PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSIONS (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, PBOP and SERP	$ 787,550,000		$ 787,550,000		$ 896,844,000
Pension Plans Defined Benefit Member
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	19,100,000	25,600,000	60,700,000	76,700,000
Defined Benefit Plan Interest Cost	56,400,000	51,700,000	169,500,000	155,000,000
Defined Benefit Plan, Expected Return on Plan Assets	(77,700,000)	(69,500,000)	(233,100,000)	(208,500,000)
Defined Benefit Plan, Amortization of Gains (Losses)	31,700,000	52,400,000	96,500,000	158,100,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	1,100,000	1,100,000	3,300,000	3,000,000
Defined Benefit Plan Net Periodic Benefit Cost	30,600,000	61,300,000	96,900,000	184,300,000
Amount Capitalized Defined Benefit Expense	8,300,000	18,300,000	26,700,000	54,900,000
PBOP Plan
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	3,100,000	4,200,000	9,300,000	12,600,000
Defined Benefit Plan Interest Cost	12,400,000	11,800,000	37,100,000	35,400,000
Defined Benefit Plan, Expected Return on Plan Assets	(15,800,000)	(13,900,000)	(47,400,000)	(41,600,000)
Defined Benefit Plan, Amortization of Gains (Losses)	3,000,000	6,500,000	9,100,000	19,500,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	(700,000)	(500,000)	(2,100,000)	(1,500,000)
Defined Benefit Plan Net Periodic Benefit Cost	2,000,000	8,100,000	6,000,000	24,400,000
Amount Capitalized Defined Benefit Expense	1,100,000	2,600,000	1,900,000	7,600,000
The Connecticut Light And Power Company [Member]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, PBOP and SERP	63,756,000		63,756,000		95,895,000
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	5,000,000	6,300,000	15,200,000	18,700,000
Defined Benefit Plan Interest Cost	12,500,000	12,100,000	38,100,000	36,300,000
Defined Benefit Plan, Expected Return on Plan Assets	(18,700,000)	(18,400,000)	(56,700,000)	(55,400,000)
Defined Benefit Plan, Amortization of Gains (Losses)	8,200,000	13,900,000	25,500,000	42,000,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	400,000	400,000	1,400,000	1,400,000
Defined Benefit Plan Net Periodic Benefit Cost	7,400,000	14,300,000	23,500,000	43,000,000
Related Intercompany Allocationsof Defined Benefit Expense	6,500,000	11,400,000	20,800,000	33,600,000
Amount Capitalized Defined Benefit Expense	4,300,000	7,000,000	13,600,000	21,000,000
The Connecticut Light And Power Company [Member] | PBOP Plan
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	600,000	900,000	1,700,000	2,600,000
Defined Benefit Plan Interest Cost	2,000,000	2,000,000	6,000,000	5,900,000
Defined Benefit Plan, Expected Return on Plan Assets	(2,600,000)	(2,500,000)	(7,900,000)	(7,600,000)
Defined Benefit Plan, Amortization of Gains (Losses)	1,000,000	1,800,000	3,200,000	5,600,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit			0
Defined Benefit Plan Net Periodic Benefit Cost	1,000,000	2,200,000	3,000,000	6,500,000
Related Intercompany Allocationsof Defined Benefit Expense	900,000	1,700,000	3,100,000	5,300,000
Amount Capitalized Defined Benefit Expense	500,000	1,300,000	1,500,000	3,700,000
NSTAR Electric Company [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Net Periodic Benefit Cost		1,200,000
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, PBOP and SERP	142,249,000		142,249,000		118,010,000
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	3,000,000	8,300,000	10,600,000	24,800,000
Defined Benefit Plan Interest Cost	10,300,000	14,500,000	31,000,000	43,500,000
Defined Benefit Plan, Expected Return on Plan Assets	(15,700,000)	(21,100,000)	(47,300,000)	(63,300,000)
Defined Benefit Plan, Amortization of Gains (Losses)	5,900,000	14,400,000	17,600,000	43,600,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	0	0	0	(200,000)
Defined Benefit Plan Net Periodic Benefit Cost	3,500,000	16,100,000	11,900,000	48,400,000
Related Intercompany Allocationsof Defined Benefit Expense	2,900,000	(2,100,000)	6,700,000	(6,200,000)
Amount Capitalized Defined Benefit Expense	2,600,000	9,800,000	5,500,000	21,600,000
