REGULATORY ACCOUNTING - Narrative (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	$ 236.1	$ 221.0
Regulatory costs	105.4	92.5
CL&P
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	124.9	116.3
Regulatory costs	55.8	47.2
NSTAR Electric
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	44.1	41.1
Regulatory costs	28.2	24.4
PSNH
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	$ 5.7	$ 4.5