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Investments in Marketable Securities - Cost, Gross Unrealized Gains and Losses and Fair Value (Details) - USD ($)
$ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2023
Dec. 31, 2022
Marketable Securities [Abstract]    
Cost $ 44.2 $ 394.4
Unrealized gains 2.6 59.9
Unrealized losses (1.8) (6.4)
Fair value $ 45.0 $ 447.9