PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSIONS (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Pension Plans Defined Benefit Member
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	$ 26.6	$ 15.3
Defined Benefit Plan Interest Cost	51.4	38.1
Defined Benefit Plan, Expected Return on Plan Assets	(70.3)	(42.5)
Defined Benefit Plan, Amortization of Gains (Losses)	52.9	30.1
Defined Benefit Plan Amortization Of Prior Service Cost Credit	1.1	2.1
Defined Benefit Plan Net Periodic Benefit Cost	61.7	43.1
Amount Capitalized Pension Expense	16.7	10.6
DescriptionOfDefinedContributionPensionAndOtherPostretirementPlans	: NU’s policy is to annually fund the NUSCO and NSTAR Pension Plans in an amount at least equal to an amount that will satisfy federal requirements. Based on the current status of the NUSCO Pension Plan, NU anticipates making a contribution of approximately $203 million in 2013, of which $107 million is required to meet minimum federal funding requirements. NSTAR Electric anticipates making a contribution of approximately $82 million in 2013 to the NSTAR Pension Plan, of which $38 million is required to meet minimum federal funding requirements. Contributions are being made in installments and began in January 2013. For the three months ended March 31, 2013, NU contributed $35.1 million to the NUSCO Pension Plan, all of which was contributed by PSNH, and NSTAR Electric contributed $4.3 million to the NSTAR Pension Plan.
Other Postretirement Benefit Plans Defined Benefit Member
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	4.8	2.3
Defined Benefit Plan Interest Cost	12.8	6.2
Defined Benefit Plan, Expected Return on Plan Assets	(13.8)	(5.6)
Defined Benefit Plan, Amortization of Gains (Losses)	8.2	5.5
Defined Benefit Plan Amortization Of Transition Obligations Assets		2.9
Defined Benefit Plan Amortization Of Prior Service Cost Credit	(0.6)	(0.1)
Defined Benefit Plan Net Periodic Benefit Cost	11.4	11.2
DescriptionOfDefinedContributionPensionAndOtherPostretirementPlans	For the PBOP Plans, it is NU’s policy to annually fund the NUSCO PBOP Plans in an amount equal to the PBOP Plans' postretirement benefit cost, excluding curtailment and termination benefits, if applicable. NU anticipates making $25.7 million in contributions in 2013. NU contributes an amount that approximates annual benefit payments to the NSTAR PBOP Plan. NU anticipates making $30 million in contributions in 2013, of which $7.5 million was contributed during the three months ended March 31, 2013.
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	6.1	5.4
Defined Benefit Plan Interest Cost	12.1	12.8
Defined Benefit Plan, Expected Return on Plan Assets	(18.5)	(17.5)
Defined Benefit Plan, Amortization of Gains (Losses)	14.1	11.9
Defined Benefit Plan Amortization Of Prior Service Cost Credit	0.5	0.9
Defined Benefit Plan Net Periodic Benefit Cost	14.3	13.5
Related Intercompany Allocationsof Defined Benefit Expense	10.7	10.6
Amount Capitalized Pension Expense	7.0	6.6
The Connecticut Light And Power Company [Member] | Other Postretirement Benefit Plans Defined Benefit Member
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	0.9	0.8
Defined Benefit Plan Interest Cost	2.0	2.4
Defined Benefit Plan, Expected Return on Plan Assets	(2.5)	(2.3)
Defined Benefit Plan, Amortization of Gains (Losses)	1.7	2.0
Defined Benefit Plan Amortization Of Transition Obligations Assets		1.5
Defined Benefit Plan Net Periodic Benefit Cost	2.1	4.4
Related Intercompany Allocationsof Defined Benefit Expense	1.6	2.1
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	9.3	7.8
Defined Benefit Plan Interest Cost	14.2	14.8
Defined Benefit Plan, Expected Return on Plan Assets	(22.0)	(16.5)
Defined Benefit Plan, Amortization of Gains (Losses)	14.5	15.7
Defined Benefit Plan Amortization Of Prior Service Cost Credit		(0.2)
Defined Benefit Plan Net Periodic Benefit Cost	16.0	21.6
Related Intercompany Allocationsof Defined Benefit Expense	(2.0)	(3.1)
Amount Capitalized Pension Expense	5.3	6.3
NSTAR Electric Company [Member] | Other Postretirement Benefit Plans Defined Benefit Member
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Related Intercompany Allocationsof Defined Benefit Expense	4.3	9.0
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	3.3	2.9
Defined Benefit Plan Interest Cost	6.0	6.1
Defined Benefit Plan, Expected Return on Plan Assets	(7.7)	(6.7)
Defined Benefit Plan, Amortization of Gains (Losses)	5.5	3.9
Defined Benefit Plan Amortization Of Prior Service Cost Credit	0.1	0.4
Defined Benefit Plan Net Periodic Benefit Cost	7.2	6.6
Related Intercompany Allocationsof Defined Benefit Expense	2.6	2.5
Amount Capitalized Pension Expense	2.2	2.0
Public Service Company Of New Hampshire [Member] | Other Postretirement Benefit Plans Defined Benefit Member
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	0.6	0.5
Defined Benefit Plan Interest Cost	1.0	1.2
Defined Benefit Plan, Expected Return on Plan Assets	(1.3)	(1.1)
Defined Benefit Plan, Amortization of Gains (Losses)	0.9	1.0
Defined Benefit Plan Amortization Of Transition Obligations Assets		0.6
Defined Benefit Plan Net Periodic Benefit Cost	1.2	2.2
Related Intercompany Allocationsof Defined Benefit Expense	0.4	0.5
Western Massachusetts Electric Company [Member] | Pension Plans Defined Benefit Member
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	1.2	1.1
Defined Benefit Plan Interest Cost	2.5	2.6
Defined Benefit Plan, Expected Return on Plan Assets	(4.3)	(4.1)
Defined Benefit Plan, Amortization of Gains (Losses)	3.0	2.5
Defined Benefit Plan Amortization Of Prior Service Cost Credit	0.1	0.2
Defined Benefit Plan Net Periodic Benefit Cost	2.5	2.3
Related Intercompany Allocationsof Defined Benefit Expense	1.8	2.0
Amount Capitalized Pension Expense	1.3	1.2
Western Massachusetts Electric Company [Member] | Other Postretirement Benefit Plans Defined Benefit Member
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	0.2	0.2
Defined Benefit Plan Interest Cost	0.4	0.5
Defined Benefit Plan, Expected Return on Plan Assets	(0.6)	(0.5)
Defined Benefit Plan, Amortization of Gains (Losses)	0.3	0.3
Defined Benefit Plan Amortization Of Transition Obligations Assets		0.3
Defined Benefit Plan Net Periodic Benefit Cost	0.3	0.8
Related Intercompany Allocationsof Defined Benefit Expense	$ 0.3	$ 0.4