COMMITMENTS AND CONTINGENCIES - Environmental Matters Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2022 USD ($) site	Dec. 31, 2021 USD ($) site	Dec. 31, 2020 USD ($)
Loss Contingencies [Line Items]
Number of sites | site	59	61
Reserve	$ 122.6	$ 115.4	$ 102.4
CL&P
Loss Contingencies [Line Items]
Number of sites | site	13	14
Reserve	$ 13.9	$ 13.9	12.3
NSTAR Electric
Loss Contingencies [Line Items]
Number of sites | site	10	11
Reserve	$ 3.4	$ 3.3	4.7
PSNH
Loss Contingencies [Line Items]
Number of sites | site	8	9
Reserve	$ 6.1	$ 6.3	$ 7.1
MGP Site accrual
Loss Contingencies [Line Items]
Reserve balances related to former MGP sites	$ 112.6	$ 105.6
Environmental Sites for Which a Range of Loss Exists
Loss Contingencies [Line Items]
Number of sites | site	7
Reserve	$ 29.9
Best estimate of potential remediation costs (up to)	$ 17.0
Environmental Sites for Which a Range of Loss Exists | CL&P
Loss Contingencies [Line Items]
Number of sites | site	1
Reserve	$ 0.6
Best estimate of potential remediation costs (up to)	$ 0.3
Environmental Site for Which a Range is Too Early to Determine
Loss Contingencies [Line Items]
Number of sites | site	15
Reserve	$ 25.8
Environmental Site for Which a Range is Too Early to Determine | CL&P
Loss Contingencies [Line Items]
Number of sites | site	7
Reserve	$ 4.4
Environmental Site for Which a Range is Too Early to Determine | NSTAR Electric
Loss Contingencies [Line Items]
Number of sites | site	1
Reserve	$ 0.1
Evironmental Sites with Best Estimate
Loss Contingencies [Line Items]
Number of sites | site	37
Reserve	$ 66.9
Evironmental Sites with Best Estimate | CL&P
Loss Contingencies [Line Items]
Reserve	$ 8.9
Environmental site quantity remaining | site	5
Evironmental Sites with Best Estimate | NSTAR Electric
Loss Contingencies [Line Items]
Reserve	$ 3.3
Environmental site quantity remaining | site	9
Evironmental Sites with Best Estimate | PSNH
Loss Contingencies [Line Items]
Reserve	$ 6.1
Environmental site quantity remaining | site	8