EMPLOYEE BENEFITS - Schedule of Other Retirement Benefits (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarially-Determined Liability	$ 42.8	$ 45.7	$ 52.0
Other Retirement Benefits Expense	2.2	3.3	2.7
The Connecticut Light and Power Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarially-Determined Liability	0.2	0.2	0.2
Other Retirement Benefits Expense	0.7	1.2	1.0
NSTAR Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarially-Determined Liability	0.1	0.1	0.1
Other Retirement Benefits Expense	0.7	1.1	0.9
Public Service Company of New Hampshire
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarially-Determined Liability	1.5	1.7	1.7
Other Retirement Benefits Expense	$ 0.3	$ 0.5	$ 0.4