PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSIONS (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan, Curtailments			$ 1.8
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year, Description			Contributions: NU’s policy is to annually fund the Pension Plans in an amount at least equal to an amount that will satisfy the requirements of ERISA, as amended by the PPA of 2006, and the Internal Revenue Code. Based on the current status of the NUSCO Pension Plan, NU is required to make a contribution to the NUSCO Pension Plan of $197.3 million in 2012 to meet minimum funding requirements under the PPA. Contributions are being made in installments and began in January 2012. NU made contributions totaling $35.2 million and $162.3 million ($87.7 million of which was contributed by PSNH in the first quarter of 2012) for the three and nine months ended September 30, 2012, respectively. NSTAR Electric made contributions to the NSTAR Electric Pension Plan of $18.7 million and $25 million for the three and nine months ended September 30, 2012. NSTAR Electric does not anticipate additional contributions to the pension plan for the remainder of 2012. The actual level of funding may differ from this estimate.
Defined Benefit Plan, Description of Settlements and Curtailments			Curtailment Expense (SERP): NU recorded curtailment expense of $1.8 million for the NUSCO SERP for the nine months ended September 30, 2012 to reflect charges related to organization changes that occurred as a result of the merger.
Pension Plans Defined Benefit Member
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	23.0	13.8	61.1	27.5
Defined Benefit Plan Interest Cost	53.3	38.3	144.7	76.5
Defined Benefit Plan, Expected Return on Plan Assets	(59.5)	(42.7)	(161.3)	(85.8)
Defined Benefit Plan, Amortization of Gains (Losses)	47.4	21.1	125.0	42.1
Defined Benefit Plan Amortization Of Prior Service Cost Credit	2.0	2.4	6.1	4.8
Defined Benefit Plan Net Periodic Benefit Cost	66.2	32.9	175.6	65.1
Amount Capitalized Pension Expense	19.2	7.8	49.5	23.5
Other Postretirement Benefit Plans Defined Benefit Member
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	4.4	2.3	11.3	6.8
Defined Benefit Plan Interest Cost	14.3	6.4	34.4	19.3
Defined Benefit Plan, Expected Return on Plan Assets	(11.1)	(5.4)	(28.1)	(16.2)
Defined Benefit Plan, Amortization of Gains (Losses)	10.3	4.8	25.5	14.3
Defined Benefit Plan Amortization Of Transition Obligations Assets	3.1	2.9	9.0	8.7
Defined Benefit Plan Amortization Of Prior Service Cost Credit	(0.5)	(0.1)	(0.9)	(0.2)
Defined Benefit Plan Net Periodic Benefit Cost	20.5	10.9	51.2	32.7
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	5.4	4.9	16.3	14.6
Defined Benefit Plan Interest Cost	12.9	13.0	38.5	39.1
Defined Benefit Plan, Expected Return on Plan Assets	(17.7)	(19.1)	(52.8)	(57.4)
Defined Benefit Plan, Amortization of Gains (Losses)	12.6	8.2	37.0	24.8
Defined Benefit Plan Amortization Of Prior Service Cost Credit	0.9	1.1	2.7	3.1
Defined Benefit Plan Net Periodic Benefit Cost	14.1	8.1	41.7	24.2
Related Intercompany Allocationsof Defined Benefit Expense	10.7	8.5	32.0	25.0
Amount Capitalized Pension Expense	6.8	4.5	20.2	13.4
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	0.8	0.7	2.2	2.2
Defined Benefit Plan Interest Cost	2.3	2.5	6.9	7.5
