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, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	$ 10.7	$ 17.5	$ 32.4	$ 52.7
Interest Cost	63.4	38.6	190.6	115.8
Expected Return on Plan Assets	(116.3)	(130.7)	(348.7)	(393.1)
Actuarial Loss	10.9	28.4	35.0	87.5
Prior Service Cost/(Credit)	0.3	0.4	0.9	1.1
Settlement Loss	8.7		12.4
Total Net Periodic Benefit Plan Income	(22.3)	(45.8)	(77.4)	(136.0)
PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	1.9	2.9	5.7	8.7
Interest Cost	8.5	5.0	25.4	15.1
Expected Return on Plan Assets	(19.3)	(22.5)	(57.9)	(67.5)
Actuarial Loss	0.0	0.0	0.0	0.0
Prior Service Cost/(Credit)	(5.4)	(5.4)	(16.2)	(16.2)
Settlement Loss	0.0		0.0
Total Net Periodic Benefit Plan Income	(14.3)	(20.0)	(43.0)	(59.9)
CL&P | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	3.1	4.7	9.2	13.9
Interest Cost	12.6	7.8	37.9	23.4
Expected Return on Plan Assets	(23.5)	(26.5)	(70.6)	(79.6)
Actuarial Loss	0.5	4.0	1.9	12.2
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)	(21.6)	(30.1)
Intercompany Expense/(Income) Allocations	0.8	(4.0)	(1.8)	(11.9)
CL&P | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.4	0.5	1.0	1.5
Interest Cost	1.5	0.9	4.6	2.7
Expected Return on Plan Assets	(2.3)	(2.8)	(7.0)	(8.4)
Actuarial Loss	0.0	0.0	0.0	0.0
Prior Service Cost/(Credit)	0.2	0.3	0.8	0.8
Settlement Loss	0.0		0.0
Total Net Periodic Benefit Plan Income	(0.2)	(1.1)	(0.6)	(3.4)
Intercompany Expense/(Income) Allocations	(0.5)	(0.9)	(1.4)	(2.7)
NSTAR Electric | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	1.9	3.4	5.9	10.4
Interest Cost	13.5	8.2	40.5	24.5
Expected Return on Plan Assets	(28.4)	(32.1)	(85.3)	(96.2)
Actuarial Loss	4.0	8.1	12.9	24.7
Prior Service Cost/(Credit)	0.1	0.1	0.3	0.3
Settlement Loss	0.0		0.0
Total Net Periodic Benefit Plan Income	(8.9)	(12.3)	(25.7)	(36.3)
Intercompany Expense/(Income) Allocations	0.9	(3.2)	(1.2)	(9.3)
NSTAR Electric | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.3	0.5	0.9	1.5
Interest Cost	2.3	1.3	6.9	3.9
Expected Return on Plan Assets	(9.2)	(10.6)	(27.7)	(31.8)
Actuarial Loss	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	(10.8)	(13.0)	(32.6)	(39.1)
Intercompany Expense/(Income) Allocations	(0.5)	(0.9)	(1.6)	(2.7)
PSNH | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	1.0	1.7	3.2	5.2
Interest Cost	6.8	4.2	20.4	12.6
Expected Return on Plan Assets	(12.3)	(14.0)	(37.1)	(42.1)
Actuarial Loss	0.4	1.9	1.2	6.0
Prior Service Cost/(Credit)	0.0	0.0	0.0	0.0
Settlement Loss	0.0		0.0
Total Net Periodic Benefit Plan Income	(4.1)	(6.2)	(12.3)	(18.3)
Intercompany Expense/(Income) Allocations	0.3	(1.0)	(0.3)	(2.7)
PSNH | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.2	0.3	0.6	0.8
Interest Cost	0.9	0.5	2.7	1.6
Expected Return on Plan Assets	(1.4)	(1.7)	(4.2)	(5.0)
Actuarial Loss	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	(0.2)	(0.8)	(0.6)	(2.3)
Intercompany Expense/(Income) Allocations	$ (0.2)	$ (0.3)	$ (0.5)	$ (0.9)