PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSIONS (Details) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Defined Benefit Plan Disclosure Line Items
Expected Pension Contribution			Due to an underfunded balance as of January 1, 2010, NU is required to make an additional contribution to the Pension Plan of approximately $145 million in 2011. A contribution of approximately $124 million was made in the nine months ended September 30, 2011 ($93 million of which was contributed by PSNH). Approximately $105 million of this amount was paid in the three months ended September 30, 2011 ($78 million of which was contributed by PSNH). The required contribution is being made in installments, which began in April 2011, to meet the current minimum funding requirements established by the PPA of 2006. Based on estimates as of December 31, 2010, additional contributions totalling $390 million are expected to be made from 2012 through 2015, subject to a variety of factors, including the performance of existing plan assets, valuation of the plan's liabilities and changes in long-term discount rates.
The Connecticut Light and Power Company [Member] | Pension Plans Defined Benefit Member
Defined Benefit Plan Disclosure Line Items
Defined Benefit Plan Service Cost	$ 4.9	$ 4.4	$ 14.6	$ 13.2
Defined Benefit Plan Interest Cost	13.0	13.2	39.1	39.2
Defined Benefit Plan, Expected Return on Plan Assets	(19.1)	(21.5)	(57.4)	(64.4)
Defined Benefit Plan Amortization Of Prior Service Cost Credit	1.1	1.0	3.1	3.0
Defined Benefit Plan, Amortization of Gains (Losses)	8.2	5.1	24.8	15.7
Defined Benefit Plan Net Periodic Benefit Cost	8.1	2.2	24.2	6.7
Related Intercompany Allocations	8.5	6.3	25.0	18.9
Amount Capitalized	4.5		13.4	5.2
The Connecticut Light and Power Company [Member] | Other Postretirement Benefit Plans Defined Benefit Member
Defined Benefit Plan Disclosure Line Items
Defined Benefit Plan Service Cost	0.7	0.7	2.2	2.0
Defined Benefit Plan Interest Cost	2.5	2.6	7.5	7.8
Defined Benefit Plan, Expected Return on Plan Assets	(2.2)	(2.2)	(6.5)	(6.5)
Defined Benefit Plan Amortization Of Transition Obligations Assets	1.5	1.5	4.6	4.6
Defined Benefit Plan, Amortization of Gains (Losses)	1.9	1.6	5.4	4.8
Defined Benefit Plan Net Periodic Benefit Cost	4.4	4.2	13.2	12.7
Related Intercompany Allocations	2.0	2.0	6.2	6.0
Public Service Company of New Hampshire [Member] | Pension Plans Defined Benefit Member
Defined Benefit Plan Disclosure Line Items
Defined Benefit Plan Service Cost	2.6	2.5	7.9	7.4
Defined Benefit Plan Interest Cost	6.1	6.2	18.4	18.3
Defined Benefit Plan, Expected Return on Plan Assets	(5.0)	(3.7)	(15.0)	(10.8)
Defined Benefit Plan Amortization Of Prior Service Cost Credit	0.5	0.4	1.5	1.1
Defined Benefit Plan, Amortization of Gains (Losses)	2.6	1.7	7.8	5.2
Defined Benefit Plan Net Periodic Benefit Cost	6.8	7.1	20.6	21.2
Related Intercompany Allocations	1.9	1.4	5.7	4.4
Amount Capitalized	2.1	2.0	5.9	6.1
Public Service Company of New Hampshire [Member] | Other Postretirement Benefit Plans Defined Benefit Member
Defined Benefit Plan Disclosure Line Items
Defined Benefit Plan Service Cost	0.5	0.4	1.4	1.4
Defined Benefit Plan Interest Cost	1.2	1.2	3.6	3.7
Defined Benefit Plan, Expected Return on Plan Assets	(1.1)	(1.0)	(3.3)	(3.2)
Defined Benefit Plan Amortization Of Transition Obligations Assets	0.6	0.6	1.9	1.8
Defined Benefit Plan, Amortization of Gains (Losses)	0.8	0.7	2.4	2.1
Defined Benefit Plan Net Periodic Benefit Cost	2.0	1.9	6.0	5.8
Related Intercompany Allocations	0.5	0.5	1.5	1.5
Western Massachusetts Electric Company [Member] | Pension Plans Defined Benefit Member
Defined Benefit Plan Disclosure Line Items
Defined Benefit Plan Service Cost	1.0	0.9	3.0	2.5
Defined Benefit Plan Interest Cost	2.7	2.7	8.1	8.0
Defined Benefit Plan, Expected Return on Plan Assets	(4.4)	(4.8)	(13.2)	(14.6)
Defined Benefit Plan Amortization Of Prior Service Cost Credit	0.2	0.2	0.6	0.7
Defined Benefit Plan, Amortization of Gains (Losses)	1.7	1.0	5.1	3.3
Defined Benefit Plan Net Periodic Benefit Cost	1.2	0	3.6	(0.1)
Related Intercompany Allocations	1.6	1.1	4.6	3.4
Amount Capitalized	0.7	0.2	2.1	0.5
Western Massachusetts Electric Company [Member] | Other Postretirement Benefit Plans Defined Benefit Member
Defined Benefit Plan Disclosure Line Items
Defined Benefit Plan Service Cost	0.2	0.2	0.4	0.4
Defined Benefit Plan Interest Cost	0.5	0.6	1.6	1.8
Defined Benefit Plan, Expected Return on Plan Assets	(0.5)	(0.5)	(1.5)	(1.6)
Defined Benefit Plan Amortization Of Transition Obligations Assets	0.3	0.3	1.0	1.0
Defined Benefit Plan, Amortization of Gains (Losses)	0.3	0.2	0.9	0.7
Defined Benefit Plan Net Periodic Benefit Cost	0.8	0.8	2.4	2.3
Related Intercompany Allocations	0.4	0.3	1.1	1.0
Pension Plans Defined Benefit Member
Defined Benefit Plan Disclosure Line Items
Defined Benefit Plan Service Cost	13.8	12.8	27.5	38.2
Defined Benefit Plan Interest Cost	38.3	38.2	76.5	114.5
Defined Benefit Plan, Expected Return on Plan Assets	(42.7)	(45.7)	(85.8)	(137.0)
Defined Benefit Plan Amortization Of Prior Service Cost Credit	2.4	2.4	4.8	7.3
Defined Benefit Plan, Amortization of Gains (Losses)	21.1	13.4	42.1	40.3
Defined Benefit Plan Net Periodic Benefit Cost	32.9	21.1	65.1	63.3
Amount Capitalized	7.8	4.4	23.5	13.2
Other Postretirement Benefit Plans Defined Benefit Member
Defined Benefit Plan Disclosure Line Items
Defined Benefit Plan Service Cost	2.3	2.1	6.8	6.3
Defined Benefit Plan Interest Cost	6.4	6.7	19.3	20.1
Defined Benefit Plan, Expected Return on Plan Assets	(5.4)	(5.4)	(16.2)	(16.2)
Defined Benefit Plan Amortization Of Transition Obligations Assets	2.9	4.2	8.7	12.5
Defined Benefit Plan Amortization Of Prior Service Cost Credit	(0.1)	(0.1)	(0.2)	(0.2)
Defined Benefit Plan, Amortization of Gains (Losses)	4.8	2.9	14.3	8.7
Defined Benefit Plan Net Periodic Benefit Cost	$ 10.9	$ 10.4	$ 32.7	$ 31.2