EMPLOYEE BENEFITS - Schedule of Defined Contribution Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	$ 34.5	$ 31.8	$ 30.4
The Connecticut Light And Power Company
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	4.6	4.5	4.8
NSTAR Electric Company
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	8.5	8.1	7.3
Public Service Company Of New Hampshire
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, cost recognized	$ 3.7	$ 3.4	$ 3.4