REGULATORY ACCOUNTING - Narrative (Details) - USD ($) $ in Millions	Jun. 30, 2022	Dec. 31, 2021
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	$ 286.8	$ 252.5
COVID-19
Regulatory Assets [Line Items]
Net incremental deferred costs	37.6	33.0
CL&P
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	123.2	114.9
CL&P | COVID-19
Regulatory Assets [Line Items]
Net incremental deferred costs	15.3	18.0
NSTAR Electric
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	91.1	85.0
NSTAR Electric | COVID-19
Regulatory Assets [Line Items]
Net incremental deferred costs	4.8	6.1
PSNH
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	$ 1.3	$ 3.4