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan		83,400,000		83,400,000	118,000,000
NSTAR Electric Company [Member] | PBOP Plan
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	800,000		2,300,000
Defined Benefit Plan Interest Cost	4,900,000		14,600,000
Defined Benefit Plan, Expected Return on Plan Assets	(6,500,000)		(19,500,000)
Defined Benefit Plan, Amortization of Gains (Losses)	(100,000)		(400,000)
Defined Benefit Plan Amortization Of Prior Service Cost Credit	(500,000)		(1,400,000)
Defined Benefit Plan Net Periodic Benefit Cost	(1,400,000)	1,200,000	(4,400,000)	3,500,000
Related Intercompany Allocationsof Defined Benefit Expense	300,000		400,000
Amount Capitalized Defined Benefit Expense	(500,000)		(1,500,000)
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	55,600,000		55,600,000
NSTAR Electric Company [Member] | Supplemental Employee Retirement Plans, Defined Benefit [Member]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Pension and Other Postretirement Defined Benefit Plans, Current Liabilities	(400,000)		(400,000)
Defined Benefit Plan, Funded Status of Plan	3,600,000		3,600,000
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	2,300,000	3,300,000	7,400,000	9,800,000
Defined Benefit Plan Interest Cost	5,700,000	5,800,000	18,000,000	17,800,000
Defined Benefit Plan, Expected Return on Plan Assets	(9,300,000)	(9,200,000)	(28,800,000)	(26,200,000)
Defined Benefit Plan, Amortization of Gains (Losses)	2,800,000	5,400,000	8,900,000	16,200,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	100,000	100,000	500,000	400,000
Defined Benefit Plan Net Periodic Benefit Cost	1,600,000	5,400,000	6,000,000	18,000,000
Related Intercompany Allocationsof Defined Benefit Expense	1,800,000	2,600,000	6,000,000	7,800,000
Amount Capitalized Defined Benefit Expense	700,000	1,700,000	2,400,000	5,600,000
Public Service Company Of New Hampshire [Member] | PBOP Plan
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	300,000	600,000	1,000,000	1,700,000
Defined Benefit Plan Interest Cost	1,100,000	1,000,000	3,200,000	3,100,000
Defined Benefit Plan, Expected Return on Plan Assets	(1,400,000)	(1,300,000)	(4,100,000)	(3,900,000)
Defined Benefit Plan, Amortization of Gains (Losses)	600,000	900,000	1,700,000	2,700,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit			0
Defined Benefit Plan Net Periodic Benefit Cost	600,000	1,200,000	1,800,000	3,600,000
Related Intercompany Allocationsof Defined Benefit Expense	200,000	400,000	800,000	1,200,000
Amount Capitalized Defined Benefit Expense	200,000	400,000	600,000	1,100,000
Western Massachusetts Electric Company [Member] | Pension Plans Defined Benefit Member
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	800,000	1,200,000	2,700,000	3,500,000
Defined Benefit Plan Interest Cost	2,500,000	2,500,000	7,800,000	7,500,000
Defined Benefit Plan, Expected Return on Plan Assets	(4,400,000)	(4,300,000)	(13,500,000)	(13,000,000)
Defined Benefit Plan, Amortization of Gains (Losses)	1,700,000	2,900,000	5,200,000	8,900,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	100,000	100,000	300,000	300,000
Defined Benefit Plan Net Periodic Benefit Cost	700,000	2,400,000	2,500,000	7,200,000
Related Intercompany Allocationsof Defined Benefit Expense	1,200,000	2,000,000	3,900,000	6,000,000
Amount Capitalized Defined Benefit Expense	600,000	1,300,000	2,000,000	3,900,000
Western Massachusetts Electric Company [Member] | PBOP Plan
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	100,000	200,000	300,000	500,000
Defined Benefit Plan Interest Cost	400,000	400,000	1,300,000	1,300,000
Defined Benefit Plan, Expected Return on Plan Assets	(600,000)	(600,000)	(1,700,000)	(1,700,000)
Defined Benefit Plan, Amortization of Gains (Losses)	100,000	300,000	300,000	800,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit			0
Defined Benefit Plan Net Periodic Benefit Cost	0	300,000	200,000	900,000
Related Intercompany Allocationsof Defined Benefit Expense	200,000	300,000	600,000	1,000,000
Amount Capitalized Defined Benefit Expense	$ 0	$ 300,000	$ 100,000	$ 700,000