COMMITMENTS AND CONTINGENCIES - Environmental Matters Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2025 USD ($) site	Dec. 31, 2024 USD ($) site	Dec. 31, 2023 USD ($)
Loss Contingencies [Line Items]
Number of sites | site	66	65
Reserve	$ 154.3	$ 128.0	$ 128.2
CL&P
Loss Contingencies [Line Items]
Number of sites | site	15	15
Reserve	$ 14.9	$ 13.4	13.8
NSTAR Electric
Loss Contingencies [Line Items]
Number of sites | site	14	14
Reserve	$ 7.0	$ 6.6	5.4
PSNH
Loss Contingencies [Line Items]
Number of sites | site	8	8
Reserve	$ 9.2	$ 6.3	$ 7.6
MGP Site accrual
Loss Contingencies [Line Items]
Reserve balances related to former MGP sites	$ 140.9	$ 115.9
Environmental Site for Which a Range is Too Early to Determine
Loss Contingencies [Line Items]
Number of sites | site	13
Reserve	$ 33.6
Environmental Site for Which a Range is Too Early to Determine | CL&P
Loss Contingencies [Line Items]
Number of sites | site	7
Reserve	$ 7.0
Environmental Sites for Which a Range of Loss Exists
Loss Contingencies [Line Items]
Number of sites | site	9
Reserve	$ 30.9
Best estimate of potential remediation costs (up to)	29.6
Environmental Sites for Which a Range of Loss Exists | CL&P
Loss Contingencies [Line Items]
Reserve	$ 0.7
Evironmental Sites with Best Estimate
Loss Contingencies [Line Items]
Number of sites | site	44
Reserve	$ 89.8
Evironmental Sites with Best Estimate | CL&P
Loss Contingencies [Line Items]
Reserve	$ 7.2
Environmental site quantity remaining | site	6
Evironmental Sites with Best Estimate | NSTAR Electric
Loss Contingencies [Line Items]
Reserve	$ 7.0
Environmental site quantity remaining | site	13
Evironmental Sites with Best Estimate | PSNH
Loss Contingencies [Line Items]
Reserve	$ 9.2
Environmental site quantity remaining | site	8