EMPLOYEE BENEFITS - Schedule of Defined Contribution Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	$ 38.4	$ 34.5	$ 31.8
The Connecticut Light And Power Company
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	5.0	4.6	4.5
NSTAR Electric Company
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	9.7	8.5	8.1
Public Service Company Of New Hampshire
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	$ 3.3	$ 3.7	$ 3.4