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DERIVATIVE INSTRUMENTS - Changes in the Level 3 Category of Derivative Assets Measured at Fair Value on a Recurring Basis (Details) - CL&P - Level 2 - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
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Sep. 30, 2023 | Sep. 30, 2023
Derivatives, Net [Rollforward] |  |
Fair Value as of Beginning of Period | $ (152.1) | $ (181.8)
Net Realized/Unrealized Losses Included in Regulatory Assets | (1.5) | (1.6)
Settlements | 17.0 | 46.8
Fair Value as of End of Period | $ (136.6) | $ (136.6)

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