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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 |  |

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