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COMMITMENTS AND CONTINGENCIES - Environmental Sites and Related Reserves (Details) $ in Millions | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2023 USD ($) site | Dec. 31, 2022 USD ($) site
Site Contingency [Line Items] |  |
Number of Sites | site | 63 | 59
Reserve | $ | $ 130.5 | $ 122.6
CL&P |  |
Site Contingency [Line Items] |  |
Number of Sites | site | 14 | 13
Reserve | $ | $ 12.6 | $ 13.9
NSTAR Electric |  |
Site Contingency [Line Items] |  |
Number of Sites | site | 12 | 10
Reserve | $ | $ 5.4 | $ 3.4
PSNH |  |
Site Contingency [Line Items] |  |
Number of Sites | site | 8 | 8
Reserve | $ | $ 7.7 | $ 6.1

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