PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSIONS (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year, Description	Based on the current status of the Pension Plan, NU is required to make a contribution to the Pension Plan of approximately $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 $92 million ($87.7 million of which was contributed by PSNH) in the first quarter of 2012.
Pension Plans Defined Benefit Member
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	$ 15.3	$ 13.7
Defined Benefit Plan Interest Cost	38.1	38.2
Defined Benefit Plan, Expected Return on Plan Assets	(42.5)	(43.1)
Defined Benefit Plan, Amortization of Gains (Losses)	30.1	21.0
Defined Benefit Plan Amortization Of Prior Service Cost Credit	2.1	2.4
Defined Benefit Plan Net Periodic Benefit Cost	43.1	32.2
Amount Capitalized Pension Expense	10.6	7.6
Other Postretirement Benefit Plans Defined Benefit Member
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	2.3	2.4
Defined Benefit Plan Interest Cost	6.2	6.4
Defined Benefit Plan, Expected Return on Plan Assets	(5.6)	(5.4)
Defined Benefit Plan, Amortization of Gains (Losses)	5.5	4.5
Defined Benefit Plan Amortization Of Transition Obligations Assets	2.9	2.9
Defined Benefit Plan Amortization Of Prior Service Cost Credit	(0.1)	(0.1)
Defined Benefit Plan Net Periodic Benefit Cost	11.2	10.7
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	4.8
Defined Benefit Plan Interest Cost	12.8	13.1
Defined Benefit Plan, Expected Return on Plan Assets	(17.5)	(19.2)
Defined Benefit Plan, Amortization of Gains (Losses)	11.9	8.4
Defined Benefit Plan Amortization Of Prior Service Cost Credit	0.9	1.0
Defined Benefit Plan Net Periodic Benefit Cost	13.5	8.1
Related Intercompany Allocationsof Defined Benefit Expense	10.6	8.3
Amount Capitalized Pension Expense	6.6	4.5
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
Defined Benefit Plan Interest Cost	2.4	2.5
Defined Benefit Plan, Expected Return on Plan Assets	(2.3)	(2.1)
Defined Benefit Plan, Amortization of Gains (Losses)	2.0	1.7
Defined Benefit Plan Amortization Of Transition Obligations Assets	1.5	1.5
Defined Benefit Plan Net Periodic Benefit Cost	4.4	4.3
Related Intercompany Allocationsof Defined Benefit Expense	2.1	2.1
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
Defined Benefit Plan Interest Cost	6.1	6.2
Defined Benefit Plan, Expected Return on Plan Assets	(6.7)	(5.3)
Defined Benefit Plan, Amortization of Gains (Losses)	3.9	2.6
Defined Benefit Plan Amortization Of Prior Service Cost Credit	0.4	0.5
Defined Benefit Plan Net Periodic Benefit Cost	6.6	6.6
Related Intercompany Allocationsof Defined Benefit Expense	2.5	2.0
Amount Capitalized Pension Expense	2.0	1.8
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
Defined Benefit Plan Interest Cost	1.2	1.2
Defined Benefit Plan, Expected Return on Plan Assets	(1.1)	(1.1)
Defined Benefit Plan, Amortization of Gains (Losses)	1.0	0.7
Defined Benefit Plan Amortization Of Transition Obligations Assets	0.6	0.6
Defined Benefit Plan Net Periodic Benefit Cost	2.2	1.9
Related Intercompany Allocationsof Defined Benefit Expense	0.5	0.4
Western Massachusetts Electric Company [Member] | Pension Plans Defined Benefit Member
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	1.1	1.0
Defined Benefit Plan Interest Cost	2.6	2.7
Defined Benefit Plan, Expected Return on Plan Assets	(4.1)	(4.4)
Defined Benefit Plan, Amortization of Gains (Losses)	2.5	1.7
Defined Benefit Plan Amortization Of Prior Service Cost Credit	0.2	0.2
Defined Benefit Plan Net Periodic Benefit Cost	2.3	1.2
Related Intercompany Allocationsof Defined Benefit Expense	2.0	1.6
Amount Capitalized Pension Expense	1.2	0.7
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	0.1
Defined Benefit Plan Interest Cost	0.5	0.6
Defined Benefit Plan, Expected Return on Plan Assets	(0.5)	(0.5)
Defined Benefit Plan, Amortization of Gains (Losses)	0.3	0.3
Defined Benefit Plan Amortization Of Transition Obligations Assets	0.3	0.3
Defined Benefit Plan Net Periodic Benefit Cost	0.8	0.8
Related Intercompany Allocationsof Defined Benefit Expense	$ 0.4	$ 0.5