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COMMITMENTS AND CONTINGENCIES - Environmental Sites and Related Reserves (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2025 USD ($) site	Dec. 31, 2024 USD ($) site
Site Contingency [Line Items]
Number of Sites | site	65	65
Reserve | $	$ 144.8	$ 128.0
CL&P
Site Contingency [Line Items]
Number of Sites | site	15	15
Reserve | $	$ 13.2	$ 13.4
NSTAR Electric
Site Contingency [Line Items]
Number of Sites | site	14	14
Reserve | $	$ 7.9	$ 6.6
PSNH
Site Contingency [Line Items]
Number of Sites | site	8	8
Reserve | $	$ 6.4	$ 6.3

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- Definition

Number of environmental sites by NU Entity in the remediation or long-term monitoring phase

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Total costs accrued as of the balance sheet date for environmental loss contingencies.

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