PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSIONS (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Pension Plans Defined Benefit Member
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	$ 24.6	$ 22.8	$ 51.1	$ 38.1
Defined Benefit Plan Interest Cost	51.9	53.2	103.5	91.3
Defined Benefit Plan, Expected Return on Plan Assets	(68.6)	(59.4)	(139.0)	(101.9)
Defined Benefit Plan, Amortization of Gains (Losses)	52.6	47.4	105.5	77.4
Defined Benefit Plan Amortization Of Prior Service Cost Credit	1.0	2.0	2.1	4.1
Defined Benefit Plan Net Periodic Benefit Cost	61.5	66.0	123.2	109.0
Amount Capitalized Defined Benefit Expense	19.9	19.7	36.6	30.3
DescriptionOfDefinedContributionPensionAndOtherPostretirementPlans			: For the six months ended June 30, 2013, NU contributed $75.7 million to the NUSCO Pension Plan, $44.2 million of which was contributed by PSNH, and NSTAR Electric contributed $22.9 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	3.7	4.5	8.5	6.8
Defined Benefit Plan Interest Cost	10.9	13.9	23.6	20.1
Defined Benefit Plan, Expected Return on Plan Assets	(13.9)	(11.3)	(27.7)	(17.0)
Defined Benefit Plan, Amortization of Gains (Losses)	4.7	9.7	13.0	15.2
Defined Benefit Plan Amortization Of Transition Obligations Assets		3.1		5.9
Defined Benefit Plan Amortization Of Prior Service Cost Credit	(0.5)	(0.4)	(1.1)	(0.4)
Defined Benefit Plan Net Periodic Benefit Cost	4.9	19.5	16.3	30.6
Amount Capitalized Defined Benefit Expense	1.5	6.5	5.0	9.8
DescriptionOfDefinedContributionPensionAndOtherPostretirementPlans			NU contributed $24.2 million to the PBOP Plans for the six months ended June 30, 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	6.3	5.4	12.4	10.9
Defined Benefit Plan Interest Cost	12.1	12.9	24.2	25.6
Defined Benefit Plan, Expected Return on Plan Assets	(18.4)	(17.7)	(36.9)	(35.2)
Defined Benefit Plan, Amortization of Gains (Losses)	13.9	12.6	28.0	24.5
Defined Benefit Plan Amortization Of Prior Service Cost Credit	0.5	0.9	0.9	1.8
Defined Benefit Plan Net Periodic Benefit Cost	14.4	14.1	28.6	27.6
Related Intercompany Allocationsof Defined Benefit Expense	11.3	10.7	22.1	21.3
Amount Capitalized Defined Benefit Expense	7.0	6.8	14.0	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	0.9	0.7	1.7	1.4
Defined Benefit Plan Interest Cost	2.0	2.3	3.9	4.6
Defined Benefit Plan, Expected Return on Plan Assets	(2.5)	(2.3)	(5.0)	(4.5)
Defined Benefit Plan, Amortization of Gains (Losses)	1.9	1.8	3.7	3.8
Defined Benefit Plan Amortization Of Transition Obligations Assets		1.5	0	3.0
Defined Benefit Plan Net Periodic Benefit Cost	2.3	4.0	4.3	8.3
Related Intercompany Allocationsof Defined Benefit Expense	1.9	1.9	3.6	4.1
Amount Capitalized Defined Benefit Expense	1.2	2.0	2.4	4.1
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	7.3	7.3	16.5	15.1
Defined Benefit Plan Interest Cost	14.7	14.7	29.0	29.5
Defined Benefit Plan, Expected Return on Plan Assets	(20.2)	(16.3)	(42.2)	(32.8)
Defined Benefit Plan, Amortization of Gains (Losses)	14.6	15.9	29.1	31.6
Defined Benefit Plan Amortization Of Prior Service Cost Credit	(0.1)	(0.1)	(0.1)	(0.3)
Defined Benefit Plan Net Periodic Benefit Cost	16.3	21.5	32.3	43.1
Related Intercompany Allocationsof Defined Benefit Expense	(2.2)	(3.0)	(4.1)	(6.2)
Amount Capitalized Defined Benefit Expense	6.5	8.9	11.8	15.2
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	(2.3)	8.0	2.6	17.0
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	3.2	2.9	6.5	5.8
Defined Benefit Plan Interest Cost	5.9	6.1	11.9	12.2
Defined Benefit Plan, Expected Return on Plan Assets	(9.2)	(7.2)	(16.8)	(13.9)
Defined Benefit Plan, Amortization of Gains (Losses)	5.4	4.1	10.8	8.0
Defined Benefit Plan Amortization Of Prior Service Cost Credit	0.1	0.4	0.3	0.8
Defined Benefit Plan Net Periodic Benefit Cost	5.4	6.3	12.7	12.9
Related Intercompany Allocationsof Defined Benefit Expense	2.6	2.4	5.2	5.0
Amount Capitalized Defined Benefit Expense	1.7	1.9	3.9	3.9
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	0.6	0.5	1.1	1.0
Defined Benefit Plan Interest Cost	1.0	1.1	2.0	2.3
Defined Benefit Plan, Expected Return on Plan Assets	(1.3)	(1.1)	(2.6)	(2.3)
Defined Benefit Plan, Amortization of Gains (Losses)	0.9	0.8	1.8	1.8
Defined Benefit Plan Amortization Of Transition Obligations Assets		0.6	0	1.2
Defined Benefit Plan Net Periodic Benefit Cost	1.2	1.9	2.3	4.0
Related Intercompany Allocationsof Defined Benefit Expense	0.4	0.5	0.8	1.0
Amount Capitalized Defined Benefit Expense	0.4	0.5	0.7	1.1
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	2.4	2.1
Defined Benefit Plan Interest Cost	2.5	2.6	5.0	5.3
Defined Benefit Plan, Expected Return on Plan Assets	(4.4)	(4.1)	(8.7)	(8.2)
Defined Benefit Plan, Amortization of Gains (Losses)	2.9	2.7	5.9	5.2
Defined Benefit Plan Amortization Of Prior Service Cost Credit	0.1	0.2	0.2	0.4
Defined Benefit Plan Net Periodic Benefit Cost	2.3	2.5	4.8	4.8
Related Intercompany Allocationsof Defined Benefit Expense	2.0	2.0	4.0	4.0
Amount Capitalized Defined Benefit Expense	1.3	1.2	2.6	2.4
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	0.4	0.3
Defined Benefit Plan Interest Cost	0.4	0.5	0.8	1.0
Defined Benefit Plan, Expected Return on Plan Assets	(0.6)	(0.5)	(1.2)	(1.1)
Defined Benefit Plan, Amortization of Gains (Losses)	0.3	0.3	0.6	0.6
Defined Benefit Plan Amortization Of Transition Obligations Assets		0.3	0	0.7
Defined Benefit Plan Net Periodic Benefit Cost	0.3	0.7	0.6	1.5
Related Intercompany Allocationsof Defined Benefit Expense	0.3	0.4	0.6	0.8
Amount Capitalized Defined Benefit Expense	$ 0.2	$ 0.3	$ 0.4	$ 0.7