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REGULATORY ACCOUNTING - Narrative (Details) - USD ($) $ in Millions | May 01, 2023 | Sep. 30, 2023 | Dec. 31, 2022
--- | --- | --- | ---
Regulatory Assets [Line Items] |  |  |
Amount of regulatory costs not yet approved |  | $ 251.4 | $ 210.8
Regulatory costs |  | 85.6 | 64.0
Regulatory assets |  | 5,856.6 | 5,578.3
Operation, maintenance and vegetation expenses |  |  |
Regulatory Assets [Line Items] |  |  |
Purchase price of regulatory assets | $ 23.3 |  |
Regulatory assets | $ 16.9 |  |
COVID-19 |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory costs |  | 29.8 | 29.8
CL&P |  |  |
Regulatory Assets [Line Items] |  |  |
Amount of regulatory costs not yet approved |  | 159.8 | 135.9
Regulatory costs |  | 65.6 | 52.8
Regulatory assets |  | 1,982.6 | 1,907.8
CL&P | COVID-19 |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory costs |  | 11.8 | 11.8
NSTAR Electric |  |  |
Regulatory Assets [Line Items] |  |  |
Amount of regulatory costs not yet approved |  | 26.7 | 19.8
Regulatory costs |  | 9.6 | 3.5
Regulatory assets |  | 1,828.6 | 1,714.4
NSTAR Electric | COVID-19 |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory costs |  | 2.2 | 2.2
PSNH |  |  |
Regulatory Assets [Line Items] |  |  |
Amount of regulatory costs not yet approved |  | 1.2 | 1.0
Regulatory assets |  | $ 916.5 | $ 696.2

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