COMMITMENTS AND CONTINGENCIES - Environmental Sites and Related Reserves (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2024 USD ($) site	Dec. 31, 2023 USD ($) site
Site Contingency [Line Items]
Number of Sites | site	65	65
Reserve | $	$ 130.5	$ 128.2
CL&P
Site Contingency [Line Items]
Number of Sites | site	15	16
Reserve | $	$ 13.7	$ 13.8
NSTAR Electric
Site Contingency [Line Items]
Number of Sites | site	13	12
Reserve | $	$ 6.7	$ 5.4
PSNH
Site Contingency [Line Items]
Number of Sites | site	8	8
Reserve | $	$ 6.4	$ 7.6