EMPLOYEE BENEFITS - Schedule of Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 76.2	$ 67.7	$ 84.8
Interest Cost	177.8	219.0	196.4
Expected Return on Plan Assets	(400.3)	(367.1)	(391.6)
Actuarial Loss	202.0	143.2	145.7
Prior Service Cost/(Credit)	1.2	0.9	4.3
Total Net Periodic Benefit Expense/(Income)	56.9	63.7	39.6
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	10.2	7.8	10.0
Interest Cost	24.6	32.7	30.7
Expected Return on Plan Assets	(73.6)	(66.8)	(72.4)
Actuarial Loss	8.4	8.3	10.3
Prior Service Cost/(Credit)	(21.2)	(23.5)	(23.6)
Total Net Periodic Benefit Expense/(Income)	(51.6)	(41.5)	(45.0)
The Connecticut Light and Power Company | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	21.8	18.0	21.4
Interest Cost	37.3	45.7	41.8
Expected Return on Plan Assets	(79.2)	(73.2)	(79.1)
Actuarial Loss	39.2	26.9	29.1
Prior Service Cost/(Credit)	0.0	0.0	1.1
Total Net Periodic Benefit Expense/(Income)	19.1	17.4	14.3
Intercompany Allocations	9.1	8.5	6.1
The Connecticut Light and Power Company | Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	1.7	1.4	1.9
Interest Cost	4.4	6.3	5.8
Expected Return on Plan Assets	(9.9)	(9.2)	(10.4)
Actuarial Loss	1.1	1.3	1.6
Prior Service Cost/(Credit)	1.1	1.1	1.1
Total Net Periodic Benefit Expense/(Income)	(1.6)	0.9	0.0
Intercompany Allocations	(1.1)	(0.9)	(1.0)
NSTAR Electric Company | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	15.4	14.6	17.4
Interest Cost	38.6	49.0	43.5
Expected Return on Plan Assets	(103.0)	(97.1)	(104.9)
Actuarial Loss	55.2	44.7	41.1
Prior Service Cost/(Credit)	0.3	0.3	0.2
Total Net Periodic Benefit Expense/(Income)	6.5	11.5	(2.7)
Intercompany Allocations	8.9	8.0	6.5
NSTAR Electric Company | Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	2.1	1.7	2.0
Interest Cost	6.6	9.5	8.7
Expected Return on Plan Assets	(34.0)	(30.2)	(32.5)
Actuarial Loss	2.5	3.3	2.3
Prior Service Cost/(Credit)	(17.0)	(16.9)	(16.9)
Total Net Periodic Benefit Expense/(Income)	(39.8)	(32.6)	(36.4)
Intercompany Allocations	(1.4)	(1.2)	(1.3)
Public Service Company of New Hampshire | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	8.2	7.1	11.2
Interest Cost	19.4	24.0	22.0
Expected Return on Plan Assets	(44.7)	(40.7)	(43.6)
Actuarial Loss	15.6	10.6	11.6
Prior Service Cost/(Credit)	0.0	0.0	0.4
Total Net Periodic Benefit Expense/(Income)	(1.5)	1.0	1.6
Intercompany Allocations	2.9	2.3	1.9
Public Service Company of New Hampshire | Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	0.9	0.7	1.1
Interest Cost	2.8	3.4	3.4
Expected Return on Plan Assets	(5.7)	(5.4)	(6.0)
Actuarial Loss	0.8	0.3	0.7
Prior Service Cost/(Credit)	0.4	0.4	0.5
Total Net Periodic Benefit Expense/(Income)	(0.8)	(0.6)	(0.3)
Intercompany Allocations	$ (0.5)	$ (0.4)	$ (0.4)