PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSION - Components of Net Periodic Benefit Expense/(Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	$ 10.8	$ 11.0
Interest Cost	62.0	63.7
Expected Return on Plan Assets	(115.6)	(115.9)
Actuarial Loss/(Gain)	19.8	12.5
Prior Service Cost/(Credit)	0.3	0.3
Settlement Loss	4.3
Total Net Periodic Benefit Plan Income	(18.4)	(28.4)
PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	1.7	1.8
Interest Cost	8.0	8.5
Expected Return on Plan Assets	(20.4)	(19.1)
Actuarial Loss/(Gain)	(0.1)	0.0
Prior Service Cost/(Credit)	(5.4)	(5.4)
Settlement Loss	0.0
Total Net Periodic Benefit Plan Income	(16.2)	(14.2)
CL&P | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	2.9	3.0
Interest Cost	12.4	12.6
Expected Return on Plan Assets	(23.3)	(23.4)
Actuarial Loss/(Gain)	2.5	0.8
Prior Service Cost/(Credit)	0.0	0.0
Settlement Loss	0.0
Total Net Periodic Benefit Plan Income	(5.5)	(7.0)
Intercompany Expense/(Income) Allocations	0.3	(2.0)
CL&P | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.3	0.3
Interest Cost	1.4	1.6
Expected Return on Plan Assets	(2.4)	(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	(0.4)	(0.2)
Intercompany Expense/(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	2.1	2.1
Interest Cost	13.1	13.5
Expected Return on Plan Assets	(28.3)	(28.4)
Actuarial Loss/(Gain)	6.4	4.9
Prior Service Cost/(Credit)	0.1	0.1
Settlement Loss	0.0
Total Net Periodic Benefit Plan Income	(6.6)	(7.8)
Intercompany Expense/(Income) Allocations	0.4	(1.7)
NSTAR Electric | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.3	0.3
Interest Cost	2.2	2.3
Expected Return on Plan Assets	(9.9)	(9.2)
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	(11.6)	(10.8)
Intercompany Expense/(Income) Allocations	(0.7)	(0.6)
PSNH | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	1.0	1.1
Interest Cost	6.7	6.8
Expected Return on Plan Assets	(12.2)	(12.3)
Actuarial Loss/(Gain)	1.1	0.4
Prior Service Cost/(Credit)	0.0	0.0
Settlement Loss	0.0
Total Net Periodic Benefit Plan Income	(3.4)	(4.0)
Intercompany Expense/(Income) Allocations	0.1	(0.5)
PSNH | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.1	0.2
Interest Cost	0.9	0.9
Expected Return on Plan Assets	(1.4)	(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	(0.3)	(0.2)
Intercompany Expense/(Income) Allocations	$ (0.2)	$ (0.2)