EMPLOYEE BENEFITS - Schedule of Funded Status (Details) - Pension Plan - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 6,669.4	$ 5,963.4
The Connecticut Light and Power Company
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	1,356.4	1,205.4
NSTAR Electric Company
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	1,449.4	1,340.8
Public Service Company of New Hampshire
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 707.2	$ 646.7