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, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	$ 17.2	$ 11.2	$ 50.8	$ 33.2
Interest Cost	64.1	62.7	191.4	187.4
Expected Return on Plan Assets	(113.7)	(115.7)	(340.7)	(347.0)
Actuarial Loss/(Gain)	11.0	22.0	32.0	63.9
Prior Service Cost/(Credit)	0.3	0.3	0.9	0.9
Settlement Loss			3.3	4.3
Total Net Periodic Benefit Plan Income	(21.1)	(19.5)	(62.3)	(57.3)
PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	1.6	1.7	4.8	5.2
Interest Cost	8.0	8.0	24.0	24.0
Expected Return on Plan Assets	(21.1)	(20.3)	(63.4)	(61.0)
Actuarial Loss/(Gain)	(0.2)	(0.1)	(0.7)	(0.3)
Prior Service Cost/(Credit)	(5.4)	(5.4)	(16.2)	(16.2)
Settlement Loss			0.0	0.0
Total Net Periodic Benefit Plan Income	(17.1)	(16.1)	(51.5)	(48.3)
CL&P | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	3.9	3.3	11.5	9.5
Interest Cost	12.9	12.6	38.6	37.7
Expected Return on Plan Assets	(23.0)	(23.4)	(68.9)	(70.2)
Actuarial Loss/(Gain)	1.3	3.2	3.8	8.9
Prior Service Cost/(Credit)	0.0	0.0	0.0	0.0
Settlement Loss			0.0	0.0
Total Net Periodic Benefit Plan Income	(4.9)	(4.3)	(15.0)	(14.1)
Intercompany Income Allocations	(0.5)	(0.6)	(0.9)	(0.9)
CL&P | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.3	0.3	0.8	0.9
Interest Cost	1.4	1.4	4.2	4.2
Expected Return on Plan Assets	(2.4)	(2.4)	(7.2)	(7.1)
Actuarial Loss/(Gain)	0.0	0.0	0.0	0.0
Prior Service Cost/(Credit)	0.3	0.3	0.9	0.8
Settlement Loss			0.0	0.0
Total Net Periodic Benefit Plan Income	(0.4)	(0.4)	(1.3)	(1.2)
Intercompany Income Allocations	(0.5)	(0.6)	(1.5)	(1.7)
NSTAR Electric | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	2.4	1.9	7.3	5.9
Interest Cost	13.3	12.8	39.7	38.7
Expected Return on Plan Assets	(27.7)	(28.0)	(83.1)	(84.3)
Actuarial Loss/(Gain)	3.7	6.4	10.6	19.2
Prior Service Cost/(Credit)	0.1	0.1	0.3	0.3
Settlement Loss			0.0	0.0
Total Net Periodic Benefit Plan Income	(8.2)	(6.8)	(25.2)	(20.2)
Intercompany Income Allocations	(0.3)	(0.6)	(0.3)	(0.8)
NSTAR Electric | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.3	0.3	0.8	0.9
Interest Cost	2.2	2.2	6.6	6.6
Expected Return on Plan Assets	(10.5)	(9.9)	(31.5)	(29.7)
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
Total Net Periodic Benefit Plan Income	(12.2)	(11.6)	(36.8)	(34.9)
Intercompany Income Allocations	(0.7)	(0.6)	(2.1)	(1.9)
PSNH | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	1.7	1.1	4.9	3.2
Interest Cost	6.8	6.7	20.4	20.1
Expected Return on Plan Assets	(12.0)	(12.2)	(35.9)	(36.6)
Actuarial Loss/(Gain)	0.5	1.3	1.5	3.7
Prior Service Cost/(Credit)	0.0	0.0	0.0	0.0
Settlement Loss			0.0	0.0
Total Net Periodic Benefit Plan Income	(3.0)	(3.1)	(9.1)	(9.6)
Intercompany Income Allocations	0.0	(0.2)	0.0	(0.3)
PSNH | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.1	0.1	0.3	0.4
Interest Cost	0.8	0.9	2.4	2.6
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
Total Net Periodic Benefit Plan Income	(0.4)	(0.3)	(1.2)	(0.9)
Intercompany Income Allocations	$ (0.2)	$ (0.2)	$ (0.6)	$ (0.6)