EMPLOYEE BENEFITS - Schedule of Defined Contribution Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	$ 67.3	$ 59.9	$ 55.5
CL&P
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	9.0	7.7	7.0
NSTAR Electric
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	13.7	12.8	12.2
PSNH
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	$ 5.4	$ 4.8	$ 4.3