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MARKETABLE SECURITIES - Narrative (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020 | Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Available-for-sale equity securities | $ 39.2 |  | $ 39.2 |  | $ 40.9
Unrealized gain (loss) recorded in other income | (0.5) | $ 1.6 | 2.5 | $ (1.0) |
CYAPC and YAEC |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Available-for-sale equity securities | 214.9 |  | 214.9 |  | 205.1
Marketable securities held in nuclear decommissioning trust | $ 195.2 |  | $ 195.2 |  | $ 192.5

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- Definition Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI), classified as current. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 45 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=123594786&loc=SL75136599-209740 + Details
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