EMPLOYEE BENEFITS - Schedule of Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 85.8	$ 76.2	$ 67.7
Interest Cost	130.0	177.8	219.0
Expected Return on Plan Assets	(437.5)	(400.3)	(367.1)
Actuarial Loss	243.9	202.0	143.2
Prior Service Cost/(Credit)	1.4	1.2	0.9
Total Net Periodic Benefit Plan Expense/(Income)	23.6	56.9	63.7
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	13.5	10.2	7.8
Interest Cost	17.4	24.6	32.7
Expected Return on Plan Assets	(79.1)	(73.6)	(66.8)
Actuarial Loss	8.9	8.4	8.3
Prior Service Cost/(Credit)	(21.2)	(21.2)	(23.5)
Total Net Periodic Benefit Plan Expense/(Income)	(60.5)	(51.6)	(41.5)
The Connecticut Light and Power Company | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	23.0	21.8	18.0
Interest Cost	27.3	37.3	45.7
Expected Return on Plan Assets	(86.8)	(79.2)	(73.2)
Actuarial Loss	45.5	39.2	26.9
Prior Service Cost/(Credit)	0.0	0.0	0.0
Total Net Periodic Benefit Plan Expense/(Income)	9.0	19.1	17.4
Intercompany Expense/(Income) Allocations	8.0	9.1	8.5
The Connecticut Light and Power Company | Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	2.3	1.7	1.4
Interest Cost	3.2	4.4	6.3
Expected Return on Plan Assets	(10.3)	(9.9)	(9.2)
Actuarial Loss	1.8	1.1	1.3
Prior Service Cost/(Credit)	1.1	1.1	1.1
Total Net Periodic Benefit Plan Expense/(Income)	(1.9)	(1.6)	0.9
Intercompany Expense/(Income) Allocations	(1.6)	(1.1)	(0.9)
NSTAR Electric Company | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	15.8	15.4	14.6
Interest Cost	26.8	38.6	49.0
Expected Return on Plan Assets	(108.1)	(103.0)	(97.1)
Actuarial Loss	61.6	55.2	44.7
Prior Service Cost/(Credit)	0.3	0.3	0.3
Total Net Periodic Benefit Plan Expense/(Income)	(3.6)	6.5	11.5
Intercompany Expense/(Income) Allocations	8.8	8.9	8.0
NSTAR Electric Company | Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	2.4	2.1	1.7
Interest Cost	4.4	6.6	9.5
Expected Return on Plan Assets	(36.9)	(34.0)	(30.2)
Actuarial Loss	2.4	2.5	3.3
Prior Service Cost/(Credit)	(17.0)	(17.0)	(16.9)
Total Net Periodic Benefit Plan Expense/(Income)	(44.7)	(39.8)	(32.6)
Intercompany Expense/(Income) Allocations	(1.9)	(1.4)	(1.2)
Public Service Company of New Hampshire | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	8.9	8.2	7.1
Interest Cost	14.5	19.4	24.0
Expected Return on Plan Assets	(47.5)	(44.7)	(40.7)
Actuarial Loss	20.7	15.6	10.6
Prior Service Cost/(Credit)	0.0	0.0	0.0
Total Net Periodic Benefit Plan Expense/(Income)	(3.4)	(1.5)	1.0
Intercompany Expense/(Income) Allocations	2.7	2.9	2.3
Public Service Company of New Hampshire | Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	1.2	0.9	0.7
Interest Cost	1.8	2.8	3.4
Expected Return on Plan Assets	(6.1)	(5.7)	(5.4)
Actuarial Loss	0.7	0.8	0.3
Prior Service Cost/(Credit)	0.4	0.4	0.4
Total Net Periodic Benefit Plan Expense/(Income)	(2.0)	(0.8)	(0.6)
Intercompany Expense/(Income) Allocations	$ (0.6)	$ (0.5)	$ (0.4)