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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	$ 18.7	$ 16.2	$ 57.0	$ 51.6
Interest Cost	44.4	54.8	133.0	163.8
Expected Return on Plan Assets	(99.2)	(91.6)	(298.8)	(275.4)
Actuarial Loss	50.7	35.6	150.7	107.9
Prior Service Cost/(Credit)	0.2	0.3	0.8	0.9
Total Net Periodic Benefit Expense/(Income)	14.8	15.3	42.7	48.8
PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	2.4	2.0	7.3	5.9
Interest Cost	6.1	8.1	18.3	24.5
Expected Return on Plan Assets	(18.3)	(16.7)	(55.1)	(50.1)
Actuarial Loss	2.2	2.1	6.3	6.2
Prior Service Cost/(Credit)	(5.3)	(5.8)	(15.9)	(17.6)
Total Net Periodic Benefit Expense/(Income)	(12.9)	(10.3)	(39.1)	(31.1)
CL&P | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	5.4	4.4	16.5	13.6
Interest Cost	9.3	11.4	28.1	34.4
Expected Return on Plan Assets	(19.8)	(18.1)	(59.6)	(55.1)
Actuarial Loss	9.7	6.3	29.4	20.6
Prior Service Cost/(Credit)	0.0	0.0	0.0	0.0
Total Net Periodic Benefit Expense/(Income)	4.6	4.0	14.4	13.5
Intercompany Allocations	2.4	2.0	6.7	6.5
CL&P | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.4	0.3	1.3	1.1
Interest Cost	1.1	1.6	3.3	4.7
Expected Return on Plan Assets	(2.5)	(2.2)	(7.4)	(6.9)
Actuarial Loss	0.3	0.3	0.9	1.0
Prior Service Cost/(Credit)	0.3	0.2	0.8	0.8
Total Net Periodic Benefit Expense/(Income)	(0.4)	0.2	(1.1)	0.7
Intercompany Allocations	(0.3)	(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	3.8	3.6	11.4	11.0
Interest Cost	9.7	12.3	29.0	36.6
Expected Return on Plan Assets	(25.7)	(24.2)	(77.2)	(72.9)
Actuarial Loss	14.0	11.8	41.3	33.1
Prior Service Cost/(Credit)	0.0	0.0	0.2	0.2
Total Net Periodic Benefit Expense/(Income)	1.8	3.5	4.7	8.0
Intercompany Allocations	2.3	1.9	6.5	6.1
NSTAR Electric | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.5	0.4	1.6	1.3
Interest Cost	1.7	2.4	5.0	7.1
Expected Return on Plan Assets	(8.5)	(7.5)	(25.5)	(22.6)
Actuarial Loss	0.6	0.8	1.8	2.5
Prior Service Cost/(Credit)	(4.2)	(4.2)	(12.7)	(12.7)
Total Net Periodic Benefit Expense/(Income)	(9.9)	(8.1)	(29.8)	(24.4)
Intercompany Allocations	(0.4)	(0.3)	(1.0)	(0.9)
PSNH | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	2.0	1.5	6.2	5.6
Interest Cost	4.8	6.0	14.5	18.2
Expected Return on Plan Assets	(11.1)	(10.1)	(33.5)	(30.6)
Actuarial Loss	3.8	2.3	11.8	8.2
Prior Service Cost/(Credit)	0.0	0.0	0.0	0.0
Total Net Periodic Benefit Expense/(Income)	(0.5)	(0.3)	(1.0)	1.4
Intercompany Allocations	0.8	0.5	2.2	1.8
PSNH | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.2	0.2	0.6	0.5
Interest Cost	0.7	0.8	2.1	2.5
Expected Return on Plan Assets	(1.4)	(1.3)	(4.2)	(4.0)
Actuarial Loss	0.2	0.1	0.6	0.3
Prior Service Cost/(Credit)	0.1	0.1	0.3	0.3
Total Net Periodic Benefit Expense/(Income)	(0.2)	(0.1)	(0.6)	(0.4)
Intercompany Allocations	$ (0.1)	$ (0.1)	$ (0.4)	$ (0.3)