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, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	$ 18.4	$ 18.7	$ 39.7	$ 38.3
Interest Cost	27.1	44.3	59.7	88.6
Expected Return on Plan Assets	(87.4)	(99.3)	(196.3)	(199.6)
Actuarial Loss	47.8	50.7	109.4	100.0
Prior Service Cost/(Credit)	0.3	0.3	0.6	0.6
Total Net Periodic Benefit Plan Expense/(Income)	6.2	14.7	13.1	27.9
PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	3.3	2.0	6.8	4.9
Interest Cost	4.2	6.4	8.6	12.2
Expected Return on Plan Assets	(19.8)	(18.7)	(39.5)	(36.8)
Actuarial Loss	1.3	2.0	3.9	4.1
Prior Service Cost/(Credit)	(5.3)	(5.2)	(10.6)	(10.6)
Total Net Periodic Benefit Plan Expense/(Income)	(16.3)	(13.5)	(30.8)	(26.2)
CL&P | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	5.5	5.4	11.8	11.1
Interest Cost	6.7	9.3	14.0	18.8
Expected Return on Plan Assets	(21.7)	(19.8)	(43.3)	(39.8)
Actuarial Loss	10.8	9.7	23.8	19.7
Prior Service Cost/(Credit)	0.0	0.0	0.0	0.0
Total Net Periodic Benefit Plan Expense/(Income)	1.3	4.6	6.3	9.8
Intercompany Expense/(Income) Allocations	2.3	2.4	3.6	4.3
CL&P | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.6	0.4	1.2	0.9
Interest Cost	0.8	1.0	1.6	2.2
Expected Return on Plan Assets	(2.6)	(2.5)	(5.2)	(4.9)
Actuarial Loss	0.4	0.2	0.8	0.6
Prior Service Cost/(Credit)	0.3	0.2	0.5	0.4
Total Net Periodic Benefit Plan Expense/(Income)	(0.5)	(0.7)	(1.1)	(0.8)
Intercompany Expense/(Income) Allocations	(0.6)	(0.2)	(0.9)	(0.6)
NSTAR Electric | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	3.9	3.8	7.9	7.6
Interest Cost	6.7	9.7	13.4	19.3
Expected Return on Plan Assets	(27.1)	(25.7)	(54.0)	(51.5)
Actuarial Loss	15.3	14.0	30.8	27.3
Prior Service Cost/(Credit)	0.1	0.1	0.2	0.2
Total Net Periodic Benefit Plan Expense/(Income)	(1.1)	1.9	(1.7)	2.9
Intercompany Expense/(Income) Allocations	2.5	2.3	4.0	4.2
NSTAR Electric | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.6	0.5	1.2	1.1
Interest Cost	1.1	1.6	2.2	3.3
Expected Return on Plan Assets	(9.2)	(8.5)	(18.5)	(17.0)
Actuarial Loss	0.5	0.5	1.1	1.2
Prior Service Cost/(Credit)	(4.2)	(4.3)	(8.4)	(8.5)
Total Net Periodic Benefit Plan Expense/(Income)	(11.2)	(10.2)	(22.4)	(19.9)
Intercompany Expense/(Income) Allocations	(0.6)	(0.2)	(1.0)	(0.6)
PSNH | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	2.2	2.0	4.4	4.2
Interest Cost	3.6	4.8	7.2	9.7
Expected Return on Plan Assets	(11.8)	(11.1)	(23.7)	(22.4)
Actuarial Loss	5.2	3.9	10.1	8.0
Prior Service Cost/(Credit)	0.0	0.0	0.0	0.0
Total Net Periodic Benefit Plan Expense/(Income)	(0.8)	(0.4)	(2.0)	(0.5)
Intercompany Expense/(Income) Allocations	0.7	0.8	1.2	1.4
PSNH | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.3	0.2	0.6	0.4
Interest Cost	0.4	0.8	0.9	1.4
Expected Return on Plan Assets	(1.5)	(1.4)	(3.0)	(2.8)
Actuarial Loss	0.1	0.3	0.3	0.4
Prior Service Cost/(Credit)	0.1	0.1	0.2	0.2
Total Net Periodic Benefit Plan Expense/(Income)	(0.6)	0.0	(1.0)	(0.4)
Intercompany Expense/(Income) Allocations	$ (0.2)	$ (0.1)	$ (0.3)	$ (0.3)