	9,100
Defined Benefit Plan Amortization Of Prior Service Cost Credit	(100)	(700)	(300)	(2,100)
Defined Benefit Plan Net Periodic Benefit Cost	600	2,000	1,700	6,000
Amount Capitalized Defined Benefit Expense	0	1,100	100	1,900
The Connecticut Light And Power Company [Member]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, SERP and PBOP	284,890		284,890		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	18,400	15,200
Defined Benefit Plan Interest Cost	12,900	12,500	38,400	38,100
Defined Benefit Plan, Expected Return on Plan Assets	(19,800)	(18,700)	(59,100)	(56,700)
Defined Benefit Plan, Amortization of Gains (Losses)	8,000	8,200	24,200	25,500
Defined Benefit Plan Amortization Of Prior Service Cost Credit	400	400	1,100	1,400
Defined Benefit Plan Net Periodic Benefit Cost	7,700	7,400	23,000	23,500
Related Intercompany Allocationsof Defined Benefit Expense	5,800	6,500	18,000	20,800
Amount Capitalized Defined Benefit Expense	4,700	4,300	14,100	13,600
The Connecticut Light And Power Company [Member] | PBOP Plan
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	500	600	1,600	1,700
Defined Benefit Plan Interest Cost	1,800	2,000	5,400	6,000
Defined Benefit Plan, Expected Return on Plan Assets	(2,800)	(2,600)	(8,300)	(7,900)
Defined Benefit Plan, Amortization of Gains (Losses)	200	1,000	500	3,200
Defined Benefit Plan Amortization Of Prior Service Cost Credit			0	0
Defined Benefit Plan Net Periodic Benefit Cost	(300)	1,000	(800)	3,000
Related Intercompany Allocationsof Defined Benefit Expense	400	900	1,400	3,100
Amount Capitalized Defined Benefit Expense	(100)	500	(200)	1,500
NSTAR Electric Company [Member]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, SERP and PBOP	211,891		211,891		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	11,200	10,600
Defined Benefit Plan Interest Cost	10,000	10,300	30,200	31,000
Defined Benefit Plan, Expected Return on Plan Assets	(17,500)	(15,700)	(52,500)	(47,300)
Defined Benefit Plan, Amortization of Gains (Losses)	8,700	5,900	27,000	17,600
Defined Benefit Plan Amortization Of Prior Service Cost Credit	0	0	100	0
Defined Benefit Plan Net Periodic Benefit Cost	4,900	3,500	16,000	11,900
Related Intercompany Allocationsof Defined Benefit Expense	3,400	2,900	10,300	6,700
Amount Capitalized Defined Benefit Expense	2,700	2,600	8,600	5,500
NSTAR Electric Company [Member] | PBOP Plan
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	1,300	800	4,000	2,300
Defined Benefit Plan Interest Cost	4,800	4,900	14,200	14,600
Defined Benefit Plan, Expected Return on Plan Assets	(6,800)	(6,500)	(20,500)	(19,500)
Defined Benefit Plan, Amortization of Gains (Losses)	600	(100)	1,800	(400)
Defined Benefit Plan Amortization Of Prior Service Cost Credit	(100)	(500)	(100)	(1,400)
Defined Benefit Plan Net Periodic Benefit Cost	(200)	(1,400)	(600)	(4,400)
Related Intercompany Allocationsof Defined Benefit Expense	200	300	700	400
Amount Capitalized Defined Benefit Expense	0	(500)	(100)	(1,500)
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	97,167		97,167		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	9,000	7,400
Defined Benefit Plan Interest Cost	6,100	5,700	18,100	18,000
Defined Benefit Plan, Expected Return on Plan Assets	(10,100)	(9,300)	(30,300)	(28,800)
Defined Benefit Plan, Amortization of Gains (Losses)	2,900	2,800	8,800	8,900
Defined Benefit Plan Amortization Of Prior Service Cost Credit	100	100	400	500
Defined Benefit Plan Net Periodic Benefit Cost	2,100	1,600	6,000	6,000
Related Intercompany Allocationsof Defined Benefit Expense	1,600	1,800	5,000	6,000
Amount Capitalized Defined Benefit Expense	800	700	2,600	2,400
Public Service Company Of New Hampshire [Member] | PBOP Plan
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	300	300	1,000	1,000
Defined Benefit Plan Interest Cost	1,000	1,100	2,900	3,200
Defined Benefit Plan, Expected Return on Plan Assets	(1,500)	(1,400)	(4,400)	(4,100)
Defined Benefit Plan, Amortization of Gains (Losses)	100	600	400	1,700
Defined Benefit Plan Amortization Of Prior Service Cost Credit			0	0
Defined Benefit Plan Net Periodic Benefit Cost	(100)	600	(100)	1,800
Related Intercompany Allocationsof Defined Benefit Expense	100	200	300	800
Amount Capitalized Defined Benefit Expense	100	200	200	600
Western Massachusetts Electric Company [Member]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, SERP and PBOP	18,320		18,320		$ 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	3,200	2,700
Defined Benefit Plan Interest Cost	2,600	2,500	7,800	7,800
Defined Benefit Plan, Expected Return on Plan Assets	(4,700)	(4,400)	(14,200)	(13,500)
Defined Benefit Plan, Amortization of Gains (Losses)	1,600	1,700	4,800	5,200
Defined Benefit Plan Amortization Of Prior Service Cost Credit	100	100	200	300
Defined Benefit Plan Net Periodic Benefit Cost	600	700	1,800	2,500
Related Intercompany Allocationsof Defined Benefit Expense	1,100	1,200	3,300	3,900
Amount Capitalized Defined Benefit Expense	500	600	1,400	2,000
Western Massachusetts Electric Company [Member] | PBOP Plan
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	100	100	300	300
Defined Benefit Plan Interest Cost	300	400	1,100	1,300
Defined Benefit Plan, Expected Return on Plan Assets	(600)	(600)	(1,900)	(1,700)
Defined Benefit Plan, Amortization of Gains (Losses)	0	100	0	300
Defined Benefit Plan Amortization Of Prior Service Cost Credit			0	0
Defined Benefit Plan Net Periodic Benefit Cost	(200)	0	(500)	200
Related Intercompany Allocationsof Defined Benefit Expense	100	200	300	600
Amount Capitalized Defined Benefit Expense	$ 0	$ 0	$ (100)	$ 100