COMMITMENTS AND CONTINGENCIES - Environmental Sites and Related Reserves (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2023 USD ($) site	Dec. 31, 2022 USD ($) site
Site Contingency [Line Items]
Number of Sites | site	63	59
Reserve | $	$ 130.5	$ 122.6
CL&P
Site Contingency [Line Items]
Number of Sites | site	14	13
Reserve | $	$ 12.6	$ 13.9
NSTAR Electric
Site Contingency [Line Items]
Number of Sites | site	12	10
Reserve | $	$ 5.4	$ 3.4
PSNH
Site Contingency [Line Items]
Number of Sites | site	8	8
Reserve | $	$ 7.7	$ 6.1