PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSION - Components of Net Periodic Benefit Expense/(Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	$ 11.0	$ 17.7
Interest Cost	63.7	38.6
Expected Return on Plan Assets	(115.9)	(131.7)
Actuarial Losses, net	12.5	30.7
Prior Service Cost/(Credit)	0.3	0.4
Total Net Periodic Benefit Plan Income	(28.4)	(44.3)
PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	1.8	2.8
Interest Cost	8.5	5.0
Expected Return on Plan Assets	(19.1)	(22.4)
Actuarial Losses, net	0.0	0.0
Prior Service Cost/(Credit)	(5.4)	(5.4)
Total Net Periodic Benefit Plan Income	(14.2)	(20.0)
CL&P | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	3.0	4.5
Interest Cost	12.6	7.8
Expected Return on Plan Assets	(23.4)	(26.6)
Actuarial Losses, net	0.8	4.2
Prior Service Cost/(Credit)	0.0	0.0
Total Net Periodic Benefit Plan Income	(7.0)	(10.1)
Intercompany Income Allocations	(2.0)	(3.8)
CL&P | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.3	0.5
Interest Cost	1.6	0.9
Expected Return on Plan Assets	(2.4)	(2.8)
Actuarial Losses, net	0.0	0.0
Prior Service Cost/(Credit)	0.3	0.2
Total Net Periodic Benefit Plan Income	(0.2)	(1.2)
Intercompany Income Allocations	(0.5)	(0.9)
NSTAR Electric | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	2.1	3.6
Interest Cost	13.5	8.1
Expected Return on Plan Assets	(28.4)	(32.0)
Actuarial Losses, net	4.9	8.5
Prior Service Cost/(Credit)	0.1	0.1
Total Net Periodic Benefit Plan Income	(7.8)	(11.7)
Intercompany Income Allocations	(1.7)	(2.9)
NSTAR Electric | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.3	0.5
Interest Cost	2.3	1.3
Expected Return on Plan Assets	(9.2)	(10.6)
Actuarial Losses, net	0.0	0.0
Prior Service Cost/(Credit)	(4.2)	(4.3)
Total Net Periodic Benefit Plan Income	(10.8)	(13.1)
Intercompany Income Allocations	(0.6)	(0.9)
PSNH | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	1.1	1.8
Interest Cost	6.8	4.2
Expected Return on Plan Assets	(12.3)	(14.1)
Actuarial Losses, net	0.4	2.2
Prior Service Cost/(Credit)	0.0	0.0
Total Net Periodic Benefit Plan Income	(4.0)	(5.9)
Intercompany Income Allocations	(0.5)	(0.8)
PSNH | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.2	0.2
Interest Cost	0.9	0.5
Expected Return on Plan Assets	(1.4)	(1.6)
Actuarial Losses, net	0.0	0.0
Prior Service Cost/(Credit)	0.1	0.1
Total Net Periodic Benefit Plan Income	(0.2)	(0.8)
Intercompany Income Allocations	$ (0.2)	$ (0.3)