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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	$ 21.5	$ 18.7	$ 64.3	$ 57.0
Interest Cost	32.5	44.4	97.5	133.0
Expected Return on Plan Assets	(109.5)	(99.2)	(328.0)	(298.8)
Actuarial Loss	60.7	50.7	183.1	150.7
Prior Service Cost/(Credit)	0.3	0.2	1.1	0.8
Total Net Periodic Benefit Plan Expense/(Income)	5.5	14.8	18.0	42.7
PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	3.3	2.4	10.1	7.3
Interest Cost	4.3	6.1	12.9	18.3
Expected Return on Plan Assets	(19.8)	(18.3)	(59.3)	(55.1)
Actuarial Loss	2.0	2.2	5.9	6.3
Prior Service Cost/(Credit)	(5.3)	(5.3)	(15.9)	(15.9)
Total Net Periodic Benefit Plan Expense/(Income)	(15.5)	(12.9)	(46.3)	(39.1)
CL&P | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	5.6	5.4	17.4	16.5
Interest Cost	6.7	9.3	20.7	28.1
Expected Return on Plan Assets	(21.7)	(19.8)	(65.0)	(59.6)
Actuarial Loss	10.8	9.7	34.6	29.4
Prior Service Cost/(Credit)	0.0	0.0	0.0	0.0
Total Net Periodic Benefit Plan Expense/(Income)	1.4	4.6	7.7	14.4
Intercompany Expense/(Income) Allocations	2.2	2.4	5.8	6.7
CL&P | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.5	0.4	1.7	1.3
Interest Cost	0.8	1.1	2.4	3.3
Expected Return on Plan Assets	(2.5)	(2.5)	(7.7)	(7.4)
Actuarial Loss	0.4	0.3	1.2	0.9
Prior Service Cost/(Credit)	0.3	0.3	0.8	0.8
Total Net Periodic Benefit Plan Expense/(Income)	(0.5)	(0.4)	(1.6)	(1.1)
Intercompany Expense/(Income) Allocations	(0.4)	(0.3)	(1.3)	(0.9)
NSTAR Electric | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	4.0	3.8	11.9	11.4
Interest Cost	6.7	9.7	20.1	29.0
Expected Return on Plan Assets	(27.1)	(25.7)	(81.1)	(77.2)
Actuarial Loss	15.3	14.0	46.1	41.3
Prior Service Cost/(Credit)	0.1	0.0	0.3	0.2
Total Net Periodic Benefit Plan Expense/(Income)	(1.0)	1.8	(2.7)	4.7
Intercompany Expense/(Income) Allocations	2.4	2.3	6.4	6.5
NSTAR Electric | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.6	0.5	1.8	1.6
Interest Cost	1.1	1.7	3.3	5.0
Expected Return on Plan Assets	(9.2)	(8.5)	(27.7)	(25.5)
Actuarial Loss	0.5	0.6	1.6	1.8
Prior Service Cost/(Credit)	(4.3)	(4.2)	(12.7)	(12.7)
Total Net Periodic Benefit Plan Expense/(Income)	(11.3)	(9.9)	(33.7)	(29.8)
Intercompany Expense/(Income) Allocations	(0.5)	(0.4)	(1.5)	(1.0)
PSNH | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	2.2	2.0	6.6	6.2
Interest Cost	3.7	4.8	10.9	14.5
Expected Return on Plan Assets	(11.9)	(11.1)	(35.6)	(33.5)
Actuarial Loss	5.3	3.8	15.4	11.8
Prior Service Cost/(Credit)	0.0	0.0	0.0	0.0
Total Net Periodic Benefit Plan Expense/(Income)	(0.7)	(0.5)	(2.7)	(1.0)
Intercompany Expense/(Income) Allocations	0.7	0.8	1.9	2.2
PSNH | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.3	0.2	0.9	0.6
Interest Cost	0.4	0.7	1.3	2.1
Expected Return on Plan Assets	(1.6)	(1.4)	(4.6)	(4.2)
Actuarial Loss	0.1	0.2	0.4	0.6
Prior Service Cost/(Credit)	0.1	0.1	0.3	0.3
Total Net Periodic Benefit Plan Expense/(Income)	(0.7)	(0.2)	(1.7)	(0.6)
Intercompany Expense/(Income) Allocations	$ (0.2)	$ (0.1)	$ (0.5)	$ (0.4)