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Investments in Marketable Securities - Debt Securities, Available-for-Sale (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Available-for-Sale
Cost	$ 99.9	$ 405.4
Unrealized gains	0.0	0.0
Unrealized losses	(0.0)	(0.1)
Fair value	99.9	405.3
Trading
Cost	19.7	17.9
Unrealized gains	0.0	0.0
Unrealized losses	(0.3)	(0.1)
Fair value	$ 19.4	$ 17.8

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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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