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Investments in Marketable Securities - Debt Securities, Available-for-Sale (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Investments, Debt and Equity Securities [Abstract]
Debt securities, available-for-sale, maturity, year one	$ 228.3
Debt securities, available-for-sale, maturity, year two	171.9
Available-for-Sale
Cost	400.7	$ 252.3
Unrealized gains	0.1	0.0
Unrealized losses	(0.6)	(1.3)
Fair value	400.2	251.0
Trading
Cost	18.3	19.7
Unrealized gains	0.0	0.0
Unrealized losses	(1.2)	(1.7)
Fair value	$ 17.1	$ 18.0

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