REGULATORY ACCOUNTING - Narrative (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	$ 243.9	$ 241.7
Regulatory costs	75.9	82.1
Regulatory assets	6,728.8	6,389.2
CL&P
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	169.2	166.7
Regulatory costs	60.7	64.0
Regulatory assets	2,427.7	2,143.2
NSTAR Electric
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	21.7	21.9
Regulatory costs	6.5	7.3
Regulatory assets	2,040.9	1,957.9
PSNH
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved	1.3	1.2
Regulatory assets	$ 999.6	$ 963.3