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Investments in Marketable Securities - Debt Securities, Available-for-Sale (Details) - USD ($) $ in Millions | Dec. 31, 2023 | Dec. 31, 2022
--- | --- | ---
Available-for-Sale |  |
Cost | $ 405.4 | $ 252.3
Unrealized gains | 0.0 | 0.0
Unrealized losses | (0.1) | (1.3)
Fair value | 405.3 | 251.0
Trading |  |
Cost | 17.9 | 19.7
Unrealized gains | 0.0 | 0.0
Unrealized losses | (0.1) | (1.7)
Fair value | $ 17.8 | $ 18.0

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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-2 + Details
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