EMPLOYEE BENEFITS - Schedule of Funded Status (Details) - Pension Plan - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 5,583.6	$ 4,829.6
The Connecticut Light And Power Company
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	1,179.2	1,065.2
NSTAR Electric Company
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	1,260.1	1,124.8
Public Service Company Of New Hampshire
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 597.2	$ 518.9