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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 |  |

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