EMPLOYEE BENEFITS - Schedule of Defined Contribution Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	$ 49.4	$ 41.6	$ 38.4
The Connecticut Light and Power Company
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	6.6	5.5	5.0
NSTAR Electric Company
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	11.8	10.3	9.7
Public Service Company of New Hampshire
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	$ 4.1	$ 3.5	$ 3.3