REGULATORY ACCOUNTING - Narrative (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	$ 262.5	$ 252.5
COVID-19
Regulatory Assets [Line Items]
Net incremental deferred costs	35.0	33.0
CL&P
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	113.4	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	84.2	85.0
NSTAR Electric | COVID-19
Regulatory Assets [Line Items]
Net incremental deferred costs	6.1	6.1
PSNH
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	$ 3.4	$ 3.4