Defined Benefit Plan, Expected Return on Plan Assets	(2.3)	(2.2)	(6.8)	(6.5)
Defined Benefit Plan, Amortization of Gains (Losses)	1.9	1.9	5.7	5.4
Defined Benefit Plan Amortization Of Transition Obligations Assets	1.5	1.5	4.6	4.6
Defined Benefit Plan Net Periodic Benefit Cost	4.2	4.4	12.6	13.2
Related Intercompany Allocationsof Defined Benefit Expense	2.0	2.0	5.9	6.2
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	2.9	2.6	8.8	7.9
Defined Benefit Plan Interest Cost	6.1	6.1	18.3	18.4
Defined Benefit Plan, Expected Return on Plan Assets	(7.2)	(5.0)	(21.1)	(15.0)
Defined Benefit Plan, Amortization of Gains (Losses)	4.2	2.6	12.1	7.8
Defined Benefit Plan Amortization Of Prior Service Cost Credit	0.4	0.5	1.1	1.5
Defined Benefit Plan Net Periodic Benefit Cost	6.4	6.8	19.2	20.6
Related Intercompany Allocationsof Defined Benefit Expense	2.4	1.9	7.5	5.7
Amount Capitalized Pension Expense	1.9	2.1	5.8	5.9
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	0.5	0.5	1.5	1.4
Defined Benefit Plan Interest Cost	1.1	1.2	3.4	3.6
Defined Benefit Plan, Expected Return on Plan Assets	(1.1)	(1.1)	(3.4)	(3.3)
Defined Benefit Plan, Amortization of Gains (Losses)	0.9	0.8	2.7	2.4
Defined Benefit Plan Amortization Of Transition Obligations Assets	0.6	0.6	1.9	1.9
Defined Benefit Plan Net Periodic Benefit Cost	2.0	2.0	6.1	6.0
Related Intercompany Allocationsof Defined Benefit Expense	0.5	0.5	1.5	1.5
Western Massachusetts Electric Company [Member] | Pension Plans Defined Benefit Member
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	1.0	1.0	3.1	3.0
Defined Benefit Plan Interest Cost	2.6	2.7	7.9	8.1
Defined Benefit Plan, Expected Return on Plan Assets	(4.1)	(4.4)	(12.3)	(13.2)
Defined Benefit Plan, Amortization of Gains (Losses)	2.7	1.7	8.0	5.1
Defined Benefit Plan Amortization Of Prior Service Cost Credit	0.2	0.2	0.6	0.6
Defined Benefit Plan Net Periodic Benefit Cost	2.4	1.2	7.3	3.6
Related Intercompany Allocationsof Defined Benefit Expense	2.1	1.6	6.0	4.6
Amount Capitalized Pension Expense	1.3	0.7	3.7	2.1
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	0.1	0.2	0.4	0.4
Defined Benefit Plan Interest Cost	0.5	0.5	1.5	1.6
Defined Benefit Plan, Expected Return on Plan Assets	(0.5)	(0.5)	(1.6)	(1.5)
Defined Benefit Plan, Amortization of Gains (Losses)	0.3	0.3	0.9	0.9
Defined Benefit Plan Amortization Of Transition Obligations Assets	0.3	0.3	1.1	1.0
Defined Benefit Plan Net Periodic Benefit Cost	0.7	0.8	2.3	2.4
Related Intercompany Allocationsof Defined Benefit Expense	0.4	0.4	1.1	1.1
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	7.6	6.5	22.7	19.5
Defined Benefit Plan Interest Cost	14.7	15.2	44.2	45.7
Defined Benefit Plan, Expected Return on Plan Assets	(16.4)	(17.8)	(49.2)	(53.5)
Defined Benefit Plan, Amortization of Gains (Losses)	15.7	12.2	47.3	36.5
Defined Benefit Plan Amortization Of Prior Service Cost Credit	(0.1)	(0.2)	(0.4)	(0.6)
Defined Benefit Plan Net Periodic Benefit Cost	21.5	15.9	64.6	47.6
Related Intercompany Allocationsof Defined Benefit Expense	(3.0)	(2.3)	(9.2)	(6.8)
Amount Capitalized Pension Expense	8.4	4.9	23.6	14.7
Pension Expense Incurred Prior to Merger			21.6
NSTAR Electric Company [Member] | Other Postretirement Benefit Plans Defined Benefit Member
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Net Periodic Benefit Cost	$ 8.5	$ 6.4	$ 25.6	$ 19.2
Pension Expense Incurred Prior to Merger			9