COMMITMENTS AND CONTINGENCIES - Environmental Sites and Related Reserves (Details) $ in Millions	3 Months Ended	9 Months Ended
	Mar. 31, 2025 site	Sep. 30, 2025 USD ($) site	Dec. 31, 2024 USD ($)
Site Contingency [Line Items]
Number of Sites | site	65	66
Reserve | $		$ 151.2	$ 128.0
CL&P
Site Contingency [Line Items]
Number of Sites | site	15	16
Reserve | $		$ 15.4	13.4
NSTAR Electric
Site Contingency [Line Items]
Number of Sites | site	14	14
Reserve | $		$ 8.1	6.6
PSNH
Site Contingency [Line Items]
Number of Sites | site	8	8
Reserve | $		$ 9.3	$ 6.3