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COMMITMENTS AND CONTINGENCIES - Environmental Matters Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2024 USD ($) site	Dec. 31, 2023 USD ($) site	Dec. 31, 2022 USD ($)
Loss Contingencies [Line Items]
Number of sites | site	65	65
Reserve	$ 128.0	$ 128.2	$ 122.6
NSTAR Electric
Loss Contingencies [Line Items]
Number of sites | site	14	12
Reserve	$ 6.6	$ 5.4	3.4
CL&P
Loss Contingencies [Line Items]
Number of sites | site	15	16
Reserve	$ 13.4	$ 13.8	13.9
PSNH
Loss Contingencies [Line Items]
Number of sites | site	8	8
Reserve	$ 6.3	$ 7.6	$ 6.1
MGP Site accrual
Loss Contingencies [Line Items]
Reserve balances related to former MGP sites	$ 115.9	$ 117.1
Environmental Site for Which a Range is Too Early to Determine
Loss Contingencies [Line Items]
Number of sites | site	17
Reserve	$ 30.7
Environmental Site for Which a Range is Too Early to Determine | NSTAR Electric
Loss Contingencies [Line Items]
Number of sites | site	2
Reserve	$ 1.4
Environmental Site for Which a Range is Too Early to Determine | CL&P
Loss Contingencies [Line Items]
Number of sites | site	9
Reserve	$ 5.7
Environmental Sites for Which a Range of Loss Exists
Loss Contingencies [Line Items]
Number of sites | site	8
Reserve	$ 29.5
Best estimate of potential remediation costs (up to)	$ 17.5
Evironmental Sites with Best Estimate
Loss Contingencies [Line Items]
Number of sites | site	40
Reserve	$ 67.8
Evironmental Sites with Best Estimate | NSTAR Electric
Loss Contingencies [Line Items]
Reserve	$ 5.2
Environmental site quantity remaining | site	11
Evironmental Sites with Best Estimate | CL&P
Loss Contingencies [Line Items]
Reserve	$ 7.7
Environmental site quantity remaining | site	6
Evironmental Sites with Best Estimate | PSNH
Loss Contingencies [Line Items]
Reserve	$ 6.3
Environmental site quantity remaining | site	8

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