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COMMITMENTS AND CONTINGENCIES - Environmental Matters Narrative (Details) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2021 USD ($) site | Dec. 31, 2020 USD ($) site | Dec. 31, 2019 USD ($)
Loss Contingencies [Line Items] |  |  |
Number of sites | site | 61 | 63 |
Reserve | $ 115.4 | $ 102.4 | $ 81.0
The Connecticut Light and Power Company |  |  |
Loss Contingencies [Line Items] |  |  |
Number of sites | site | 14 | 15 |
Reserve | $ 13.9 | $ 12.3 | 11.4
NSTAR Electric Company |  |  |
Loss Contingencies [Line Items] |  |  |
Number of sites | site | 11 | 12 |
Reserve | $ 3.3 | $ 4.7 | 8.0
Public Service Company of New Hampshire |  |  |
Loss Contingencies [Line Items] |  |  |
Number of sites | site | 9 | 9 |
Reserve | $ 6.3 | $ 7.1 | $ 7.5
MGP Site accrual |  |  |
Loss Contingencies [Line Items] |  |  |
Reserve balances related to former MGP sites | $ 105.6 | $ 92.2 |
Environmental Sites for Which a Range of Loss Exists |  |  |
Loss Contingencies [Line Items] |  |  |
Number of sites | site | 7 |  |
Reserve | $ 25.9 |  |
Best estimate of potential remediation costs (up to) | $ 10.0 |  |
Environmental Sites for Which a Range of Loss Exists | The Connecticut Light and Power Company |  |  |
Loss Contingencies [Line Items] |  |  |
Number of sites | site | 2 |  |
Reserve | $ 3.2 |  |
Best estimate of potential remediation costs (up to) | $ 0.6 |  |
Environmental Site for Which a Range is Too Early to Determine |  |  |
Loss Contingencies [Line Items] |  |  |
Number of sites | site | 13 |  |
Reserve | $ 16.1 |  |
Environmental Site for Which a Range is Too Early to Determine | The Connecticut Light and Power Company |  |  |
Loss Contingencies [Line Items] |  |  |
Number of sites | site | 7 |  |
Reserve | $ 3.9 |  |
Environmental Site for Which a Range is Too Early to Determine | NSTAR Electric Company |  |  |
Loss Contingencies [Line Items] |  |  |
Number of sites | site | 2 |  |
Reserve | $ 0.2 |  |
Evironmental Sites with Best Estimate |  |  |
Loss Contingencies [Line Items] |  |  |
Number of sites | site | 41 |  |
Reserve | $ 73.4 |  |
Evironmental Sites with Best Estimate | The Connecticut Light and Power Company |  |  |
Loss Contingencies [Line Items] |  |  |
Reserve | $ 6.8 |  |
Environmental site quantity remaining | site | 5 |  |
Evironmental Sites with Best Estimate | NSTAR Electric Company |  |  |
Loss Contingencies [Line Items] |  |  |
Reserve | $ 3.1 |  |
Environmental site quantity remaining | site | 9 |  |
Evironmental Sites with Best Estimate | Public Service Company of New Hampshire |  |  |
Loss Contingencies [Line Items] |  |  |
Reserve | $ 6.3 |  |
Environmental site quantity remaining | site | 9 |  |

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