PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSION - Components of Net Periodic Benefit Expense/(Income) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	$ 11.2	$ 10.7	$ 22.0	$ 21.7
Interest Cost	62.7	63.5	124.7	127.2
Expected Return on Plan Assets	(115.7)	(116.5)	(231.3)	(232.4)
Actuarial Loss/(Gain)	22.1	11.6	41.9	24.1
Prior Service Cost/(Credit)	0.3	0.3	0.6	0.6
Settlement Loss		3.7	4.3	3.7
Total Net Periodic Benefit Plan Income	(19.4)	(26.7)	(37.8)	(55.1)
PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	1.8	2.0	3.5	3.8
Interest Cost	8.0	8.4	16.0	16.9
Expected Return on Plan Assets	(20.3)	(19.5)	(40.7)	(38.6)
Actuarial Loss/(Gain)	(0.1)	0.0	(0.2)	0.0
Prior Service Cost/(Credit)	(5.4)	(5.4)	(10.8)	(10.8)
Settlement Loss		0.0	0.0	0.0
Total Net Periodic Benefit Plan Income	(16.0)	(14.5)	(32.2)	(28.7)
CL&P | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	3.3	3.1	6.2	6.1
Interest Cost	12.7	12.7	25.1	25.3
Expected Return on Plan Assets	(23.5)	(23.7)	(46.8)	(47.1)
Actuarial Loss/(Gain)	3.2	0.6	5.7	1.4
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)	(9.8)	(14.3)
Intercompany Income Allocations	(0.6)	(0.6)	(0.3)	(2.6)
CL&P | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.3	0.3	0.6	0.6
Interest Cost	1.4	1.5	2.8	3.1
Expected Return on Plan Assets	(2.3)	(2.3)	(4.7)	(4.7)
Actuarial Loss/(Gain)	0.0	0.0	0.0	0.0
Prior Service Cost/(Credit)	0.2	0.3	0.5	0.6
Settlement Loss		0.0	0.0	0.0
Total Net Periodic Benefit Plan Income	(0.4)	(0.2)	(0.8)	(0.4)
Intercompany Income Allocations	(0.5)	(0.4)	(1.1)	(0.9)
NSTAR Electric | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	1.9	1.9	4.0	4.0
Interest Cost	12.8	13.5	25.9	27.0
Expected Return on Plan Assets	(28.0)	(28.5)	(56.3)	(56.9)
Actuarial Loss/(Gain)	6.4	4.0	12.8	8.9
Prior Service Cost/(Credit)	0.1	0.1	0.2	0.2
Settlement Loss		0.0	0.0	0.0
Total Net Periodic Benefit Plan Income	(6.8)	(9.0)	(13.4)	(16.8)
Intercompany Income Allocations	(0.6)	(0.4)	(0.2)	(2.1)
NSTAR Electric | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.3	0.3	0.6	0.6
Interest Cost	2.2	2.3	4.4	4.6
Expected Return on Plan Assets	(9.9)	(9.3)	(19.8)	(18.5)
Actuarial Loss/(Gain)	0.0	0.0	0.0	0.0
Prior Service Cost/(Credit)	(4.3)	(4.3)	(8.5)	(8.5)
Settlement Loss		0.0	0.0	0.0
Total Net Periodic Benefit Plan Income	(11.7)	(11.0)	(23.3)	(21.8)
Intercompany Income Allocations	(0.6)	(0.5)	(1.3)	(1.1)
PSNH | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	1.1	1.1	2.1	2.2
Interest Cost	6.7	6.8	13.4	13.6
Expected Return on Plan Assets	(12.2)	(12.5)	(24.4)	(24.8)
Actuarial Loss/(Gain)	1.3	0.4	2.4	0.8
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)	(4.2)	(6.5)	(8.2)
Intercompany Income Allocations	(0.2)	(0.1)	(0.1)	(0.6)
PSNH | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.2	0.2	0.3	0.4
Interest Cost	0.8	0.9	1.7	1.8
Expected Return on Plan Assets	(1.4)	(1.4)	(2.8)	(2.8)
Actuarial Loss/(Gain)	0.0	0.0	0.0	0.0
Prior Service Cost/(Credit)	0.1	0.1	0.2	0.2
Settlement Loss		0.0	0.0	0.0
Total Net Periodic Benefit Plan Income	(0.3)	(0.2)	(0.6)	(0.4)
Intercompany Income Allocations	$ (0.2)	$ (0.1)	$ (0.4)	$ (0.3)