PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSIONS (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, PBOP and SERP	$ 825,001,000		$ 825,001,000		$ 896,844,000
Pension Plans Defined Benefit Member
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	19,100,000	24,600,000	41,500,000	51,100,000
Defined Benefit Plan Interest Cost	56,300,000	51,900,000	113,000,000	103,500,000
Defined Benefit Plan, Expected Return on Plan Assets	(77,700,000)	(68,600,000)	(155,400,000)	(139,000,000)
Defined Benefit Plan, Amortization of Gains (Losses)	31,700,000	52,600,000	64,700,000	105,500,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	1,100,000	1,000,000	2,100,000	2,100,000
Defined Benefit Plan Net Periodic Benefit Cost	30,500,000	61,500,000	65,900,000	123,200,000
Amount Capitalized Defined Benefit Expense	8,700,000	19,900,000	18,400,000	36,600,000
PBOP Plan
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	3,200,000	3,700,000	6,300,000	8,500,000
Defined Benefit Plan Interest Cost	12,100,000	10,900,000	24,700,000	23,600,000
Defined Benefit Plan, Expected Return on Plan Assets	(15,800,000)	(13,900,000)	(31,600,000)	(27,700,000)
Defined Benefit Plan, Amortization of Gains (Losses)	3,000,000	4,700,000	6,000,000	13,000,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	(700,000)	(500,000)	(1,400,000)	(1,100,000)
Defined Benefit Plan Net Periodic Benefit Cost	1,800,000	4,900,000	4,000,000	16,300,000
Amount Capitalized Defined Benefit Expense	400,000	1,500,000	800,000	5,000,000
The Connecticut Light And Power Company [Member]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, PBOP and SERP	66,543,000		66,543,000		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	10,200,000	12,400,000
Defined Benefit Plan Interest Cost	12,400,000	12,100,000	25,700,000	24,200,000
Defined Benefit Plan, Expected Return on Plan Assets	(18,700,000)	(18,400,000)	(38,000,000)	(36,900,000)
Defined Benefit Plan, Amortization of Gains (Losses)	8,200,000	13,900,000	17,300,000	28,000,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	500,000	500,000	900,000	900,000
Defined Benefit Plan Net Periodic Benefit Cost	7,400,000	14,400,000	16,100,000	28,600,000
Related Intercompany Allocationsof Defined Benefit Expense	7,500,000	11,300,000	14,300,000	22,100,000
Amount Capitalized Defined Benefit Expense	4,400,000	7,000,000	9,300,000	14,000,000
The Connecticut Light And Power Company [Member] | PBOP Plan
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	500,000	900,000	1,100,000	1,700,000
Defined Benefit Plan Interest Cost	1,900,000	2,000,000	4,000,000	3,900,000
Defined Benefit Plan, Expected Return on Plan Assets	(2,500,000)	(2,500,000)	(5,200,000)	(5,000,000)
Defined Benefit Plan, Amortization of Gains (Losses)	1,000,000	1,900,000	2,100,000	3,700,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit			0
Defined Benefit Plan Net Periodic Benefit Cost	900,000	2,300,000	2,000,000	4,300,000
Related Intercompany Allocationsof Defined Benefit Expense	1,100,000	1,900,000	2,200,000	3,600,000
Amount Capitalized Defined Benefit Expense	500,000	1,200,000	1,000,000	2,400,000
NSTAR Electric Company [Member]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, PBOP and SERP	150,151,000		150,151,000		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	7,300,000	7,600,000	16,500,000
Defined Benefit Plan Interest Cost	10,300,000	14,700,000	20,600,000	29,000,000
Defined Benefit Plan, Expected Return on Plan Assets	(15,700,000)	(20,200,000)	(31,500,000)	(42,200,000)
Defined Benefit Plan, Amortization of Gains (Losses)	5,900,000	14,600,000	11,700,000	29,100,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	0	(100,000)	0	(100,000)
Defined Benefit Plan Net Periodic Benefit Cost	3,500,000	16,300,000	8,400,000	32,300,000
Related Intercompany Allocationsof Defined Benefit Expense	1,400,000	(2,200,000)	3,800,000	(4,100,000)
