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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

Debt Securities, Trading, Accumulated Gross Unrealized Gain Before Tax

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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).

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