PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSION - Components of Net Periodic Benefit Expense/(Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	$ 17.7	$ 21.3
Interest Cost	38.6	32.6
Expected Return on Plan Assets	(131.7)	(108.9)
Actuarial Loss	30.7	61.6
Prior Service Cost/(Credit)	0.4	0.3
Total Net Periodic Benefit Plan Expense/(Income)	(44.3)	6.9
PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	2.8	3.5
Interest Cost	5.0	4.4
Expected Return on Plan Assets	(22.4)	(19.7)
Actuarial Loss	0.0	2.6
Prior Service Cost/(Credit)	(5.4)	(5.3)
Total Net Periodic Benefit Plan Expense/(Income)	(20.0)	(14.5)
CL&P | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	4.5	6.3
Interest Cost	7.8	7.3
Expected Return on Plan Assets	(26.6)	(21.6)
Actuarial Loss	4.2	13.0
Prior Service Cost/(Credit)	0.0	0.0
Total Net Periodic Benefit Plan Expense/(Income)	(10.1)	5.0
Intercompany Expense/(Income) Allocations	(3.8)	1.3
CL&P | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.5	0.6
Interest Cost	0.9	0.8
Expected Return on Plan Assets	(2.8)	(2.6)
Actuarial Loss	0.0	0.4
Prior Service Cost/(Credit)	0.2	0.2
Total Net Periodic Benefit Plan Expense/(Income)	(1.2)	(0.6)
Intercompany Expense/(Income) Allocations	(0.9)	(0.3)
NSTAR Electric | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	3.6	4.0
Interest Cost	8.1	6.7
Expected Return on Plan Assets	(32.0)	(26.9)
Actuarial Loss	8.5	15.5
Prior Service Cost/(Credit)	0.1	0.1
Total Net Periodic Benefit Plan Expense/(Income)	(11.7)	(0.6)
Intercompany Expense/(Income) Allocations	(2.9)	1.5
NSTAR Electric | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.5	0.6
Interest Cost	1.3	1.1
Expected Return on Plan Assets	(10.6)	(9.3)
Actuarial Loss	0.0	0.6
Prior Service Cost/(Credit)	(4.3)	(4.2)
Total Net Periodic Benefit Plan Expense/(Income)	(13.1)	(11.2)
Intercompany Expense/(Income) Allocations	(0.9)	(0.4)
PSNH | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	1.8	2.2
Interest Cost	4.2	3.6
Expected Return on Plan Assets	(14.1)	(11.9)
Actuarial Loss	2.2	4.9
Prior Service Cost/(Credit)	0.0	0.0
Total Net Periodic Benefit Plan Expense/(Income)	(5.9)	(1.2)
Intercompany Expense/(Income) Allocations	(0.8)	0.5
PSNH | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.2	0.3
Interest Cost	0.5	0.5
Expected Return on Plan Assets	(1.6)	(1.5)
Actuarial Loss	0.0	0.3
Prior Service Cost/(Credit)	0.1	0.1
Total Net Periodic Benefit Plan Expense/(Income)	(0.8)	(0.3)
Intercompany Expense/(Income) Allocations	$ (0.3)	$ (0.1)