EMPLOYEE BENEFITS - Schedule of Funded Status (Details) - Pension Plan - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 5,070.8	$ 5,583.6
The Connecticut Light And Power Company
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	1,031.0	1,179.2
NSTAR Electric Company
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	1,144.7	1,260.1
Public Service Company Of New Hampshire
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 543.1	$ 597.2