REGULATORY ACCOUNTING - Narrative (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	$ 176.3	$ 146.0
CL&P
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	85.2	51.8
PSNH
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	15.0	18.0
NSTAR Electric
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	49.0	$ 55.7
COVID-19
Regulatory Assets [Line Items]
Deferred costs	6.6
Deferred costs related to uncollectible expense incurred at our natural gas distribution segment	$ 4.1