PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSION - Components of Net Periodic Benefit Expense/(Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	$ 11.2	$ 10.7	$ 33.2	$ 32.4
Interest Cost	62.7	63.4	187.4	190.6
Expected Return on Plan Assets	(115.7)	(116.3)	(347.0)	(348.7)
Actuarial Loss/(Gain)	22.0	10.9	63.9	35.0
Prior Service Cost/(Credit)	0.3	0.3	0.9	0.9
Settlement Loss		8.7	4.3	12.4
Total Net Periodic Benefit Plan Income	(19.5)	(22.3)	(57.3)	(77.4)
PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	1.7	1.9	5.2	5.7
Interest Cost	8.0	8.5	24.0	25.4
Expected Return on Plan Assets	(20.3)	(19.3)	(61.0)	(57.9)
Actuarial Loss/(Gain)	(0.1)	0.0	(0.3)	0.0
Prior Service Cost/(Credit)	(5.4)	(5.4)	(16.2)	(16.2)
Settlement Loss		0.0	0.0	0.0
Total Net Periodic Benefit Plan Income	(16.1)	(14.3)	(48.3)	(43.0)
CL&P | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	3.3	3.1	9.5	9.2
Interest Cost	12.6	12.6	37.7	37.9
Expected Return on Plan Assets	(23.4)	(23.5)	(70.2)	(70.6)
Actuarial Loss/(Gain)	3.2	0.5	8.9	1.9
Prior Service Cost/(Credit)	0.0	0.0	0.0	0.0
Settlement Loss		0.0	0.0	0.0
Total Net Periodic Benefit Plan Income	(4.3)	(7.3)	(14.1)	(21.6)
Intercompany Income Allocations	(0.6)	0.8	(0.9)	(1.8)
CL&P | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.3	0.4	0.9	1.0
Interest Cost	1.4	1.5	4.2	4.6
Expected Return on Plan Assets	(2.4)	(2.3)	(7.1)	(7.0)
Actuarial Loss/(Gain)	0.0	0.0	0.0	0.0
Prior Service Cost/(Credit)	0.3	0.2	0.8	0.8
Settlement Loss		0.0	0.0	0.0
Total Net Periodic Benefit Plan Income	(0.4)	(0.2)	(1.2)	(0.6)
Intercompany Income Allocations	(0.6)	(0.5)	(1.7)	(1.4)
NSTAR Electric | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	1.9	1.9	5.9	5.9
Interest Cost	12.8	13.5	38.7	40.5
Expected Return on Plan Assets	(28.0)	(28.4)	(84.3)	(85.3)
Actuarial Loss/(Gain)	6.4	4.0	19.2	12.9
Prior Service Cost/(Credit)	0.1	0.1	0.3	0.3
Settlement Loss		0.0	0.0	0.0
Total Net Periodic Benefit Plan Income	(6.8)	(8.9)	(20.2)	(25.7)
Intercompany Income Allocations	(0.6)	0.9	(0.8)	(1.2)
NSTAR Electric | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.3	0.3	0.9	0.9
Interest Cost	2.2	2.3	6.6	6.9
Expected Return on Plan Assets	(9.9)	(9.2)	(29.7)	(27.7)
Actuarial Loss/(Gain)	0.0	0.0	0.0	0.0
Prior Service Cost/(Credit)	(4.2)	(4.2)	(12.7)	(12.7)
Settlement Loss		0.0	0.0	0.0
Total Net Periodic Benefit Plan Income	(11.6)	(10.8)	(34.9)	(32.6)
Intercompany Income Allocations	(0.6)	(0.5)	(1.9)	(1.6)
PSNH | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	1.1	1.0	3.2	3.2
Interest Cost	6.7	6.8	20.1	20.4
Expected Return on Plan Assets	(12.2)	(12.3)	(36.6)	(37.1)
Actuarial Loss/(Gain)	1.3	0.4	3.7	1.2
Prior Service Cost/(Credit)	0.0	0.0	0.0	0.0
Settlement Loss		0.0	0.0	0.0
Total Net Periodic Benefit Plan Income	(3.1)	(4.1)	(9.6)	(12.3)
Intercompany Income Allocations	(0.2)	0.3	(0.3)	(0.3)
PSNH | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.1	0.2	0.4	0.6
Interest Cost	0.9	0.9	2.6	2.7
Expected Return on Plan Assets	(1.4)	(1.4)	(4.2)	(4.2)
Actuarial Loss/(Gain)	0.0	0.0	0.0	0.0
Prior Service Cost/(Credit)	0.1	0.1	0.3	0.3
Settlement Loss		0.0	0.0	0.0
Total Net Periodic Benefit Plan Income	(0.3)	(0.2)	(0.9)	(0.6)
Intercompany Income Allocations	$ (0.2)	$ (0.2)	$ (0.6)	$ (0.5)