EMPLOYEE BENEFITS - Schedule of Defined Contribution Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	$ 41.6	$ 38.4	$ 34.5
CL&P
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	5.5	5.0	4.6
NSTAR Electric
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	10.3	9.7	8.5
PSNH
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	$ 3.5	$ 3.3	$ 3.7