COMMITMENTS AND CONTINGENCIES - Environmental Sites and Related Reserves (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2024 USD ($) site	Dec. 31, 2023 USD ($) site
Site Contingency [Line Items]
Number of Sites | site	66	65
Reserve | $	$ 128.0	$ 128.2
CL&P
Site Contingency [Line Items]
Number of Sites | site	16	16
Reserve | $	$ 13.8	$ 13.8
NSTAR Electric
Site Contingency [Line Items]
Number of Sites | site	13	12
Reserve | $	$ 5.9	$ 5.4
PSNH
Site Contingency [Line Items]
Number of Sites | site	8	8
Reserve | $	$ 7.7	$ 7.6