PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSION - Components of Net Periodic Benefit Expense/(Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	$ 18.4	$ 16.4
Interest Cost	59.1	63.2
Expected Return on Plan Assets	(117.3)	(113.3)
Actuarial Loss/(Gain)	8.7	10.0
Prior Service Cost/(Credit)	0.2	0.3
Settlement Loss		3.3
Total Net Periodic Benefit Plan Income	(30.9)	(20.1)
PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	1.6	1.6
Interest Cost	7.2	8.0
Expected Return on Plan Assets	(22.7)	(21.2)
Actuarial Loss/(Gain)	(0.4)	(0.3)
Prior Service Cost/(Credit)	(5.4)	(5.4)
Settlement Loss		0.0
Total Net Periodic Benefit Plan Income	(19.7)	(17.3)
CL&P | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	3.9	3.8
Interest Cost	12.1	12.8
Expected Return on Plan Assets	(23.9)	(22.9)
Actuarial Loss/(Gain)	1.1	1.2
Prior Service Cost/(Credit)	0.0	0.0
Settlement Loss		0.0
Total Net Periodic Benefit Plan Income	(6.8)	(5.1)
Intercompany Income Allocations	(1.8)	0.1
CL&P | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.3	0.2
Interest Cost	1.3	1.4
Expected Return on Plan Assets	(2.6)	(2.4)
Actuarial Loss/(Gain)	0.0	0.0
Prior Service Cost/(Credit)	0.3	0.3
Settlement Loss		0.0
Total Net Periodic Benefit Plan Income	(0.7)	(0.5)
Intercompany Income Allocations	(0.6)	(0.5)
NSTAR Electric | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	3.2	2.5
Interest Cost	12.2	13.0
Expected Return on Plan Assets	(28.9)	(27.7)
Actuarial Loss/(Gain)	2.8	3.2
Prior Service Cost/(Credit)	0.1	0.1
Settlement Loss		0.0
Total Net Periodic Benefit Plan Income	(10.6)	(8.9)
Intercompany Income Allocations	(1.5)	0.3
NSTAR Electric | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.3	0.2
Interest Cost	1.9	2.2
Expected Return on Plan Assets	(11.6)	(10.5)
Actuarial Loss/(Gain)	0.0	0.0
Prior Service Cost/(Credit)	(4.2)	(4.2)
Settlement Loss		0.0
Total Net Periodic Benefit Plan Income	(13.6)	(12.3)
Intercompany Income Allocations	(0.8)	(0.7)
PSNH | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	1.6	1.6
Interest Cost	6.2	6.8
Expected Return on Plan Assets	(12.4)	(11.9)
Actuarial Loss/(Gain)	0.4	0.5
Prior Service Cost/(Credit)	0.0	0.0
Settlement Loss		0.0
Total Net Periodic Benefit Plan Income	(4.2)	(3.0)
Intercompany Income Allocations	(0.5)	0.1
PSNH | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.1	0.1
Interest Cost	0.8	0.8
Expected Return on Plan Assets	(1.5)	(1.4)
Actuarial Loss/(Gain)	0.0	0.0
Prior Service Cost/(Credit)	0.1	0.1
Settlement Loss		0.0
Total Net Periodic Benefit Plan Income	(0.5)	(0.4)
Intercompany Income Allocations	$ (0.3)	$ (0.2)