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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 3 - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020
Derivatives, Net [Rollforward] |  |  |  |
Fair Value as of Beginning of Period | $ (279.7) | $ (322.0) | $ (293.1) | $ (329.2)
Net Realized/Unrealized Gains/(Losses) Included in Regulatory Assets | 0.7 | (4.7) | (10.9) | (22.3)
Settlements | 13.7 | 14.9 | 38.7 | 39.7
Fair Value as of End of Period | $ (265.3) | $ (311.8) | $ (265.3) | $ (311.8)

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- Definition Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19207-110258 + Details
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