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, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	$ 10.7	$ 17.5	$ 21.7	$ 35.2
Interest Cost	63.5	38.6	127.2	77.2
Expected Return on Plan Assets	(116.5)	(130.7)	(232.4)	(262.4)
Actuarial Loss	11.6	28.4	24.1	59.1
Prior Service Cost/(Credit)	0.3	0.3	0.6	0.7
Settlement Loss	3.7		3.7
Total Net Periodic Benefit Plan Income	(26.7)	(45.9)	(55.1)	(90.2)
PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	2.0	3.0	3.8	5.8
Interest Cost	8.4	5.1	16.9	10.1
Expected Return on Plan Assets	(19.5)	(22.6)	(38.6)	(45.0)
Actuarial Loss	0.0	0.0	0.0	0.0
Prior Service Cost/(Credit)	(5.4)	(5.4)	(10.8)	(10.8)
Settlement Loss	0.0		0.0
Total Net Periodic Benefit Plan Income	(14.5)	(19.9)	(28.7)	(39.9)
CL&P | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	3.1	4.7	6.1	9.2
Interest Cost	12.7	7.8	25.3	15.6
Expected Return on Plan Assets	(23.7)	(26.5)	(47.1)	(53.1)
Actuarial Loss	0.6	4.0	1.4	8.2
Prior Service Cost/(Credit)	0.0	0.0	0.0	0.0
Settlement Loss	0.0		0.0
Total Net Periodic Benefit Plan Income	(7.3)	(10.0)	(14.3)	(20.1)
Intercompany Income Allocations	(0.6)	(4.1)	(2.6)	(7.9)
CL&P | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.3	0.5	0.6	1.0
Interest Cost	1.5	0.9	3.1	1.8
Expected Return on Plan Assets	(2.3)	(2.8)	(4.7)	(5.6)
Actuarial Loss	0.0	0.0	0.0	0.0
Prior Service Cost/(Credit)	0.3	0.3	0.6	0.5
Settlement Loss	0.0		0.0
Total Net Periodic Benefit Plan Income	(0.2)	(1.1)	(0.4)	(2.3)
Intercompany Income Allocations	(0.4)	(0.9)	(0.9)	(1.8)
NSTAR Electric | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	1.9	3.4	4.0	7.0
Interest Cost	13.5	8.2	27.0	16.3
Expected Return on Plan Assets	(28.5)	(32.1)	(56.9)	(64.1)
Actuarial Loss	4.0	8.1	8.9	16.6
Prior Service Cost/(Credit)	0.1	0.1	0.2	0.2
Settlement Loss	0.0		0.0
Total Net Periodic Benefit Plan Income	(9.0)	(12.3)	(16.8)	(24.0)
Intercompany Income Allocations	(0.4)	(3.2)	(2.1)	(6.1)
NSTAR Electric | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.3	0.5	0.6	1.0
Interest Cost	2.3	1.3	4.6	2.6
Expected Return on Plan Assets	(9.3)	(10.6)	(18.5)	(21.2)
Actuarial Loss	0.0	0.0	0.0	0.0
Prior Service Cost/(Credit)	(4.3)	(4.2)	(8.5)	(8.5)
Settlement Loss	0.0		0.0
Total Net Periodic Benefit Plan Income	(11.0)	(13.0)	(21.8)	(26.1)
Intercompany Income Allocations	(0.5)	(0.9)	(1.1)	(1.8)
PSNH | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	1.1	1.7	2.2	3.5
Interest Cost	6.8	4.2	13.6	8.4
Expected Return on Plan Assets	(12.5)	(14.0)	(24.8)	(28.1)
Actuarial Loss	0.4	1.9	0.8	4.1
Prior Service Cost/(Credit)	0.0	0.0	0.0	0.0
Settlement Loss	0.0		0.0
Total Net Periodic Benefit Plan Income	(4.2)	(6.2)	(8.2)	(12.1)
Intercompany Income Allocations	(0.1)	(0.9)	(0.6)	(1.7)
PSNH | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.2	0.3	0.4	0.5
Interest Cost	0.9	0.6	1.8	1.1
Expected Return on Plan Assets	(1.4)	(1.7)	(2.8)	(3.3)
Actuarial Loss	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
Total Net Periodic Benefit Plan Income	(0.2)	(0.7)	(0.4)	(1.5)
Intercompany Income Allocations	$ (0.1)	$ (0.3)	$ (0.3)	$ (0.6)