COMMITMENTS AND CONTINGENCIES - Environmental Sites and Related Reserves (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2026 USD ($) site	Dec. 31, 2025 USD ($) site
Site Contingency [Line Items]
Number of Sites | site	67	66
Reserve | $	$ 154.1	$ 154.3
CL&P
Site Contingency [Line Items]
Number of Sites | site	15	15
Reserve | $	$ 15.8	$ 14.9
NSTAR Electric
Site Contingency [Line Items]
Number of Sites | site	15	14
Reserve | $	$ 7.0	$ 7.0
PSNH
Site Contingency [Line Items]
Number of Sites | site	8	8
Reserve | $	$ 9.2	$ 9.2