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Investments in Marketable Securities - Cost, Gross Unrealized Gains and Losses and Fair Value (Details) - USD ($)
$ in Millions
6 Months Ended 12 Months Ended
Jun. 30, 2024
Dec. 31, 2023
Investments, Debt and Equity Securities [Abstract]    
Cost $ 44.0 $ 35.3
Unrealized gains 4.2 2.6
Unrealized losses (0.2) (0.0)
Fair value $ 48.0 $ 37.9