REGULATORY ACCOUNTING - Narrative (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	$ 216.6	$ 241.7
Regulatory costs	86.3	82.1
CL&P
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	115.9	166.7
Regulatory costs	46.7	64.0
NSTAR Electric
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	39.1	21.9
Regulatory costs	21.5	7.3
PSNH
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	$ 2.6	$ 1.2