EMPLOYEE BENEFITS - Schedule of Funded Status (Details) - Pension Plan - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 5,963.4	$ 5,070.8
CL&P
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	1,205.4	1,031.0
NSTAR Electric
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	1,340.8	1,144.7
PSNH
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 646.7	$ 543.1