COMMITMENTS AND CONTINGENCIES - Environmental Matters Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($) site	Dec. 31, 2022 USD ($) site	Dec. 31, 2021 USD ($)
Loss Contingencies [Line Items]
Number of sites | site	65	59
Reserve | $	$ 128.2	$ 122.6	$ 115.4
CL&P
Loss Contingencies [Line Items]
Number of additional sites | site	3
Number of sites | site	16	13
Reserve | $	$ 13.8	$ 13.9	13.9
NSTAR Electric
Loss Contingencies [Line Items]
Number of additional sites | site	2
Number of sites | site	12	10
Reserve | $	$ 5.4	$ 3.4	3.3
PSNH
Loss Contingencies [Line Items]
Number of sites | site	8	8
Reserve | $	$ 7.6	$ 6.1	$ 6.3
NSTAR Gas
Loss Contingencies [Line Items]
Number of additional sites | site	1
MGP Site accrual
Loss Contingencies [Line Items]
Reserve balances related to former MGP sites | $	$ 117.1	$ 112.6
Environmental Sites for Which a Range of Loss Exists
Loss Contingencies [Line Items]
Number of sites | site	7
Reserve | $	$ 29.4
Best estimate of potential remediation costs (up to) | $	$ 17.7
Environmental Sites for Which a Range of Loss Exists | CL&P
Loss Contingencies [Line Items]
Number of sites | site	1
Reserve | $	$ 0.4
Best estimate of potential remediation costs (up to) | $	$ 0.5
Environmental Site for Which a Range is Too Early to Determine
Loss Contingencies [Line Items]
Number of sites | site	19
Reserve | $	$ 39.9
Environmental Site for Which a Range is Too Early to Determine | CL&P
Loss Contingencies [Line Items]
Number of sites | site	11
Reserve | $	$ 12.6
Environmental Site for Which a Range is Too Early to Determine | NSTAR Electric
Loss Contingencies [Line Items]
Number of sites | site	1
Reserve | $	$ 0.3
Evironmental Sites with Best Estimate
Loss Contingencies [Line Items]
Number of sites | site	39
Reserve | $	$ 58.9
Evironmental Sites with Best Estimate | CL&P
Loss Contingencies [Line Items]
Reserve | $	$ 0.8
Environmental site quantity remaining | site	4
Evironmental Sites with Best Estimate | NSTAR Electric
Loss Contingencies [Line Items]
Reserve | $	$ 5.1
Environmental site quantity remaining | site	11
Evironmental Sites with Best Estimate | PSNH
Loss Contingencies [Line Items]
Reserve | $	$ 7.6
Environmental site quantity remaining | site	8