Amount Capitalized Defined Benefit Expense	1,000,000	6,500,000	2,900,000	11,800,000
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	85,800,000		85,800,000		118,000,000
NSTAR Electric Company [Member] | PBOP Plan
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	800,000		1,600,000
Defined Benefit Plan Interest Cost	4,800,000		9,700,000
Defined Benefit Plan, Expected Return on Plan Assets	(6,500,000)		(13,000,000)
Defined Benefit Plan, Amortization of Gains (Losses)	(200,000)		(300,000)
Defined Benefit Plan Amortization Of Prior Service Cost Credit	(500,000)		(900,000)
Defined Benefit Plan Net Periodic Benefit Cost	(1,600,000)	(2,000,000)	(2,900,000)	2,300,000
Related Intercompany Allocationsof Defined Benefit Expense	0		100,000
Amount Capitalized Defined Benefit Expense	(500,000)		(1,000,000)
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	61,200,000		61,200,000
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]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, PBOP and SERP	15,055,000		15,055,000		15,272,000
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	3,200,000	5,100,000	6,500,000
Defined Benefit Plan Interest Cost	5,800,000	5,900,000	12,300,000	11,900,000
Defined Benefit Plan, Expected Return on Plan Assets	(9,300,000)	(9,200,000)	(19,500,000)	(16,800,000)
Defined Benefit Plan, Amortization of Gains (Losses)	2,800,000	5,400,000	6,000,000	10,800,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	100,000	100,000	300,000	300,000
Defined Benefit Plan Net Periodic Benefit Cost	1,700,000	5,400,000	4,200,000	12,700,000
Related Intercompany Allocationsof Defined Benefit Expense	2,100,000	2,600,000	4,200,000	5,200,000
Amount Capitalized Defined Benefit Expense	800,000	1,700,000	1,700,000	3,900,000
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	700,000	1,100,000
Defined Benefit Plan Interest Cost	1,000,000	1,000,000	2,100,000	2,000,000
Defined Benefit Plan, Expected Return on Plan Assets	(1,300,000)	(1,300,000)	(2,700,000)	(2,600,000)
Defined Benefit Plan, Amortization of Gains (Losses)	600,000	900,000	1,100,000	1,800,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit			0
Defined Benefit Plan Net Periodic Benefit Cost	600,000	1,200,000	1,200,000	2,300,000
Related Intercompany Allocationsof Defined Benefit Expense	300,000	400,000	600,000	800,000
Amount Capitalized Defined Benefit Expense	200,000	400,000	400,000	700,000
Western Massachusetts Electric Company [Member]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, PBOP and SERP	2,805,000		2,805,000		3,911,000
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	1,900,000	2,400,000
Defined Benefit Plan Interest Cost	2,500,000	2,500,000	5,200,000	5,000,000
Defined Benefit Plan, Expected Return on Plan Assets	(4,400,000)	(4,400,000)	(9,000,000)	(8,700,000)
Defined Benefit Plan, Amortization of Gains (Losses)	1,700,000	2,900,000	3,500,000	5,900,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	100,000	100,000	200,000	200,000
Defined Benefit Plan Net Periodic Benefit Cost	700,000	2,300,000	1,800,000	4,800,000
Related Intercompany Allocationsof Defined Benefit Expense	1,400,000	2,000,000	2,700,000	4,000,000
Amount Capitalized Defined Benefit Expense	600,000	1,300,000	1,400,000	2,600,000
Western Massachusetts Electric Company [Member] | PBOP Plan
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	100,000	200,000	200,000	400,000
Defined Benefit Plan Interest Cost	400,000	400,000	800,000	800,000
Defined Benefit Plan, Expected Return on Plan Assets	(600,000)	(600,000)	(1,100,000)	(1,200,000)
Defined Benefit Plan, Amortization of Gains (Losses)	100,000	300,000	200,000	600,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit			0
Defined Benefit Plan Net Periodic Benefit Cost	0	300,000	100,000	600,000
Related Intercompany Allocationsof Defined Benefit Expense	200,000	300,000	400,000	600,000
Amount Capitalized Defined Benefit Expense	$ 0	$ 200,000	$ 100,000	$ 400,000