REGULATORY ACCOUNTING - Narrative (Details) - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	$ 288.9	$ 252.5
COVID-19
Regulatory Assets [Line Items]
Net incremental deferred costs	34.6	33.0
CL&P
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	121.0	114.9
CL&P | COVID-19
Regulatory Assets [Line Items]
Net incremental deferred costs	16.5	18.0
NSTAR Electric
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	94.6	85.0
NSTAR Electric | COVID-19
Regulatory Assets [Line Items]
Net incremental deferred costs	2.2	6.1
PSNH
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	$ 2.8	$ 3.4