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REGULATORY ACCOUNTING - Narrative (Details) - USD ($) $ in Millions | Mar. 31, 2022 | Dec. 31, 2021
--- | --- | ---
Regulatory Assets [Line Items] |  |
Amount of regulatory costs not yet approved | $ 262.5 | $ 252.5
COVID-19 |  |
Regulatory Assets [Line Items] |  |
Net incremental deferred costs | 35.0 | 33.0
CL&P |  |
Regulatory Assets [Line Items] |  |
Amount of regulatory costs not yet approved | 113.4 | 114.9
CL&P | COVID-19 |  |
Regulatory Assets [Line Items] |  |
Net incremental deferred costs | 15.3 | 18.0
NSTAR Electric |  |
Regulatory Assets [Line Items] |  |
Amount of regulatory costs not yet approved | 84.2 | 85.0
NSTAR Electric | COVID-19 |  |
Regulatory Assets [Line Items] |  |
Net incremental deferred costs | 6.1 | 6.1
PSNH |  |
Regulatory Assets [Line Items] |  |
Amount of regulatory costs not yet approved | $ 3.4 | $ 3.4

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