REGULATORY ACCOUNTING - Narrative (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	$ 171.3	$ 146.0
CL&P
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	82.6	51.8
PSNH
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	16.9	18.0
NSTAR Electric
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	$ 50.3	$ 55.7